From Earth to Mercury - the Messenger mission writes history

Moderators: florinlita, xenocratus

Site Admin

Posts: 808

Joined: Thu Jun 30, 2011 7:46 am

Post Thu Jul 21, 2011 9:11 am

From Earth to Mercury - the Messenger mission writes history

Recently, the international press agencies spread the news about the Mercury’s exploration arousing the interest of space subjects’ enthusiasts. As there were many sources for these news but all commented from a journalistic point of view, we reply with this article which wants to be a more complete analysis of the Messenger mission, in the specific style of SpaceAlliance.
We will try to realize a professional commentary and to come with an impartial opinion over the obstacles which may appear when you want to travel to the Solar System’s planets and over the solutions offered by engineers for making possible the Mercury’s exploration.

Launching for Mercury

The Messenger probe was launched on August 3rd 2004, at 06:15 UTC, by a Delta 2 rocket from the 17 B launch complex from Cape Canaveral. The Delta 2 rocket, built initially by McDonell Douglas, was transferred afterwards to Boeing Integrated Defense Systems and it is operated now by the ULA (United Launch Alliance).

For the Messenger’s launch, the 7925H configuration was used, the same that NASA used for other interplanetary missions: the Mars Opportunity Rover (July 8th 2003) and Dawn (September 27th 2007). Other versions (7425 or 7925) successfully served the American Agency in its goal of exploring the red planet: Mars Global Surveyor (1996), Mars Pathfinder (1996), Mars Climate Orbiter (1998), Mars Polar Lander (1999), Mars Odyssey (2001) and Mars Phoenix Lander (2007).

In this configuration, the rocket has an additional traction force coming from the 9 GEM-46 “boosters” attached to the first stage. The rocket, with a height of 36 meters, a diameter of 2.4 meters and a weight of 231 tons has three stages in total: an RS-27A engine developing 1054 kN in the first stage, an AJ-10-118K-ITIP in the second one developing 46.3 kN and a PAM-D/STAR-48B developing 66 kN in the final one.

We have to say that the Delta 2 rocket was extremely active not only in the scientific launches, but also in the US Air Force service, where it has been used for more than 20 years. In 2009 it was retired from service, after it placed on the orbit its last satellite, GPS 2R-21.

The maiden flight of the Delta 2 rocket took place on February 14th 1989 from Cape Canaveral 18A, when the loading was also a GPS Navstar 2A satellite (indicative GPS 14) and when it flew in a type 6925 configuration. In its long operation period, the rocket had 148 successful launches, of which 46 dedicated to the GPS system and 55 to the Iridium system. As it had just one total fail (GPS 2R-1 in January 1997) and a partial one (Koreasat-1 in August 1995) in its whole career, we could say that it had an impressive reliability for this field.
A total of 108 flights were performed from the Cape Canaveral Complex, with an estimated cost of almost 36 million dollars per launch. Although it was disabled by the American Army, the rocket will continue to fly also in 2011 (in missions for NASA or for commercial operators), with a total of 3 flights being programmed and 5 satellites included in the program: Aquarius (SAC-D) (launching date: 9th of June), GRAIL A/B (launching date: 8th of September) and NPP/Lightsail 1 (launching date: 25th of October).

Mission’s motivation

Messenger, or “MErcury Surface, Space Environment, GEochemistry and Ranging' is the second probe to arrive near Mercury (after Mariner 10 in 1975) and the first one to orbit the planet.
Mariner 10, launched by NASA on November 3rd, 1973 was the first probe to use gravity-assisted technique. This maneuver consists of using the forces of attraction which are exerted on any body that passes through the vicinity of a planet, specially applied in interplanetary missions (in which there are huge distances to be traveled and the possibility of a direct orbital injection is limited by the current launchers’ capability).
Thus, by a careful selection of the orbit, a natural acceleration of the orbital velocity of a satellite can be achieved, and then a simple orbit correction angle can change the trajectory direction.

Using this technique Mariner 10 managed to pass three times around Mercury: first time on March 29th 1974 (the smallest distance recorded being 703 km), then on September 21st 1974 (at a distance of 480.69 km) and finally on March 16th 1975 (at a distance of 327 km). This was enough to make more than 2,800 photos and to map about 45% of the planet’s surface.
Since then, however, 30 years had to pass before a new mission is ready to explore Mercury.

In summary, with the help of the Messenger probe, scientists have proposed:
• To determine accurately the surface composition of Mercury;
• To measure and map out the local magnetic field;
• To investigate the geological history;
• To try to understand the causes of radar reflection that occurs near the poles and the possible presence of water in solid state, despite the proximity to the Sun;
• To measure Mercury's libration phenomenon and to determine indirectly the presence of a liquid core inside the planet
• To study the atmosphere and look for the presence of water or other volatile substances near the poles that would have a direct influence in the establishment of this rarefied environment.

Mercury

Mercury is the smallest planet of the Solar System with a radius of only 2439.7 km, a value 0.38 times smaller than the medium radius of the Earth.
Despite the size, Mercury is the second planet in the solar system after Earth, considering the density (which has a value of 5427g/cm3 compared with 5515g/cm3 of the Earth).
Mercury aroused the interest of the terrestrial observers since old times, but its unique position in space was the main break in gathering detailed information. All despite that, observed from Earth, Mercury appears at luminous magnitudes between 2.3 and 5.7.
Why is that so little known about Mercury?

First of all because of its orbit – being the closest planet to the Sun, Mercury appears most of the time in the direction of the Sun (the biggest angular separation from a terrestrial observer does not exceed 28.3 degrees), therefore the occasions when the planet can be visualized in optimal conditions by the terrestrial observers are quite rare.
Mercury, despite the hypothesis from antiquity doesn’t have natural satellites, has an elliptical orbit inclined with 7 degrees from the Earths, with a apogee at 69.816.900 km (approximately 0.46 astronomical units) and perigee at 46.001.200 km (or 0.30 AU), having an eccentricity of 0.21 and an orbital period of 87.969 days.
Mercury has its own rotational axis almost perpendicular (89.97 degrees) from the orbital plan, and it spins with very low speed, managing 3 complete rotations in 2 orbital periods and that’s why a solar day there lasts for approximately 176 terrestrial days, while a sidereal day lasts 58.7 terrestrial days.

It is believed that the core of the planet represents 42% of its volume, and is mostly made of iron. Around the core is a mantle of silicate with the width of 500-700km, over it a crust of 100-300 km, the surface presenting craters and large crevasse which give rather a lunar aspect.
The medium temperature on the surface is approximately 442.5 Kelvin degrees, but varies in the gamma 100K-700K. For example the side exposed to solar action presents a temperature between 700K at perigee (when the planet is closest to the Sun) and 550K at apogee (max distance).
Meanwhile the temperature on the other side being in shadow reaches 110K.

The craters from the polar region have temperatures below 100K, which favors the existence of some ice caps. Scientists estimate that these polar regions might host approximately 10e14-10e15 kg of solid water.
Mercury has an extremely rarefied and instable atmosphere loosing continuously atoms and replacing them with others, either from space (most of them brought by the solar wind) or the radioactive activity of the crust. Technically the atmosphere contains 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, 0.5% potassium, but also presents small percents of argon, nitrogen, CO2, xenon, krypton, neon.
Mercury has a magnetic field of low intensity (approximately 1.1% of the one recorded on Earth) reaching values of 300nT at equator but strong enough to create a magnetosphere around the planet.
The interaction between this magnetosphere and the solar wind creates magnetic whirlwinds in the nearby space.

As we said at the beginning of the article, exploring this planet using our satellite technique is only in the infant stage, with only 45% of the planet’s surface being mapped at this moment, and there are still many questions the scientists would like to get an answer after the Messenger will end its investigating activity near Mercury.

Scientific instruments

• Mercury Atmospheric and Surface Composition Spectrometer (MASCS), built by Laboratory for Atmospheric and Space Physics, University of Colorado, weights 3.1 kg and consumes 6.7 W.
The instrument combines two elements - UVVS (Ultraviolet and Visible Spectrometer) and VIRS (Visible and Infrared Spectrograph) and will return about 2.7 GB of data in its 12 months of operation.
UVVS will help to understand the processes that have generated and maintained the atmosphere of Mercury, as well as the causal link between the surface and atmospheric composition, the dynamics of volatile materials on and near the planet, and the nature of the substances from poles region (it has been observed that those regions reflect radar waves). However, the instrument is not limited only to determine the composition and structure of the exosphere. It is also able to make observations at ground level.
Its visual field varies between 1 x 0.5 degrees (in the atmosphere) and 0.023 x 0.023 degrees at ground level and the instrument is helped by three multiplier tubes, operating in the ultraviolet range (115-190 nm), medium ultraviolet (160-320 nm) and visible spectrum (250-600 nm).

VIRS will measure the light emissions, visible or near visible infrared range (0.3-1.45 µm) in search of certain materials as Fe and Ti.
The visual field is 0.023 x 0.023 degrees and the instrument will provide a ground resolution between 100 m and 7.5 km and a spectral resolution of 4nm. The two spectra will be observed by two different detectors - the infrared detector in line with 256 pixels built with InGaAs technology and the visible light detector in line with 512 pixels with built-in Si technology.

• X-ray Spectrometer (XRS)
XRS was built by the Applied Physics Laboratory of “Johns Hopkins” University after a similar model that flew on NEAR-Shoemaker satellite and it weights 3.4 kg and consumes 6.9 W. It is expected to return about 1.5 GB.
It consists of 3 components – 3 MXU (Mercury X-ray unit) detectors, SAX (Solar Assembly for X rays) and MEX (Main Electronics for X-rays). The instrument has a field of view of 12 degrees and can measure emissions in the range 1 - 10 keV.
With the help of its three MXU detectors, scientists hope to measure, at resolutions of 20 km, X-ray emission of the elements accumulated in the planets' crust (Mg, Al, S, Ca, Ti, Fe) and, thus, to determine the chemical composition and the geological past of Mercury.
In parallel, the SAX sensor is able to measure the solar flux variation that is affecting the planet.

• The Mercury Dual Imaging System (MDIS)
MDIS is an instrument that weights 8 kg and has a consumption of 7.6 W. It was built by the same JHU / APL. It consists of 2 WAC cameras (Wide-Angle Camera) and one NAC (Narrow-Angle Camera), both having CCD sensors and both being placed on mobile platforms that facilitates the image capture, without the need of rotating the entire satellite to a certain target.
WAC has a field of view of 10.5 x 10.5 degrees and can observe Mercury through 11 different colored and monochrome filters at wavelengths between 395 and 1040 nm whose comparison will be able to distinguish different types of rocks that make up the planet’s surface. NAC has a visual field of 1.5 x 1.5 degrees with a ground resolution of up to 18 m and captures black and white images.

• Gamma-Ray and Neutron Spectrometer (GRNS)
GRNS is also an instrument composed of two elements – GRS (Gamma Ray Spectrometer) and NS (Neutron Spectrometer). GRS is built by JHU / APL, Patriot Engineering, Lawrence Berkeley National Laboratory and Lawrence Livermore National Laboratory. It weights 9.2 kg, consumes 16.5 W and is expected to retrieve up to 3.9 GB of data.
GRS measures gamma radiation emissions sent the planet’s surface under the influence of cosmic rays, or the natural radioactive emission of atoms such as K, Th, U – seeking to conduct a geological analysis based on the individual signatures of each chemical element that is present there (H, Mg , Si, O, Fe, Ti, Na, Ca).
GRS has a sensor based on a cryogenically cooled Ge crystal up to -183 degrees Celsius.
NS is an instrument built by Johns Hopkins University, Applied Physics Laboratory, Patriot Engineering and Los Alamos National Laboratory. It weights 3.9 kg and consumes about 6 W. It measures the energy of the neutrons that have been emitted by the planet’s surface, under the influence of cosmic rays. The measurements are made especially for determining the hydrogen atoms’ concentration that signals the potential presence of water (which is based on the physical principle that the neutron energy decreases sharply in collision with light atoms such as hydrogen atoms).

• Mercury Laser Altimeter (MLA)
MLA, built by NASA Goddard Space Flight Center weights 7.4 kg, uses 16.4 W and was inspired by a similar experiment that flew on the Mars Global Surveyor platform. In the 12 months in which it will operate, approximately 1.5 GB of data will be transmitted, consisting of data taken for topological measurements of the northern hemisphere which will help at creating a geological history, at accurately determining the shape, the rotation axis and the libration of Mercury and at measuring the size and current condition of the nucleus.
The operating principle is simple: an infrared laser emitter transmits 8 pulses with a wavelength of 1064 nm, 20 mJ of energy and a divergence of 50 mrad in every second. The reflected waves are caught by 4 special receivers which will measure the amount of time passed since it was emitted (with an accuracy of 3.3 ns, meaning a precision of 0.5 m).
On the ground, the pulse has a scattering area of 10-50m, with a 100-300m distance between the spots (in the direction of movement). MLA can work in a nominal state at up to 1500 km (from where it assures a 30 cm resolution).

• Energetic Particle and Plasma Spectrometer (EPPS)
The EPPS is built by Johns Hopkins University Applied Physics Laboratory and University of Michigan, weights 3.1 kg and uses 7.8 W, being mounted on the upper side of the satellite. The instrument, which was supposed to return 4.4 GB of data in its operational year around the planet, is made up of 2 components: FIPS (Fast Imaging Plasma Spectrometer) and EPS (Energetic Particle Spectrometer).
FIPS detects H, 3He, 4He, O, Ne, Na, K, S, Ar and Fe ions, measuring the ratio between the energy and the electrical charge of the particles (0-10 keV/q), and also their speed an spatial distribution, estimated indirectly through the time in which the particles get to the main detector (50-500 ns).
EPS will measure the energetic spectrum, the chemical composition and the spatial distribution of ions with more than 10 keV – 5 MeV of energy (H, He, CNO, Fe), and also of the electrons with 20 to 700 keV of energy.

• Radio Science (RS) experiments
RS uses the onboard communication system (two X band transponders, MGA – medium gain antenna, LGA – low gain antenna and the 2 so-called “phased arrays”) for a very precise measurement of the speed and position of the satellite (with an accuracy of 0.1 mm/s). The variation of the speed is then used for determining gravitational effects from which the mass distribution inside the planet can later be determined. Similarly, from a precise determination of Messenger's position, Mercury's libration can be determined, and from radio observations, the exact shape of the planet can be estimated.

The Satellite

Messenger, the seventh of the Discovery program which started in 1992 as an idea of doing cheap exploration of the Solar System, had a cost of 446 million dollars (including the design, construction of the satellite, the launch, operation and finally the scientific investigation segment together with modeling the collected data).

The satellite has a parallelepiped form with the dimensions 1.27 x 1.42 x 1.85 m and weights 1093 kg from which 607 kg represents the reserve of helium and fuel.
The structure is built from a composite material – GrCE (graphite-cyanate-ester) - which lowers significantly the weight and brings a better rigidity and thermal constant.
Four vertical panels represent the central cell on which the internal components are fixed.

The thermal control system is vital for the success of the mission, considering that the satellite will approach the Sun at a minimum distance of 46 million km (0.3 AU), where it will be seen 11 times bigger than on Earth.
In these conditions the thermal radiation received from the Sun would bring temperatures of up to 370 degrees Celsius, which can be fatal for the electronics on board.

To avoid such a scenario, the engineers came up with the solution of equipping the satellite with an exterior protection panel whose role will be to prevent the sunlight to damage the satellite itself.
This panel has a semi-cylindrical form with the length of 2.54 m and a diameter of 1.82 m, is built of several successive layers of Kapton (a plastic material) and Nextel (a ceramic material) on a rigid frame made from titanium. The whole system weights only 20 kg, despite its size.

In this way, the electronics shaded by the panel, remain at an ambient temperature of 20 degrees Celsius. In theory, under the protection of the panel, the satellite can rotate but only between some certain limits – the so called SKI (Sun keep-in-zone) - dictated by the constructive characteristics i.e. the protection panel being closed only on one side.

This optimization isn’t sufficient because in every orbit, for approximately 25 minutes the satellites reaches a low distance from the planet (200 km), time when the satellite passes a phase of increasing temperature, caused by the solar rays reflected by the surface of Mercury. This problem requires another solution to compensate the effects of coming close to Mercury.

The thermal equilibrium is maintained further inside the structure of the satellite itself with the help of two systems – on one side a passive system (built from overlapped layers of isolating thermal materials ) and on the other side an active system which uses radiators and heat radiant diodes.

These can be activated progressively (depending on the amount of heat that must be dissipated in order to maintain constant the temperature on board).
On the other hand, when the satellite faces an eclipse (on the shaded side of Mercury) the temperature can reach even -135 degrees Celsius, conditions that require the activation of the thermal system.

However the satellite must continuously monitor the flight position and avoid the dangerous orientation called HPKO (planet “hot pole” keep-out zone) where the inferior panel of the satellite – on which the battery is attached – would be exposed to the radiation reflected by Mercury.

The 2 solar panels, which grant a supplementary shade, have a secondary role in maintaining the thermal control. They are the most important part of the electric system of the satellite, being the source of energy which ensures the correct function of all sub-systems.
The company responsible with the design is Northrop Grumman Space Technology but they used components from other producers as well, like we will show further.

The engineers used a particularly adapted solution, specific to this mission, attaching 2 mobile panels (they can be rotated on two directions with the help of some mechanisms called SADA-solar array drive assemblies) - each panel having the dimensions of 1.54 x 1.75 m.
Their structure is made of aluminum on which some special panels made of composite material RS-3/K13C2U built by AASC (Applied Aerospace Structures Corporation) were adapted. They are light but at the same time they have a great resistance to thermal stress (a necessity for Messenger considering the fast and significant temperature variations it will face).

Built in the GaAs technology with an efficiency of minimum 28% (reaching 30%) they can produce between 385 and 485 W during the cruise phase towards Mercury and 640 W in normal conditions of the orbit around the planet.
In theory, due to their dimension, the 2 panels could produce up to 2000 W, but this would be a useless surplus of electric energy, the nominal 640 W being more than enough for the onboard consume.

Again the designers came with a pretty ingenious solution mounting 2/3 of the panel’s surface with reflective mirrors. Thus each row of solar cells is surrounded by two rows of reflective cells concluding to an active surface (which generates electrical energy) of only 28%.
Each solar panel contains a total of 18 rows of solar cells (each 3 by 4 cm) and the rows are electrical connected via the Solar Array Junction Box (SAJB) equipment. The manufacturer of these cells is the EMCORE Corporation.
The rest of 72% of reflective surface is not less important because it should prevent the overheating. Due to it the side exposed to the action of solar activity will reflect approximately 60% of the incident radiation and will limit the temperature to 150 degrees Celsius.
This reflective surface is composed of small cells called OSR (optical solar reflectors) manufactured by the Pilkington company. They are in fact some mini reflective mirrors which incorporate a special substrate of glass called CMX.
The energy is stored (for the eclipse periods) in a Ni-H battery having 23 Ah and 11 distinctive elements. These, built in CPV (Common Pressure Vessel) technology, are manufactured by EaglePicher Space Energy Production Division and are mounted in a three rooms Aluminum structure.
The structure is specially positioned to avoid the overheating (shadowed by the structure of the satellite) and is thermally connected at a radiator with an area of 0.13 m2 which maintains the battery between -5 and 0 degrees Celsius. Also the difference between the battery’s cells should not be more than 3 degrees Celsius in order to avoid a supplementary thermal stress.
Another part of the electric system is the PSE (Power System Electronics) and the PDU (Power Distribution Unit). The electronic of PSE is designed to operate at temperatures between -34 and +65 degrees Celsius and can dissipate between 15 and 40 W. This extra heat is transported through dissipative lines (in fact Aluminum made cylinders filled with Ammonia) to the radiators and later to the external space.
The communication system should ensure the permanent connection between the satellite and the ground stations on Earth – independently of the orbital position and the attitude of the satellite.
The normal communication is established via the DSN (deep Space Network) in a bidirectional manner: the telemetry is received at rates of 9.9 bps-109 kbps and the telecommands are transmitted at rates of 7.8-500 bps, both in the X band.
For a facile communication onboard the spacecraft have been mounted 2 HGA (High Gain Antenna), 2 MGA (Medium Gain Antenna) and 4 LGA (Low Gain Antenna).
The two HGA are “phased array antenna” and can acquire the transmission of a big volume of data (as the transmission of scientific data collected by the instruments).
The two MGA - “gain fanbeam antenna” – are the standard elements of communication, they are fixed but have an access angle of 45 degrees ensuring the Earth visibility under the nominal conditions.
The LGA’s are specially positioned onboard the spacecraft and they ensure the transmission of the commands from the control centre, the essential housekeeping data coming from the satellite and the emergency communication in the case the spacecraft enters the safe-mode.

The onboard computer of Messenger called IEM (Integrated Electronics Module) has a double configuration (main and backup) for a safe operation. When a failure occurs on the main module (i.e. a SW corruption by the solar flux) the backup module takes over autonomously as soon as possible in order to isolate the problem and to ensure the functionality of the spacecraft.
Each of the IEM incorporates 2 processors RAD6000 one running at 25 MHz and the other at 10 MHz. The first one runs the basic routines - i.e. command and data handling or attitude and orbit control both sharing the physical resources of the processor while the second one only takes care of the FDRI (Failure Detection Isolation and Recovery) routines.
The 2 IEMs incorporate two 1GB solid state hard-disks (one main and one backup) for storing the data when the satellite is not in direct communication with the Earth.
The IEM has also a data bus which establishes the connection between the processor and the instruments, and some power convertors for the local use.

We will provide in the next paragraph some explanations for the 3 routine processes ran by the IEM processors.
The FDIR mechanism is an essential one for every satellite – which is by definition a complex machine that should operate most of the time autonomously, without the possibility of human intervention. In these conditions (specially in ‘deep space’ missions where communications are even more difficult), but also taking into account the hard operating environment which can affect the proper functioning of the satellite, a robust system is therefore required. FDIR accomplishes the monitoring and protection of the satellite’s systems by turning some components on/off or by interchanging primary/secondary modules for each subsystem (in case of a malfunction).
The ‘data handling’ mechanism is responsible for collecting, transferring and storing data and for receiving and distributing commands to on-board devices.
The commands – a vital element for each satellite – are executed either in real-time or through “time-tagged” commands which are stored for later use (in this way being kept the control for the periods without visibility from the command center). Judging by the execution mode, there are normal commands (which are executed exclusively by using the software) and high-priority commands executed directly (through dedicated electrical interfaces) – the equivalent of usual PCs ‘reset’ command.
The ‘attitude and orbit control’ routine is a component of the software system with the same name (AOCS) and incorporates all the mathematical algorithms used for orbital control.
Maintaining a correct flight position is a critical aspect for Messenger - any deviation from the nominal position (for example, if the solar shield isn’t pointed towards the Sun for protecting the spacecraft) can induce a major fault in the electronic devices. As an example, it should be said that the so-called SKI zone (Sun keep-in, the zone in which the satellite can spin without internal thermal changes) is between ± 10degrees for the z axis and ± 12 degrees for the x axis. One of the disruptive factors to which the algorithms must pay attention is the solar radiation and, as we will recall later, Messenger takes into account a model of the radiation, not only to compensate its effects, but also to use it actively on the road to Mercury. The AOCS system that assures a precise three axes stabilization is composed of a sensor part and an actuator part.
Messenger is equipped with 2 stellar cameras (the popular model A-STR built by Galileo Avionica) which measures at a 10 Hz frequency, the spatial inertial orientation of the satellite in respect to the stars observed in the visual field, 2 solar sensors which measure the Sun’s direction and 1 SIRU (Space Inertial Reference Unit) – which is actually a classic IMU (inertial measurement unit). The SIRU is built by the Northrop-Gumman and includes 4 gyroscopes and 4 accelerometers (Honeywell QA3000 type) and is able to measure speed variations and accelerations very precisely at a 100 Hz frequency. The actual satellite positioning control is provided in two ways: small corrections are performed with the help of a 4 flywheel, Teldix RSI 7-75/601 system which provides 0.0075 Nm and can store up to 7.5 Nms, and important corrections (as well as the momentum RWL discharge) are made by on-board engines.
Messenger has a primary engine named LVA (large velocity adjust) used for an orbital transfer, generating 667 N, then a set of 4 engines providing 22 N each, used for orbital corrections and 12 engines providing 4.4 N for in-flight stabilization. The primary engine is bipropellant, using a combination of hydrazine as fuel and 'nitrogen tetroxide as oxidant. The two are kept in separate tanks while a helium tank helps maintaining a constant pressure and proper functioning of the system. The other engines are monopropellant, using only hydrazine.
The satellite is equipped with 3 main fuel tanks (2 with N2H4 and 1 with N2O4), a secondary tank (N2H4) and an auxiliary tank (He). The connection between the fuel tanks and the engines is made by a complex pipe system and its flow can be controlled by 9 valves. Also, the thermal control of the pipes (an essential element for preventing hydrazine, which has a low freezing point, from freezing) is performed by a number of dedicated thermistors.

Going back to the algorithms which control the satellite’s position, it must be said that the engineers have grouped them into three big classes/flight scenarios, each having its own characteristics and capabilities, with the intention of assuring the best protection to the equipment and an autonomous functioning with the least human intervention.
The 3 categories are called: OP (operational mode), SH (safe hold mode) and EA (Earth acquisition mode). In turn, the 3 categories also have 2 distinct sub-modes: normal and autonomous.
The shift is made either automatic (the decision belongs to the on-board software) or commanded (the decision belongs to the engineers).
The AOCS system combines the measurements taken from the sensors and its orbital predictions for the planets (it incorporates a mathematical model dedicated for this kind of activity) and then uses a Kalman filter for estimates and for a propagation of the satellite’s position.
We will perform a detailed analysis of the system's performance as they resulted on the road to Mercury, in the next paragraph dedicated to Messengers' orbit and the maneuvers performed on its long trip.

Image
credit JHU-APL

The road to Mercury

As we said in the first article of the series, the Messenger spacecraft was launched on the 3rd August 2004 at 06:15 UTC with the help of a Delta 2 rocket, flying the 7925H configuration.
Although adapted to interplanetary missions, the rocket couldn’t provide a high enough acceleration so that a satellite like Messenger could not reach an appropriate speed for the huge distances that need to be completed for a direct placement into an orbit around Mercury.
The engineers used again a classical technique, mostly utilized for interplanetary missions – assisted gravity technique – which uses the attraction force applied by all the planets in the Solar System to every object which passes near them.
Therefore, by right selecting the orbit, a natural acceleration of the orbital speed of a satellite can be obtained and later a simple correction of the orbit’s angle (with the help of the onboard engines) can change the trajectory on the desired direction.

Image

Messenger’s flight to Mercury was a very complicated one, requiring 6.6 years just as the next table illustrates.

Image

Immediately after the launch and the deploying of the solar panels, the onboard engines were activated to stabilize the satellite and to reduce the rotational movement induced after the separation of the carrying rockets. A series of TCM (trajectory correction maneuvers) followed right after, which prepared the first flyby of Earth (August 2005 at an altitude of 2347 km).
Six months later, December 2005, the first major propelled maneuver (DSM or ‘deep space maneuver’) took place – which was also the biggest; it consumed approximately 106 kg of fuel – followed by CMD type maneuver (commanded momentum dump) which reduced again the rotation speed of the satellite.

Several other maneuvers brought Messenger in position to fly over the planet Venus - first time in October, 2006, at an altitude of 2987 km, then in June, 2007, at an altitude of 338 km. In both cases, the scientists had the opportunity to obtain additional observations regarding Venus; the registered data will be used by the international community along with the data provided by the ESA’s Venus Express probe.

Soon after, a second DSM maneuver (being also the second largest, consuming 68 kg of fuel) moved the Messenger spacecraft from Venus, in order to approach Mercury.

The preparations for the first flyby of Mercury began in December 2007 and in a short time (January 2008) the probe came at a distance of 201 km from the planet.
DSM’s third maneuver in March 2008, the second flyby of Mercury (October 2008 at an altitude of 199 km), the fourth DSM maneuver (December 2008) and finally the third flyby of Mercury (September 2009 to an altitude of 231 km) followed after.
DSM’s last maneuver, before enrolling in the final trajectory around the planet, took place in November 2009, 45 kg of fuel being consumed, leaving onboard, thus, a reserve of only 41% compared to the amount the spacecraft had when it left the Earth.

The critical point was, though, the final maneuver, called “Mercury orbit insertion maneuver”, or MOIM. Any mistake could have been, most certainly, the end of the mission. NASA engineers had provided, of course, alternatives, but if they could not succeed orbital injection (as happened before, several times in the case of the JAXA’s missions), considering the amount of the remaining fuel and the life span of only a year it would make hard to believe that Messenger would have been effectively used afterwards to collect scientific data.
Also, considering the effort spent to bring a probe near Mercury, it would have probably been a huge loss for NASA and the scientific community, in general.
But things went almost perfect, proving once again the professionalism of the people that stand behind these operations.
Our table clearly illustrates the events sequence and finally the first estimates of deviation from the desired orbit (which are very small, as it can be seen) - a deviation of 0.02 degrees with respect to the orbital plane inclination and a few kilometers in terms of altitude perigee and apogee.
The actual maneuver lasted only 15 minutes (it was executed on March 18, 2011 at 00:45), but the preparations have begun about 48 hours before.
The command center configured the satellite, taking into account all the technical constraints; NASA allocated their DSN (Deep Space Network) for this important event. In total, four DSN stations have served the command and the telemetry of Messenger.
After receiving the first data and the confirmation of successful injection in a stable orbit around Mercury, a period of intense checking of equipment followed. On April 4th, 2011, using the scientific instrumentation that has been prepared, an observation campaign began, which will last for one year.

How does Messenger operate?
Messenger evolves on a high elliptical orbit around Mercury (15193 km x 200 km x 82.5 degrees inclination, and with an orbital period of about 12 hours).

Because of perturbations (especially due to solar gravity) this orbit isn't stable without corrections and in a short while would appear changes of the altitude at perigee (the satellite would slowly drift away from the planet, making observations difficult), changes of the orbital inclination and also of the perigee projection spot on the surface of Mercury (a spot which is normally situated at the 60 degree latitude North, close to the Caloris crater).
For correcting this problem, engineers operating Messenger came up with two sets of orbital maneuvers: the first type is intending to lower the altitude in the perigee point to its nominal value of 200 km and is performed periodically, once every 88 days (the orbital period of Mercury), and the second type is destined to increase the orbital period to 12 hours (the nominal period time) and comes as a complement of the first type of orbital maneuvers (normally, at every drop in altitude, the orbital period shrinks with about 15 minutes out of the 12 hours).

Our table illustrates once again the 5 maneuvers which will be executed in June, July, September, October and December.

As we can see, Messenger's flight inside the solar system was extremely complicate and our short summary, as well as the correct data illustration gives us a clear image of how hard it is for a probe to be sent from Earth to Mercury and why Messenger is the only satellite to have surpassed these constraints.
Beneath, we have made a number of tables for different missions (interplanetary or not) which can serve for comparisons. They illustrate very well the distanced traveled by each probe, the heliocentric speed, their position in the solar system etc. and Messenger's positioning between these probes.

Image

Image

Image

Image

First pictures

As we have remembered previously, after reaching a stable orbit around Mercury, Messenger has used the first days for calibrating the scientific instruments before starting effectively the observation phase- a phase which will last approximately 1 year.
In this short period since then, 6 weeks in total, the operation command and control centre started to collect the first science data. Couple of images have been offered already to the public and can be accessed on the official website of the mission.

The first image ever captured from an orbit around Mercury (excluding the ones taken during the previous short fly-bys of Mariner 10 and respectively Messenger) has been produced on 29th of March 2011. It has been taken using the WAC and MDIS instruments over the location -53.3 degrees East latitude and 13.0 degrees East longitude where is the Debussy crater (a crater which is 80 km in diameter).

Image

On the same day came also the first measurements from Mercury Laser Altimeter (MLA) which counted the differences of the relief during the first two orbits.
Image
Then, the first estimations of the Mercury’s magnetic field have been published on 30th of March 2011.
Image
The first NAC picture has been produced on 29th of March at -53 degrees latitude and 13 degrees longitude east.
Image


Future exploration of Mercury

In this final part of our series about the Messenger mission we will try to conclude and to see how the Mercury exploration will look like in the next decade.
Under the pressure exerted by the latest financial cuts, the aerospace field does not seem to move into the right direction thus for the moment a single satellite has been announced to succeed the Messenger. We speak about the European-Japanese satellite BepiColombo which is at the present under the construction and testing, with a launch most probably in July 2014 and with an arrival at the destination somewhere in 2020.

Without going back to the characteristics of a flight around Mercury we will try to compare the two missions by putting in the mirror the technical solutions found by the engineering teams in both cases.

First let allow us to give some information about BepiColombo.
Named after the Italian scientist Giuseppe Colombo, who dedicated most of his life for studying the Mercury, the BepiColombo mission will improve the Messenger exploration bringing into a direct collaboration, first of this kind, the two space agencies ESA and JAXA.
It will be in fact 2 satellites- Mercury Planetary Orbiter (MPO) built by ESA and Mercury Magnetosphere Orbiter (MMO) built by JAXA- grouped inside the Mercury Transfer Module (MTM) which is responsible for the transfer from Earth to Mercury and the placement of the two probes in a stable orbit.
This platform, which can be seen as a container carrying the two satellites (inactive during the transfer phase) will ensure the protection for the thermal variations and for the radiation exposure this flight implies, but the main role will be, as we said, to assure the propulsion necessary to reach Mercury.
Once it will arrive to the destination, MTM will be separated and it will leave the two satellites in two independent orbits.
We have to remind that initially it was taken into account a third module- Mercury Surface Element (MSE)- in fact a small lander who was supposed to operate for about a week at the surface of the planet, but this one has been later cancelled due to the financial constraints.
The BepiColombo mission has been considered for the first time in May 1993. In 2000 ESA has reaffirmed the interest for building a satellite to explore Mercury, and in February 2007 it has entered officially in the Cosmic Vision Program. The mission will cost Europe approximately 1 billion euro including here the construction of the satellite but also the launch and the ground operations.

The launch will take place from the Kourou space base in French Guyana aboard an Ariane 5 rocket. The official launching date is today 19th of July 2014 but a second window is available for August 2015 in the case when the first opportunity is missed.
The orbital transfer will use as for the Messenger, some flybys around the Earth, Venus and Mercury- as we will detail later, but also the solar pressure and the electrical propulsion.
ESA will be responsible for the interplanetary journey and the injection of the two satellites in orbits around Mercury, and later the two agencies will operate individually their spacecrafts.

The scientific results and the ground infrastructure will be used in common, the BepiColombo mission having at it disposal the Cebreros (35 meters in diameter) station and the Usuda (64 meters in diameter) station, but also the powerful American DSN network for emergency cases.

MPO is a 1.6 x 1.7 x 1.9 m satellite, weighting 1147 kg (dry) from which 80 kg is the weight of the instruments. In total 11 experiments are hold onboard:
• BELA – BepiColombo Laser Altimeter
• ISA – Italian Spring Accelerometer
• MERMAG – Mercury Magnetometer
• MERTIS-TIS – Mercury Thermal Infrared Spectrometer
• MGNS – Mercury Gamma ray and Neutron Spectrometer
• MIXS – Mercury Imaging X-ray Spectrometer
• MORE – Mercury Orbiter Radio science Experiment
• PHEBUS – Probing of Hermean Exosphere by Ultraviolet Spectroscopy
• SERENA – Search for Exosphere Refilling and Emitted Neutral Abundances (Neutral and ionised particle analyser)
• SIMBIO-SYS – Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (High resolution and stereo cameras, Visual and NIR spectrometer)
• SIXS – Solar Intensity X-ray Spectrometer

The three axes stabilized spacecraft will travel in a polar, 90 degrees inclination orbit with the perigee at 400 km and the apogee at 1508 km. In this configuration the satellite will have an orbital period of 2.3 hours.
The control will be done by using four 22 N thrusters and the satellite will be equipped with 3 star cameras.

The power generated by the solar panels is estimated between 935 and 1565 W, enough for the consumption of the instruments- which will go in the range 100-174 W.
In total is expected the reception of 1550Gb of data for a nominal operating period of about 1 year and at a rate of 50kbps (using the X and Ka communication).

MMO- the Japanese satellite- will measure 1.1 m height by 1.9 m in diameter, weighting 275 kg (dry) from which 45 kg of scientific instruments:
• MERMAG-M/MGF – Mercury Magnetometer
• MPPE – Mercury Plasma Particle Experiment
• PWI – Plasma Wave Instrument
• MSASI – Mercury Sodium Atmospheric Spectral Imager
• MDM – Mercury Dust Monitor

It will be a spinning stabilized spacecraft (spinner) at the rotational speed of 15 rpm, with a rotational axis position at 90 degrees compared with the Sun.
The flying orbit will be again polar with a 90 degrees inclination, the perigee at 400 km and the apogee at 11824 km. The orbital period will thus be 9.3 hours.
The AOCS system will handle 6 GN2 thrusters (each 4.25 kg) and 0.2 N in thrust, 2 solar sensors, a star camera and a de-spinning mechanism.
The generated electrical power will vary between 348 and 450 W based on the distance to the Sun but it will cover the 90 W necessary for the instruments.
The satellite will communicate with the ground in the X band at a rate of 5kbps which will produce a total of 160Gb of data per year.
As we previously mentioned, the orbital transfer of the BepiColombo, and the critical part of the mission, will benefit from the help of the gravity of Earth, Venus and Mercury similarly with the scenario used for Messenger.

Next table summarizes the mission’s timeline:

Image

In this configuration the orbital transfer has decreased in comparison with Messenger from 6.5 years to 6 years and this mainly due to the electrical propulsion.
SEP or the “solar electric propulsion” is a system inherited from the European mission Smart 1 – the one who has used it for the first time around the Moon.

MTM (Mercury Transfer Module) the one who will get it will have a launching mass of 4.2 tons – with a proportion of 32% fuel- respectively 816 kg of chemical conventional fuel and 500 kg of xenon to be used for the electrical thrusters.
The classical chemical propulsion system will be powered by 8 bi-propellant (N2O4-MMH) 10N each thrusters, while the electrical system comprises 4 thrusters.

In this conditions BepiColombo can reach a delta-v value of 5.025 km/s (for the electrical) and 1.065 km/s (for chemical), these values being anyway far superior to the ones of Messenger (2.2 km/s).

MTM will be able to generate through its 40m2 solar panels, an energy varying from 7kW (at the maximal distance to Sun of 1.13 AU) and 14kW (at the minimal distance of 0.62 AU), these numbers being decisive for the final performance of SEP (which has an estimated power demand of 10.6 kW).
For example, at the nominal distance of 1 AU, the entire produced electricity will be used (is enough) for operating a single thruster at the time, thus obtaining a thrust of 100-130 mN, but once the distance to the Sun decreases, due the extra energy coming from the solar panels the spacecraft could power two thrusters in parallel resulting a traction force of about 290 mN.

Despite the fact it is not coming with spectacular results in comparison with the chemical propulsion (in the sense of the resulting thrust) the electrical version has a number of advantages in the case of interplanetary trips. There, where is no need for short time planed maneuvers, but instead the things are carefully planed long time before the events happen, a constant propulsion, even at low values, can accelerate a satellite to appropriate speeds necessary for interplanetary journeys (compensated by the long action time).
The use of electrical propulsion onboard BepiColombo significantly decreases the risk of a single major maneuver scenario and offers the flexibility to modify the parameters in flight. By its intensive use one can save a lot of the classical fuel reserve.
In principle if the things go in the right direction at arrival to the Mercury, BepiColombo will have a speed low enough to ease the natural capture by the planet’s gravity. There will still be needed some extra maneuvers accomplished with the chemical propulsion but these will be small and anyway not having the same criticality as for Messenger- respectively a delta-v of 325 m/s for MMO and 620 m/s for MPO.

Thus it is very probable that most of the fuel stored onboard the two satellites will not be used and if the electronics will perform also well there is a good chance that the mission will get an extension of one year by the end of the nominal lifetime.
This can only be a good thing for the scientific community because it means the data collection will increase and there are better chances to understand our neighbor planet from the solar system.

If we have to speak about the technical constraints imposed to the two satellites they are the same as for Messenger. First the radiation coming form the Sun and which can reach up to 14 kW/m2 and the one coming from Mercury (with values up to 6 kW/m2).
In order to sustain these values, the Messenger’s designers have projected a special orbit- picking up a high elliptical trajectory such that the satellite comes close to the surface of the planet only for 5% of the orbital period and thus limiting the thermal stress. For the rest of the orbit is then enough time to get rid of the heat excess from the satellite’s structure.
However this has a big disadvantage because the periods of maximum resolution observations are limited.

For MPO, the European designers have pushed the limits, lowering the apogee of the orbit to 1508 km and forcing the satellite to spend a lot more time close to the planet. In this scenario the flight control will be essential and as 5 out of the 6 sides of the spacecraft will be illuminated while is an out of the eclipse condition, they have chosen to put a heat radiator on the sixth side. To have a good performance this side of the satellite should be kept away form the direct action of the Sun and the AOCS system will be responsible to solve autonomously this problem.

For MMO the Japanese designers have copied the American solution with solar panels playing a double role- capturing the energy but at the same time reflecting it- with 50% of the panel area used for solar cells and the other 50% for small mirror cells (OSR or optical solar reflectors). Also mounting a thermal scout for limiting the action of the solar rays is another solution inherited from Messenger.
For both satellites we are speaking about a complex thermal protective system- with an active and a passive component- which involves new composite materials, resistant to the temperature variations and to the degradation with time.
With BepiColombo launched we will be able to test these new materials in the conditions of a real flight and Europe hopes that these new technologies will confirm the financial investment and will soon spread to some other fields of the industry on the old continent, bringing back money.

Concluding our virtual trip to Mercury we only can say that 2020 is still far away in the future and since then we are restricted to the images and data coming from the Messenger probe.
As people who share a passion for science we hope that these two missions will not remain singular and despite the financial constraints the scientific curiosity will win the battle and they will be soon followed by some other explorations.
SpaceAlliance.ro

Posts: 1

Joined: Fri Nov 22, 2013 6:28 am

Post Tue Nov 26, 2013 10:42 am

Re: From Earth to Mercury - the Messenger mission writes his

spacesys wrote:Recently, the international press agencies spread the news about the Mercury’s exploration arousing the interest of space subjects’ enthusiasts. As there were many sources for these news but all commented from a journalistic point of view, we reply with this article which wants to be a more complete analysis of the Messenger mission, in the specific style of SpaceAlliance.
We will try to realize a professional commentary and to come with an impartial opinion over the obstacles which may appear when you want to travel to the Solar System’s planets and over the solutions offered by engineers for making possible the Mercury’s exploration.

Launching for Mercury

The Messenger probe was launched on August 3rd 2004, at 06:15 UTC, by a Delta 2 rocket from the 17 B launch complex from Cape Canaveral. The Delta 2 rocket, built initially by McDonell Douglas, was transferred afterwards to Boeing Integrated Defense Systems and it is operated now by the ULA (United Launch Alliance).

For the Messenger’s launch, the 7925H configuration was used, the same that NASA used for other interplanetary missions: the Mars Opportunity Rover (July 8th 2003) and Dawn (September 27th 2007). Other versions (7425 or 7925) successfully served the American Agency in its goal of exploring the red planet: Mars Global Surveyor (1996), Mars Pathfinder (1996), Mars Climate Orbiter (1998), Mars Polar Lander (1999), Mars Odyssey (2001) and Mars Phoenix Lander (2007).

In this configuration, the rocket has an additional traction force coming from the 9 GEM-46 “boosters” attached to the first stage. The rocket, with a height of 36 meters, a diameter of 2.4 meters and a weight of 231 tons has three stages in total: an RS-27A engine developing 1054 kN in the first stage, an AJ-10-118K-ITIP in the second one developing 46.3 kN and a PAM-D/STAR-48B developing 66 kN in the final one.

We have to say that the Delta 2 rocket was extremely active not only in the scientific launches, but also in the US Air Force service, where it has been used for more than 20 years. In 2009 it was retired from service, after it placed on the orbit its last satellite, GPS 2R-21.

The maiden flight of the Delta 2 rocket took place on February 14th 1989 from Cape Canaveral 18A, when the loading was also a GPS Navstar 2A satellite (indicative GPS 14) and when it flew in a type 6925 configuration. In its long operation period, the rocket had 148 successful launches, of which 46 dedicated to the GPS system and 55 to the Iridium system. As it had just one total fail (GPS 2R-1 in January 1997) and a partial one (Koreasat-1 in August 1995) in its whole career, we could say that it had an impressive reliability for this field.
A total of 108 flights were performed from the Cape Canaveral Complex, with an estimated cost of almost 36 million dollars per launch. Although it was disabled by the American Army, the rocket will continue to fly also in 2011 (in missions for NASA or for commercial operators), with a total of 3 flights being programmed and 5 satellites included in the program: Aquarius (SAC-D) (launching date: 9th of June), GRAIL A/B (launching date: 8th of September) and NPP/Lightsail 1 (launching date: 25th of October).

Mission’s motivation

Messenger, or “MErcury Surface, Space Environment, GEochemistry and Ranging' is the second probe to arrive near Mercury (after Mariner 10 in 1975) and the first one to orbit the planet.
Mariner 10, launched by NASA on November 3rd, 1973 was the first probe to use gravity-assisted technique. This maneuver consists of using the forces of attraction which are exerted on any body that passes through the vicinity of a planet, specially applied in interplanetary missions (in which there are huge distances to be traveled and the possibility of a direct orbital injection is limited by the current launchers’ capability).
Thus, by a careful selection of the orbit, a natural acceleration of the orbital velocity of a satellite can be achieved, and then a simple orbit correction angle can change the trajectory direction.

Using this technique Mariner 10 managed to pass three times around Mercury: first time on March 29th 1974 (the smallest distance recorded being 703 km), then on September 21st 1974 (at a distance of 480.69 km) and finally on March 16th 1975 (at a distance of 327 km). This was enough to make more than 2,800 photos and to map about 45% of the planet’s surface.
Since then, however, 30 years had to pass before a new mission is ready to explore Mercury.

In summary, with the help of the Messenger probe, scientists have proposed:
• To determine accurately the surface composition of Mercury;
• To measure and map out the local magnetic field;
• To investigate the geological history;
• To try to understand the causes of radar reflection that occurs near the poles and the possible presence of water in solid state, despite the proximity to the Sun;
• To measure Mercury's libration phenomenon and to determine indirectly the presence of a liquid core inside the planet
• To study the atmosphere and look for the presence of water or other volatile substances near the poles that would have a direct influence in the establishment of this rarefied environment.

Mercury

Mercury is the smallest planet of the Solar System with a radius of only 2439.7 km, a value 0.38 times smaller than the medium radius of the Earth.
Despite the size, Mercury is the second planet in the solar system after Earth, considering the density (which has a value of 5427g/cm3 compared with 5515g/cm3 of the Earth).
Mercury aroused the interest of the terrestrial observers since old times, but its unique position in space was the main break in gathering detailed information. All despite that, observed from Earth, Mercury appears at luminous magnitudes between 2.3 and 5.7.
Why is that so little known about Mercury?

First of all because of its orbit – being the closest planet to the Sun, Mercury appears most of the time in the direction of the Sun (the biggest angular separation from a terrestrial observer does not exceed 28.3 degrees), therefore the occasions when the planet can be visualized in optimal conditions by the terrestrial observers are quite rare.
Mercury, despite the hypothesis from antiquity doesn’t have natural satellites, has an elliptical orbit inclined with 7 degrees from the Earths, with a apogee at 69.816.900 km (approximately 0.46 astronomical units) and perigee at 46.001.200 km (or 0.30 AU), having an eccentricity of 0.21 and an orbital period of 87.969 days.
Mercury has its own rotational axis almost perpendicular (89.97 degrees) from the orbital plan, and it spins with very low speed, managing 3 complete rotations in 2 orbital periods and that’s why a solar day there lasts for approximately 176 terrestrial days, while a sidereal day lasts 58.7 terrestrial days.

It is believed that the core of the planet represents 42% of its volume, and is mostly made of iron. Around the core is a mantle of silicate with the width of 500-700km, over it a crust of 100-300 km, the surface presenting craters and large crevasse which give rather a lunar aspect.
The medium temperature on the surface is approximately 442.5 Kelvin degrees, but varies in the gamma 100K-700K. For example the side exposed to solar action presents a temperature between 700K at perigee (when the planet is closest to the Sun) and 550K at apogee (max distance).
Meanwhile the temperature on the other side being in shadow reaches 110K.

The craters from the polar region have temperatures below 100K, which favors the existence of some ice caps. Scientists estimate that these polar regions might host approximately 10e14-10e15 kg of solid water.
Mercury has an extremely rarefied and instable atmosphere loosing continuously atoms and replacing them with others, either from space (most of them brought by the solar wind) or the radioactive activity of the crust. Technically the atmosphere contains 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, 0.5% potassium, but also presents small percents of argon, nitrogen, CO2, xenon, krypton, neon.
Mercury has a magnetic field of low intensity (approximately 1.1% of the one recorded on Earth) reaching values of 300nT at equator but strong enough to create a magnetosphere around the planet.
The interaction between this magnetosphere and the solar wind creates magnetic whirlwinds in the nearby space.

As we said at the beginning of the article, exploring this planet using our satellite technique is only in the infant stage, with only 45% of the planet’s surface being mapped at this moment, and there are still many questions the scientists would like to get an answer after the Messenger will end its investigating activity near Mercury.

Scientific instruments

• Mercury Atmospheric and Surface Composition Spectrometer (MASCS), built by Laboratory for Atmospheric and Space Physics, University of Colorado, weights 3.1 kg and consumes 6.7 W.
The instrument combines two elements - UVVS (Ultraviolet and Visible Spectrometer) and VIRS (Visible and Infrared Spectrograph) and will return about 2.7 GB of data in its 12 months of operation.
UVVS will help to understand the processes that have generated and maintained the atmosphere of Mercury, as well as the causal link between the surface and atmospheric composition, the dynamics of volatile materials on and near the planet, and the nature of the substances from poles region (it has been observed that those regions reflect radar waves). However, the instrument is not limited only to determine the composition and structure of the exosphere. It is also able to make observations at ground level.
Its visual field varies between 1 x 0.5 degrees (in the atmosphere) and 0.023 x 0.023 degrees at ground level and the instrument is helped by three multiplier tubes, operating in the ultraviolet range (115-190 nm), medium ultraviolet (160-320 nm) and visible spectrum (250-600 nm).

VIRS will measure the light emissions, visible or near visible infrared range (0.3-1.45 µm) in search of certain materials as Fe and Ti.
The visual field is 0.023 x 0.023 degrees and the instrument will provide a ground resolution between 100 m and 7.5 km and a spectral resolution of 4nm. The two spectra will be observed by two different detectors - the infrared detector in line with 256 pixels built with InGaAs technology and the visible light detector in line with 512 pixels with built-in Si technology.

• X-ray Spectrometer (XRS)
XRS was built by the Applied Physics Laboratory of “Johns Hopkins” University after a similar model that flew on NEAR-Shoemaker satellite and it weights 3.4 kg and consumes 6.9 W. It is expected to return about 1.5 GB.
It consists of 3 components – 3 MXU (Mercury X-ray unit) detectors, SAX (Solar Assembly for X rays) and MEX (Main Electronics for X-rays). The instrument has a field of view of 12 degrees and can measure emissions in the range 1 - 10 keV.
With the help of its three MXU detectors, scientists hope to measure, at resolutions of 20 km, X-ray emission of the elements accumulated in the planets' crust (Mg, Al, S, Ca, Ti, Fe) and, thus, to determine the chemical composition and the geological past of Mercury.
In parallel, the SAX sensor is able to measure the solar flux variation that is affecting the planet.

• The Mercury Dual Imaging System (MDIS)
MDIS is an instrument that weights 8 kg and has a consumption of 7.6 W. It was built by the same JHU / APL. It consists of 2 WAC cameras (Wide-Angle Camera) and one NAC (Narrow-Angle Camera), both having CCD sensors and both being placed on mobile platforms that facilitates the image capture, without the need of rotating the entire satellite to a certain target.
WAC has a field of view of 10.5 x 10.5 degrees and can observe Mercury through 11 different colored and monochrome filters at wavelengths between 395 and 1040 nm whose comparison will be able to distinguish different types of rocks that make up the planet’s surface. NAC has a visual field of 1.5 x 1.5 degrees with a ground resolution of up to 18 m and captures black and white images.

• Gamma-Ray and Neutron Spectrometer (GRNS)
GRNS is also an instrument composed of two elements – GRS (Gamma Ray Spectrometer) and NS (Neutron Spectrometer). GRS is built by JHU / APL, Patriot Engineering, Lawrence Berkeley National Laboratory and Lawrence Livermore National Laboratory. It weights 9.2 kg, consumes 16.5 W and is expected to retrieve up to 3.9 GB of data.
GRS measures gamma radiation emissions sent the planet’s surface under the influence of cosmic rays, or the natural radioactive emission of atoms such as K, Th, U – seeking to conduct a geological analysis based on the individual signatures of each chemical element that is present there (H, Mg , Si, O, Fe, Ti, Na, Ca).
GRS has a sensor based on a cryogenically cooled Ge crystal up to -183 degrees Celsius.
NS is an instrument built by Johns Hopkins University, Applied Physics Laboratory, Patriot Engineering and Los Alamos National Laboratory. It weights 3.9 kg and consumes about 6 W. It measures the energy of the neutrons that have been emitted by the planet’s surface, under the influence of cosmic rays. The measurements are made especially for determining the hydrogen atoms’ concentration that signals the potential presence of water (which is based on the physical principle that the neutron energy decreases sharply in collision with light atoms such as hydrogen atoms).

• Mercury Laser Altimeter (MLA)
MLA, built by NASA Goddard Space Flight Center weights 7.4 kg, uses 16.4 W and was inspired by a similar experiment that flew on the Mars Global Surveyor platform. In the 12 months in which it will operate, approximately 1.5 GB of data will be transmitted, consisting of data taken for topological measurements of the northern hemisphere which will help at creating a geological history, at accurately determining the shape, the rotation axis and the libration of Mercury and at measuring the size and current condition of the nucleus.
The operating principle is simple: an infrared laser emitter transmits 8 pulses with a wavelength of 1064 nm, 20 mJ of energy and a divergence of 50 mrad in every second. The reflected waves are caught by 4 special receivers which will measure the amount of time passed since it was emitted (with an accuracy of 3.3 ns, meaning a precision of 0.5 m).
On the ground, the pulse has a scattering area of 10-50m, with a 100-300m distance between the spots (in the direction of movement). MLA can work in a nominal state at up to 1500 km (from where it assures a 30 cm resolution).

• Energetic Particle and Plasma Spectrometer (EPPS)
The EPPS is built by Johns Hopkins University Applied Physics Laboratory and University of Michigan, weights 3.1 kg and uses 7.8 W, being mounted on the upper side of the satellite. The instrument, which was supposed to return 4.4 GB of data in its operational year around the planet, is made up of 2 components: FIPS (Fast Imaging Plasma Spectrometer) and EPS (Energetic Particle Spectrometer).
FIPS detects H, 3He, 4He, O, Ne, Na, K, S, Ar and Fe ions, measuring the ratio between the energy and the electrical charge of the particles (0-10 keV/q), and also their speed an spatial distribution, estimated indirectly through the time in which the particles get to the main detector (50-500 ns).
EPS will measure the energetic spectrum, the chemical composition and the spatial distribution of ions with more than 10 keV – 5 MeV of energy (H, He, CNO, Fe), and also of the electrons with 20 to 700 keV of energy.

• Radio Science (RS) experiments
RS uses the onboard communication system (two X band transponders, MGA – medium gain antenna, LGA – low gain antenna and the 2 so-called “phased arrays”) for a very precise measurement of the speed and position of the satellite (with an accuracy of 0.1 mm/s). The variation of the speed is then used for determining gravitational effects from which the mass distribution inside the planet can later be determined. Similarly, from a precise determination of Messenger's position, Mercury's libration can be determined, and from radio observations, the exact shape of the planet can be estimated.

The Satellite

Messenger, the seventh of the Discovery program which started in 1992 as an idea of doing cheap exploration of the Solar System, had a cost of 446 million dollars (including the design, construction of the satellite, the launch, operation and finally the scientific investigation segment together with modeling the collected data).

The satellite has a parallelepiped form with the dimensions 1.27 x 1.42 x 1.85 m and weights 1093 kg from which 607 kg represents the reserve of helium and fuel.
The structure is built from a composite material – GrCE (graphite-cyanate-ester) - which lowers significantly the weight and brings a better rigidity and thermal constant.
Four vertical panels represent the central cell on which the internal components are fixed.

The thermal control system is vital for the success of the mission, considering that the satellite will approach the Sun at a minimum distance of 46 million km (0.3 AU), where it will be seen 11 times bigger than on Earth.
In these conditions the thermal radiation received from the Sun would bring temperatures of up to 370 degrees Celsius, which can be fatal for the electronics on board.

To avoid such a scenario, the engineers came up with the solution of equipping the satellite with an exterior protection panel whose role will be to prevent the sunlight to damage the satellite itself.
This panel has a semi-cylindrical form with the length of 2.54 m and a diameter of 1.82 m, is built of several successive layers of Kapton (a plastic material) and Nextel (a ceramic material) on a rigid frame made from titanium. The whole system weights only 20 kg, despite its size.

In this way, the electronics shaded by the panel, remain at an ambient temperature of 20 degrees Celsius. In theory, under the protection of the panel, the satellite can rotate but only between some certain limits – the so called SKI (Sun keep-in-zone) - dictated by the constructive characteristics i.e. the protection panel being closed only on one side.

This optimization isn’t sufficient because in every orbit, for approximately 25 minutes the satellites reaches a low distance from the planet (200 km), time when the satellite passes a phase of increasing temperature, caused by the solar rays reflected by the surface of Mercury. This problem requires another solution to compensate the effects of coming close to Mercury.

The thermal equilibrium is maintained further inside the structure of the satellite itself with the help of two systems – on one side a passive system (built from overlapped layers of isolating thermal materials ) and on the other side an active system which uses radiators and heat radiant diodes.

These can be activated progressively (depending on the amount of heat that must be dissipated in order to maintain constant the temperature on board).
On the other hand, when the satellite faces an eclipse (on the shaded side of Mercury) the temperature can reach even -135 degrees Celsius, conditions that require the activation of the thermal system.

However the satellite must continuously monitor the flight position and avoid the dangerous orientation called HPKO (planet “hot pole” keep-out zone) where the inferior panel of the satellite – on which the battery is attached – would be exposed to the radiation reflected by Mercury.

The 2 solar panels, which grant a supplementary shade, have a secondary role in maintaining the thermal control. They are the most important part of the electric system of the satellite, being the source of energy which ensures the correct function of all sub-systems.
The company responsible with the design is Northrop Grumman Space Technology but they used components from other producers as well, like we will show further.

The engineers used a particularly adapted solution, specific to this mission, attaching 2 mobile panels (they can be rotated on two directions with the help of some mechanisms called SADA-solar array drive assemblies) - each panel having the dimensions of 1.54 x 1.75 m.
Their structure is made of aluminum on which some special panels made of composite material RS-3/K13C2U built by AASC (Applied Aerospace Structures Corporation) were adapted. They are light but at the same time they have a great resistance to thermal stress (a necessity for Messenger considering the fast and significant temperature variations it will face).

Built in the GaAs technology with an efficiency of minimum 28% (reaching 30%) they can produce between 385 and 485 W during the cruise phase towards Mercury and 640 W in normal conditions of the orbit around the planet.
In theory, due to their dimension, the 2 panels could produce up to 2000 W, but this would be a useless surplus of electric energy, the nominal 640 W being more than enough for the onboard consume.

Again the designers came with a pretty ingenious solution mounting 2/3 of the panel’s surface with reflective mirrors. Thus each row of solar cells is surrounded by two rows of reflective cells concluding to an active surface (which generates electrical energy) of only 28%.
Each solar panel contains a total of 18 rows of solar cells (each 3 by 4 cm) and the rows are electrical connected via the Solar Array Junction Box (SAJB) equipment. The manufacturer of these cells is the EMCORE Corporation.
The rest of 72% of reflective surface is not less important because it should prevent the overheating. Due to it the side exposed to the action of solar activity will reflect approximately 60% of the incident radiation and will limit the temperature to 150 degrees Celsius.
This reflective surface is composed of small cells called OSR (optical solar reflectors) manufactured by the Pilkington company. They are in fact some mini reflective mirrors which incorporate a special substrate of glass called CMX.
The energy is stored (for the eclipse periods) in a Ni-H battery having 23 Ah and 11 distinctive elements. These, built in CPV (Common Pressure Vessel) technology, are manufactured by EaglePicher Space Energy Production Division and are mounted in a three rooms Aluminum structure.
The structure is specially positioned to avoid the overheating (shadowed by the structure of the satellite) and is thermally connected at a radiator with an area of 0.13 m2 which maintains the battery between -5 and 0 degrees Celsius. Also the difference between the battery’s cells should not be more than 3 degrees Celsius in order to avoid a supplementary thermal stress.
Another part of the electric system is the PSE (Power System Electronics) and the PDU (Power Distribution Unit). The electronic of PSE is designed to operate at temperatures between -34 and +65 degrees Celsius and can dissipate between 15 and 40 W. This extra heat is transported through dissipative lines (in fact Aluminum made cylinders filled with Ammonia) to the radiators and later to the external space.
The communication system should ensure the permanent connection between the satellite and the ground stations on Earth – independently of the orbital position and the attitude of the satellite.
The normal communication is established via the DSN (deep Space Network) in a bidirectional manner: the telemetry is received at rates of 9.9 bps-109 kbps and the telecommands are transmitted at rates of 7.8-500 bps, both in the X band.
For a facile communication onboard the spacecraft have been mounted 2 HGA (High Gain Antenna), 2 MGA (Medium Gain Antenna) and 4 LGA (Low Gain Antenna).
The two HGA are “phased array antenna” and can acquire the transmission of a big volume of data (as the transmission of scientific data collected by the instruments).
The two MGA - “gain fanbeam antenna” – are the standard elements of communication, they are fixed but have an access angle of 45 degrees ensuring the Earth visibility under the nominal conditions.
The LGA’s are specially positioned onboard the spacecraft and they ensure the transmission of the commands from the control centre, the essential housekeeping data coming from the satellite and the emergency communication in the case the spacecraft enters the safe-mode.

The onboard computer of Messenger called IEM (Integrated Electronics Module) has a double configuration (main and backup) for a safe operation. When a failure occurs on the main module (i.e. a SW corruption by the solar flux) the backup module takes over autonomously as soon as possible in order to isolate the problem and to ensure the functionality of the spacecraft.
Each of the IEM incorporates 2 processors RAD6000 one running at 25 MHz and the other at 10 MHz. The first one runs the basic routines - i.e. command and data handling or attitude and orbit control both sharing the physical resources of the processor while the second one only takes care of the FDRI (Failure Detection Isolation and Recovery) routines.
The 2 IEMs incorporate two 1GB solid state hard-disks (one main and one backup) for storing the data when the satellite is not in direct communication with the Earth.
The IEM has also a data bus which establishes the connection between the processor and the instruments, and some power convertors for the local use.

We will provide in the next paragraph some explanations for the 3 routine processes ran by the IEM processors.
The FDIR mechanism is an essential one for every satellite – which is by definition a complex machine that should operate most of the time autonomously, without the possibility of human intervention. In these conditions (specially in ‘deep space’ missions where communications are even more difficult), but also taking into account the hard operating environment which can affect the proper functioning of the satellite, a robust system is therefore required. FDIR accomplishes the monitoring and protection of the satellite’s systems by turning some components on/off or by interchanging primary/secondary modules for each subsystem (in case of a malfunction).
The ‘data handling’ mechanism is responsible for collecting, transferring and storing data and for receiving and distributing commands to on-board devices.
The commands – a vital element for each satellite – are executed either in real-time or through “time-tagged” commands which are stored for later use (in this way being kept the control for the periods without visibility from the command center). Judging by the execution mode, there are normal commands (which are executed exclusively by using the software) and high-priority commands executed directly (through dedicated electrical interfaces) – the equivalent of usual PCs ‘reset’ command.
The ‘attitude and orbit control’ routine is a component of the software system with the same name (AOCS) and incorporates all the mathematical algorithms used for orbital control.
Maintaining a correct flight position is a critical aspect for Messenger - any deviation from the nominal position (for example, if the solar shield isn’t pointed towards the Sun for protecting the spacecraft) can induce a major fault in the electronic devices. As an example, it should be said that the so-called SKI zone (Sun keep-in, the zone in which the satellite can spin without internal thermal changes) is between ± 10degrees for the z axis and ± 12 degrees for the x axis. One of the disruptive factors to which the algorithms must pay attention is the solar radiation and, as we will recall later, Messenger takes into account a model of the radiation, not only to compensate its effects, but also to use it actively on the road to Mercury. The AOCS system that assures a precise three axes stabilization is composed of a sensor part and an actuator part.
Messenger is equipped with 2 stellar cameras (the popular model A-STR built by Galileo Avionica) which measures at a 10 Hz frequency, the spatial inertial orientation of the satellite in respect to the stars observed in the visual field, 2 solar sensors which measure the Sun’s direction and 1 SIRU (Space Inertial Reference Unit) – which is actually a classic IMU (inertial measurement unit). The SIRU is built by the Northrop-Gumman and includes 4 gyroscopes and 4 accelerometers (Honeywell QA3000 type) and is able to measure speed variations and accelerations very precisely at a 100 Hz frequency. The actual satellite positioning control is provided in two ways: small corrections are performed with the help of a 4 flywheel, Teldix RSI 7-75/601 system which provides 0.0075 Nm and can store up to 7.5 Nms, and important corrections (as well as the momentum RWL discharge) are made by on-board engines.
Messenger has a primary engine named LVA (large velocity adjust) used for an orbital transfer, generating 667 N, then a set of 4 engines providing 22 N each, used for orbital corrections and 12 engines providing 4.4 N for in-flight stabilization. The primary engine is bipropellant, using a combination of hydrazine as fuel and 'nitrogen tetroxide as oxidant. The two are kept in separate tanks while a helium tank helps maintaining a constant pressure and proper functioning of the system. The other engines are monopropellant, using only hydrazine.
The satellite is equipped with 3 main fuel tanks (2 with N2H4 and 1 with N2O4), a secondary tank (N2H4) and an auxiliary tank (He). The connection between the fuel tanks and the engines is made by a complex pipe system and its flow can be controlled by 9 valves. Also, the thermal control of the pipes (an essential element for preventing hydrazine, which has a low freezing point, from freezing) is performed by a number of dedicated thermistors.

Going back to the algorithms which control the satellite’s position, it must be said that the engineers have grouped them into three big classes/flight scenarios, each having its own characteristics and capabilities, with the intention of assuring the best protection to the equipment and an autonomous functioning with the least human intervention.
The 3 categories are called: OP (operational mode), SH (safe hold mode) and EA (Earth acquisition mode). In turn, the 3 categories also have 2 distinct sub-modes: normal and autonomous.
The shift is made either automatic (the decision belongs to the on-board software) or commanded (the decision belongs to the engineers).
The AOCS system combines the measurements taken from the sensors and its orbital predictions for the planets (it incorporates a mathematical model dedicated for this kind of activity) and then uses a Kalman filter for estimates and for a propagation of the satellite’s position.
We will perform a detailed analysis of the system's performance as they resulted on the road to Mercury, in the next paragraph dedicated to Messengers' orbit and the maneuvers performed on its long trip.

Image
credit JHU-APL

The road to Mercury

As we said in the first article of the series, the Messenger spacecraft was launched on the 3rd August 2004 at 06:15 UTC with the help of a Delta 2 rocket, flying the 7925H configuration.
Although adapted to interplanetary missions, the rocket couldn’t provide a high enough acceleration so that a satellite like Messenger could not reach an appropriate speed for the huge distances that need to be completed for a direct placement into an orbit around Mercury.
The engineers used again a classical technique, mostly utilized for interplanetary missions – assisted gravity technique – which uses the attraction force applied by all the planets in the Solar System to every object which passes near them.
Therefore, by right selecting the orbit, a natural acceleration of the orbital speed of a satellite can be obtained and later a simple correction of the orbit’s angle (with the help of the onboard engines) can change the trajectory on the desired direction.

Image

Messenger’s flight to Mercury was a very complicated one, requiring 6.6 years just as the next table illustrates.

Image

Immediately after the launch and the deploying of the solar panels, the onboard engines were activated to stabilize the satellite and to reduce the rotational movement induced after the separation of the carrying rockets. A series of TCM (trajectory correction maneuvers) followed right after, which prepared the first flyby of Earth (August 2005 at an altitude of 2347 km).
Six months later, December 2005, the first major propelled maneuver (DSM or ‘deep space maneuver’) took place – which was also the biggest; it consumed approximately 106 kg of fuel – followed by CMD type maneuver (commanded momentum dump) which reduced again the rotation speed of the satellite.

Several other maneuvers brought Messenger in position to fly over the planet Venus - first time in October, 2006, at an altitude of 2987 km, then in June, 2007, at an altitude of 338 km. In both cases, the scientists had the opportunity to obtain additional observations regarding Venus; the registered data will be used by the international community along with the data provided by the ESA’s Venus Express probe.

Soon after, a second DSM maneuver (being also the second largest, consuming 68 kg of fuel) moved the Messenger spacecraft from Venus, in order to approach Mercury.

The preparations for the first flyby of Mercury began in December 2007 and in a short time (January 2008) the probe came at a distance of 201 km from the planet.
DSM’s third maneuver in March 2008, the second flyby of Mercury (October 2008 at an altitude of 199 km), the fourth DSM maneuver (December 2008) and finally the third flyby of Mercury (September 2009 to an altitude of 231 km) followed after.
DSM’s last maneuver, before enrolling in the final trajectory around the planet, took place in November 2009, 45 kg of fuel being consumed, leaving onboard, thus, a reserve of only 41% compared to the amount the spacecraft had when it left the Earth.

The critical point was, though, the final maneuver, called “Mercury orbit insertion maneuver”, or MOIM. Any mistake could have been, most certainly, the end of the mission. NASA engineers had provided, of course, alternatives, but if they could not succeed orbital injection (as happened before, several times in the case of the JAXA’s missions), considering the amount of the remaining fuel and the life span of only a year it would make hard to believe that Messenger would have been effectively used afterwards to collect scientific data.
Also, considering the effort spent to bring a probe near Mercury, it would have probably been a huge loss for NASA and the scientific community, in general.
But things went almost perfect, proving once again the professionalism of the people that stand behind these operations.
Our table clearly illustrates the events sequence and finally the first estimates of deviation from the desired orbit (which are very small, as it can be seen) - a deviation of 0.02 degrees with respect to the orbital plane inclination and a few kilometers in terms of altitude perigee and apogee.
The actual maneuver lasted only 15 minutes (it was executed on March 18, 2011 at 00:45), but the preparations have begun about 48 hours before.
The command center configured the satellite, taking into account all the technical constraints; NASA allocated their DSN (Deep Space Network) for this important event. In total, four DSN stations have served the command and the telemetry of Messenger.
After receiving the first data and the confirmation of successful injection in a stable orbit around Mercury, a period of intense checking of equipment followed. On April 4th, 2011, using the scientific instrumentation that has been prepared, an observation campaign began, which will last for one year.

How does Messenger operate?
Messenger evolves on a high elliptical orbit around Mercury (15193 km x 200 km x 82.5 degrees inclination, and with an orbital period of about 12 hours).

Because of perturbations (especially due to solar gravity) this orbit isn't stable without corrections and in a short while would appear changes of the altitude at perigee (the satellite would slowly drift away from the planet, making observations difficult), changes of the orbital inclination and also of the perigee projection spot on the surface of Mercury (a spot which is normally situated at the 60 degree latitude North, close to the Caloris crater).
For correcting this problem, engineers operating Messenger came up with two sets of orbital maneuvers: the first type is intending to lower the altitude in the perigee point to its nominal value of 200 km and is performed periodically, once every 88 days (the orbital period of Mercury), and the second type is destined to increase the orbital period to 12 hours (the nominal period time) and comes as a complement of the first type of orbital maneuvers (normally, at every drop in altitude, the orbital period shrinks with about 15 minutes out of the 12 hours).

Our table illustrates once again the 5 maneuvers which will be executed in June, July, September, October and December.

As we can see, Messenger's flight inside the solar system was extremely complicate and our short summary, as well as the correct data illustration gives us a clear image of how hard it is for a probe to be sent from Earth to Mercury and why Messenger is the only satellite to have surpassed these constraints.
Beneath, we have made a number of tables for different missions (interplanetary or not) which can serve for comparisons. They illustrate very well the distanced traveled by each probe, the heliocentric speed, their position in the solar system etc. and Messenger's positioning between these probes.

Image

Image

Image

Image

First pictures

As we have remembered previously, after reaching a stable orbit around Mercury, Messenger has used the first days for calibrating the scientific instruments before starting effectively the observation phase- a phase which will last approximately 1 year.
In this short period since then, 6 weeks in total, the operation command and control centre started to collect the first science data. Couple of images have been offered already to the public and can be accessed on the official website of the mission.

The first image ever captured from an orbit around Mercury (excluding the ones taken during the previous short fly-bys of Mariner 10 and respectively Messenger) has been produced on 29th of March 2011. It has been taken using the WAC and MDIS instruments over the location -53.3 degrees East latitude and 13.0 degrees East longitude where is the Debussy crater (a crater which is 80 km in diameter).

Image

On the same day came also the first measurements from Mercury Laser Altimeter (MLA) which counted the differences of the relief during the first two orbits.
Image
Then, the first estimations of the Mercury’s magnetic field have been published on 30th of March 2011.
Image
The first NAC picture has been produced on 29th of March at -53 degrees latitude and 13 degrees longitude east.
Image


Future exploration of Mercury

In this final part of our series about the Messenger mission we will try to conclude and to see how the Mercury exploration will look like in the next decade.
Under the pressure exerted by the latest financial cuts, the aerospace field does not seem to move into the right direction thus for the moment a single satellite has been announced to succeed the Messenger. We speak about the European-Japanese satellite BepiColombo which is at the present under the construction and testing, with a launch most probably in July 2014 and with an arrival at the destination somewhere in 2020.

Without going back to the characteristics of a flight around Mercury we will try to compare the two missions by putting in the mirror the technical solutions found by the engineering teams in both cases.

First let allow us to give some information about BepiColombo.
Named after the Italian scientist Giuseppe Colombo, who dedicated most of his life for studying the Mercury, the BepiColombo mission will improve the Messenger exploration bringing into a direct collaboration, first of this kind, the two space agencies ESA and JAXA.
It will be in fact 2 satellites- Mercury Planetary Orbiter (MPO) built by ESA and Mercury Magnetosphere Orbiter (MMO) built by JAXA- grouped inside the Mercury Transfer Module (MTM) which is responsible for the transfer from Earth to Mercury and the placement of the two probes in a stable orbit.
This platform, which can be seen as a container carrying the two satellites (inactive during the transfer phase) will ensure the protection for the thermal variations and for the radiation exposure this flight implies, but the main role will be, as we said, to assure the propulsion necessary to reach Mercury.
Once it will arrive to the destination, MTM will be separated and it will leave the two satellites in two independent orbits.
We have to remind that initially it was taken into account a third module- Mercury Surface Element (MSE)- in fact a small lander who was supposed to operate for about a week at the surface of the planet, but this one has been later cancelled due to the financial constraints.
The BepiColombo mission has been considered for the first time in May 1993. In 2000 ESA has reaffirmed the interest for building a satellite to explore Mercury, and in February 2007 it has entered officially in the Cosmic Vision Program. The mission will cost Europe approximately 1 billion euro including here the construction of the satellite but also the launch and the ground operations.

The launch will take place from the Kourou space base in French Guyana aboard an Ariane 5 rocket. The official launching date is today 19th of July 2014 but a second window is available for August 2015 in the case when the first opportunity is missed.
The orbital transfer will use as for the Messenger, some flybys around the Earth, Venus and Mercury- as we will detail later, but also the solar pressure and the electrical propulsion.
ESA will be responsible for the interplanetary journey and the injection of the two satellites in orbits around Mercury, and later the two agencies will operate individually their spacecrafts.

The scientific results and the ground infrastructure will be used in common, the BepiColombo mission having at it disposal the Cebreros (35 meters in diameter) station and the Usuda (64 meters in diameter) station, but also the powerful American DSN network for emergency cases.

MPO is a 1.6 x 1.7 x 1.9 m satellite, weighting 1147 kg (dry) from which 80 kg is the weight of the instruments. In total 11 experiments are hold onboard:
• BELA – BepiColombo Laser Altimeter
• ISA – Italian Spring Accelerometer
• MERMAG – Mercury Magnetometer
• MERTIS-TIS – Mercury Thermal Infrared Spectrometer
• MGNS – Mercury Gamma ray and Neutron Spectrometer
• MIXS – Mercury Imaging X-ray Spectrometer
• MORE – Mercury Orbiter Radio science Experiment
• PHEBUS – Probing of Hermean Exosphere by Ultraviolet Spectroscopy
• SERENA – Search for Exosphere Refilling and Emitted Neutral Abundances (Neutral and ionised particle analyser)
• SIMBIO-SYS – Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (High resolution and stereo cameras, Visual and NIR spectrometer)
• SIXS – Solar Intensity X-ray Spectrometer

The three axes stabilized spacecraft will travel in a polar, 90 degrees inclination orbit with the perigee at 400 km and the apogee at 1508 km. In this configuration the satellite will have an orbital period of 2.3 hours.
The control will be done by using four 22 N thrusters and the satellite will be equipped with 3 star cameras.

The power generated by the solar panels is estimated between 935 and 1565 W, enough for the consumption of the instruments- which will go in the range 100-174 W.
In total is expected the reception of 1550Gb of data for a nominal operating period of about 1 year and at a rate of 50kbps (using the X and Ka communication).

MMO- the Japanese satellite- will measure 1.1 m height by 1.9 m in diameter, weighting 275 kg (dry) from which 45 kg of scientific instruments:
• MERMAG-M/MGF – Mercury Magnetometer
• MPPE – Mercury Plasma Particle Experiment
• PWI – Plasma Wave Instrument
• MSASI – Mercury Sodium Atmospheric Spectral Imager
• MDM – Mercury Dust Monitor

It will be a spinning stabilized spacecraft (spinner) at the rotational speed of 15 rpm, with a rotational axis position at 90 degrees compared with the Sun.
The flying orbit will be again polar with a 90 degrees inclination, the perigee at 400 km and the apogee at 11824 km. The orbital period will thus be 9.3 hours.
The AOCS system will handle 6 GN2 thrusters (each 4.25 kg) and 0.2 N in thrust, 2 solar sensors, a star camera and a de-spinning mechanism.
The generated electrical power will vary between 348 and 450 W based on the distance to the Sun but it will cover the 90 W necessary for the instruments.
The satellite will communicate with the ground in the X band at a rate of 5kbps which will produce a total of 160Gb of data per year.
As we previously mentioned, the orbital transfer of the BepiColombo, and the critical part of the mission, will benefit from the help of the gravity of Earth, Venus and Mercury similarly with the scenario used for Messenger.

Next table summarizes the mission’s timeline:

Image

In this configuration the orbital transfer has decreased in comparison with Messenger from 6.5 years to 6 years and this mainly due to the electrical propulsion.
SEP or the “solar electric propulsion” is a system inherited from the European mission Smart 1 – the one who has used it for the first time around the Moon.

MTM (Mercury Transfer Module) the one who will get it will have a launching mass of 4.2 tons – with a proportion of 32% fuel- respectively 816 kg of chemical conventional fuel and 500 kg of xenon to be used for the electrical thrusters.
The classical chemical propulsion system will be powered by 8 bi-propellant (N2O4-MMH) 10N each thrusters, while the electrical system comprises 4 thrusters.

In this conditions BepiColombo can reach a delta-v value of 5.025 km/s (for the electrical) and 1.065 km/s (for chemical), these values being anyway far superior to the ones of Messenger (2.2 km/s).

MTM will be able to generate through its 40m2 solar panels, an energy varying from 7kW (at the maximal distance to Sun of 1.13 AU) and 14kW (at the minimal distance of 0.62 AU), these numbers being decisive for the final performance of SEP (which has an estimated power demand of 10.6 kW).
For example, at the nominal distance of 1 AU, the entire produced electricity will be used (is enough) for operating a single thruster at the time, thus obtaining a thrust of 100-130 mN, but once the distance to the Sun decreases, due the extra energy coming from the solar panels the spacecraft could power two thrusters in parallel resulting a traction force of about 290 mN.

Despite the fact it is not coming with spectacular results in comparison with the chemical propulsion (in the sense of the resulting thrust) the electrical version has a number of advantages in the case of interplanetary trips. There, where is no need for short time planed maneuvers, but instead the things are carefully planed long time before the events happen, a constant propulsion, even at low values, can accelerate a satellite to appropriate speeds necessary for interplanetary journeys (compensated by the long action time).
The use of electrical propulsion onboard BepiColombo significantly decreases the risk of a single major maneuver scenario and offers the flexibility to modify the parameters in flight. By its intensive use one can save a lot of the classical fuel reserve.
In principle if the things go in the right direction at arrival to the Mercury, BepiColombo will have a speed low enough to ease the natural capture by the planet’s gravity. There will still be needed some extra maneuvers accomplished with the chemical propulsion but these will be small and anyway not having the same criticality as for Messenger- respectively a delta-v of 325 m/s for MMO and 620 m/s for MPO.

Thus it is very probable that most of the fuel stored onboard the two satellites will not be used and if the electronics will perform also well there is a good chance that the mission will get an extension of one year by the end of the nominal lifetime.
This can only be a good thing for the scientific community because it means the data collection will increase and there are better chances to understand our neighbor planet from the solar system.

If we have to speak about the technical constraints imposed to the two satellites they are the same as for Messenger. First the radiation coming form the Sun and which can reach up to 14 kW/m2 and the one coming from Mercury (with values up to 6 kW/m2).
In order to sustain these values, the Messenger’s designers have projected a special orbit- picking up a high elliptical trajectory such that the satellite comes close to the surface of the planet only for 5% of the orbital period and thus limiting the thermal stress. For the rest of the orbit is then enough time to get rid of the heat excess from the satellite’s structure.
However this has a big disadvantage because the periods of maximum resolution observations are limited.

For MPO, the European designers have pushed the limits, lowering the apogee of the orbit to 1508 km and forcing the satellite to spend a lot more time close to the planet. In this scenario the flight control will be essential and as 5 out of the 6 sides of the spacecraft will be illuminated while is an out of the eclipse condition, they have chosen to put a heat radiator on the sixth side. To have a good performance this side of the satellite should be kept away form the direct action of the Sun and the AOCS system will be responsible to solve autonomously this problem.

For MMO the Japanese designers have copied the American solution with solar panels playing a double role- capturing the energy but at the same time reflecting it- with 50% of the panel area used for solar cells and the other 50% for small mirror cells (OSR or optical solar reflectors). Also mounting a thermal scout for limiting the action of the solar rays is another solution inherited from Messenger.
For both satellites we are speaking about a complex thermal protective system- with an active and a passive component- which involves new composite materials, resistant to the temperature variations and to the degradation with time.
With BepiColombo launched we will be able to test these new materials in the conditions of a real flight and Europe hopes that these new technologies will confirm the financial investment and will soon spread to some other fields of the industry on the old continent, bringing back money.

Concluding our virtual trip to Mercury we only can say that 2020 is still far away in the future and since then we are restricted to the images and data coming from the Messenger probe.
As people who share a passion for science we hope that these two missions will not remain singular and despite the financial constraints the scientific curiosity will win the battle and they will be soon followed by some other explorations.


Best thread I have ever came across. Each point is mention in detail and I am gald to find the post.. I hope you will keep more such useful stuff in future..

Return to Fourth edition of the Space Alliance magazine

Who is online

Users browsing this forum: No registered users and 2 guests

cron