Post Tue Jul 19, 2011 9:47 am

What happened with the Japanese Akatsuki probe

Recently, news about a Japanese attempt to insert the Akatsuki probe into an observation orbit around Venus spread around the globe. The attempt was a failure because the satellite escaped the gravitational field and remained on a large orbit around the Sun. We will try in this short article based on the little information released by the officials to see what could be the reasons for this anomaly.

First of all, we should give some basic details about the Japanese mission to Venus.
The satellite was sent on 20th of May 2010 from the Tanegashima space centre onto a 482 million kilometer flight to its destination, aboard a H II A rocket, on its 17th flight.
Five other platforms were sent with the same rocket, each of them designed to carry out different technological experiments: IKAROS, UNITEC 1, WASEDA-SAT2, KSAT and Negai.
The initial name (Planet-C or VCO “Venus Climate Experiment”) referred directly to the mission's scientific purpose, this being the 24th scientific platform built-up by Japan.
The prism shaped satellite, having the approximate dimensions 1.04 x 1.45 x 1.4 m, weights about 640 kilograms at launch (including 320 kilograms of fuel) and costs JAXA 25.2 billion yens (about 300 million Euros).
On board there are 6 scientific experiments: UVI (“Ultraviolet Imager”), LAC (“Lightening and airglow camera”), IR1(“1 µm infrared camera”), IR2 (“2 µm infrared camera”), LIR (“Long wave IR camera”) and USO (“Ultra stable oscillator”) which along with DE (Sensor Digital Electronics Unit) providing control and data processing- weight 37 kilograms out of the satellite's total mass.

The power system was designed to make use of an area of 2 x 1.4 m2 and generates a nominal power of 1200W (with a minimum of 500W for the chosen orbit around Venus, calculated for the end of the mission taking into account the degradation of the solar cells). The satellite is equipped with two solar panels whose position can be adjusted on one axis, allowing them to follow sunlight even when the rest of the platform is directed towards a particular target (namely the surface of the planet).

The chosen orbit is an elliptical one with a high eccentricity, the satellite having to travel between 300 km for the perigee and 79.000 km (equivalent to 13 average Venusians rays) to apogee, with an orbital period of about 30 hours. Traveling in a westerly direction, the satellite has for 20 hours out of one period the same speed as the rotational speed of the atmosphere, allowing a better synchronization for observations of physical phenomena that take place there.
For comparison with the Venus orbital plane which is inclined at 3 degrees from the ecliptic, the Akatsuki probe should travel at an inclination of about 172 degrees.

The satellite command and control system, named by the Japanese AOCU (“attitude and orbit control subsystem with processor”) assures a three axes stabilization using an engine system (RCS or “reaction control system”), used in conjunction with the reaction wheels (RW). These momentum “storage devices”, four in total, are of two types: some bigger and able to develop 20Nms and some smaller, able to develop 4Nms.
The RCS system is composed of 12 monopropellant engines (N2H4) for attitude control, and one bipropellant engine (N2H4+NTO), or OME (“orbital maneuvering engine”) for orbital corrections.
Keeping a constant pressure in the two fuel tanks is provided by a compressor that pumps Helium from a special tank so that it compensates the loss and eliminates the compression and displacement effects induced from acceleration.
Normal engines are grouped in pairs and develop a traction force of 23N (8 of them) and respectively 3N (the remaining 4). As we mentioned earlier, these are the ones meant to do the small corrections necessary for attitude control and for the discharge of the momentum stored in the RW. OME is used only in two situations, but it plays a major role in the mission's success. Firstly, OME must pull the probe out of Earth's gravitational field and place it on a transfer orbit around Venus. Secondly, it must put the probe on an elliptical Venusian orbit.

The sensors consisted of IRU (“inertial reference unit with gyro”), two stellar cameras (STT-“star trackers”), solar sensors (TSAS and CSAS “sun aspect sensors”) and accelerometers (ACM).

The thermal control system had to be specially designed for operating near Venus, where the solar radiation has a value twice as big as the one registered around Earth. Thus, despite the MLI (“multi layer isolation”), it is expected that 140 W/m2 of the solar energy will overcome the barrier and go inside the satellite. In addition to that, the electronics on-board dissipate about 500W and scientific instrumentation another 1000W, in nominal operating conditions.
Because of this, passive insulation was not enough and a number of dissipative panels were added. Another big problem to overcome was the internal temperature distribution which is not uniform, because in some areas a temperature of 20 degrees Celsius has to be maintained (board electronics) and in parallel there are regions where the temperature must remain at 0 degrees Celsius (the optical zone).
Part of the thermal control system there is the possibility of heating (using a thermistor system with a maximum capacity of 300W) internal areas that are prone to low temperatures.

Communication with Earth is made in the X band, at a frequency of the transponders of 8 Ghz and a transmission power of 20W. Three types of antennas are installed on board: one HGA (“high gain antenna”), two MGA (“medium gain antenna”) and two LGA (“low gain antenna”).
HGA is circular, with a diameter of 1.6 m and has a gain of 37 dBi providing scientific data transmission to ground stations at a rate ranging from 4kbps at 1.5 AU, 8 kbps at 1.1 AU, 16kbps at 0.7 AU and 32kbps at 0.5 AU.
MGA is horn-type antenna, mounted on a panel that allows 180 degrees rotation, has a gain of 14 dBi and is used for “housekeeping” telemetry transmission (the one that includes essential data regarding on-board electronics) when the probe is performing observation missions and HGA is no longer oriented towards Earth. Transmission rates were calculated for values greater then 6bps (at a distance of 1.7 AU).
LGA is a wide coverage antenna and does not require precise positioning to aquire signal. The two antennas are mounted in tandem in the +x/-x directions, assuring in this way a spherical coverage. LGA is used for receiving remote commands sent from the ground station.
For nominal operation, two Japanese ground stations have been designated to communicate with the satellite – Uchinoure and Usuda, but the support of DSN (“deep space network”) is also available through Goldstone, Canberra or Madrid in case of an emergency.

Some words about the scientific instruments onboard the spacecraft.
IR1 has been designed to study the cloud movement in the low atmosphere of Venus, the vapors distribution, the mineral composition of the soil or the existence of volcanic activity. Weighting 6.7 kg it has an optic system with a field of view of 12 degrees and a Si central sensor type CSD/CCD (charge sweeping device/charge coupled device) with 1024 x 1024 pixels. The instrument is able to scan Venus at three different wavelengths: 1.01 µm, 0.97 µm and 0.9 µm.

IR2 can go through the deep clouds of the planets and study the physics of it, the concentration and dimension of the component particles, the CO distribution and the phenomena which drive the circulation in the low atmosphere. A secondary role is played in studying (while traveling from Earth to Venus) the clouds of interplanetary dust based on the light emission passing them.
Weighting 18 kg the instrument –with a field of view of 12 degrees- is carrying a CSD/CCD detector of type PtSi and can scan at 4 different wavelengths: 1.735, 2.26, 2.32 and 1.65 µm.

LIR (long wave infrared camera) intends to study the movement and the convection current, the distribution and the speed of the wind above the clouds based on the images taken at the wavelength of 10 µm. It is actually a bolometer weighting 3.3 kg, with a field of view of 12 degrees and a 320 x 240 pixels sensor.

UVI (ultraviolet imager) captures the ultraviolet radiation and from its variation can measure the SO2 level which is involved in the formation of the clouds or other substances from their composition and which absorb the radiation. Based on this it can be realized the diagram of the wind speed above those clouds. UVI, weighting 4.1 kg has an optic system with a 12 degrees field of view and a 1024 x 1024 pixels CCD sensor on Si technology, being able to scan at the wavelength of 283 and 365 nm.

LAC (lightning and airglow camera) weights 2.3 kg, has a field of view of 16 degrees and works at 4 distinct wavelengths: 777.4, 480-650, 557.7 and 545 nm. It is designed for the study of the airglow phenomena (the light emission of the oxygen from the upper atmosphere) and for the study of the electrical discharges which take place on the planet.

USO (ultra stable oscillator), 2 kg in weight, will observe the radio transmission through the Venus atmosphere, the temperature distribution based on the altitude and the density of the electrons. By emitting the radio waves at 8.4 GHz, the ground stations will measure the change of frequency in the received waves and the modification of intensity in the radio signal after passing the Venus atmosphere.

What is so special at our neighbor Venus to attract the attention of the scientists?
Venus, the closest planet to us and similar in size (the medium radius being 0.949 of the Earth’s radius) is a very good source to understand the process which produced the Earth formation and its natural conditions.
The atmosphere is an extremely dense one, with a pressure around the value of 93 bar, mainly containing CO2 which induces a greenhouse effect with temperatures of 740 K and high altitudes H2SO4-H2O clouds.
Venus is at a distance of 0.7 AU from the Sun and executes a complete orbit in 224.65 Earth days. The Venus rotation is the slowest from all the planets with values of 6.5 km/h at the equator (compared with 1670 km/h for the Earth) and with a period of 243 days. Despite this the upper atmosphere is moving at much faster speeds of 360 km/h inducing very powerful tornados. Previous observations have seen two major vortexes placed on the poles of the planet and moving on the local vertical with a periodicity of 3 days.
Despite the previous explorations the atmospheric circulation and the meteorological phenomena or the fluid dynamics which sustain these mechanisms are unknown meaning there is enough place for major discoveries in this field.
Another point of interest for the scientists is the exploration of the surface of the planet specially the composition of the soil and the volcanic activities which take place all over the places.

Until now several missions have searched on Venus: Venera, Mariner, Pioneer Venus, Vega, Magellan and Venus Express from ESA which is the only active satellite at this location. The big agencies have kept their interest for future Venus explorations- ESA with the BepiColombo which will be launched in 2014 and which in its way to Mercury will take the opportunity to observe Venus, NASA-through the New Frontiers program which intends to send a lander on the surface of the planet and Roscosmos- which will launch in 2016 a new Venera mission, the Venera D.

The first program has been started by USSR in February 1961 when the first Venera 1 mission was launched with the purpose of a direct collision with Venus. Some days later the satellite has been declared lost from the communication point of view and later measurements have shown it failed reaching the target passing at a distance of 100.000 km from Venus.
USSR did not stopped here and the exploration continued with the Venera 3 in 1966 (marking the first human object passing the atmosphere of a planet and hitting the surface), Venera 4 in October 1967, Venera 5 and 6 in 1969, Venera 7 in 1970 which survived the surface impact and successfully transmitted the first data from the planet, Venera 8, 9 and 10 which transmitted the first images, the Venera 11 and 12 which made measurements on the Venusian storms, Venera 13 and 14 which realized the first investigations on the soil and transmitted the first color images, Venera 15 and 16 which made use for the first time of the SAR (synthetic aperture radar) for mapping the planet.
The Venera missions continued in 1985 when two new satellites from the Vega program focused on the Venus atmosphere.

On the other side NASA started the exploration with the Mariner program and for a short time even an Apollo mission with human crew has been considered. After the loss of the first probe Mariner 1, the second one Mariner 2 succeeded to observe the physical conditions on Venus from a distance of 34.000 km, followed by the Mariner 5 in 1967 which approaches to 4000 km from Venus and the Mariner 10 in 1974.
Investigations continued with the Pioneer Venus – a complex project including the satellite itself (which orbited the planet between 1978 and 1992) and 4 special probes which collected information from atmosphere.
In 1989 NASA was sending the Magellan spacecraft which in the 4.5 years of operations around Venus and with the help of the onboard radar succeeded to scan 98% of the surface and to make the most complete map.

For short periods some other satellites have been performing Venus observations i.e. Galileo, Cassini or Messenger.

Still, currently, as mentioned before the only active satellite around Venus is the Venus Express of ESA. VEX has been launched in November 2005 and reached the desired orbit (89.99 degrees x 24 hours orbital period) in April 2006.
It is a 1270 kg satellite (of which 570 kg of fuel), equipped with the following scientific instruments: VMC (Venus monitoring camera), ASPERA (analyzer of space plasma and energetic atoms), PFS (planetary Fourier spectrometer), VIRTIS (visible/ultraviolet/near infrared mapping spectrometer), MAG (Venus Express magnetometer), VeRa (Venus radio science experiment), SPICAV/SOIR (ultraviolet and infrared atmospheric spectrometer). Theoretically, from the scientific point of view, VEX should be complementary to the Japanese solution meaning that the data collected by JAXA should complement the VEX observations.

So, what happened with the Japanese probe? On the 6th of December Akatsuki started the maneuvers for orbital injection around Venus. The satellite had loaded onboard the sequence of commands necessary for this autonomous maneuver (there was no possibility to control it from ground as the reentry did not occurred on the visible side of the planet).
In general, the entry in a dense atmosphere as the one from Venus and the orbital injection can be made in 2 ways: using a big maneuver or using a succession of small maneuvers, both techniques having advantages and disadvantages.
For the Akatsuki satellite the first solution has been considered, the principal engine OME being ignited at 23:49 GMT for a burn of 12 minutes, a communication blackout of 22 minutes (another 2 smaller maneuvers on 11 and 13 December intended to perform the latest orbital corrections).
Unfortunately the ground stations re-established the connection with the spacecraft lately after 90 minutes at 01:28 GMT when they have seen the failure of the orbital injection, the spacecraft being found in safe mode and with telemetry at low rate via LGA.
From the analysis of the data collected in the internal memory it has been found that the main engine functioned for only 152 seconds while the acceleration progressively decreased in parallel with the decrease of the pressure in the fuel tank. In the interval 152-158 s the acceleration dropped suddenly from 0.8 to 0.5 and the satellite lost its stabilization and rotated by 42 degrees. Almost immediately AOCS switched the stabilization from the RCS system (by commanding the closure of the fuel valves) to the RWL system.

However due to the high rotation previously induced and because of the limited momentum of the RWL, the spacecraft could not stabilize itself and went down autonomously to the safe mode at the moment 375s.

The engineers are still doing investigations. It appears that the uncontrolled rotation has been produced by a malfunction of the main engine. OME has a special ceramic expansion cone and from the first observation it is thought that this one has been somehow damaged. Initially it was taken into account the possibility of an impact with small space debris which could break the ceramic nozzle but this has a very low probability. Then the engineers made the connection between the decrease of the pressure in the tank and the OME. Thus, because of an inconstant fuel supply, the engine started to work intermittently and this added on top of the heat produced by the atmosphere re-entry could induce a thermal stress on the ceramic nozzle breaking it. When this thing finally happened the engine’s jet has been suddenly redirected to another vector, inducing a major perturbation and the loss of the stabilization. In the end the cause of this anomaly could be a defective valve of the Helium pressure installation or even worst a pipe that could be broken. Encountering this kind of anomaly is not new for JAXA which already has a negative score in the field of propulsion technology.

This is the second failure of the Japanese agency after the one of the Nozomi spacecraft which had as destination the planet Mars. The first orbital injection attempt in 1999 has failed due to a defective valve the spacecraft being left in a large orbit around the Sun. The second attempt was intended to take place in 2003 but in 2002 a solar flare hit the onboard equipments and canceled any possibility to run the maneuver.

Later on major propulsion problems affected the Hayabusa satellite, the success of the maneuvers performed above the Itokawa asteroid being a subject of debate until the last moment when, after returning to Earth, the specialists analyzed the probes collected by the onboard capsule.

JAXA has no other choice and as with the satellites mentioned earlier it will go further and try to recover the mission. The Akatsuki spacecraft, despite the failure of the orbital injection, could be used for some other experiments and for gaining experience which in the end could prove vital for the future, as with the lesson learned from the Nozomi and which helped operating the Hayabusa spacecraft in its way back to Earth.

Still a question stays in place- how difficult if not impossible it will be to perform a future orbital injection around Venus. Despite of the more than enough onboard reserve of fuel, even if the defective valve will be isolated, there is still a suspicion over the functionality of the OME engine and which should be clarified in the following days by performing small activations of the thruster and by monitoring the performances of the propulsion. If indeed the nozzle is broken JAXA will have a very difficult mission to calculate the next orbital injection when a very precise direction for thruster firing is requested. A good chance would be if the hole in the ceramic nozzle has increased in the past days, something that should cause its complete cease and detach from the engine. If this is the case- with no nozzle left, even if the thrust force decreases at least will be easier to keep the direction.

On long term, considering the current orbit which will bring the satellite back to Venus somewhere in December 2016 or January 2017 it will be interesting to see if the batteries, the solar panels and the rest of the electronics will last until then, taking into account that they have been designed for a 2 years lifetime and for a different kind of orbit.