The American satellite Wise ends the mission

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Post Tue Jul 19, 2011 9:47 am

The American satellite Wise ends the mission

On 17th of February 2011, after more than a year of space operation, the American satellite Wise ended officially the mission. At 20:00 GMT the command centre has sent a sequence of commands which have sent the satellite in hibernation state until further notice.

Despite the fact that the satellite is completely functional from the perspective of the onboard equipment, it is however useless from the scientific point of view. The entire mission is based on sensible infrared observations and for this the satellite needs to cool its detectors (using liquid hydrogen circulated through special cryogenic installation).
The mission was estimated initially to have a lifetime of 10 months, enough to perform 1.5 full scans of the celestial sphere and to produce approximately 2.7 millions images, but in Mai 2010, as the satellite was approaching the end of the liquid hydrogen provision and because the satellite was still working fine, a special commission took into account a possible extension of the mission for another 3 months. At that time, the commission considered inadequate to operate the mission in the classical scenario which would spend another 6.5 millions dollars but without a scientific importance. It found however a compromise by using 2 of the onboard detectors (able to work without cryogenic cooling) for the search of the near Earth asteroids.
This additional phase called “NEOWISE Post-Cryogenic Mission” costs NASA 400.000 dollars/month, the satellite being able to focus its attention on the comets and asteroids from our solar system. With this occasion it has found 20 new comets, more than 33500 asteroids (from which 134 have orbits potentially dangerous for the Earth).
The first catalog containing the data collected by the Wise mission (including the first 14 weeks spent in space) will be made public in April 2011, but more interesting will be the complete catalog from March 2012, because it will confirm or not the speculations in the astronomical world about the presence of the so called Tyche planet.
Bellow we will quote the original article published on SpaceAlliance in December 2009 when the Wise satellite has been launched.

The infrared observatories series has increased after the launch of the Wise satellite on 15th of December 2009.

The launch has been performed from the Vandenberg base at 14:09 GMT marking the 92nd consecutive success of a Delta 2 rocket starting with Mai 1997. The first stage ignited for 4 minutes and 39 seconds, followed by the burn of the second and then the third stage. This one worked until T0+10:26 bringing the satellite in an intermediate parking, 97.5 degrees inclination orbit, the apogee at 553 km and the perigee at 185 km. The second orbital correction has been done by a short 8.5 seconds burn at T0+51:40 followed at T0+55:57 by the separation and the confirmation of the mission’s success.

The entire flight has been coordinated using the TDRS system (Tracking and Data Relay Satellite System).
For 16 days the satellite will keep the protection of the telescope’s mirror, but in middle January, it will unfold it and after the entire instrument’s calibration will take place, it will start to work and to deliver the first scientific data.

Wise or “Wide field infrared survey explorer” has started initially in 2004 as NGSS (Next generation sky survey) as the sixth satellite from the Midex class (Medium Class Explorer) which had to be developed by NASA and to be launched aboard the Taurus rocket.
Later on the launcher has been changed for a Delta 2 type 7320 rocket. This is equipped with 3 solid fuel type GEM-40 boosters and has two active stages: a RS-27A for the first stage and an AJ-10-118K-ITIP for the second one. The flight configuration 10C is able to carry into orbit a load of up to 2703 kg for a LEO and 1579 kg for a SSO.

The satellite, which is belonging to NASA, will be coordinated by the JPL on the technical side and by UCLA and Caltech on the scientific side- a team which has been involved previously in the COBE/WMAP/Spitzer projects. The first contractor is the Ball Aerospace division but the scientific instrumentation is built by Space Dynamics Laboratory.

The entire contract has a cost of 320 millions dollars and has been developed in less than 5 years (in August 2004 NASA selected the project and in April 2005 has started the design of the satellite).
The satellite is built on a RS300 spacecraft bus, is three axes stabilized, equipped with fixed solar panels attached directly to the structure and playing also a role in the thermal protection. It weights 661 kg and has a 2.85 m height x 1.73 m diameter Aluminum structure.

The system of attitude and orbit control has to precisely stabilize the in flight position during the observing periods. It is equipped with two star cameras mounted on opposite sides of the satellite in order to minimize the effect of perturbations induced by the bright objects near the satellite (Earth, Moon or Sun). The active control is realized using 4 reaction wheels.

The principal instrument is a 40 cm diameter telescope connected to 4 infrared detectors each of 1 million pixels. The telescope contains 10 curved mirrors and 2 plate ones each built from Aluminum covered by gold which increases the reflexive characteristic. One of the mirrors, called scan mirror, moves opposite the satellite’s movement at a synchronized speed in such a way to compensate the rotational effect and to ensure precise successive images. The visual field obtained in this way is approximately 47 arcmin.

The 4 detectors function at different wavelengths (3.4, 4.6 and 12 µm with a resolution of 6 arcsec and respectively 22 µm with a resolution of 12 arcsec) and at 4 predefined scenarios: “one frame”, “one orbit”, “two orbits” and “many orbits”.

The detector itself incorporates the last generation technologies. For example the ones from the 3.4 and 4.6 gammas are made from Te, Hg and Ca alloy which radiates electrons under the light coming from the stars. These electrons are further captured by the electronics of the instrument. The detectors from the 12 and 22 gamma are based on an As doped Si semiconductor.

Because Wise is going to capture the infrared radiation coming from cold space objects it is important that the telescope and the detectors to be kept at a cold temperature and not to be contamined by its own radiation emission. Therefore they have very strict requirements: 12K for the telescope, 8K for the 12&22 detectors and 32K for the 3.4 & 4.6 detectors.

For keeping the operating conditions, both the telescope and the detectors have been placed in a giant thermal recipient- a cryostat system built by Lockheed Martin and supplied by two containers containing liquid hydrogen. The temperature is later regulated by circulating the cryogenic agent through a special pipes system or if heating is wanted, by activating a system of thermistors.

Wise will ensure a complete observation of the sky hoping to create a specialized infrared catalog after more than 25 years since the precedent NASA experiment – the IRAS (Infrared Astronomical Satellite) mission. In comparison with the previous missions the sensitivity increased by approximately 500 times compared with COBE (which observes in the 3.5-4.9 µm), and some hundred times more comparing with IRAS (12-25 µm) (probably the best characteristic which reflects the technological advancement is the number of pixels on the detector which increased from 62 for IRAS to 1 million in Wise’s case). The data collected by Wise will be further used by the future JWST mission.

So how is cataloged the new satellite within the infrared observatories already in the orbit taking into account that its instruments are modest compared with some other satellites?
Wise is a wide view surveyor so a sky mapper – generating permanent observations of the celestial sphere. Based on this information the astronomers can ask for detailed punctual investigations (only on a specific target) with more powerful instruments. One of them is the huge telescope installed onboard the European satellite Herschel.
This is equipped with a 3.5 m diameter mirror which is 4 times bigger than the previous infrared telescopes and 1.5 times bigger than the one of Hubble.
With these characteristics the Herschel telescope is able to capture 12 times more light than its predecessor the ISO satellite.
This is not the only difference with Wise but also the spectral range which varies from 60 to 670 µm.
The two platforms have also two different orbits: a Lissajous 800.000 km amplitude orbit around the Lagrange point L2 for Herschel and a circular sun-synchronous 500 km altitude orbit around Earth for Wise.
The Lissajous orbits around the L2 point are today preferred against the orbits around the Earth because they ensure a better thermal stabilization (up to some degrees Kelvin compared with 30-40 K for the other) and a smaller orbital drift (0.1 AU/year for a heliocentric observatory). A mission around L2 is however much more expensive but for its niche (medium resolution space observer) Wise was preferred to make a compromise and to be placed in a LEO orbit.

Wise will orbit around the poles following the separator line between eclipse and illumination, with the telescope placed in the direction opposite to that of the Earth. The side exposed to the solar radiation will be protected by a thermal shield while the other one will benefit from the low natural temperature in the eclipse condition.

The satellite will have around 15 orbits per day, 4 being used to download the data produced by the instruments.
Flying around the Earth and following it in its natural orbit around the Sun, it will take half of the orbital period i.e. 6 months to completely scan the sky. For comparison the cryogenic reserve is calculated for 10 months so the satellite is able basically to perform 1.5 scans during the predicted lifetime (and to cover eventually gaps).

Wise will find the brightest galaxies in the Universe, it will perform observations on the closest stars to the Sun, on the near Earth asteroids and comets (in total 100.000 new objects are expected), it will allow studies over the planetary evolution or how the stars formed in the galaxies, it will do investigations over the cold stars so called “brown dwarfs”.

With an image acquired every 11 seconds, the specialists expect a number of 1.500.000 pictures for the end of the mission.

Six months after the mission ends the first consolidated data should be offered to the astronomers (the first version of the expected infrared atlas) while the final version will appear 17 months latter.

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Post Tue Jul 16, 2013 10:14 am

Re: The American satellite Wise ends the mission

The adavancement in Space research and knowledge always keeps us exciting. thanks for your contribution to that fiels and sharing such a post.
Cryostat have many practical applications in many industries like science,engineering,medicine and other such fields.

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