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Mars Express Orbiter

From Academic Kids

Oblique view of the Reull Vallis near the Hellas basin, rendered from data obtained by the Mars Express orbiter's High Resolution Stereo Camera (HRSC)
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Oblique view of the Reull Vallis near the Hellas basin, rendered from data obtained by the Mars Express orbiter's High Resolution Stereo Camera (HRSC)

The Mars Express Orbiter is part of the Mars Express program, a European Space Agency (ESA) mission to Mars. It consists of the orbiter and a lander, Beagle 2.

The scientific objectives of the Mars Express Orbiter are to obtain global high-resolution photo-geology (10 m resolution), mineralogical mapping (100 m resolution) and mapping of the atmospheric composition, study the subsurface structure, the global atmospheric circulation, and the interaction between the atmosphere and the subsurface, and the atmosphere and the interplanetary medium.

The Beagle 2 lander objectives were to characterize the landing site geology, mineralogy, and geochemistry, the physical properties of the atmosphere and surface layers, collect data on martian meteorology and climatology, and search for possible signatures of life. However, the landing attempt was unsuccessful and the lander was declared lost.

Contents

Spacecraft and subsystems

The Mars Express Orbiter is a cube-shaped spacecraft with two solar panel wings extending from opposite sides. The launch mass of 1123 kg includes a main bus with 113 kg of payload, the 60 kg lander, and 457 kg of propellant. The main body is 1.5 m × 1.8 m × 1.4 m in size, with an aluminium honeycomb structure covered by an aluminum skin. The solar panels measure about 12 m tip-to-tip. A 1.8 m diameter high-gain antenna is mounted on one face, pointing in the same direction as the solar panels. Two 20 m long wire dipole antennas extend from opposite side faces perpendicular to the solar panels as part of the radar sounder. A 4 m tubular monopole low-gain antenna extends from the upper face. The body is built around the main propulsion system, which consists of a bipropellant 400 N main engine. The two 267-liter propellant tanks have a total capacity of 595 kg. Approximately 370 kg are needed for the nominal mission. Pressurized helium from a 35 liter tank is used to force fuel into the engine. Trajectory corrections will be made using a set of eight 10 N thrusters, one attached to each corner of the spacecraft bus. Attitude control (3-axis stabilization) is achieved using two 3-axis inertial measurement units, a set of two star cameras and two Sun sensors, gyroscopes, accelerometers, and four 12 N·m·s reaction wheels. Pointing accuracy is 0.04 degree with respect to the inertial reference frame and 0.8 degree with respect to the Mars orbital frame. Thermal control is maintained through the use of radiators, multi-layer insulation, and actively controlled heaters. The spacecraft configuration is optimized for a Soyuz/Fregat, but is fully compatible with a Delta II launch vehicle if necessary.

Spacecraft power is provided by the solar panels which contain 11.42 square meters of silicon cells. The originally planned power was to be 660 W at 1.5 AU but a faulty connection has reduced the amount of power available by 30%, to about 460 W. This loss of power is not expected to significantly impact the science return of the mission. Power is stored in three lithium-ion batteries with a total capacity of 64.8 A·h for use during eclipses. The power is fully regulated at 28 V, the peak power requirement at Mars is 450 W. Telecommunications are via the high-gain parabolic antenna and two omnidirectional low-gain S-band antennas. These provide X-band (7.1 GHz) and S-band (2.1 GHz) uplink and downlink. Two Mars lander relay UHF antennas are mounted on the top face for communication with the Beagle 2. The spacecraft is run by two Control and Data management Units with a 10 gigabit solid state mass memory for storage of data and housekeeping information for transmission.

The science payload consists of seven experiments. The high resolution stereoscopic camera (HRSC) is mounted inside the spacecraft body, aimed through the top face of the spacecraft, which is nadir pointing during Mars operations. A visible and near-infrared spectrometer (OMEGA), infrared spectrometer (PFS) and ultraviolet spectrometer (SPICAM) are also mounted inside pointing out the top face. The neutral and charged particle sensors (ASPERA) are mounted on the top face. The subsurface radar and altimeter is mounted in the body and is nadir pointing, and also incorporates the two 20 m antennas. The radio science experiment (MaRS) uses the communications subsystem. The total mass budgeted for the science payload is 116 kg.

Mission profile

Launch took place on a Soyuz/Fregat from Baikonur Cosmodrome on June 2 2003 at 23:45 local time (17:45 UT, 1:45 p.m. EDT), with the Mars Express and Fregat booster put into a 200 km Earth parking orbit. The Fregat was fired again at 19:14 UT to put the spacecraft into a Mars transfer orbit, and the Fregat and Mars Express separated at approximately 19:17 UT. The solar panels have been deployed and a trajectory correction maneuver was performed on June 4 to aim Mars Express towards Mars and allow the Fregat booster to coast into interplanetary space. Mars Express arrived at Mars after a 400 million km journey and a course correction in September, in December 2003. The Beagle 2 lander was released on December 19 at 8:31 UTC (9:31 CET) on a ballistic cruise towards the surface. On December 20, Mars Express fired a short thruster burst to put it into position to orbit the planet. The Mars Express Orbiter the fired its main engine and go into a highly elliptical 250 km × 150,000 km initial capture orbit with an inclination of 25 degrees on December 25 at 03:00 UT (10:00 p.m., December 24 EST).

The Beagle 2 lander was supposed to coast for five days after release and enter the martian atmosphere on the morning of 25 December. Landing was expected to occur at about 02:45 UT on 25 December (9:45 p.m. EST 24 December). A signal was to be sent to Mars Express after landing and another the next (local) morning to confirm that Beagle 2 survived the landing and the first night on Mars. No signals were received and the lander was declared lost.

First evaluation of the orbital insertion showed that the orbiter reached its first milestone at Mars. The orbit was later adjusted by four more main engine firings to the desired 259 km × 11,560 km near-polar (86 degree inclination) orbit with a period of 7.5 hours. Near periapsis the top deck is pointed down towards the Martian surface and near apoapsis the high gain antenna will be pointed towards Earth for uplink and downlink.

After 440 days the apoapsis will be lowered to 10,107 km and periapsis will be raised to 298 km to give an orbital period of 6.7 hours. Aerobraking can be used to modify the orbit if there are any problems with the main engine. Nominal mission duration is planned to be 1 Martian year (687 Earth days).

The overall Mars Express budget excluding the lander is 150 million (roughly US$185 million)

Scientific discoveries and important events

2004

  • January 23
    • ESA announced the discovery of water ice in the South Polar ice cap, using data taken on January 18 with the OMEGA instrument.
  • January 28
    • Mars Express Orbiter reaches final science orbit around Mars.
  • March 30
    • A press release announces that the orbiter has detected methane in the Martian atmosphere. Although the amount is small, about 10 parts in a thousand million, it has excited scientists ask about its source. Since methane is removed from the Martian "air" very fast, there needs to be a current source that releases fresh methane still today. Because one of the possible sources could be microbial life, it is planned to verify the reliability of this data and especially watch for difference in the concentration in various places on Mars. It is hoped that the source of this gas can be discovered by finding its location of release.
  • April 28
    • ESA announced that the deployment of the boom carrying the radar based MARSIS antenna was delayed. It described concerns with the motion of the boom during deployment, which can cause the spacecraft to be struck by elements of it. Further investigations are planned to make sure that this will not happen.
  • July 15
    • Scientis working with the PFS instrument announced that they tentatively discovered the spectral features of the compound ammonia in the Martian atmosphere. Just like methane discovered earlier (see above), ammonia breaks down rapidly in Mars' atmosphere and needs to be constantly replenished. This points towards the existence of active life or geological activity; two contending phenomena whose presence so far have remained undetected.

2005

  • February 8
    • The delayed deployment of the MARSIS antenna has been given a green light by ESA [1] (http://www.esa.int/esaCP/SEM42PXEM4E_index_0.html). It is planned to take place in early May 2005.
  • May 5
    • The first boom of the MARSIS antenna was successfully deployed [2] (http://news.bbc.co.uk/2/hi/science/nature/4516535.stm). At first, there was no indication of any problems, but later it was discovered that one segment of the boom did not lock [3] (http://news.bbc.co.uk/2/hi/science/nature/4528763.stm). The deployment of the second boom was delayed to allow for further analysis of the problem.
  • May 11
    • Using the Sun's heat to expand the segments of the MARSIS antenna, the last segment locked in successfully [4] (http://www.esa.int/esaCP/SEMY3R5TI8E_index_0.html).
  • June 14
    • The second boom was deployed, and on June 16 ESA announced it was a success [5] (http://www.esa.int/esaCP/SEMT1T1DU8E_index_0.html).
  • June 22
    • ESA announces that MARSIS is fully operational and will soon begin acquiring data. This comes after the deployment of the third boom on June 17, and a successful transmission test on June 19. [6] (http://www.esa.int/esaCP/SEMEV82DU8E_index_0.html)

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