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An artist's concept of the Mars Observer in orbit around Mars. This mission would have been NASA's first spacecraft to Mars in two decades.

Mars Observer, launched by NASA in September 25, 1992, was the first of the proposed Observer series of planetary missions, and was designed to study the geoscience and climate of Mars. Contact with the spacecraft was lost three days before scheduled orbit insertion and was never re-established.

The primary science objectives for the mission were to:

1. determine the global elemental and mineralogical character of the surface material;
2. define globally the topography and gravitational field;
3. establish the nature of the Martian magnetic field;
4. determine the temporal and spatial distribution, abundance, sources, and sinks of volatiles and dust over a seasonal cycle;
5. explore the structure and circulation of the atmosphere.

The bus and electronics of the Observer series of spacecraft, used to study the terrestrial planets and near-Earth asteroids, were derived from the Satcom-K and DMSP/TIROS spacecraft. The rectangular bus section was 2.1 x 1.5 x 1.1 m. During the cruise phase of the mission, the high-gain antenna and the booms for the magnetometer (MAG/ER) and gamma-ray spectrometer (GRS) were partially deployed. When fully deployed, the two booms were each 6 m long. The 1.5 m diameter high-gain antenna was, when fully deployed, on a 5.5 m boom to allow for clearance over the solar array when the antenna was pointed toward Earth.

Pointing control for the spacecraft was maintained through the use of four reaction wheels. Attitude information was provided by a horizon sensor (which defined the direction of the nadir when the spacecraft is in Martian orbit), a 6-slit star scanner (for inertial attitude), gyros and accelerometers (for measuring angular rates and linear accelerations), and five sun sensor heads tied into a single sun sensor electronics box.

Mars Observer launch preparations

Power was provided through a six-panel solar array which, when fully deployed, measured 7.0 x 3.7 m. During the cruise phase, however, only four panels were deployed (due to the proximity of the spacecraft to the sun) to reduce the amount of power generated. During periods when the spacecraft was in Mars' shadow, energy was to be provided by two Ni-Cd batteries, each with a capacity of 43 ampere-hours.

The interplanetary cruise phase of the mission was intended primarily for spacecraft and instrument checkout and calibration. Two periods of data collection for the MAG/ER and GRS and one for the gravity wave experiment were planned for this phase as well. During the four month period from Mars orbital insertion until the spacecraft was to achieve its final mapping orbit, only data collection for the MAG/ER, GRS, and Thermal Emission Spectrometer (TES) were scheduled. The mapping phase of the mission was scheduled to nominally last one Martian year. Mars Observer was also to support the acquisition of data from the Russian Mars 1994 mission through the use of the joint French-Russian-American Mars Balloon Relay instrument.

Contact with Mars Observer was lost on August 21, 1993, three days before scheduled orbit insertion, for unknown reasons and has not been re-established. It is not known whether the spacecraft was able to follow its automatic programming and go into Mars orbit or if it flew by Mars and is now in a heliocentric orbit. It was speculated that there may have been an explosion in a propellant line during pressurisation procedures just before the orbital insertion engine burn. It is believed that hypergolic fuel may have leaked past valves in the system during cruise to Mars, allowing the fuel and oxidiser to combine prematurely before reaching the combustion chamber. The engine was derived from an Earth orbital satellite's and not really designed to lie dormant for many months before being fired.

Although none of the primary objectives of the mission were achieved, cruise mode data were collected up to loss of contact. The total cost of the Mars Observer mission including development, construction, launch, and ground support is estimated at US$980 million. However, science instruments originally developed for Mars Observer are being used by three other orbiters, the Mars Global Surveyor, launched in 1996, Mars Odyssey, launched in 2001, and the Mars Reconnaissance Orbiter, launched in 2005.

• Exploration of Mars
• Space exploration
• Unmanned space missions
• Planetary Observer program

• Mars Observer at NSSDC Master Catalog
• The Loss of Mars Observer at Malin Space Science Systems
• NASA Spacelink - Mars Observer