Mars Observer

The Mars Observer, formally known as the Mars Geoscience–Climatology Orbiter, was a NASA robotic spacecraft launched on 25 September 1992 to conduct an extensive scientific survey of Mars. The probe was designed to study the planet’s surface, atmosphere, climate system and magnetic field, building upon data obtained from the Viking missions. However, communication with the spacecraft was lost on 21 August 1993, just three days before its scheduled insertion into Martian orbit, and all attempts to re-establish contact were unsuccessful.

Mission background and development

Plans for a new high-priority Mars mission emerged in 1984 following recommendations from the Solar System Exploration Committee. Initially conceived as the Mars Geoscience–Climatology Orbiter, the spacecraft was intended to expand knowledge of Martian geophysics and climatology, including high-resolution imaging at scales approaching one metre per pixel and acquisition of global elevation data. Early designs considered a Space Shuttle launch in 1990, although an expendable rocket alternative was also explored.
After the Challenger disaster, NASA rescheduled several missions, and in March 1987 the launch was deferred to 1992. During the redesign, cost overruns required the removal of two planned instruments. These were the Visual and Infrared Mapping Spectrometer (VIMS) and the Pressure Modulation Radiometer (PMR), both of which exceeded mass and cost constraints.
By the late 1980s the mission’s scientific goals were firmly established. The objectives included mapping global mineralogy, defining planetary topography and gravity fields, characterising the magnetic environment, documenting atmospheric volatiles and dust cycles, and observing the Martian meteorological structure. The total mission cost was estimated at approximately US$813 million.

Spacecraft structure and subsystems

The Mars Observer spacecraft had a bus derived from Earth-orbiting satellite designs, particularly the RCA AS4000 Ku-band platform. The spacecraft measured roughly 1.1 metres in height, 2.2 metres in width and 1.6 metres in depth, excluding its deployable appendages. Elements from RCA’s Television Infrared Observation Satellite and the Defense Meteorological Satellite Program informed the design of key subsystems including attitude control, command and data handling, and power distribution.
Attitude control and propulsionThe spacecraft employed three-axis stabilisation using four reaction wheels and twenty-four small thrusters. Propellant capacity totalled around 1,346 kilograms. A bipropellant system utilising monomethyl hydrazine and nitrogen tetroxide provided high-thrust capability, including 490-newton engines for major trajectory adjustments and Mars orbit insertion. Additional 22-newton thrusters supported roll control. A hydrazine monopropellant system offered lower-thrust manoeuvring and reaction-wheel desaturation. Orientation sensing relied on horizon sensors, a multi-slit star scanner and Sun sensors.
CommunicationsA gimballed 1.5-metre high-gain antenna, mounted on a six-metre boom, provided X-band communication with the Deep Space Network. The system incorporated dual transponders and redundant command detection units. Six low-gain antennas and a medium-gain antenna provided coverage during cruise and as contingency options. Data transmission rates could reach roughly 1,066 kilobytes per second, while command reception operated at up to 625 bytes per second. A low-power research payload, the Mars Observer Ka-Band Link Experiment, was included to evaluate future high-frequency communication technologies.
Power and onboard computingElectrical power was supplied by a six-panel solar array measuring approximately 7 metres by 3.7 metres, capable of generating around 1,147 watts in Martian orbit. Two nickel–cadmium batteries provided storage for eclipse periods. The onboard computer, adapted from systems used in the TIROS and DMSP programmes, processed stored commands autonomously for up to sixty days. Redundant digital tape recorders, each with a capacity of about 1.875 gigabytes, were used to store scientific data for later transmission.

Scientific payload

The mission carried an ambitious suite of instruments designed to investigate Martian geology, climate, atmosphere and magnetism.
Mars Observer Camera (MOC)Consisted of wide-angle and narrow-angle cameras for global and high-resolution imaging. The instrument supported meteorological, climatological and geologic studies, providing data on surface–atmosphere interactions and temporal surface changes.
Mars Orbiter Laser Altimeter (MOLA)Measured planetary topography with high vertical accuracy, generating elevation, slope and surface reflectivity data.
Thermal Emission Spectrometer (TES)Mapped surface mineralogy, rock and ice composition, atmospheric dust properties and the energy balance of polar caps. Measurements employed a Michelson interferometer, solar reflectance sensor and broadband radiometer.
Pressure Modulation Infrared Radiometer (PMIRR)Profiled atmospheric temperature, pressure, condensates and dust loading up to altitudes of approximately 80 kilometres. Although originally planned in earlier forms, PMIRR remained part of the final payload despite earlier instrument reductions.
Gamma-Ray Spectrometer (GRS)Determined elemental composition and hydrogen distribution in upper subsurface layers. Also monitored cosmic gamma-ray bursts and assessed atmospheric column density.
Magnetometer/Electron Reflectometer (MAG/ER)Characterised the Martian magnetic field, analysed crustal remanent magnetisation and examined interactions between the solar wind, plasma environment and Martian ionosphere using in situ and remote-sensing techniques.

Loss of spacecraft and mission legacy

During the final phase of cruise, engineers prepared Mars Observer for orbital insertion. On 21 August 1993, shortly after pressurisation of the propellant system, all telemetry ceased. Multiple communication attempts over succeeding weeks and months yielded no response. The precise cause of failure remains uncertain, although rupture or leakage in the pressurised fuel lines is a widely discussed possibility.