Spacecraft and Subsystems
The Mars Polar Lander consists of a hexagonal base composed of aluminum honeycomb with composite graphite epoxy face sheets supported on three aluminum landing legs. The launch mass of the lander will be approximately 618 kg, including 56 kg of fuel and the two 2 kg microprobes. A thermally regulated interior component deck holds temperature sensitive electronic components and batteries and the thermal control system. Two solar panels extend out from opposite sides of the base. Mounted on top of the base are the robotic arm, the stereo imager and mast, a UHF antenna, the LIDAR, the MVACS electronics, the meteorology mast and the medium gain dish antenna. The MARDI is mounted at the base of the lander, and the propellant tanks are affixed to the sides. During cruise, the lander is attached to a cruise stage and enclosed in a 2.4 meter diameter aeroshell.
The spacecraft is three-axis stabilized during cruise using star cameras and sun sensors in conjunction with inertial measurement units. Four hydrazine cruise reaction engine modules, each consisting of one 5-lbf trajectory correction maneuver thruster and one canted 1-lbf reaction control system thruster, provide attitude control. The descent and landing propulsion system consists of three groups of four pulse modulated 266 N hydrazine engines. Control and knowledge for descent and landing is provided by a 4 beam doppler radar system and an AACS subsystem. The hydrazine is stored in two diaphragm tanks with a total capacity of 64 kg for both cruise and descent systems.
Communications between Earth and the spacecraft during cruise to Mars are via X-band using two solid state power amplifiers and a fixed medium gain antenna mounted on the cruise stage and backed up by a receive-only low gain antenna. During surface operations communications (downlink and uplink) will be via the UHF antenna on the lander to the Mars Climate Surveyor orbiter, which will function as a relay to Earth. The Mars Global Surveyor can also be used as a data downlink relay. Eight to ten relay passes over the lander will be available from each orbiter each day, but the number of communications sessions will be limited by power demands. A back up link to Earth is provided by the medium gain DTE (direct to Earth) 2-axis articulated antenna.
Power is provided during cruise phase by two gallium arsenide solar array wings with a total area of 3.1 square meters attached to the cruise stage. After landing, two gallium arsenide solar array wings with a total area of 2.9 square meters are deployed. Power is stored in 16 amp-hr nickel-hydride common pressure vessel batteries for peak load operations and night time heating. The payload is allocated 25 W of continuous power when operating.