The Sojourner rover was a vehicle that reached Mars on July 4, 1997 as part of the Mars Pathfinder mission. Sojourner was operational on Mars for 92 sols (95 Earth days), and was the first wheeled vehicle to operate on an astronomical object other than the Earth or Moon. The landing site was in the Ares Vallis channel in the Chryse Planitia region of the Oxia Palus quadrangle. The rover was equipped with front and rear cameras, and hardware that was used to conduct several scientific experiments. It was designed for a mission ending 8 sols, with a possible extension to 30 sols, and was active for 83 sols (85 Earth days). The rover communicated with Earth through the Pathfinder base station, which had its last successful communication session with Earth at 3:23 a.m. PDT on September 27, 1997. The last signal from the rover was received on the morning of October 7, 1997. Sojourner traveled just over 100 meters (330 ft) by the time communication was lost. Its final confirmed command was to remain stationary until October 5, 1997, (sol 91) and then drive around the lander; there is no indication it was able to do so. The Sojourner mission formally ended on March 10, 1998, after all further options were exhausted.
Mission
Sojourner was an experimental vehicle whose main mission was to test in the Martian environment technical solutions that were developed by engineers from different NASA research laboratories. It was necessary to verify whether the design strategy followed had resulted in the construction of a vehicle suitable for the environment it would encounter, despite the limited knowledge of it. Careful analysis of the operations on Mars would make it possible to develop solutions to critical problems identified and to introduce improvements for subsequent planetary exploration missions. One of the mission's main aims was to prove the "faster, better, cheaper" approach embraced by the NASA administration. Development took three years and cost under $150 million for the lander, and $25 million for the rover; development was faster and less costly than all previous missions. These objectives required careful selection of the landing site to balance the technical requests with the scientific ones. A large plain was needed for the probe to land and rocky terrain to verify the rover's systems. The choice fell on Ares Vallis in Chryse Planitia, which is characterized by alluvial-looking rock formations. Scholars believed the analysis of the rocks, which lie in what appears to be the outlet of a huge drainage channel, could have confirmed the past presence of liquid water on the surface of Mars and provide details of the surrounding areas, from which the rocks were eroded.
Technical characteristics
Sojourner was developed by NASA's Jet Propulsion Laboratory (JPL). It is a six-wheeled, 65 cm (26-inch) long, 48 cm (19-inch) wide and 30 cm (12-inch) high vehicle. In the mission's cruise phase, it occupied an 18 cm (7.1-inch) high space and has a mass of 10.6 kilograms (23 lb). It was supported by a lander, a tetrahedron-shaped structure with a mass of 250 kg (550 lb), and had a camera, scientific instrumentation, three petals of solar panels, a meteorology mast, and 6 kg (13 lb) of equipment that was required to maintain communications between the rover and the lander. Hardware included a steerable, high-gain X-band antenna that could send approximately 5.5 kilobits per second into a 70 m (230 ft) Deep Space Network antenna, 3.3 m2 (36 sq ft) gallium-arsenide solar arrays that generated 1.1 kW⋅h/day and were capable of providing enough power to transmit for 2–4 hours per sol and maintain 128 megabytes of dynamic memory through the night.
Lander
One of the lander's main tasks was to support the rover by imaging its operations and sending data from the rover to Earth. The lander had rechargeable batteries and over 2.5 m (8.2 ft) of solar cells on its petals. The lander contained a stereoscopic camera with spatial filters on an expandable pole called Imager for Mars Pathfinder (IMP), and the Atmospheric Structure Instrument/Meteorology Package (ASI/MET) which acted as a Mars meteorological station, collecting data about pressure, temperature, and winds. The MET structure included three windsocks mounted at three heights on a pole, the topmost at about one meter (3.3 ft) and generally registered winds from the west. To provide continuous data, the IMP imaged the windsocks once every daylight hour. These measurements allowed the eolian processes at the landing site, including the particle threshold and the aerodynamic surface roughness, to be measured. The square eyes of the IMP camera are separated by 15 cm (5.9 in) to provide stereoscopic vision and ranging performance to support rover operations. The dual optical paths are folded by two sets of mirrors to bring the light to a single charge-coupled device (CCD). To minimize moving parts, the IMP is electronically shuttered; half of the CCD is masked and used as a readout zone for the electronic shutter. The optics had an effective pixel resolution of one milliradian per pixel which gives 1 mm (0.039 in) per pixel at a range of one meter (3.3 ft). The camera cylinder is mounted on gimbals that provide rotation freedom of 360° in azimuth and −67° to +90° in elevation. This assembly is supported by an extendible mast that was designed and built by AEC Able Engineering. The mast holds the camera at approximately 1.5 m (4.9 ft) above the Martian surface and extends Pathfinder's horizon to 3.4 km (2.1 miles) on a featureless plane.
Power system
Sojourner had solar panels and a non-rechargeable lithium-thionyl chloride (LiSOCl2) battery that could provide 150 watt-hours and allowed limited nocturnal operations. Once the batteries were depleted, the rover could only operate during the day. The batteries also allowed the rover's health to be checked while enclosed in the cruise stage while en route to Mars. The rover had 0.22 m2 (2.4 sq ft) of solar cells, which could produce a maximum of about 15 watts on Mars, depending on conditions. The cells were GaAs/Ge (Gallium Arsenide/Germanium) with approximately 18 percent efficiency. They could survive temperatures down to about −140 °C (−220 °F). After about its 40th sol on Mars, the lander's battery no longer held a charge so it was decided to shut off the rover before sunset and wake it up at sunrise.
Locomotion system
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