Sojourner (rover)
The robotic Sojourner rover reached Mars on July 4, 1997 as part of the Mars Pathfinder mission. Sojourner was operational on Mars for 92 sols, 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. 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 by the time communication was lost. Its final confirmed command was to remain stationary until October 5, 1997, 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 of the 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. It is a six-wheeled, long, wide and high vehicle. In the mission's cruise phase, it occupied an high space and has a mass of. It was supported by a lander, a tetrahedron-shaped structure with a mass of, and had a camera, scientific instrumentation, three petals of solar panels, a meteorology mast, and 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 Deep Space Network antenna, 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 of solar cells on its petals. The lander contained a stereoscopic camera with spatial filters on an expandable pole called Imager for Mars Pathfinder, and the Atmospheric Structure Instrument/Meteorology Package 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 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 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. 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 per pixel at a range of. 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 above the Martian surface and extends Pathfinders horizon to on a featureless plane.
Power system
Sojourner had solar panels and a non-rechargeable lithium-thionyl chloride 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 of solar cells, which could produce a maximum of about 15 watts on Mars, depending on conditions. The cells were GaAs/Ge with approximately 18 percent efficiency. They could survive temperatures down to about. 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
The rover's wheels were made of aluminum and were in diameter and wide. They had serrated, stainless steel tracks that could generate a pressure of in optimal conditions on soft ground. No such need arose during the operational phase. Each wheel was driven by its own independent motor. The first and third wheels were used for steering. A six-wheel-steering configuration was considered, but this was too heavy. As the rover rotated on itself, it drew a wide circle.The wheels were connected to the frame through specially developed suspension to ensure all six were in contact with the ground, even on rough terrain. JPL's Don Bickler developed the wheels, which were referred to as "Rocker-bogie", for the experimental "Rocky" vehicles, of which the Sojourner is the eighth version. They consisted of two elements; "Bogie" connected the front wheel with the central one and "Rocker" connected the rear wheel with the other two. The system did not include springs or other elastic elements, which could have increased the pressure exerted by each wheel. This system allowed the overcoming of obstacles up to high but theoretically would have allowed the rover to overcome obstacles of, or about 30% of the rover's length. The suspension system was also given the ability to collapse on itself so the rover would occupy in the cruising configuration.
The locomotion system was found to be suitable for the environment of Mars—being very stable, and allowing forward and backward movements with similar ease—and was adopted with appropriate precautions in the subsequent Spirit and Opportunity rover missions.
In the ten-year development phase that led to the realization of Sojourner, alternative solutions that could take advantage of the long experience gained at JPL in the development of vehicles for the Moon and Mars were examined. The use of four or more legs was excluded for three reasons: a low number of legs would limit the rover's movements and the freedom of action, and increasing the number would lead to a significant increase in complexity. Proceeding in this configuration would also require knowledge of the space in front—the ground corresponding to the next step—leading to further difficulties. The choice of a wheeled vehicle solved most of the stability problems, led to a reduction in weight, and improved efficiency and control compared to the previous solution. The simplest configuration was a four-wheel system that, however, encounters difficulties in overcoming obstacles. Better solutions were the use of six or eight wheels with the rear ones able to push, allowing the obstacle to be overcome. The lighter, simpler, six-wheeled option was preferred.
The rover could travel from the lander—the approximate limit of its communication range— and had a maximum speed of.
Hardware and software
Sojourner's central processing unit was an Intel 80C85 with a 2 MHz clock, addressing 64 kilobytes of memory, and running a cyclic executive. It had four memory stores; 64 Kb of RAM made by IBM for the main processor, 16 Kb of radiation-hardened PROM made by Harris, 176 Kb of non-volatile storage made by Seeq Technology, and 512 Kb of temporary data storage made by Micron. The electronics were housed inside the rover's warm electronics box. The WEB is a box-like structure formed from fiberglass facesheets bonded to aluminum spars. The gaps between facesheets were filled with blocks of aerogel that worked as thermal insulation. The aerogel used on the Sojourner had a density of approximately 20 mg/cc. This insulator was designed to trap heat generated by rover's electronics; this trapped heat soaked at night through the passive insulation maintaining the electronics in the WEB at between, while externally the rover experienced a temperature range between.The Pathfinder lander's computer was a Radiation Hardened IBM Risc 6000 Single Chip with a Rad6000 SC CPU, 128 megabytes of RAM and 6 Mb of EEPROM memory, and its operating system was VxWorks.
The mission was jeopardised by a concurrent software bug in the lander that had been found in preflight testing but was deemed a glitch and given a low priority because it only occurred in certain unanticipated heavy-load conditions, and the focus was on verifying the entry and landing code. The problem, which was reproduced and corrected from Earth using a laboratory duplicate, was due to computer resets caused by priority inversion. No scientific or engineering data was lost after a computer reset but all of the following operations were interrupted until the next day. Resets occurred on July 5, 10, 11 and 14 during the mission before the software was patched on July 21 to enable priority inheritance.