The Mars Pathfinder was
the second of NASA's low-cost planetary Discovery missions to be launched. The
mission consists of a stationary lander and a surface rover. The mission had the primary
objective of demonstrating the feasibility of low-cost landings on and exploration of the
Martian surface. This objective was met by tests of communications between the rover
and lander, and the lander and Earth, tests of the imaging devices and sensors, and tests
of the maneuverability and systems of the rover on the surface. The scientific
objectives include atmospheric entry science, long-range and close-up surface imaging,
rock and soil composition and material properties experiments,
and meteorology, with the general objective being to characterize the Martian environment
for further exploration. (Mars Pathfinder was formerly known as the Mars Environmental
Survey (MESUR) Pathfinder.)
The rover, which has been named "Sojourner" is a six-wheeled vehicle, 280 mm high, 630
mm long, and 480 mm wide with a ground clearance of 130 mm, mounted on a "rocker-bogie" suspension. The rover was
stowed on the lander at a height of 180 mm. At deployment, the rover extended to its
full height and rolled down a deployment ramp at about 05:40 UT on 6 July 1997 (1:40 a.m.
EDT). The rover was controlled by an Earth-based operator who used images obtained
by both the rover and lander systems. Note that the time delay was between 10 and
15 minutes depending on the relative position of Earth and Mars over the course of the
mission, requiring some autonomous control, provided by a hazard avoidance system on the
rover. The on-board control system is an Intel 80C85 8-bit processor which runs
about 100,000 instructions per second. The computer is capable of compressing and
storing a single image on-board. The rover is powered
by 0.2 square meters of solar cells, which will provide energy for several hours of
operations per sol (1 Martian day = 24.6 Earth hours). Non-rechargeable lithium thionyl
chloride (LiSOCl2) D-cell batteries provide backup. All rover communications were done
through the lander.
The rover is equipped with black and white
and color imaging systems which were used to image the lander in order to assess its
condition after touchdown. The goal was to acquire three black and white images spaced 120
degrees apart of the lander. Images of the surrounding terrain were also acquired to study
size and distribution of soils and rocks, as well as locations of larger features. Imaging
of the rover wheel tracks will be used to estimate soil properties. Imaging of the rover
by the lander was also done to assess rover performance and soil and site properties. The
rover's performance was monitored to determine tracking capabilities, drive performance,
thermal behavior, and sensor performance. UHF Communications between the rover and lander
were studied to determine the effectiveness of the link between the rover and lander.
Assessments of rock and soil mechanics will be made based on abrasion of the wheels and
adherence of dust. An alpha-proton-X-ray spectrometer (APXS)
is on-board the rover to assess the composition of rocks and soil. Images of all samples
tested are transmitted to Earth. The primary objectives were scheduled for the first seven
sols, all within about 10 meters of the lander. The extended mission included slightly
longer trips away from the lander, and even longer journeys were planned. Images were
taken and experiments performed by the lander and rover until 27 September 1997 when
communications were lost for unknown reasons.
The landing site in the Ares Vallis region of Mars is at 19.33 N, 33.55 W. The lander has
been named the Sagan Memorial Station. The Ares Vallis region of Mars is a large outwash
plain near Chryse Planitia. This region is one of the largest outflow channels on Mars,
the result of a huge flood (possibly an amount of water equivalent to the volume of all
five Great Lakes) over a short period of time flowing into the martian northern lowlands.