Mars Pathfinder was the first mission since Viking to land on Mars. It
was truly a "pathfinder" in this aspect, paving the way for
future missions to Mars. Launched on December 4, 1996 by a Delta II rocket,
the Pathfinder lander rocketed to Mars in its cruise stage capsule with
a mini-rover, Sojourner, snug inside. The mission was a success, landing
at Ares Vallis (Mars Valley), an ancient floodplain, on July 4, 1997 --
to the favorability of the Americans. The versatile Sojourner rover traversed
off the lander to "sniff" out the composition of the Martian
surface. The final communications with Pathfinder happened in October,
when the electronics are believed to have broken because of the bitter
cold of the upcoming Martian winter or because of the degradation of the
rechargeable batteries. Although the communications failure was a disappointment,
Pathfinder had completed all of its science goals except for the Super
Panorama image which was 83% done. Pathfinder had exceeded its one month
lifespan by three times.
The Pathfinder lander -- now renamed the Sagan Memorial
Station after the late astronomer, Carl Sagan -- was built to take images
and carry out a few experiments on the Martian surface. Another job was
to communicate with the Sojourner rover. It then relayed information via
a high or low-gain antenna to one of Earth's tracking stations, called
the Deep Space Network. Pathfinder landed on Mars mid-morning July 4 when
its airbags were deployed for the 870 kg craft to bounce to a stop on
the surface. After a few minutes, the airbags deflated and the petals
were opened, revealing the Sojourner rover perched on top one of the petals.
Pathfinder is made up of three hexagonal-shaped petals attached to a base.
On the base are the Instrument Electronics Assemblies which housed the
electronics. They were also the white cover that supported the low and
high-gain antennas and the IMP (imager). To insulate the electronics,
2 inch thick phenolic honeycomb, ecofoam and graphite epoxy face sheets
were used. The assemblies were then painted with a substance called "flame-master
white" to absorb and keep heat in. On each of the petals, solar panels
were installed. These panels, along with the rechargeable batteries, were
used to power the lander.
One of the instruments on Pathfinder was the IMP (Imager
for Mars Pathfinder). It was a black and white, and color camera set on
a deployable mast 1 metre high. The camera's resolution was 256 x 524
pixels and included 12 filter wheels. Its mission was to take pictures
of the Martian surface, keep track of and communicate with the rover,
and to image the magnetic experiments and windsocks. The magnetic experiments
consisted of a magnet attached to various points of the lander; if there
were any magnetic particles in the Martian dust, it would be imaged by
the IMP. The IMP provided excellent resolution of the nearby area, and
was assigned the job to take the "super-pan" which was nearly
ASI/MET (Atmospheric Structure Instrument/Meteorology Package)
are the weather instruments. They contain pressure sensors, thermocouples
and windsocks. The pressure sensors measured the atmospheric pressure
at landing and on the surface. Three thermocouples -- placed at 25, 50
and 100 cm intervals -- on a mast measured the surface temperature. The
three windsocks, also spaced out on the mast, were hard cones. Windsocks
that are used on Earth would be too soft to detect the light Martian wind.
To find out the wind speed, the IMP was used to image the windsocks.
The Sojourner rover was an example of innovative technology.
Made at a cost of only $25 million US, Sojourner ambled out onto the Martian
floodplain, inspecting and imaging the nearby landscape. It was to demonstrate
low-cost rovers made with materials you could easily find at an electronics
shop. Being only 23 pounds and 65 x 48 cm in dimension, the foot high
rover seemed midget the large boulders. In addition to its minute size,
the communications were quite simple, with a UHF link and a 9600 baud
modem; the computer had a 80C85 processor and 0.5 megabyte of RAM, which
is considerably slower than most people's computers. It is powered by
0.25 m2 of solar panels (16 watt hours)
and non-rechargeable batteries (150 watt hours). Because of the low power,
Sojourner was only able to travel at about 1 cm/s and only from 10 a.m.
to 2 p.m. on each sol (Martian day). Because the rover could only communicate
with the lander via the antenna, it could not stray off within the lander's
line of sight.
Although Sojourner was quite simple in ways, the rover did have some high-tech
technology. Its aluminum wheels, three on each side, were each powered
by a motor. The "rocker-bogie" suspension system allowed each
wheel to follow the contour of the landscape to prevent tip-overs. Also,
Sojourner was semi-autonomous (could partly run by itself). There were
5 lasers at the front of the rover (side opposite the APXS) which detected
any hazards the rover could encounter. If an obstacle was in the way,
Sojourner would just back up and take a different route. This way, the
engineers at JPL did not have to always keep track and "drive"
the rover. They told it where to go and it would find a way to get there
itself. Engineers used the IMP to tell where the rover had gone. One of
the science instruments was the APXS, which was used to determine the
composition of the Martian surface. Furthermore, there were three cameras
mounted on the rover; two black and white cameras were on the front to
make stereo images and one color camera was on the back. To roll off the
lander petal, Sojourner had to traverse down one of two ramps which unfurled
on the lander.
Sojourner's mission was a success, lasting 12 times its expected week-long
lifetime. Its mission was cut off by the failure of the lander. Perhaps,
the rover could still be driving around the lander, trying to communicate
with its "mother-ship."
The WEB (Warm Electronics Box) is the gold-foiled box of
the rover. It, like the Instrument Electronics Assemblies of the the lander,
contained the electronics of the rover. The WEB is made out of 1.5 inches
of aerogel (a lightweight smoke-like solid) and low-density silicon to
insulate the rover. Gold foil was used on the outside of the box to absorb
any reflected heat.
The APXS (Alpha Proton X-Ray Spectrometer) is the main science
instrument on the rover. It is retractable from the back chassis of Sojourner.
The purpose of the APXS was to determine the composition of the Martian
rocks and soil by use of X-rays provided by alpha particle radiation.
This 560 g and 0.4 watt spectrometer measured all the elements except
hydrogen (a constituent of water). The APXS was powerful, measuring up
to 1/10 of a percent of a rock's composition when it was deployed up on
Wheel Abrasion Experiment
Sojourner was involved with the Wheel Abrasion Experiment.
This experiment consisted of imaging the reflectivity of a special rover
wheel to see how abrasive the soil was. A low reflectivity meant abrasive
soil. Also, at times, all the rover wheels except the special one were
stalled to increase the abrasion of the special wheel digging into the
Material Adherence Experiment
The Material Adherence Experiment is located at one of the
corners of Sojourner's solar arrays. It has a solar cell which measures
the amount of dust that settled on the solar cells. The less light, the
more dust. There was also a mass sensitive crystal on it which measured
the mass of the dust on the cell.