Mars Pathfinder

The Mars Pathfinder probe landed on Mars on July 4, 1997. Pathfinder was second in the Discovery series of robotic spacecraft, which the United States National Aeronautics and Space Administration (NASA) began to develop in the mid 1990s. Costing an average of $150 million per project, the Discovery shift to faster, cheaper, less-ambitious probes was prompted by the catastrophic failure in 1993 of the $1-billion Mars Observer mission. Pathfinder was the third spacecraft ever to land successfully on Mars; NASA's Viking I and Viking II spacecraft were the first and second, in 1976. There have been no successful landings since Pathfinder.

Mars Pathfinder left Cape Canaveral, Florida, on December 4, 1996 for a seven-month journey to Mars. Lofted by a Delta-II rocket, Pathfinder consisted of a 795–lb (360–kg) lander and a 25–lb (11.5–kg) surface rover named Sojourner in honor of Sojourner Truth (1797?–1883), ex-slave and abolitionist. Pathfinder had two main objectives: first, to demonstrate that certain new and economical technologies could be used to explore the Martian surface, and second, to perform scientific study of the area around the landing site. Although spacecraft preparation and testing were completed at the Jet Propulsion Laboratory (JPL) in Pasadena, California, many components, including the main scientific instrument aboard the rover (the α -proton x-ray spectrometer or APXS), were created through international cooperation. Spacecraft tracking and communication were provided by the giant dish antennas of the NASA/JPL Deep Space Network.

The complex process of landing safely on Mars began several days before arrival, when ground controllers at JPL used the Deep Space Network to tell the spacecraft to prepare for landing. Because radio signals require approximately 40 minutes to travel from Earth to Mars and back again, Earth-based control of Mars landings—which require real-time coordination of a complex series of events—is impractical; therefore, for Pathfinder, just as for Viking 21 years earlier, all critical actions had to be performed by the spacecraft itself during the landing phase. On-board software was in control of the landing sequence from about 30 minutes before landing until about three hours after.

The approach phase began with venting of the heat rejection system's coolant fluid about 90 minutes prior to landing. This fluid is circulated around the cruise stage (upper stage of the Delta-II rocket, crowned by Pathfinder) during the seven-month journey through space to Mars and also through the lander to keep it and the rover cool. About 30 minutes before landing, the cruise stage, having fulfilled its purpose of sustaining and protecting the lander and rover during their journey through space, was jettisoned. The remaining entry vehicle consisted of the lander and rover inside a saucer-shaped protective aeroshell.

Several minutes before landing, the spacecraft entered the thin Martian atmosphere at a shallow angle. If a steep angle of approach had been used, powerful rockets would have been needed to achieve a soft landing; as it was, a low-angle entry made a cheaper landing possible by allowing Pathfinder to convert most of its kinetic energy to heat through friction with the atmosphere.

Though the Martian atmosphere is thinner than Earth's (e.g., Martian surface is pressure only 0.001 that of Earth's), the amount of force and heat generated by atmospheric braking was great. In only a few minutes, atmospheric friction slowed the entry vehicle from about 17,000 MPH (27,000 km/h) to only 900 MPH (1,400 km/h). On-board instruments detected this rapid deceleration and triggered further preprogrammed events.

First, a parachute 24 ft (7.3 m) in diameter was opened to slow the vehicle from 900 MPH to 145 MPH (230 km/h). Twenty seconds after parachute deployment, the heat shield (i.e., the bottom half of the aeroshell) was blasted free. Up to this point, the landing scenario had been similar to that for Viking, but from here onward events comprised a test of new lander technologies.

Soon after the heat shield was released the lander (with rover attached) dropped away from the backshell (upper half of the aeroshell). The lander then lowered itself down a metal tape using a built-in braking system. This descent placed the lander at the bottom end of 65–ft (20–m) braided-Kevlar tether termed the bridle, providing space for airbags to inflate around the lander and a safe margin of distance from the hot exhaust that would soon come from the solid rocket motors on the back-shell. Once the lander was in position at the end of the bridle, the radar altimeter (altitude-measuring instrument) on board the lander was activated. This aided in timing airbag inflation, backshell rocket firing, and cutting of the bridle. Three roughly triangular airbags attached to Pathfinder, each approximately 17 ft (5.2 m) in diameter about eight seconds before landing. Four seconds later, electrical wires running up bridle then ignited the three backshell rockets simultaneously. The firing of these rockets, starting at an altitude of 250–300 ft (75–90 m), brought the lander to an approximate stop about 40 ft (12 m) above the surface. The bridle connecting the lander and backshell was then snipped in the lander, causing the backshell (with its rockets still firing) to fly rapidly upward, carrying itself and the parachute away from the landing site. Meanwhile the lander, encased in its airbags, fell directly to the surface.

Because of the possibility of the backshell being at a small angle when its rockets fired, rather than perfectly level, the lander's speed upon impacting the surface could have been as high as 56 MPH (25 m/sec) at a 30° angle to the ground. The spacecraft designers planned for the lander to bounce at least 40 ft (12 m) above the ground and soar 330–660 ft (100–200 m) between bounces. (In fact, took a series of much smaller bounces.) This landing system provided an economical means of getting the probe safely to the surface without the high cost of heavy landing engines and fuel.

Once the lander stopped bouncing and rolling, pyrotechnic devices in the latches holding its sides together were blown off to allow three sides to open like petals of a flower. Soon after, small winches began reeling the airbags toward the lander, deflating them in the process. As the lander might, after rolling, have come to rest on its side, a motor was built into each petal with enough power to roll the whole probe right over if necessary. The lander was designed to sense its orientation and to open one of its petals first, if necessary, to right itself before opening the other two.

During the three hours allotted for retracting the airbags and opening the lander petals, the lander's radio transmitter was turned off. This saved battery power (the lander was powered both by solar panels and by batteries) and allowed the transmitter electronics to cool down after reentry. It also allowed the Earth to rise well above the horizon and be in a better position for communications. At this point, while the lander still had many tasks to fulfill, it had successfully completed one of its main objectives—it was now the first of the new low-cost probes to land on Mars.

Information on the Martian atmosphere had already been gathered by measuring changes in Pathfinder's radio signal during its descent, but Pathfinder's primary scientific data-gathering activities began only after the lander was safely deployed on the surface. Radio signals from the lander enabled scientists on Earth to precisely locate the spacecraft and, therefore, Mars itself. Pathfinder's tasks also included taking pictures of the area around the lander so that scientists could study the geology of the surface, measuring atmospheric conditions, and relaying measurements gathered by Sojourner, the rover, back to Earth. The imager for Mars Pathfinder (IMP) was a sophisticated stereoscopic camera system that produced both black-and-white and color images. The camera was attached to an extendable mast that allowed it to be raised, lowered, swiveled, and aimed up or down. Detailed panoramic images were created with the IMP, giving a full view of the landing site and of the activities of Sojourner.

In addition to normal and stereoscopic pictures, the IMP was able to study the atmosphere and surface geology through filters, providing valuable information about the chemical makeup of both. A close-up lens allowed observation of wind-blown dust captured by a small magnet, revealing information about the magnetic properties of the dust. By imaging the sun through filters, the content of the atmosphere was observed. Characteristics of dust particles in the atmosphere were measured by observing one of the moons of Mars at night. Images of windsocks located at different heights above the ground were used to measure wind speed and direction. Since the lander was designed to make these kind of observations for over a month, daily cycles could be observed and compared. Because the Pathfinder lander continued to work for over three months, some seasonal changes were also observed. Comparison of Pathfinder's data with Viking's made both more scientifically valuable.

Perhaps the most exciting aspect of the Mars Pathfinder mission was the small Sojourner rover mounted to the main lander. Looking like a tiny (just under 11 in [4.3 cm] high, 2 ft [61 cm] long and 19 in [48.26 cm] wide), flat-topped all-terrain-vehicle with six independent wheels, this device moved down a ramp from its perch on top of the lander onto the soil of Mars. Primarily a technology experiment, like the landing mechanisms of the lander, Sojourner was also equipped with an on-board communications system, laser detection devices to keep it from running into obstacles or going places that might make it fall over, and an α-proton x-ray spectrometer (APXS).

The first objective for the Sojourner was to show that it could function in the little-known environment on the surface of Mars and to observe its behavior in order to make design improvements in future rovers. Sojourner moved around the immediate area of the lander, butting the APXS up against rocks. Detectors measured interactions between a radioactive source in the APXS and the surface materials by obtaining an energy spectrum of the alpha particles, protons, and x rays produced by the exposure. This instrument could determine the chemical composition of materials, including the amounts present of most major elements except hydrogen.

After operating on the surface of Mars three times longer than planned and returning a large amount of information about the red planet, the Pathfinder lander—officially renamed the Sagan Memorial Station after landing, in honor of American scientist Carl Sagan (1934–1996)—finally became inactive in November 1997. Sojourner operated 12 times longer than the seven days it was designed to. Both Pathfinder and Sojourner drew their power from a combination of pre-charged batteries and solar panels, validating another new technology; the Viking probes had used plutonium as an energy source.

Pathfinder broadened our understanding of Mars and paved the way for future exploration by showing that the new, inexpensive exploration technologies can work. However, no lander has managed to land successfully on Mars since Pathfinder. The U.S. mission Mars Polar Lander crashed near the Martian south pole in 2000, prompting widespread questioning of the claim—seemingly proved by Pathfinder's spectacular success—that Mars missions could be built faster, cheaper, and better.

The next scheduled lander mission to Mars, Mars Exploration Rover, is due to attempt touchdown in January, 2004. Twin Mars Exploration Rovers will land at separate locations using the Pathfinder-tested airbag method. Each probe, if successful, will deploy a rover that is approximately ten times heavier than Sojourner and carries far more elaborate instrumentation.