Jet Propulsion Laboratory (U.S.). Mars Pathfinder Project.

The Mars Pathfinder landed on Mars July 4, 1997 after a twenty-year hiatus of landers. It was the first spacecraft ever to send a rover out to independently explore the Martian landscape. Mars Pathfinder also was the second of NASA's planetary Discovery missions designed to foster low-cost spacecraft with highly focused science objectives. The Mars Pathfinder (formerly known as the Mars Environmental Survey, or MESUR, Pathfinder) was launched atop a Delta 7925, a Delta II Lite launch vehicle with nine strapped-on solid-rocket boosters and a Star 48 PAM-D upper third stage booster, at 1:58 a.m. EST on 4 December 1996, from the launch complex 17B at Cape Canaveral, Florida.

The cruise stage was jettisoned 30 minutes before atmospheric entry trajectory as Mars Pathfinder approached Mars at 26, 460 kilometers per hour (16,600 miles per hour) and a mean flight path of 14.2 degrees. The lander took atmospheric measurements as it descended. The entry vehicle's heat shield slowed the craft to 400 m/s in about 160 seconds. A 12.5-meter billowing parachute was deployed at that time, slowing the craft to about 70 m/s. The heat shield was released 20 seconds after parachute deployment, and the bridle, a 20-meter long braided Kevlar tether, deployed below the spacecraft.

The lander separated from the backshell and slid down to the bottom of the bridle over about 25 seconds, at an altitude of about 1.6-km. The radar altimeter acquired the ground and about 10 seconds before landing, four air bags inflated in about 0.3 seconds forming a 5.2 meter diameter protective 'ball' around the lander. Four seconds later at an altitude of 98 meters, the three solid rockets, mounted in the back-shell, fired to slow the descent and about two seconds later the bridle was cut 21.5 meters above the ground, releasing the airbag-encased lander.

The lander dropped to the ground in 3.8 seconds, impacted at a velocity of 18 m/s, approximately 14 m/s vertical and 12 m/s horizontal. Pathfinder rebounded about 12 meters (40 feet) into the air and bounced at least another 15 times before coming to rest approximately 2.5 minutes after impact and about 1 km from the initial impact site. After landing, the airbags deflated and retracted. Pathfinder opened its three metallic triangular solar panels (petals) 87 minutes after landing.

The lander first transmitted the engineering and atmospheric science data collected during entry and landing, the first signal being received at Earth at 2:34 p.m. EDT. The imaging system obtained views of the rover and immediate surroundings and a panoramic view of the landing area and transmitted it to Earth.

The landing site, Ares Vallis, was chosen because scientists believe it was a relatively safe surface to land on and contained a wide variety of rocks washed down into this flood basin during a catastrophic flood. During its exploration of the surface, Sojourner did rely on the lander primarily for communications with Earth and for imaging support. After some maneuvers to clear an airbag out of the way, ramps were deployed and the rover, stowed against one of the petals, rolled onto the surface on 6 July 1997 at about 1: 40 a.m. EDT. This began the exploration of the Martian northern lowlands.

The mission consisted of a surface rover and a stationary lander controlled by a derivative of the commercially available IBM 6000 computer that had a processor and associated components that were radiation-hardened and mounted on a single electronics board. The computer had a 32-bit architecture that executed about 20 million instructions per second. The computer stored flight software as well as engineering and science data, including images and rover information, in 128 megabytes of dynamic random access memory.

The lander's camera, the IMP (Imager for Mars Pathfinder), provided much of the images and science knowledge that came from the Mars Pathfinder mission. The mission's primary objective was to demonstrate 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, the lander and Earth and tests of the imaging devices and sensor.

Also, the maneuverability and systems of the free-ranging robotic rover (named Sojourner, after African-American abolitionist Sojourner Truth; who lived during the tumultuous era of the American Civil War, in 1861-1867), as a technology experiment on the red planet's surface. The rover was a six-wheeled vehicle, which was controlled and operated by remote control by an Earth-based operator, who used images obtained by both the rover and lander systems to guide the rover across the planet's terrain.

Sojourner was powered by a 0.2-square-meter (1.9-square-foot) solar array, sufficient to power the rover for several hours per day, even in the worst dust storms. As a backup and augmentation, lithium thionol chloride-D-cell sized batteries were enclosed in the rover's thermally protected warm electronics box. Thermal insulation was provided by a nearly weightless material called silica aerogel.

Three radioisotope heater units (RHUs) - each about the size of a flashlight C-cell battery - contained small amounts of plutonium-238 (about 2.6 grams, less than 1/10th of an ounce each) which gave off about 1 watt of heat each to keep the rover's electronics warm.

The rover's wheels and suspension used a rocker-bogie system that was unique that it did not use springs. Rather, its joints rotated and conformed to the contour of the ground, providing the greatest degree of stability for traversing rocky, uneven surfaces. A six-wheel chassis was chosen over a four-wheel design because it provided greater stability and obstacle-crossing capability.

Six-wheeled vehicles can overcome obstacles three times larger than those crossable by four-wheeled vehicles. For instance, one side of Sojourner could tip as much as 45 degrees as it climbed over a rock without tipping over. The wheels were 13 centimeters (5 inches) in diameter and made of aluminum. Stainless steel treads and cleats on the wheels provided traction and each wheel could move up and down independently of all the others.

Three motion sensors along Sojourner's frame could detect excessive tilt and stop the rover before it got dangerously close to tipping over. Sojourner was capable of scaling a boulder on Mars that was more than 20 centimeters (8 inches) high and could keep on going.

The Pathfinder Lander and the Sojourner Rover were great successes in July and August 1997. During its initial thirty-day mission, the lander returned 1.2 Gbits of data and 9600 images. Sojourner returned 384 images. On September 27, 1997, communication was lost with the Pathfinder, after meeting the August predictions that its battery would be the first thing to fail. Pathfinder had a design lifetime of thirty days, Sojourner of seven days. Both exceeded expectations.

The Mars Pathfinder mission cost approximately $265 million including launch and operations. Development and construction of the lander cost $150 million and the rover about $25 million. Missions operations for Mars Pathfinder were conducted at the Jet Propulsion Laboratory; Dr. Matthew Golombek was project scientist. Science data, both raw and processed, was transferred after a period of validation to NASA's Planetary Data System. The Deep Space Network's 70-meter (230-foot) and 34-meter (110-foot) antennas in Madrid, Spain were used to support entry communications.