The Mars Pathfinder, launched in December 4, 1996 by NASA, was a hugely successful Mars exploration probe.
The landing site was an ancient flood plain in Mars' northern hemisphere called "Ares Vallis" and is among the rockiest parts of Mars. It was chosen because scientists found it to be a relatively safe surface to land on and one which contained a wide variety of rocks deposited during a catastrophic flood.
The probe consisted of a lander and of a lightweight (10.6 kilograms/23 pounds) wheeled robot (Rover) called Sojourner ("one in a break from journeying"), after the sometime slave, abolitionist, and women's-rights activist Sojourner Truth.
Mars Pathfinder used an innovative method of directly entering the Martian atmosphere, assisted by a parachute to slow its descent through the thin Martian atmosphere and a giant system of airbags to cushion the impact.
The lander relayed transmissions to and from the robot, allowing it to operate independently of the probe body. The robot was remotely controlled, but had a basic camera-assisted autonomous control system allowing it to navigate and negotiate minor obstacles without operator intervention.
The robot's freedom of movement allowed the exploration team to closely analyze many more rocks and soil samples than with a traditional probe. From its landing in July 4, 1997 until the final data transmission on September 27, 1997, Mars Pathfinder returned 16,500 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and soil and extensive data on winds and other weather factors. Findings from the investigations carried out by scientific instruments on both the lander and the rover suggest that Mars was at one time in its past warm and wet, with water existing in its liquid state and a thicker atmosphere.
The exact reason for the final failure of the lander is not certain, but probably it was a failure of a battery, resulting in night-time cooling of the spacecraft that would render it inoperable. The rover was instructed by automatic backup procedures to return to the lander, but its exact location and state are unknown. However, the lander and rover performed much longer and better than expected. NASA efforts to recontact Pathfinder ended March 10, 1998.