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Pathfinder: NASA sends its first rover to Mars

Paul Sutherland, Feature writer
Feb 29, 2016, 14:17 UTC

Sen—Independence Day in the United States was celebrated for an extra reason on July 4, 1997, because it was the day that the nation’s latest mission to Mars landed on the planet.

At the time, the movie Independence Day had been a big hit, a tongue-in-cheek story of how an American President defeated an alien invasion. For Mars, an alien spacecraft had invaded, in the shape of Mars Pathfinder. It carried the first robotic rover to explore the Red Planet.

Pathfinder had been launched seven months earlier, on Dec. 4, 1996, atop a Delta II rocket from Cape Canaveral, Florida. It was a low-cost mission, at $265 million, and simpler than the Vikings had been. And it used a novel airbag system of cushioning the lander as it hit the Martian surface.

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The little rover Sojourner takes a look at a rock that was dubbed Yogi. Image credit: NASA

Unlike the Viking landers that had been despatched from orbit 20 years previously, Mars Pathfinder plunged directly into the Martian atmosphere, 30 minutes after jettisoning its cruise stage, as it completed its long journey from Earth. As instruments began gathering data, the spacecraft’s heat shield protected it for nearly three minutes before a 12.5-meter parachute slowed the craft and the heat shield was ejected.

Next, a 20-meter long tether was deployed, and the lander separated from its protective shell and slid to the end of it. Ten seconds before landing, four airbags inflated around the lander. Three solid rockets in the backshell fired to slow the descent further before the tether was cut releasing the lander. It bounced at least 16 times before coming to rest in the Ares Vallis region of Mars at a latitude of 19.33 degrees N. The region was chosen because it appeared safe for a landing and a region where ancient flooding deposited a wide variety of rock samples. The landing site was renamed Carl Sagan Memorial Station after the famous populariser of science.

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A diagram shows the design of Pathfinder, with Sojourner attached to one of the open petals. Image credit: NASA

The airbags were deflated and pulled back after the landing, allowing Pathfinder to open its three solar panels like the petals of a flower. The probe’s next task was to send data back to Earth about its descent and landing, including scientific observations of the atmosphere. This was followed by the first pictures to build up a panorama of its landing site.

Deployment of the Pathfinder rover, named Sojourner after civil rights campaigner Sojourner Truth, was delayed slightly because one of the airbags had failed to retract completely, but the problem was solved by lifting one of the “petals” and commanding it to retract again. Finally, and nearly 41 hours after landing, Sojourner was released and rolled down one of the ramps and onto the surface of Mars.

The little runabout, with a top speed of 1 cm per second, made more than 15 chemical analyses of rocks and soil, including three large boulders nicknamed Barnacle Bill, Scooby-Doo and Yogi. The lander took more than 16,500 pictures of its surroundings, while Sojourner took 550. Images of rounded pebbles, along with other scientific data collected, told scientists that Mars was probably warm and wet several billion years ago. The rocks were high in silica, but the chemistry of the soil was found to be similar to that at the two Viking landing sites. Whirlwinds, called dust devils, were imaged and early morning clouds of water ice spotted in the lower atmosphere which soon evaporated.

Though the mission enjoyed great success, it came to a sudden end on Sep. 27, 1997, when all communications with Pathfinder and Sojourner were lost. Despite repeated attempts to renew contact, nothing more was ever heard from them and the mission was officially ended on March 10, 1998.

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A diagram of Sojourner, showing its experiments and features. Image credit: NASA

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