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Let's go to Mars

Pathfinder bounces onto Mars and lets Sojourner out

Airbags took the shock: the lander rebounded twelve metres, bounced at least fifteen more times across Ares Vallis, then opened to release Sojourner, 10.6 kg of rover.

A small boxy rover with a flat, dark solar panel across its top stands on reddish, rock-strewn ground at the left of the frame, three cleated wheels visible along its near side. It is pushed up against a large rounded tan boulder that stands taller than the rover is long. A pale, jointed arm reaches from one end of the rover to the boulder's shadowed face. Smaller stones are scattered across the plain behind, which rises to a low horizon under a hazy tan sky.
Sojourner places its alpha proton X-ray spectrometer on the surface of Yogi, a boulder the mission's controllers nicknamed after a cartoon character. The Imager for Mars Pathfinder took this view over sols 8, 9 and 10, in July 1997. · Image: NASA/JPL

At 16:56:55 UT on 4 July 1997 a tetrahedron wrapped in twenty-four airbag lobes hit Ares Vallis at 18 metres per second — 14 straight down, 12 sideways — bounced at least fifteen more times, the first about twelve metres high, and stopped roughly a kilometre from first contact. The bags deflated, winches dragged them in, and three petals folded open. Nothing had worked on the surface of Mars since Viking 1 fell silent in 1982.

Pathfinder cost about $265 million, against roughly a billion 1970s dollars for Viking. With no orbital reconnaissance and no orbit insertion, the cruise stage was jettisoned and the capsule entered the atmosphere at 7.3 kilometres per second, deployed a parachute, dropped its heat shield, lowered the lander down a twenty-metre braided Kevlar bridle, inflated the bags in about three-tenths of a second, fired three solid retrorockets at 98 metres, and cut the bridle 21.5 metres up. Every step was one-shot, the full sequence unrehearsed. Luna 9 had bounced onto the Moon in 1966; nothing like this had been tried at Mars.

Sojourner rolled out: 10.6 kilograms, 65 centimetres long, six wheels on rocker-bogie suspension, a top speed of about one centimetre per second. A signal took more than ten minutes each way, so it was never driven live; operators sent waypoints and hazard rules and it felt its way. Its one composition instrument, an alpha proton X-ray spectrometer, was pressed against rocks named Barnacle Bill, Yogi and Shark. The readings suggested compositions more silica-rich than the basalts Viking implied, indicating a differentiated crust — still argued over, because dust coatings and weathering rinds skew the numbers.

The lander was built for 30 sols, the rover for seven; the lander returned data for 83, until contact was lost on 27 September 1997 for reasons never established. Sojourner covered about a hundred metres in all, never more than a dozen from the lander. The mission's website took tens of millions of hits a day.