Sojourner

A wheeled robot on Mars

Sojourner was a small robotic rover carried to the surface of Mars by the Mars Pathfinder lander, which touched down on July 4, 1997. NASA describes Sojourner as the first wheeled vehicle operated on another planet in the solar system ^[1]. It weighed 23 pounds ^[1]; NASA's Jet Propulsion Laboratory (JPL) gives the rover's mass as 10.6 kilograms ^[2].

The name came out of a competition. JPL and The Planetary Society organized a contest for a name for the rover, and NASA chose to name it after Sojourner Truth, inspired by the winning essay written by 12-year-old Valerie Ambrose of Bridgeport, Connecticut ^[1].

What it carried

According to JPL, the 10.6-kilogram rover carried an Alpha Proton X-ray Spectrometer and a three-camera imaging system consisting of two black-and-white cameras and one colour camera ^[2].

The spectrometer is an instrument for working out what a rock or patch of soil is made of. In general terms, it bombards a sample with particles and reads the particles and X-rays that come back out; the energies and numbers of those signals reveal which chemical elements are present. A rover carrying such a device can therefore measure composition at many different spots instead of only where it happens to sit. The cameras served a different purpose: two black-and-white and one colour camera recorded the terrain around the vehicle, giving both a detailed and a colour view of the same landscape ^[2].

Driving around the lander

Spacecraft are built to a design life — the period for which engineers expect the hardware to keep working. It is a planning figure rather than a hard limit, and missions sometimes run well past it. On this mission both the lander and the rover outlived their design lives, and the rover did so by 12 times ^[2].

A NASA technical report records how the rover used its mobility: it traversed 100 metres clockwise around the lander and explored about 200 square metres of the surface ^[3]. Keeping the lander as a fixed reference point, the rover worked its way around it and examined the ground at many places within that area rather than only at the touchdown site ^[3].

Where it sits in Mars exploration

NASA says the landing of Mars Pathfinder and its Sojourner rover on July 4, 1997 began an uninterrupted period of Mars exploration ^[1]. The distinction that matters here is mobility. A lander that stays put can only study what lies within reach of its own position, however good its instruments are. A wheeled vehicle turns a single landing site into a region: instruments can be brought to different rocks and soils, and the same patch of ground can be looked at from several directions. That change is what makes a rover a different kind of mission from a stationary lander.

Sojourner was small and its traverse was measured in tens of metres, but NASA describes it as the forerunner of later rovers including Curiosity and Perseverance ^[1]. The line from a 23-pound experimental vehicle to the much larger rovers that followed runs through the design questions Sojourner had to answer first: how a wheeled robot is carried to another planet, how it is driven on a surface nobody has inspected up close, and how it keeps working after its planned lifetime has passed.