Mars Pathfinder lander

A fixed platform with a rover

The Mars Pathfinder lander was the half of the Mars Pathfinder mission that stayed on the ground. It weighed 793 pounds, and it carried a 23-pound rover called Sojourner, which it deployed on July 4, 1997 ^[1]. The two machines split the work. The lander remained in one spot as a platform for cameras, weather instruments and communications, while the rover drove a short distance away to look at individual rocks and patches of soil. Splitting a mission this way lets engineers put heavy, power-hungry instruments on the lander and keep the moving vehicle light.

Landing inside airbags

Pathfinder reached the surface under a parachute and then used a giant system of airbags to cushion its impact; this was the first time the airbag technique had been used ^[2]. A parachute slows the fall through a thin atmosphere, but not enough for a gentle touchdown on legs. The airbags instead let the spacecraft hit the ground and bounce: inflated bags wrapped around the structure absorb the impact, and the craft rolls until it comes to rest. That removes the need for a prepared, flat landing site and for the very precise final descent that a rocket-powered landing on legs demands. Once the craft was down, the bags could be deflated and the rover sent out.

Instruments on the mast

The lander carried a mast-mounted Imager for Mars Pathfinder, built around a stereoscopic colour camera together with a magnetometer and an anemometer ^[1]. A stereoscopic camera takes two views from slightly different positions, and comparing them yields depth, which is how the shape of distant rocks and hills can be measured rather than merely photographed. A magnetometer measures magnetic fields, and an anemometer measures wind speed. The lander also carried the Atmospheric Structure Instrument and Meteorology package, which recorded conditions in the atmosphere and at the surface during the descent and the stay on the ground ^[1].

A name for the site

After the touchdown succeeded, the lander was formally named the Carl Sagan Memorial Station ^[2]. Giving the landing site its own name is a way of marking a place on another world as a location rather than just a set of coordinates, and it ties the hardware to the person the name remembers.

What it sent back

The mission returned more than 16,500 images from the lander ^[2]. Both the lander and the rover outlived their design lives, the lander by nearly three times ^[2]. A design life is the period the hardware is expected to keep working under the cold, dusty conditions of the surface; lasting longer meant more images and more weather records than had been planned for, and more time to drive the rover around the landing site.

Where it fits

The airbag landing was the mission's main technical departure: it showed that a spacecraft could arrive on Mars without a prepared pad, a rocket-powered descent or fixed legs ^[2]. That matters because the surface is rough and the atmosphere is too thin for a parachute to do the whole braking job on its own. The lander's other contribution is quieter: a long stream of pictures and weather measurements from a single spot, and the pairing of a fixed station with a small rover, each doing what the other cannot.