Prime real estate. The landing zone for Curiosity (NASA/JPL)

As the world waits with bated breath for NASA's Curiosity rover to attempt a safe landing on Mars on August 6th (EDT), it's interesting to recall the rovers of times past. We've all heard about Spirit (R.I.P. 2010) and Opportunity (still kicking), and their immediate technological precursor Sojourner (part of the Mars Pathfinder mission), but long before these plucky robots turned their wheels in the martian regolith there was an earlier generation of explorers trying to make their mark.

In 1971 the Soviet Union launched two nearly identical spacecraft towards the red planet; Mars 2 and Mars 3. Each consisted of an orbiter and a lander, with the latter carrying remarkable little machines - the first Mars rovers.

Mars 2/3 lander, petals out

The plan was for the descent modules to enter the martian atmosphere, use their engines and attitude jets to control and slow their motion, and to deploy parachutes before making a fairly hard impact - instruments cushioned only by foam in the nearly spherical landing capsules. Each of these capsules was about four feet across, and together with their aero-braking heat shields and fueled retro rockets they amounted to about 2,500 lbs Earth-weight. On landing the capsules would open 4 'petals' to help right themselves (an approach duplicated by later rover missions) and orientate their instruments.

A Prop-M rover

The special passenger in each was a tiny 10 lb Earth-weight rover, known simply as 'Prop-M'. A Prop-M rover was attached to the landing capsule by a fifteen-meter umbilical, carrying power and telemetry. The method of locomotion was simple and ingenious; cross-country skiing. Each rover sat on a pair of metal skids, or skis, that would shuffle back and forth to inch the rover forward across Mars. Whoever designed this was clearly familiar with getting around in long snow-covered winters - and given that no-one knew what the surface consistency was on Mars this method of travel made a great deal of sense. It was also mechanically straightforward, barely more than a child's wind-up toy, with little to go wrong.

Except, sadly, surviving the rigors of actually trying to land on another planet. The Mars 2 lander failed due to an over-steep entry angle and became the first piece of human produced wreckage on Mars. Mars 3 fared somewhat better, achieving a historic soft landing - but transmitting for barely 15 seconds before failing. Had the landings gone better the Prop-M rovers would have been placed on the regolith by a mechanical arm before skiing off. Each had rudimentary obstacle detection mechanisms - simple metal rods that if pushed would cause the rover to turn, and each carried instruments to test for regolith density and penetration depth at regular intervals. The lander capsule itself carried a scanning electronic camera (not unlike that on the later Viking landers) and atmospheric measuring devices (including a mass spectrometer). It also had the ability to examine surface samples for chemistry and constitution - and particularly to test for the presence of organic (carbon) compounds.

Not Mars, just noise (probably) - the first and only image transmitted by the Mars 3 lander in 1971

Unfortunately the only image transmitted by Mars 3 appears to have consisted of noise. Had the mission lasted for a few minutes more Prop-M could have become the first tourist to be photographed on the surface of Mars. The Soviet scientists also knew that by observing the tracks left by the Prop-M rovers they would learn about the composition of the martian regolith, a trick that was eventually used decades later.

Despite the failures in landing, the Mars 2 and 3 orbiters sent back a wealth of information on the martian environment and topography, a remarkable achievement coming barely 2 years after humans set foot on the Moon.

So on August 6th 2012, as we wait nervously to hear whether Curiosity has made it safely to the interior of Gale Crater, spare a thought for those pioneering rovers that almost managed to ski on Mars more than forty years ago.