Compared To Past Mars Rovers, Curiosity Is Cruising

Sixteen sols (solar days on Mars) after landing safely on Mars, Curiosity went for a drive. Engineers wiggled the rover's wheels, checked out all of its systems, and then set off. This first drive took 16 minutes; less than four was actual roving, the rest was spent angling the Mastcam down so it could see and take pictures of the wheels at every stage of the drive. The rover pulled forward about 15 feet, turned 120 degrees in place, then backed up 8 feet. It ended up about 20 feet from its original landing spot, which is now called Bradbury landing.

As far as Mars roving goes, Curiosity had an easy first drive. Not that getting to, landing on, and roving around another planet is simple. But Curiosity had the advantage over its predecessors of landing directly on its wheels. Previous Mars rovers – Sojourner, Spirit, and Opportunity – landed on platforms. To make their first drives and leave their first marks on the surface, they had to get off their landers. That can be a real tricky bit of time delayed remote controlled driving.

The Mars Pathfinder mission was the first to land on Mars by airbags. On July 4, 1997, the pyramid-shaped payload bounced along the surface and rolled to a stop at Ares Vallis. The airbags deflated and the sides of the pyramid opened like flower petals revealing the small suite of instruments inside. Attached to one of those petals was Sojourner, the two feet long, foot and a half wide, foot high, 23 pound rover.

In line with the rover's wheels on either side were two tightly rolled ramps. Made of reinforced kevlar mesh with steel guides for the wheels, they unfurled to give Sojourner two options to egress from the lander. There was nothing stopping it. Less than two Earth days after landing, the little rover rolled down the ramp to investigate interesting rocks in its vicinity. The camera on the lander caught the first image of Sojourner's tracks on Mars, clearly leading to the ramp that it rolled down.

The Mars Exploration Rovers (MER) Spirit and Opportunity followed (figuratively) in Sojourners tracks seven years later. Spirit landed in Gusev Crater on January 4 and Opportunity landed on January 25. Both used the same airbag landing system as Pathfinder, though the airbags and pyramid-landers were considerably larger to accommodate the larger rovers. The main difference in payloads on these missions was that Spirit and Opportunity were the main payload. When both landing pyramids opened after landing, there was nothing inside but the rovers.

Before engineers could drive either MER rover, they had to unstow and check all the systems. They also imaged the area around the lander to make sure they didn't drive the rover off into a ditch. Spirit landed first so was the first to reach the surface. On sol 8, the pyrotechnic devices holding the rover in place fired, freeing the wheels and releasing the pins that held its instrument arm in place. It was free to roll down one of the petals that doubled as an egress ramp and reach the surface.

But this was easier said than done. The kevlar mesh ramps that guided Sojourner were designed to flatten any stray airbag fabric out of the rover's way. The petals couldn't do that, and Spirit opened its eyes (well, took pictures and sent them back to engineers on Earth) to find that one of its ramps was blocked with fabric. Of course, it happened to be the forward facing ramp relative to the rover's orientation.

So the team had to turn Spirit around 120º before they could start driving. It was a tricky manoeuvre owing to all the pieces of material on the lander's surface. Luckily, the mission's scientists had put it there so knew exactly what to watch out for in executing the turn. On the 12th sol, Spirit relayed to Earth that it had just under 10 feet on its odometer and very little tilt – it looked like a perfect egress. But the real proof came from Hazcam pictures, pictures from the the rover's hazard cameras it uses to detect roadblocks on a drive. The team on Earth saw clear rover tracks in the soil leading right to the landing petal.

Opportunity had a different situation. It landed in a small crater the team named Eagle Crater; it was basically an interplanetary hole in one. After unfolding the rover's instruments and looking around with its cameras the team saw that the soil directly in front of the rover was flat and unobstructed. As was the egress ramp. So the team did something that almost never happens: they drove off the lander early. On it's 7th sol, Opportunity sent back a Hazcam image showing its tracks in the Martian soil. It was confirmation that the twin MER rovers were safe and operational on the surface.

Engineers behind Curiosity had that Hazcam confirmation, and celebration, immediately; within minutes of landing NASA had an image showing the rover's wheels safely on the surface. But it's that first drive, making the first tracks, that really signifies the start of a rover's mission. Curiosity is the fourth Mars rover to pass this mission milestone with flying colours. Now, it's onward to Glenelg.