Engineers are deploying a lightweight version of a test rover (foreground) that will better simulate Mars’s gravity, which is 38% that of Earth’s (Image: NASA/JPL-Caltech)

In the struggle to free the Mars rover Spirit from a sand trap, NASA engineers are bringing out the reserve troops. A second, lighter duplicate rover slid into a sandbox for testing this week, delaying any attempt to free Spirit by as much as three weeks, to mid-September.

Spirit has been stuck in a sandpit for nearly four months. Since late June, engineers have been trying to determine the best moves to extricate it by driving a test rover around a sandbox at NASA’s Jet Propulsion Laboratory (JPL) in California.

Rover engineers had earlier announced that they were almost done with the testing and would be ready to move Spirit around 10 August, but they backtracked in a meeting on 6 August.


“We’ve come up with additional tests that we want to do, and additional computer modelling that we want to do as well,” says rover project manager John Callas at JPL. “Now we’re looking at the middle of September.”

Coarser sand

One cause for the delay is the difference between the duplicate rover’s performance in the sandbox and the actual rover’s driving on Mars. The test bed contains a sand cocktail that supposedly approximates the flour-like soil in which Spirit is stalled.

But engineers noticed that the test rover actually did worse driving through the simulated sand than Spirit did during its last few drives on Mars.

“The loose, fluffy soil we have here on the ground is worse than what there is on Mars,” Callas told New Scientist.

Engineers are filling a second test bed at JPL with crushed aggregate, a heavier, courser material. “It gives us a second data point,” Callas says. “We don’t know exactly what we have on Mars, so we’re trying to explore as best we can with the resources we have on the ground.”

Another question is how the weaker Martian gravity affects the rover’s driving. The first duplicate rover, which comes fully equipped with a scientific payload and a robotic arm, “is as close to flight [i.e., Spirit] as anything we have on the ground”, Callas says.

On Earth, Spirit weighs about 180 kilograms, but on Mars – whose gravity is about three-eighths that on Earth, Spirit weighs just 64 kilograms.

Lower gravity

So engineers have decided to test a second mock rover. That rover is lighter because it does not carry scientific instruments or a robotic arm. “It’s what we call SSTV-Lite, or Surface System Testbed Vehicle-Lite,” Callas says.

It has been around since the beginning of the five-year mission, and was used to help Spirit’s twin, Opportunity, enter Endurance Crater in 2004. This “lesser-known fourth sibling” weighs in at 73 kilograms, so it should act more like the rovers do under Martian gravity, says Callas.

“We’ll be able to explore the effects of gravity and the effects of soil choice on the results that we’ve collected,” Callas says.

With a failed front-right wheel, a rock under its belly, and a history of amnesia, Spirit is in delicate shape. So scientists are dragging their feet to make sure its first step is the strongest possible.

“Our very best chance at getting Spirit out will be our very first chance at getting Spirit out,” Callas says. “We want that first try to be the highest probability success manoeuvre… That’s why we’re doing all this testing.”