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Emily LakdawallaDecember 15, 2009

No, they can't push with the arm to free Spirit

I've gotten this question about once a week since Spirit got stuck, but yesterday, two different readers asked the same question within an hour of each other, so I figured it was time for a blog entry.

The question: Can't they use the robotic arm to help get Spirit out?

This question has many variants, including but not limited to: can they use the arm to push Spirit out? Can they use it to tilt Spirit? Can they just use it as a brace to help provide traction? Can they swing it to one side or another to shift Spirit's center of gravity in one direction or another? Can they use it to dig behind the wheels or push rocks under the wheels?

In brief, the answer to almost all of these questions is "no." Fortunately, there's an excellent resource online that explains why, the Spirit Extrication FAQ at Here are the highlights though.

The arm is a precision instrument placement tool. As such it is as heavy and strong as it needs to be to precisely position the instruments at the end of the arm onto targets of scientific interest. The whole arm weighs 2 or 3 kilograms, while the rover weighs 185 kilograms. So its weight is basically negligible -- its motion can't appreciably shift the rover's center of gravity. As for being used to push, the arm is only as strong as its joints. The joints can hold up against about 30 or 40 Newton-meters of static force; once you push the arm harder than that, the joints will simply bend under the force you impose on them. Gravity exerts a total force on the rover of nearly 700 Newtons. So the arm can't even take a tenth of the weight of the rover.

Interestingly, this is in marked contrast to the arm on Phoenix. The arm on Phoenix was not a precision scientific tool, it was a brute tool for excavation; there was a camera on the tip, and probes, but the main scientific instruments were on Phoenix' body, and required soil from the trenches to be funneled into them. I remember that there was some discussion that the Phoenix arm could theoretically be used to push against the ground and tilt the rover deck toward the faint southern sun, though I can't find an online reference for this; it may be one of those things that mission folks thought but didn't ever want to commit to paper. (Or maybe I'm just making things up. I'd appreciate it if someone could point me to a reference on this, or refute it.)

There are a few things that could theoretically be done with Spirit's arm that would sacrifice the arm as a scientific tool (leaving Spirit with only its mast-mounted cameras and Mini-TES for science), but practically speaking they would probably take too long, and they may cause other problems that would prevent it from moving after it was used for the extrication effort, anchoring the rover in place. Digging with the arm is not really possible, as there's nothing on its tip that would work as a scoop and in any case there's no software for the arm that can instruct it to dig -- new software would have to be developed, tested, debugged, tested again, simulated on Earth, uploaded to Mars, and then tested on Mars -- this is not a fast process. It might be possible to push rocks underneath the wheels using one of the scientific instruments on the arm, but there aren't many rocks nearby. Plus you'd need to write new software for rock-pushing, and since you can't drive it in real time it would probably take weeks of work just to shift one rock under one wheel. Actually the biggest thing around that you could stick under a wheel would be the tip of the arm itself; but driving over it would definitely break your instruments, and what's the point of a scientific rover without science instruments?

So, for all of you folks who want to see Spirit use the arm to get out of her mess, I'm sorry, but it's just not going to happen. And if Spirit remains stuck, we want that arm to work as well as possible so that that we can wring every last drop of science out of the mission before we finally turn the lights out. Read the Spirit Extrication FAQ for more answers to common questions.

Read more: Spirit, Phoenix, Mars Exploration Rovers, explaining technology

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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