On Sol 1892 of its mission (01/May/2009) Spirit rover stopped its travel on Mars surface due to a "sand trap", initially (optimistically) defined as "difficult terrain".

Spirit at home plate

I have some questions:

  1. Why wasn't its robotic arm used to attempt rising it and change "sand configuration" and/or get away from the pointy rock under its belly?

pointy rock


  1. I remember at that time I read a press release saying something like "there is some little energy to attempt a last move before night, but we will wait tomorrow morning before attempting"; I can't find it anymore. Does anybody have the link to that page? Most of these links are dead, and anyway date back only to 2014, 5 years after the events

  2. Is the sand trap visible in at least one of these 3d pictures?

  3. Is there any video/gif of last moves before "falling" in the trap (link to raw images/frames)?

  4. Did anybody plot the values of temperature and solar power during last days? I looked around, also on github, with no luck. .

List of updates: https://www.nasa.gov/mission_pages/mer/spirit-update.html

  • 4
    $\begingroup$ Better question would be, why did Spirit continue driving into the sand trap instead of stopping as soon as it began to sink in? $\endgroup$
    – Vikki
    May 26 at 20:35
  • 2
    $\begingroup$ @vikki, there is possibly an interesting question there in terms of exact timeline, but the summary seems to be that they were driving backwards due to the failed motors and broke through a hard crust, so 'backing out' was a problem because they could really only travel in one direction and motion at all was hampered by the surface disintegrating under them. $\endgroup$ May 27 at 5:52
  • $\begingroup$ Regarding the question in the title and looking at images of the rovers a companion question for biology.SE pops up in my mind: Why didn't T-Rex scratch his back? SCNR $\endgroup$
    – asdfex
    May 27 at 15:52

3 Answers 3


Answering the question in the title, because the arm could not reach anywhere but the front wheels.

The description here indicates the left/right swing of the shoulder joint was 160 degrees, so the arm can only swing forwards of the rover, not back towards the wheels. Folding the elbow would allow a little bit of under travel, but only through the area under the shoulder joint because there is no ability to 'roll' the elbow joint to reach areas off the line of the upper. To visualize, explore your arm range of motion while only bending your elbow in and out, with no twisting.

This constrains the reach area to ahead of the front wheels only, useful for potentially poking terrain before motion, less so once sunk in.

In terms of trying to lift the rover, the description indicates a design strength of 6G while out of the stow position. Only one of the four instruments has a easily locatable mass at 720 grams with the others being presumably lighter, but even if we take the total payload at 2000 grams, 2000 times 6G only gets 12000 grams or 12kg of lift for a 155kg rover on earth. Even in the lower mars gravity and if arm is payload is higher than 1500g that is not unloading the wheels much, and risks the arm digging in stranding the rover on the wedged and/or broken arm.

In terms of rover design, the sand trap happened after two of the six wheels had failed, it is plausible that while fully operation Spirit either would have not bogged at all, or been able to drive out. Assuming similar endurance a bigger arm capable of digging/lifting but with two of six joints failed would probably not have helped either years into a 90 day mission.

  • 14
    $\begingroup$ "and it can withstand shocks of 6 G's while roving" I don't think this means how many Newton the arm can produce. $\endgroup$
    – jumpjack
    May 26 at 15:24
  • $\begingroup$ @jumpjack, correct but it does sent an upper bound unless the design document giving motor torques etc is available somewhewhere. $\endgroup$ May 27 at 5:45
  • $\begingroup$ @GremlinWranger an upper bound on acceleration, not on force $\endgroup$
    – Tim
    May 28 at 22:48

I actually received a reply from NASA when I sent this question to them at the time. Unfortunately I can't find the exact email anymore, but their official reply was that the motors in the arm were nowhere near powerful enough.


Found specification of all the motors of the IDD (robotic arm): motors torque table


Rover mass is 176.5 kg

A mass "m" produces a vertical force given by

F = m * a

where "a" is local gravitational acceleration, which is 9.81 m/s2 on Earth and 3.721 on Mars.

Hence to lift 176.5 kg you need:

  • on Earth: 176.5 * 9.81 = 1731 Newton
  • on Mars: 176.5 * 3.721 = 657 Newton

An arm produces at its end a force given by:

Force = Torque / Radius

45Nm of torque for the motor operating on the elevation angle means:

  • 1 meter length: F = 45 Nm / 1m = 45 N
  • 0.5 meters length: F = 45 Nm / 0.5 m = 90 N

Hence the robotic arm is not powerful enough to lift the rover.

  • 2
    $\begingroup$ 1) 176.5 kg mass on Earth are still 176.5 kg mass on Mars. 2) You can't convert a torque to a mass lifted. 3) The elevation actuator doesn't help much if the elbow can't stand the force $\endgroup$
    – asdfex
    May 28 at 8:46
  • $\begingroup$ 1) True for inertial mass, not for gravitational mass. ______2) Torque is Force * Radius, weight is a Force (Mass * GravitationalAcceleration), so I think we should calculate how much linear (vertical) force the motor torque can be converted into, but it's pointless: 45 Nm torque gives 45 N at 1 meter distance, which can raise just 45/(9.81*0.37) = 12 kg on Mars; _____3) Pointless: if the strongest motor can't do it, you don't need to do calculation for weaker ones. $\endgroup$
    – jumpjack
    May 28 at 8:59
  • 2
    $\begingroup$ 1) intertial mass and gravitational mass are the same everywhere (and as far as we know equal). Only gravitational force changes. 2) You need the length of the lever arm to get any meaningful number out of it $\endgroup$
    – asdfex
    May 28 at 9:12
  • $\begingroup$ do you think you need same force to pull up same mass on different planets?? $\endgroup$
    – jumpjack
    May 28 at 13:46
  • $\begingroup$ Mass and force are two different things. The only thing that varies is the force needed to lift a mass depending on the local gravitational field. Mass is a constant (at least until we need to take relativity into account) $\endgroup$
    – asdfex
    May 28 at 14:46

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