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All the Mars rovers tend to move very slowly. For example, Curiosity moves at an average speed of 30 metres per hour and a maximum speed of 90 metres per hour. Why is this so? Is it because of safety reasons? Does the time lag between Earth and Mars necessitate such caution? Then again, the rovers are equipped with hazcams to avoid accidents.

Alternatively, do they travel so slowly for reasons of stability? Does the science equipment on board have such requirements?

Or is it simply because there is no need for haste? How much time does the Curiosity spend idle as it moves from one location to another?

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    $\begingroup$ You could find the answer [here]( robotics.stackexchange.com/questions/679/…) $\endgroup$
    – Hash
    Commented Aug 16, 2013 at 6:27
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    $\begingroup$ Those answers on Robotics are fine -- from robotics perspective -- but there's more to it than that and I belive Space Exploration can do better. ;) $\endgroup$
    – TildalWave
    Commented Aug 16, 2013 at 7:19
  • $\begingroup$ @TildalWave - the answers pick up the main reasons quite well. If you want higher speeds, the actuators must be larger. Please note that MSL uses brushless custom-made DC motors from Aeroflex. $\endgroup$
    – Deer Hunter
    Commented Aug 16, 2013 at 7:34
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    $\begingroup$ @DeerHunter - Yes, but there are other considerations that I didn't read there being addressed properly (or at all), like driving directions uploaded up to several days ahead, delicate on board scientific equipment, raising dust on itself due to low atmospheric pressure, no need to hurry due to landing site selection, autonumous obstacle avoidance system, stopping to examine surroundings, day/night cycles, unpredictable stops,... $\endgroup$
    – TildalWave
    Commented Aug 16, 2013 at 7:45
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    $\begingroup$ "Fun Fact: Each drive actuator has enough torque to drive the rover straight up a wall on Mars" Source: trs-new.jpl.nasa.gov/dspace/bitstream/2014/43242/1/… $\endgroup$
    – Deer Hunter
    Commented Aug 16, 2013 at 8:09

1 Answer 1

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The main reason for the MSL driving slower than physically possible is hazard avoidance (and also navigation/visual odometry):

Source: Introduction to mobility for MSL Heverly, Matthew C. 10-Sep-2012

Hazard avoidance and navigation

Drive rates (m/hour):

Absolute top speed:   151.2 (On rigid terrain with zero slip)
Blind drive:          139.5 (top speed for straight driving
                             with pause between steps)
Blind drive
with slip checks:     116.0 (Averaged over a significant number
                             of slip checks)
Visual Odometry (VO):  64.5 (At every step, look around, 
                             see how far the rover has driven)
Autonomous
navigation (Autonav):  40.1
Autonav 
with slip checks:      38.5
Autonav with VO:       30.0 (Assumes VO driving rate with 
                             additional time for map imaging 
                             at the appropriate spacing)

What is Visual Odometry? Taking stereo images "before" and "after" the step to track true motion, calculate slip and embedding. Accuracy of VO is 3% of the step distance, so you wouldn't want steps to be too long.

Why is it capable of moving slowly?

Actuator design constraints

The mission design logic (see http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38317/1/03-2974.pdf) is as follows:

  • Interesting landing site => Technically feasible landing error ellipse => Traverse length
  • Number of samples
  • Nominal mission duration (one martian year)

Hence, $Nominal\ Speed = Traverse\ Length/(Mission\ Duration - Number\ of\ Samples * Time\ per\ Sample)$.

Tradeoff logic

enter image description here

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  • $\begingroup$ The implication is of course that one needs to keep the mass of the structural and mobility system at a minimum for the given constraints on G loads, scientific payload mass, and rough terrain capability. $\endgroup$
    – Deer Hunter
    Commented Aug 16, 2013 at 9:13
  • $\begingroup$ Please also bear in mind that night driving is off the table for hazard avoidance and thermal reasons. $\endgroup$
    – Deer Hunter
    Commented Aug 16, 2013 at 9:36
  • $\begingroup$ What about power budget? Apart from a need to avoid hazards, speed needs power - big motors which add weight and require big batteries and solar panels. $\endgroup$
    – Anthony X
    Commented Sep 26, 2017 at 1:32

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