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This answer mentions that the Prop-M rovers that were carried to the surface of Mars by the Mars-2 and Mars-3 missions used "skis" for propulsion, rather than wheels.

Looking at the GIF below frame by frame suggest that the actuation is likely done by pure rotational motion. Is there anything written or known about the rationale at the time why skis driven by a motor would be better than wheels driven by motors?

I'm not so interested in what one might choose do today. I'm really asking about the state of the art and decision making back then. Why was "walking" on rotary-driven "skis" chosen over putting wheels on those motors, and either a tail (third point of contact dragging, a known good solution for some terrain types) or one or two more passive wheels?

According to Wikipedia, the rover was to remain attached by an umbilical to the lander, and do some local exploration while monitored and sometimes filmed by the lander.

enter image description here

above: GIF of how the Prop-M rover uses it's "skis" to walk. Linked here, from Giphy.

enter image description here

above: Mars Prop-M rover from here.

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I believe it was because the Soviets at the time did not know the composition of the Martian soil, and how wheels would cope with it. They thought that the soil was too fine for wheels and that they would become easily stuck in the soil.

The robots were sent by the Russians to study Martian soil

Due to many previous lander failures, they could never know truly what the surface soil was like. Sleds however, work in many different terrains. Although they are slower and more cumbersome, it was the least riskiest method to get movement in the soil. Two more massive wheels would would still have a chance to become stuck in the Martian soil. Based on what the Soviets knew about Mars at the time (which was very little) they decided on this approach.

The traces of movement in the martian soil would also be recorded to determine material properties.

The sleds on the Prop-M rover when in contact with the soil would leave a distinct imprint, and it was hoped that these imprints could be used to determine the properties of the surface soil.

First quote: https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1971-049F

Second quote: http://unofficialnetworks.com/2012/11/15/robots-skied-mars/ (https://en.m.wikipedia.org/wiki/Mars_sample_return_mission)

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  • $\begingroup$ This makes so much sense that it is almost certainly the right answer. Let's see if some supporting reference can be found.Thanks! $\endgroup$
    – uhoh
    May 29 '17 at 23:59
  • $\begingroup$ @uhoh got two quotes that may help support! $\endgroup$
    – Jake Blocker
    May 30 '17 at 0:10
  • $\begingroup$ That's great, can you add the name and link to source of those quotes? It seems to be from the NASA Space Science Data Coordinated Archives; Mars-2: 1971-045D and Mars-3: 1971-049F The Wikipedia article(s) have copied verbatim without properly citing or quoting. $\endgroup$
    – uhoh
    May 30 '17 at 0:18
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    $\begingroup$ @uhoh Didn't use Wikipedia, so the source must have copied it from them or vice versa. Nevertheless, linking both websites $\endgroup$
    – Jake Blocker
    May 30 '17 at 0:39
  • $\begingroup$ OK thanks. I have a hunch all of the other sources have lifted from the NASA pages, since both quotes are found verbatim on both the 1971-045D and 1971-047F pages. $\endgroup$
    – uhoh
    May 30 '17 at 0:45

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