Watching this video of Perserverance being tested on Earth, it occurred to me that requiring it to stand, drive, climb, etc. under 1g is, in some sense, overkill. Could this (or other) rovers be made more capable or less massy if it was not required to support itself on Earth at all? Has this ever been considered?
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asked May 19, 2020 at 13:27
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11$\begingroup$ The rover also has to survive the launch process $\endgroup$– Dan PichelmanMay 19, 2020 at 13:35
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4$\begingroup$ @DanPichelman true, but not standing on its wheels $\endgroup$– Steve LintonMay 19, 2020 at 14:02
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9$\begingroup$ Where else are you going to test it? $\endgroup$– GdDMay 19, 2020 at 14:03
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4$\begingroup$ I continue to be dismayed at questions which include "has this ever been considered" after 30 years of painstaking design, analysis, testing, reliability tradeoffs have been done by massive teams of expert engineers. $\endgroup$– Carl WitthoftMay 20, 2020 at 12:11
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3$\begingroup$ @SolomonSlow - if speaking of Mars - it's technically entry, right, rather than re-entry - Since it's not coming from Mars ;) $\endgroup$– NKCampbellMay 20, 2020 at 15:06
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3 Answers
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The video shows only tests within the clean room on smooth clean test floors. The vibration test at first should simulate the conditions during launch.
Drive tests on a simulated Martian rough and dirty surface on Earth (see JPL Mars Yard) are not done with the version built for the Mars surface. The flight-ready version is built in a clean room and only leaves it to be transported to the launchpad.
For drive tests on Earth, special test versions are built. A drive test version is built without a Radioisotope Thermoelectric Generator and not with the full set of scientific equipment. So, a lot of the mass of the Mars rover is not there for the 1-g tests.
The structure of the rover needs some factor of safety to survive the launch at several g as well as the entry to the Martian atmosphere, the parachute unfolding and the landing with the sky crane. Driving on Mars probably induces far from the highest structural loads the rover needs to endure.
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1$\begingroup$ @CourageousPotato Thanks for editing and the link to JPL Mars Yard. $\endgroup$– UweMay 21, 2020 at 15:20
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Faster? No. The speeds of all Mars rovers so far have been limited by the navigation software or the available electrical power, not by physical capability. Spirit and Opportunity were both physically capable of driving more than 2000 meters per day, but rarely actually traveled more than 60. Curiosity is capable of moving 2200 meters per day, but can only navigate at 200 meters per day.
Lighter? Maybe a little, but not much. Most of the mass goes into non-structural elements such as batteries, solar panels, and instruments, structural elements where the major stresses don't come from gravity (instrument arms), or structural elements where gravity can be compensated for during testing (wheels and suspension). About the only thing that needs to carry Earth gravity at all times during testing is the body of the rover.
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$\begingroup$ … which also needs to carry the impact load of landing. (Unless you put some kind of strut under it, which then prevents it from driving off the platform. So, you need the strut retractable, which is just one more thing to go wrong, and a very stupid way to get yourself stuck.) $\endgroup$ May 20, 2020 at 7:08
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Speed is not the issue, but navigation. If would be better if a mapping-satellite were placed in polar orbit over Mars, to produce a precise topographical map; along with some GPS-type satellites. This would allow the rovers to find their Waze.
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2$\begingroup$ Welcome to Space Exploration Stack Exchange! Your first answer here is correct, but it is lower on information than we prefer. Please try to reference specific objects, missions, technologies, etc, and cite sources! The best answers expand beyond qualitative statements and include some original analysis. $\endgroup$ May 20, 2020 at 7:54
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4$\begingroup$ In response to your answer: I disagree that mapping satellites or GPS are the solution here. The Curiosity rover already has great local area mapping through its Object Avoidance Cameras surrounding the rover, so the operators use the close-up 3D maps derived from this for navigation. While more mapping satellites would help with long-term route planning, local navigation is what affects speed. The true upper bounds on Curiosity's speed are the risk of damage from going fast, the limits of the local mapping, and the onboard power usage. $\endgroup$ May 20, 2020 at 7:59
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$\begingroup$ A mapping satellite wouldn't help. The best Earth-based satellites have a resolution of around 7 cm, or in other words, the sorts of obstacles that the rovers are trying to avoid show up as one or two pixels on the map, and only if the area was photographed at the correct time of day. $\endgroup$– MarkMay 20, 2020 at 20:43
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$\begingroup$ @Mark 'Earth-based' satellites, you say? Well good thing Mars' atmosphere is much thinner, with much fewer clouds. Mars' lower diameter and gravity would let a satellite orbit much closer to its ground than with Earth. So I'd expect an order of magnitude or two better clarity of maps. Directed lidar may even be possible. Though it would make more sense for the rover to do it's own sensing. $\endgroup$ May 21, 2020 at 23:33
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$\begingroup$ @BillyC., if you're looking down at the Earth, the atmosphere isn't much of an issue. The limiting factor is mirror size -- to get your "two orders of magnitude" resolution increase, you'd need to go from a 2.5-meter primary mirror to a 250-meter primary mirror, or go from orbiting at 500 km to orbiting at 5 km. (Or combine the two, giving a 25-meter mirror 50 km up.) See diffraction limit. $\endgroup$– MarkMay 21, 2020 at 23:37