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Moon's synodic day is 2551443 seconds. Its circumference is 10921 km. At sustained 15.4km/hour a rover traveling along its equator would retain full sunlight at all times; at higher latitudes that speed would be even lower. On earth, we have all-terrain vehicles for which this kind of speed is trivial to maintain - and making the speed a little higher, there would be time for stops, detours, sample pickup, data transmission and so on. The 3s radio roundtrip and good maps would allow for fully interactive remote control, so no autonomous drive problems at these speeds. And the rover would be able to analyze many locations and spot "anomalies" which are simply impossible to find with a lander due to its limited area coverage.

Were there ever plans for such a rover? Specific obstacles, or reasons why it would not be viable?

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    $\begingroup$ this reminds me of the Race the Sun game. $\endgroup$ – John Dvorak Jan 26 '17 at 12:57
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    $\begingroup$ The rover would be at the other side of the moon (in regards to the earth) around 50% of the time. So unless you have a satellite in orbit to relay radio communication, you wouldn't be able to have radio contact 50% of the time. $\endgroup$ – Gerben Jan 26 '17 at 15:23
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    $\begingroup$ Try to imagine yourself running with sustained 15,4 km/h cross country without a road or path and with a delay of vision of 3 seconds. If you are not able to run with such speed biking would be an alternative. Would you really dare to do so? There would be no interaction with other runners, bikers, cars. Just the interaction with a difficult terrain with rocks and boulders. Of course there is no direct view of the surroundings, only over some cameras and monitors mounted to a helmet and an electronic delay of 3 seconds inserted between camera and monitor. Only a small bandwidth for video. $\endgroup$ – Uwe Jan 26 '17 at 16:08
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    $\begingroup$ @supercat: Three - quite hard, although coverage of whole equator with a broad latitude range would be easy. Four forming a tetrahedral constellation would provide whole surface coverage. $\endgroup$ – SF. Jan 26 '17 at 17:00
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    $\begingroup$ Dealing with the heat of a continuously running motor would present a bit of a challenge, I would think. $\endgroup$ – John Bode Jan 26 '17 at 18:49
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3s radio roundtrip and good maps would allow for fully interactive remote control, so no autonomous drive problems at these speeds.

15.4km/hour is 4.28m/s, so during that 3s round trip the rover travels nearly 13m. It would make an interesting video game to try driving a rover at that speed with that input latency, but I suspect you'd find people crashing a lot.

The good news is that autonomous driving is improving all the time. It's being used on Mars, and in various driver assist technologies for earth cars at highway speeds. I think any such high-speed rover would have similar driver assist to avoid potholes, boulders, and control stability in dustbowls.

Note also that, while CoM stability is the same in static analysis on the moon, it has much less righting ability in dynamic analysis because the gravitational force is lower. If you take the same vehicle on Earth and the Moon to a 45 degree angle then drop it, the Earth one will level out much faster.

Another consideration in continuous driving is power. If you're going for solar rather than nuclear then the amount you get from vehicle-sized panels tends to be disappointing. Especially if you need to divert it to heaters, instruments, LIDAR, computers, etc. This is part of the reason that Spirit and Opportunity are so slow.

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  • $\begingroup$ There is more information about Curiosity's autonomous capabilities in this and this answer as well. $\endgroup$ – uhoh Jan 26 '17 at 15:10
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    $\begingroup$ In addition to righting force you can look at just plain traction; a vehicle of a given mass has the same inertia anywhere, but 1/6 the force holding its wheels to the ground on the moon. $\endgroup$ – hobbs Jan 26 '17 at 16:40
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    $\begingroup$ Good point on power. For example, a Nissan Leaf has enough roof area to generate maybe about 300 watts, during the 4 central hours of the day. It uses up to 80 kw during acceleration and will deplete its 24kWh battery pack in about an hour at freeway speeds. $\endgroup$ – Phil Jan 26 '17 at 18:38
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    $\begingroup$ You wouldn't need heaters on the sunny side of the moon. $\endgroup$ – Burgi Jan 26 '17 at 19:03
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    $\begingroup$ I can see it now... "Twitch plays lunar rover" $\endgroup$ – David Starkey Jan 26 '17 at 20:08
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The Soviets tried direct remote control in the Lunokhod program. The radio delay was found to be a huge obstacle. They used a driver plus a spotter and a commander. The driver had to continuously integrate looking ahead (predicting the future, with only low-resolution B&W images to guide him) with controlling the rover via a delay. This was exhausting work and could only be done in short stints.

This is shown in a Lunokhod documentary (Tank on the moon?), I'll see if I can find it.

This article has more detail:

Only one member of each crew would drive the rover. Behind him would sit the crew commander, who would oversee the driver’s handling of the rover. Joining them in the control room would be a navigator, a radio antenna operator, and the flight engineer, who would monitor the rover’s systems. Each crew would operate the rover for two hours; then the other crew would take control.

...

Latypov and Dovgan’s [the drivers] only guidance came from a monitor, which displayed images from Lunokhod’s two low-resolution television cameras. To any video game enthusiast it sounds simple—but this was nothing like a video game. The cameras did not send a continuous stream of images, but rather single frames, like a slide show, at intervals that varied from seven to 20 seconds. And because radio signals took three seconds to travel round trip between Earth and the moon, the driver didn’t see the results of his actions for many long moments. For this reason, if crew commanders Nikolai Yeremenko and Igor Fyodorov saw Lunokhod heading toward catastrophe, they could push a button to halt the rover.

Dovgan, now 66, was well prepared by intensive training. “Driving on the moon felt even easier than it was in the lunodrome,” he says, but his comment belies the difficulties of navigating the rover. The low resolution of the slide show made it difficult to spot craters and boulders, especially at high sun angles, and there was a “dead zone”—a three-foot-wide area immediately in front of the rover that Lunokhod’s cameras could not see. The only solution, according to Dogvan, was to memorize the features in this area from the previous image, before the rover reached it. “When we were looking ahead and thinking of the obstacles that we did see, we also had to remember what was just behind,” he says.

(emphasis mine)

Also, like the Apollo crews, they found it difficult to estimate distances due to the lack of landmarks that have a known size.

Now, some of these problems can be alleviated with modern technology. You can get better imaging than 1 frame/10 seconds. You can improve the view around the vehicle. You can add computer analysis, and maybe integration of ground-based images with overhead map data to get a better idea of distances, slopes etc. But the fundamental problems of delay and having to interpret alien terrain remain.

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    $\begingroup$ Lonokhod 2 had two speeds: ~1 km/h and ~2 km/h. It had a really high CoM, as result rather easy to flip over if driven over a slope or obstacle. Definitely not a "speedy rover". $\endgroup$ – SF. Jan 26 '17 at 11:27
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    $\begingroup$ You mean control from Earth? At the time, wouldn't that only work for the line-of-sight hemisphere, or did they have an orbiter to with it? $\endgroup$ – uhoh Jan 26 '17 at 11:29
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    $\begingroup$ @uhoh: Lunokhod landed on the line of sight hemisphere (and Moon is tidally locked), and never traveled far enough to approach the edges of the hemisphere. It hibernated through lunar night though. If the "speedy" was to stay on sunlit side at all times, it would require a relay in orbit. $\endgroup$ – SF. Jan 26 '17 at 11:38
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    $\begingroup$ That's easy enough to do in an RC vehicle that weighs a few hundred grams, much more difficult in a one-ton vehicle. $\endgroup$ – Hobbes Jan 26 '17 at 13:52
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    $\begingroup$ The problem is not weight, but impact resistance (G-forces due to a crash). It's the square-cube law (the bigger something is, the more fragile). $\endgroup$ – Hobbes Jan 26 '17 at 13:56
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A rover like this might be technically possible, however it's not viable because there's no benefit in doing it. The science being performed on the surface of bodies in the solar system requires staying in place for quite some time, drilling, taking samples, taking pictures, lasing things, etc. Having a rover which would have to remain almost constantly on the move would make little scientific sense.

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    $\begingroup$ Such a rover could give you a Google "Street" View of the Moon. ;) $\endgroup$ – called2voyage Jan 26 '17 at 18:10

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