4
$\begingroup$

Geology Dr. Paul Spudis (who was on the Clementine Lunar orbiter mission) has an interesting step-by-step proposal for exploration of potentially extractable water ice at the Lunar poles. It includes communication satellites in Lunar polar orbit. I wonder if it would be practical to replaced them with a cable. I think that the communication satellites raise the threshold for such a project, since they would have to be the very first investment.

A stationary lander on a crater ridge in (nearly) constant sunlight with a large turning photovoltaic array could produce tens of kilowatts (tens times the power of the Curiosity rover on Mars). I presume that it could have constant line-of-sight to Earth too and hence constant communication. A rover in an eternally shadowed crater has none of that on its own. I wonder if it would be practical to have a rover connected to such a stationary lander with a cable for power supply and communication with, and real time teleoperation from, Earth?

The rover would have to carry the entire cable and unwind it, or else it would have to be dragged from the station. If the rover turns back in a curve, the cable would have to be rewound or dragged. Dragging a cable means that it will get stuck at rocks. And the lunar soil might be electrostatic and interfere with power and communication in the cable. Does a corded rover sound like a good idea, or would the mass and hassle with the cable be worse than communication satellites and big batteries or RTG (which are pretty expensive alternatives)?

I imagine that after the first surveyor rover has struck gold, a heavy extraction rover could precision land near the power/communication station and connect its own cable to it. Of all the unrealized plans for space exploration, has something like this been proposed and analyzed?

$\endgroup$
  • 1
    $\begingroup$ You don't need a cable. So long as the rover can remain in line of sight, it can use the lander on the ridge as a relay, and the lander can use microwaves to beam power to the rover. $\endgroup$ – Mark Adler Dec 17 '14 at 16:36
  • 2
    $\begingroup$ @Mark Adler Is that really a mature technology, beaming power in microwaves? Does Earth's atmosphere prevent usage here, or why isn't this commonly practiced in our everyday lives? For example as a phone charger, I hate managing that cord. Is it maybe dangerous to people who get in the way? $\endgroup$ – LocalFluff Dec 17 '14 at 16:38
  • 2
    $\begingroup$ @LocalFluff If you don't have to test it for possible humans and other wildlife standing in the way between the two ends, then yes, it's a mature technology. $\endgroup$ – TildalWave Dec 17 '14 at 16:56
  • 1
    $\begingroup$ Yes, it was mature in 1975. 30 kW was transmitted over a mile with on the order of 50% efficiency. Earth's atmosphere is not a significant factor in the efficiency -- it would be about the same in vacuum. You don't see this in power transmission because the efficiency is so low compared to wires and transformers. You don't see it on cell phones, since you would need two little high-gain dish antennas that track each other. You will see inductive charging of cell phones very soon, where you just set the thing down on a charging pad. $\endgroup$ – Mark Adler Dec 17 '14 at 19:22
  • 1
    $\begingroup$ 50% is very good for the application you're asking about. See the answer below for some of the reasons that a cable is a really bad idea for a rover. A cable might be useful for a short traverse down a very steep cliff, in which case the cable would also be providing mechanical support. $\endgroup$ – Mark Adler Dec 17 '14 at 19:55
7
$\begingroup$

There are several problems with this idea:

  1. Snagging the cable on a protrusion. You'd have to constantly monitor the cable to make sure it didn't snag, and/or be very careful when driving: always return along the same path you took to get somewhere. If you've ever tried to mow the lawn with an electric mower, you know how annoying this gets.
  2. Moon rock and dust are incredibly abrasive. Any movement of the cable would wear down the insulation. This includes winding the cable on and off a storage drum, so this would happen even if you're very careful not to disturb the cable.
  3. Weight of the cable. Power cable is thick and heavy. I've got a 16 Amp cable that's maybe 25 m long, it weighs something like 7 kg. Because of 1. and 2. you need to install the cable drum on the rover, adding a lot of weight to the rover.

This method is used sometimes on Earth, e.g. in mining: some bucket wheel excavators are cable-powered. The one in the picture weighs some 10,000 tons. Schaufelradbagger
In these applications, the machine is stationary for longer periods and when it moves, personnel can keep an eye on the cable.

$\endgroup$
  • 1
    $\begingroup$ That rover in your picture there might be a bit larger than I would expect from a first Lunar ISRU-mission. But hey, ask a multi-billion mining company and they might say: Yes, why not build a mine on the Moon? It'd be cheaper than their average mining project on Earth. NASA's budget is like a subdivision of their daily operations. $\endgroup$ – LocalFluff Dec 17 '14 at 19:59
  • $\begingroup$ Power cable does not need to be heavy if higher voltage and lower current is used. Instead of 120 V and 16 A you may use 1.2 kV and only 1.6 A. $\endgroup$ – Uwe Sep 7 at 18:54
  • $\begingroup$ Most of the cable weight of that 16 A cable is in insulation already, and a 1.2 kV cable needs more of that. $\endgroup$ – Hobbes Sep 7 at 19:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.