update 1: NASA wants ideas for keeping Moon missions powered in the dark

update 2: NASA’s Artemis Rover to Land Near Nobile Region of Moon’s South Pole which is informative here

enter image description here

Source NASA

TechCrunch's NASA’s VIPER lunar rover will hunt water on the Moon in 2022 says:

VIPER is a limited-time mission; operating at the poles means there’s no sunlight to harvest with solar panels, so the rover will carry all the power it needs to last about a hundred days there. That’s longer than the U.S. has spent on the Moon’s surface in a long time — although China has for the last few years been actively deploying rovers all over the place.

yet the image there and in the NASA.gov's Moon-to-Mars feature page it links to; New VIPER Lunar Rover to Map Water Ice on the Moon show solar panels, suggesting that if the hundred-day power source is rechargeable, it could be recharged by going back to wherever those obvious solar panels were used.

Question: What is the VIPER lunar rover's 100-day power source, and why can't it recharge it using its solar panels?

Please cite authoritative sources and not just a blurb in a popular news outlet. My question was inspired by a confusing and potentially self-inconsistent blurb in a popular news outlet, but I'm looking for an authoritative and reliable resolution to this conflict. Thanks!

  • 4
    $\begingroup$ South Pole sun angle depends on time of year, local altitude and slope. It's not very large and could be stuck in the shadow of ridges, rocks, etc. I don't know the power source, but it may be fuel cells. $\endgroup$
    – mothman
    Commented Oct 26, 2019 at 7:46
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    $\begingroup$ It could be that the solar panel is part of the Solar Cell Demonstration Platform, one of the selected technology demonstrations. nasa.gov/press-release/… $\endgroup$
    – Cornelis
    Commented Oct 26, 2019 at 9:29
  • $\begingroup$ I suggest we put this question on hold until an official statement on the power source is given to the public. So we don't get a bunch of guess in the answers. $\endgroup$ Commented Oct 28, 2019 at 18:28
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    $\begingroup$ @SCLASethKurkowski I think "nobody can know an answer to this question" is an overreach. Perhaps there is an answer out there already. Perhaps the linked article simply misconstrued something else that lasts 100 days (e.g. the total mission time), perhaps there is a call for proposals out there that offers some information, perhaps something can be found in Lunar Discovery and Exploration Program material. I don't know of any cases in the past where we've put questions "on hold for later". Have faith in SE's ability to manage low quality answers, and the community's instinct not to write them. $\endgroup$
    – uhoh
    Commented Oct 28, 2019 at 23:05
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    $\begingroup$ @uhoh Not saying there isn't an answer, just saying it looks like they will be talking more about the lander soon and we should wait till then for more details. $\endgroup$ Commented Oct 29, 2019 at 20:32

2 Answers 2


An abstract for the "An Overview of the Volatiles Investigating Polar Exploration Rover (VIPER) Mission" talk, scheduled to be given in December at AGU, states

VIPER is a solar and battery powered rover mission designed to operate over multiple lunar days, traversing several kilometers as it continuously monitors for subsurface hydrogen and other surface volatiles.

UPDATE: The Sep 20 2021 press release you link to also indicates it's battery + solar:

NASA’s team evaluated viable rover traverse paths, taking into account where VIPER could use its solar panels to charge and stay warm during its 100-day journey.

It's a bit of an inference to take "charge" to mean battery, but given the previous statements it's at least consistent.

  • $\begingroup$ I suspect the answer to the rest of the question is something about the risk of permanent shadowing, but I'm not sure answers will be available to the public prior to the time of that talk. I don't see any published papers about the VIPER mission design yet. $\endgroup$
    – Erin Anne
    Commented Oct 28, 2019 at 14:34
  • $\begingroup$ Thanks for the link! Wow, lasting one hundred days on a single battery charge (presumably self-warming by powering its own heaters) would be quite a challenge. The meeting is in December so I guess we'll have to wait a little longer before more details come out. $\endgroup$
    – uhoh
    Commented Oct 28, 2019 at 16:24
  • $\begingroup$ I added an interim answer for now. If you hear anything more please feel free to update your answer. Thanks! $\endgroup$
    – uhoh
    Commented Nov 25, 2019 at 1:30
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    $\begingroup$ @uhoh Updated. Still looks like solar and battery to me $\endgroup$
    – Erin Anne
    Commented Sep 21, 2021 at 4:27
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    $\begingroup$ oh gosh, I didn't even look back at the actual question. Sorry, I'll look harder at that and try to edit the answer to answer the question, but yes I think that your conclusion in this comment is probably correct $\endgroup$
    – Erin Anne
    Commented Sep 21, 2021 at 18:49

I'll post this interim answer for now. Hopefully a primary source can be found and at that time someone can post a thorough answer.

The following paragraph in Space.com's NASA Will Launch a Lunar VIPER to Hunt Moon Water in 2022 seems to summarize how VIPER will operate. However I am not certain that this is correct. This 2006 Lunar Polar Volatiles Explorer (LPVE) Mission Concept Study also mentions a 4.4 day lifetime non-rechargeable battery and contrasts it with a 140 watt ASRG (radioisotope supplied Stirling engine powered electrical generator). The proposed non-rechargeability may have been a ploy to try to put pressure on ASRG development or selection. We'll still have to wait for a better answer to be certain.

VIPER will work for about 100 days, according to the statement, addressing one of the agency's key goals at the moon: to develop machinery that can work even in the dark. Because VIPER will land so close to the lunar south pole, it should never experience the bitterly cold, two-week darkness that characterizes the lunar night in more-equatorial regions. But it will be designed to withstand 96 hours of darkness.

So the side-mounted solar panels shown in the artwork make sense. The topography at lunar poles means that there are extremely cold areas where water can be trapped and also areas exposed to the sun, albeit obliquely. But a solar panel facing to the side will receive sunlight just as strong as a horizontal panel on the Moon's equator would. Since there's no atmosphere, a horizon-hugging Sun will be no different than at it would be at the zenith.

It seems that the rover's batteries will be large enough to power it for a few days in total darkness, running drills, intermittent lights and the internal heaters as required. Then it will drive back into the Sun and recharge its batteries before going back into darkness.

  • $\begingroup$ Where is the proof from NASA that VIPER is battery powered ? That solar panel could be just for the Solar Cell Demonstration Platform. $\endgroup$
    – Cornelis
    Commented Nov 25, 2019 at 10:28
  • $\begingroup$ @Cornelisinspace please re-read the first sentence $\endgroup$
    – uhoh
    Commented Nov 25, 2019 at 10:47
  • $\begingroup$ Well, you've asked explicitally for an answer with authoritative sources ! $\endgroup$
    – Cornelis
    Commented Nov 25, 2019 at 12:38
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    $\begingroup$ @Cornelisinspace it's certainly a real answer. It provides two supporting sources and explains what can be learned from those sources. But it also acknowledges that an even better answer will be possible in the future when additional information becomes available. $\endgroup$
    – uhoh
    Commented Nov 25, 2019 at 22:05
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    $\begingroup$ new article with different information: "The rover will take daring trips into the eternally dark craters, relying on battery power for up to 50 hours during each traverse that moves VIPER beyond the rays of the Sun, always near the horizon at the lunar poles. The 1,000-pound (450-kilogram) rover will go into hibernation when the wobble of the Moon’s rotation causes the south pole to shift out of the view of Earth for two weeks, cutting the direct communications link." arstechnica.com/space/2023/07/… $\endgroup$
    – Erin Anne
    Commented Jul 19, 2023 at 0:50

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