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Bruce Dorminey wrote an article for Forbes with the title of "Earth And Moon May Be On Long-Term Collision Course". The article notes:

"For now, our anomalously large Moon is spinning away from us at a variable rate of 3.8 centimeters per year. But, in fact, the Earth and Moon may be on a very long-term collision course --- one that incredibly some 65 billion years from now, could result in a catastrophic lunar inspiral."

It seems a little illogical that the Moon would suddenly start running toward earth at a far distance. Multiply 3.8 by 65 billion you get 2,470,000,000(May be innacurate). The article also notes this:

"As a result, the timing of the Moon’s ongoing recession is hard to precisely predict. That’s because, as Barnes1 points out, the Earth goes through glacial and interglacial cycles, causing the area of shallow seas to change as sea level rises and falls."

Is it possible to understand how/why the author believes that the Moon could fall into a lunar inspiral? Please cite sources, thanks!


1Jason Barnes, a planetary scientist at the University of Idaho

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    $\begingroup$ Welcome to Stack Exchange! I've adjusted your title to better match the question you've actually asked in the body of your posted, and adjusted some other wording to better match the style of the site. Feel free to edit further, thanks! $\endgroup$ – uhoh Feb 26 at 15:08
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    $\begingroup$ I believe the Moon's outward migration is very gradually slowing, so 3.8 cm/s $\times$ 65 billion years will probably overestimate things. $\endgroup$ – Peter Erwin Feb 26 at 15:24
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    $\begingroup$ If/(when?) The Earth becomes tidally locked to the Moon, then there will no longer be a force slowly moving the Moon away and the solar tides may actually reverse the movement. But the timing of such things is into the many billions of years. $\endgroup$ – userLTK Feb 26 at 16:11
  • $\begingroup$ The Sun will become a red giant in about 5 billion years. By that time the Earth will have been consumed by the Sun. If not, drag from the Sun's atmosphere will have caused the Moon to crash into Earth. If not, further drag will cause both the Earth and the Moon to be destroyed without any contact with each other. $\endgroup$ – fasterthanlight Feb 26 at 16:11
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    $\begingroup$ Relevant publication? Sasaki et al. 2012 $\endgroup$ – Mike G Feb 26 at 17:59
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Jason Barnes is certainly a highly credible source. He is a planetary scientist and one of the leads on the Dragonfly drone now located on Mars.

The idea that the Moon will someday reconnect with the Earth is not new to him, however. Here is a link to a Space.com article from 2007 which explains the same mechanism.

Earth's Moon Destined to Disintegrate

It is safe to accept that the article you link to reflects current thinking on the fate of the Moon-Earth system.

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  • $\begingroup$ That's not "the same mechanism", though. Barnes seems to be suggesting tidal interaction between the Sun and a tidally locked Earth-Moon system tens of billions of years in the future, while the article you link to is about friction from the Moon moving through the outer atmosphere of the (expanded) Sun in its red giant phase, about 5 billion years in the future (something I think Barnes is probably ignoring). $\endgroup$ – Peter Erwin Feb 27 at 14:12
  • $\begingroup$ I agree that the mechanisms vary somewhat. My citation was addressed to the idea that the Moon returning to hit the Earth was not a new idea. Both citations confirm the end of tidal distancing due to tidal locking. That is half of the OP’s question and therefore relevant. One article places the emphasis of return on solar tidal forces, the other on friction from solar particulates. Both solar tides and friction will, no doubt, play a role in the return of the Moon if it does, indeed, survive the Sun’s expansion. $\endgroup$ – JohnHunt Feb 28 at 6:18

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