Naked Science Forum
On the Lighter Side => That CAN'T be true! => Topic started by: thedoc on 02/08/2012 16:10:28
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edison jmorais asked the Naked Scientists:
The Moon is moving 3.8 cm further away from the Earth every year.
I would like to know if the Moon, some day, will collide with another planet in our solar system?
If yes is, is there any calculation to know when and with which planet will it collide?
Thank You
Edison
What do you think?
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I'm seeing various estimates, so I'm not sure what is correct. As the moon recedes from the Earth, it will also have less energy transfer from the tides, so the rate of receding should decrease over time (but, perhaps it will also take less energy to move it to a higher obit).
If liberated from the Earth, it would still be closer to the Earth than any other planet, so there would be a risk of it impacting Earth, although it would likely enter a shared, resonant orbit with the Earth around the sun, perhaps something similar to 3753 Cruithne (http://en.wikipedia.org/wiki/3753_Cruithne). Also see Horseshoe Orbit (http://en.wikipedia.org/wiki/Horseshoe_orbit) and Co-orbital Moon (http://en.wikipedia.org/wiki/Co-orbital_moon).
However, estimates of this happening seem to be AFTER the sun will likely enter into a Red Giant phase, incinerating Earth, possibly engulfing the Earth, followed by the sun becoming a white Dwarf.
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Assuming nothing destroyed the Earth in the meantime, the Moon's rate of receding would slow down and eventually stop. It would then proceed to orbit increasingly closer to the Earth (a decaying orbit) until it passed within the Roche limit. Once it does, tidal forces will break it up into rocky debris which would most likely form a ring around the Earth (similar to the one on Saturn). Eventually, all components of the ring would fall into the atmosphere and burn up (assuming the atmosphere hadn't mostly escaped into space by then) or impact the surface. Thus, no more Moon. This will take billions of years to occur.
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Assuming nothing destroyed the Earth in the meantime, the Moon's rate of receding would slow down and eventually stop. It would then proceed to orbit increasingly closer to the Earth (a decaying orbit) until it passed within the Roche limit.
That can only happen to a body that is inside a geosynchronous orbit. Being outside the geosynchronous orbit, the tidal forces will continue to push the moon further away.
Paradoxically, the more tidal energy imparted to the moon, the slower the orbit.
There are theories that eventually the moon and earth will become tidally locked, at which point it would stop receding from the Earth. If tidally locked, it would likely be a fairly stable orbit, unless there would be movement for the Earth/Moon system to become tidally locked with the sun.
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If gravitational radiation exists, would that not preclude the possibility of a truly stable orbit? Or are there special prerequisites for emitting gravitational radiation that the Earth-Moon orbital system lacks?