Will the Moon ever stop drifting away from Earth? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Answer by Corey S. Powell, former editor in chief of Discover, on Quora:
There is both a simple answer to this question, and a rather complicated one.
The simple answer is: The Moon is probably never going to leave us. There is no well-defined scientific scenario in which the Moon ever escapes from the Earth, and even the long-shot possibility emerges only long after Earth has been largely destroyed by the Sun.
When you try to get into the details of what exactly happens to the Moon in the future, however—then things get complicated.
Right now, the Moon is moving away from the Earth at a rate of about four centimeters per year, due to the tidal interaction between the Earth and the Moon. At a basic level, the Moon’s gravity exerts a drag on the Earth that slows its rotation, and the Earth’s gravity exerts a pull on the Moon that expands its orbit. The two effects balance out, conserving angular momentum.
The four centimeter-per-year motion is only the current rate of movement. The strength of the tidal effect varies greatly depending on the configuration of the continents and the oceans, and even on the Earth’s internal structure. For a more reliable prediction, scientists look at the inferred average rate of change in the Moon’s orbit over the past four billion years.
Projecting forward, there will come a day about 50 billion years from now when the Moon’s orbit will reach its maximum size. At that point, one lunar orbit (one month) will take 47 days. Earth’s rotation period will also be 47 days, meaning that one side of the Earth will face the Moon at all times, just as one side of the Moon currently faces the Earth at all times. With the whole system synchronized, the Earth and Moon will no longer have any tidal effects on each other, and the Moon will stop moving away.
But not all tides will be gone at that point! The Sun will continue to exert tides on the Earth, slowing its rotation further. Now the effect will be reversed and the Earth will drag on the Moon, pulling it steadily closer. After a long time (perhaps a few hundred billion years), the Moon would spiral in so close that it would be torn up by Earth’s gravity, forming a ring.
Now for an added set of complications. Starting in about five billion years, the Sun will swell up into a red giant. At the peak of its expansion, it may swallow the Earth and the Moon. Then again, it may have shed enough mass by that point that our orbit will expand and we will escape destruction. Either way, something dramatic will have happened long before we reach that hypothetical point of synchronized 47-day month/days.
Friction within the Sun’s outer layers could conceivably strip the Moon away from the Earth. It will surely have some effect on the Moon’s orbit, an effect that is impossible to forecast this far ahead. But if the Earth and Moon survive intact, and if the Moon remains in something roughly resembling its orbit from before the Sun’s expansion, the spiral of death will eventually set in.
By then the Sun will be a fading white dwarf with perhaps 2/3 its current mass. The Earth will be, at best, a scorched and chilled ball of rock. But hey, it will eventually get a really pretty set of Moon rings.
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