ASK Space

SOLAR SYSTEM

Will the Moon ever leave us?

Ocean tides driven by the Moon release energy, but that energy has to come from somewhere: the spin of Earth, which must slow down. Physics dictates that the total amount of angular momentum has to stay the same between Earth and the Moon, so the orbit of the Moon has been getting larger for billions of years. But can it ever escape and become its own planet?

If Earth was to spin down completely, the Moon would get to the distance of 87 Earth radii – now it is at 60 Earth radii. Calculations show that orbits closer than about 94 Earth radii are stable, so it looks like the Moon cannot escape, even theoretically. But to actually get to that distance, the Moon may need about 15 billion years, much longer than the 5 billion years Earth has left, in which the