The idea of the Moon spiraling inward until it crashes into Earth is a staple of science fiction, but this scenario contradicts the fundamental laws of orbital mechanics. The Earth and Moon are engaged in a stable, long-term relationship, though it is one that is slowly changing over immense timescales. Instead of approaching our planet, the Moon is currently moving in the opposite direction, increasing the distance between the two bodies.
The Moon is Moving Away
Far from heading toward a collision, the Moon is actually receding from Earth at a measurable rate of approximately 3.8 centimeters every year. This precise measurement is possible thanks to the Lunar Laser Ranging (LLR) experiment. During the Apollo missions, astronauts placed specialized retroreflectors on the Moon’s surface. Terrestrial observatories aim high-powered lasers at these reflectors and measure the exact round-trip time it takes for the light to return. Scientists can track the Earth-Moon distance to within a few centimeters.
While this recession rate seems constant now, it has not remained the same over the 4.5 billion-year history of the Earth-Moon system. If the current rate were projected backward in time, it would suggest the Moon was close to Earth only about 1.5 billion years ago, which is not consistent with the system’s age. This discrepancy means the rate of recession was slower in the distant past than it is now.
The Physics of Tidal Acceleration
The force driving the Moon away is a consequence of the gravitational interaction known as tidal acceleration. The Moon’s gravity creates bulges in Earth’s oceans and, to a lesser extent, in the solid crust. Because the Earth rotates much faster than the Moon orbits, this rotation drags the tidal bulge slightly ahead of the direct line between the Earth and Moon. This misalignment means the mass in the bulge exerts a slight gravitational pull on the Moon that is directed forward along its orbital path, accelerating it.
For any orbiting body, a forward acceleration results in the object moving into a higher, larger orbit, which causes it to move slower. This mechanism is an example of the conservation of angular momentum within the Earth-Moon system. As the Moon gains orbital angular momentum by moving farther out, the Earth must lose a corresponding amount of rotational angular momentum.
This transfer of momentum causes Earth’s spin to slow down, a process called tidal braking. The energy lost from Earth’s rotation is mostly dissipated as heat through friction, but a small portion is used to push the Moon into its higher orbit. The result is a lengthening of our day, currently increasing by about one to two milliseconds per century.
The Ultimate End for the Earth-Moon System
The process of tidal acceleration will not continue indefinitely, nor will it result in the Moon drifting away entirely. The recession will eventually slow down as the Moon gets farther away and the tidal forces weaken. The system will stop changing when the Earth’s rotation period matches the Moon’s orbital period.
When this happens, the Earth and Moon will be tidally locked, meaning the Earth’s day and the lunar month are equal, estimated to be around 47 of our current days. At this point, the Moon will hang stationary in the sky over only one hemisphere of Earth.
Long before this tidal lock can be achieved, the Sun will begin its transition into a red giant star in approximately five billion years. As the Sun exhausts the hydrogen fuel in its core, its outer layers will expand dramatically. The Sun’s expanded atmosphere will likely swell out to or beyond the Earth’s current orbit, engulfing and vaporizing our planet.
Even if the Earth were to survive being swallowed, the heat and radiation from the red giant Sun would sterilize the entire system, evaporating the oceans within about one billion years and eliminating the tides that drive the Moon’s recession. The final end for the Earth-Moon system will not be a collision, but destruction or sterilization caused by the evolution of our star.