When Moons Are Torn Apart
Gravity, tides, and the Roche limit
Our solar system abounds with natural satellites. Planets orbit the Sun, and most of the planets have moons orbiting them. The moons in our solar system exhibit a wide range of materials and sizes, from tiny balls of ice a few hundred meters across, to rocky bodies with atmospheres, large enough that they would be considered planets if they orbited the Sun on their own.
The orbits of moons also vary widely in radius. Phobos sails a mere 7,270 kilometers above the surface of Mars, while the giants Jupiter and Saturn command dozens of moons ranging beyond 20 million kilometers. For comparison, the Earth’s moon has a moderate orbital radius of 384,400 kilometers.
However, there are physical limits to orbits. If a moon strays too far from a planet, gravitational disturbances from other objects in the solar system can destabilize its orbit and steal it away.
Conversely, there is a different problem if a moon approaches too close to a planet. Without even touching the host planet or its atmosphere, such a moon can be destroyed. I will focus on this second limit, where a moon can be ripped apart when its orbit becomes too small.
