Doomed Moons

Some moons lead dangerous lives. Two moons of the planet Uranus, for example, are likely to be destroyed in a million years or so when they smash into each other.

Uranus has 27 known moons, and nine of them form the most tightly packed system of satellites in the solar system. Their orbits all lie within just 11,000 miles of one another.

In that busy environment, accidents are inevitable. Although the moons are small, each moon’s gravity still tugs at all the others. Over long periods of time, that can make a moon swerve into the wrong lane, causing a collision.

Astronomers recently deduced the mass of the moon Cressida by studying its effects on one of the rings of Uranus. This work indicates that the moon is massive enough to perturb another moon, Desdemona. That will make them crash into each other in about a million years — not long compared with the solar system’s age of four-and-a-half billion years.

These moons aren’t the only ones whose days are numbered —the moons Cupid and Belinda are likely to collide as well.

Impacts like these may explain why Uranus has a couple of rings near the moons. The rings may contain the debris from earlier collisions — the remnants of moons that crashed into their neighbors.

Uranus is too faint to see with the eye alone. But it’s easy to see its position tonight. It’s above the Moon, by less than the width of a fist held at arm’s length. Binoculars will show the planet as a tiny orb amidst the stars.

Script by Ken Croswell