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takyon writes:

Uranus's moon Cressida could collide with another moon within the next million years:

The Voyager 2 spacecraft discovered Cressida in 1986. It is just 82 kilometres across, dark in colour and orbits close to Uranus but beyond most of its rings. It belongs to the most tightly packed group of satellites in the solar system, nine moons whose orbits all lie within 18,000 kilometres of one another.

Now, Robert Chancia at the University of Idaho and his colleagues have deduced the small moon's mass – and from it discovered the probable shape of its demise. [...] This link enabled the team to use the ring to deduce the moon's mass, the first time anyone has weighed such a small moon of Uranus. They found that Cressida is about 1/300,000th as massive as Earth's moon and about 86 per cent as dense as water. That's much denser than the small moons of Saturn, which are mostly made of water ice but are porous, making them lighter than ice.

[...] The findings spell trouble for Cressida. The denser Uranus's closely packed moons, the more their gravity tugs at one another, raising the spectre that one will swerve into the wrong lane. "Some of these moons are probably going to crash into each other," says team member Matthew Hedman, also at the University of Idaho. In only about a million years, Cressida will probably strike Desdemona, a moon that orbits just 900 kilometres outside Cressida's path, says the team. A similar fate awaits the moons Cupid and Belinda, which will hit each other.

Also at Science News.

Weighing Uranus' moon Cressida with the η ring

Original Submission