In just a million years' time, two of Uranus’ moons will smash into each as they stray out of orbit and wander into each other’s paths, obliterating them both. This is probably not the first time that this has occurred around the planet, and is unlikely to be the last.
In a paper to be published in The Astronomical Journal, researchers have for the first time calculated the mass and density of one of Uranus’ diminutive moons, Cressida, that orbits close to the planet in the densest group of satellites observed in the Solar System, and from this have worked out what its future movements are likely to be. And unfortunately, the news for the little satellite isn’t great.
Known as the Portia Group, these nine moons circle Uranus in an amazingly tight orbit, all coming within just 18,000 kilometers (11,200 miles) of one another. To put this into perspective, our Moon is orbiting at a distance of 380,000 kilometers (236,000 miles).
It turns out that Cressida is not particularly dense, and because the orbit in which it moves is packed with other denser moons that in turn have more pull, the likelihood is that the gravity of these larger bodies will pull Cressida out of its orbit and into the path of another, known as Desdemona.
Apart from its orbit and diameter, almost nothing else is known about Cressida, but the scientists were able to calculate the tiny celestial body’s weight by looking at what effect it has on one of Uranus’ rings. Known as Eta, the team studied the ring's trajectory, and to their surprise found that rather than being circular in shape, the ring was actually slightly triangular. It turns out that this distortion is most likely down to the gravity of little Cressida.
They found the particles that make up the ring were traveling around Uranus three times for every two orbits made by Cressida, enabling the researchers to infer the mass of the moon. It turns out that the satellite has a mass just 1/300,000th that of our own Moon, and is only around 86 percent as dense as water. They think that it is probably rocky, but because of its small size, there is unlikely to be enough gravity to pull it all together, hence the moon's low density.
This is not good news for poor Cressida. It is now suspected that the larger and denser moon Desdemona, which orbits just a mere 900 kilometers (560 miles) away from Cressida, will pull it out of orbit and the two moons will smash together. They also think that this will be the likely fate of another pair in the Portia Group, Cupid and Belinda, while the rocky remains in one ring probably represent a past moon smash event.
[H/T: New Scientist]