From the space probe Cassini, in orbit around Saturn:

Image credit: NASA/JPL/Space Science Institute

Bad Astronomer Phil Plait says:

The moon at the top is Rhea, which is about 1500 km (950 miles) across. We’re looking past its south pole here. The moon farther away is Dione, which is 1100 km (700 miles) in size. And since Cassini was very nearly in the plane of Saturn’s equator, the rings are nearly edge-on. Note that Dione is on the other side of the rings as seen by Cassini, so the bottom of the moon is obscured by the rings. We can’t see Saturn itself, but it’s off to the left in this shot.

Rhea is only a little bigger than Dione, but is a lot closer in this shot: 61,000 km versus 924,000 for Dione! That’s why Dione looks so much smaller. As seen by Cassini in this shot, it’s actually more than twice as far as our Moon is from the Earth. Both moons are composed of mostly water ice, with some rock. Both have been heavily battered by impacts, as you can see.

Advertisements