Counting boulders on tiny worlds tells us how ancient and fragile they are

Astronomy rocks!

Well, planetary science rocks. Literally.

Let’s explore two small worlds, each similar yet wildly different to the other, but both with a bolder tale to tell. Or a boulder one.

We’ll start with Ceres, the largest object in the main asteroid belt between Mars and Jupiter, and so big it was well on its way to becoming a true planet before it ran out of building material billions of years ago. For that reason, scientists call it a protoplanet. 

The Dawn spacecraft entered orbit...