When I first arrived at the Vatican Observatory, I decided to take advantage of its extensive meteorite collection by doing a systematic measurement of meteorite densities. No one before had had both access to such a collection, and the time to make such a survey. It’s taken me more than 20 years, and the work continues.

Soon after I started publishing my first results, spacecraft began visiting the asteroids that were the source of these meteorites, also measuring their densities. The first surprise was that the asteroids were far less dense than the meteorites derived from them … showing that the asteroids were not solid rocks but, rather, loose piles of meteoritic rubble. Our data allow us to make specific estimates of the actual rock and metal content to be found within a given asteroid.

Both their detailed spectral colours and the actual samples returned by the Japanese Hayabusa probe confirm that these asteroids are made of the same material as the meteorites in our lab.