Gamma-ray emitters bounce atomic particles off rock formations to see the size, density and flow of oil reservoirs.

FORBES: Magazine Article

In cases where the mass is known, scientists can determine the density, ultimately hinting at the internal structure (gas, rock and so on).

ENGADGET: Alt-week 27.10.12: ancient texts, super-Earths and special-ops mice

Unlike any other asteroid known today, Vesta has an iron core surrounded by layers of mantle rock and crust, similar to Earth and Mars, preliminary gravity and density measurements suggest.

WSJ: Asteroid Vesta Gets a Close Study