Credit: NASA/JPL-Caltech

Asteroids can differ in the degree of porosity, or the amount of empty space that makes up their structures. At one end of the spectrum is a single solid rock and, at the other end, is a pile of rubble held together by gravity. Observations from NASA's Spitzer Space Telescope, taken in infrared light, have helped to reveal that a small asteroid called 2011 MD is made-up of two-thirds empty space, which means it has essentially the same density as water.