In the years since NEAR visited Mathilde, there haven't been too many publications on it, and they (or at least their abstracts) seem to indicate puzzlement about its internal structure. Its density is barely higher than that of water at 1.3 grams per cubic centimeter (water is 1 g/cc) so it has to be highly porous -- a rubble pile -- but the images that NEAR took indicate that at least some parts of the asteroid are locally fairly cohesive. I mean no disrespect to asteroid scientists when I say that I don't think we understand the internal structure of these little worlds very well at all. Partially because of that, I am dying to witness (virtually anyway) the adventures of the first astronauts to explore an asteroid up close, an event that I hope will happen in my lifetime.
NASA / JHUAPL / Cornell / colorized image by Daniel Macháček
Asteroid 253 Mathilde in color
NEAR captured this photo of asteroid 253 Mathilde on June 27, 1997; Mathilde was the target of a flyby encounter nearly three years before the mission went on to orbit asteroid 433 Eros.