Among the many surprises revealed by Deep Impact is it's low density, less than that of water:

The Planetary Society Weblog
by Emily Lakdawalla
Sep 7, 2005 | 10:45 PDT | 17:45 UTC
DPS: Day 3: Deep Impact
"The trajectories traced out by the ejecta allowed them to measure the force of gravity on the comet directly, and it came out to 50 milligal, or 50 parts in a million of Earth's gravity. That, in turn, lets you back out the mass of the comet, about 7 x 1013 kilograms. That, with the shape model, lets you calculate a density, 0.6 grams per cubic centimeter, but the density has large error bars, about plus or minus 0.35. But if you believe the 0.6 g/cc number, A'Hearn said, "the porosity of Tempel 1 must be at least 75%. The comet's empty."
http://planetary.org/blog/20050905.html

A'Hearn concludes the comet is porous all the way through.
But another possibility is that it is actually hollow....

Bob Clark