Deep Impact space collision reveals comets to be fluffy balls of powder

To medieval observers, they were mysterious harbingers of doom, but thanks to an unprecedented act of celestial vandalism, scientists have unveiled some of the innermost secrets of comets!

Most striking is that the comet is not made up of very much at all. "It's mostly empty," said Prof A'Hearn. The fine particles of dust and ice are held together extremely loosely, with pores thought to run throughout. "We have deduced that around 75% to 80% of the nucleus is empty and that tells me there is probably no solid nucleus. That is a significant advance in our understanding," said Prof A'Hearn.