The much smaller size of the impactor suggests the impact didn't cause a global event.

BBC: Impact led to dino rule

Rocks blasted off Mercury by a large impactor would have a difficult journey to reach the Earth, say the researchers - but not impossible.

BBC: Mercury meteorite puzzle

The impactor used simpler versions of the flyby spacecraft's hardware and software and sent back images of Tempel 1 up to three seconds from impact.

BBC: Q&A: Deep Impact comet mission

But the early models indicated that much of this debris would have originated from the impactor, whose composition would most probably have differed substantially from that of Earth.

BBC: Moon formation: New spins put on old questions

To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater.

BBC: Dinosaur-killing space rock 'was a comet'