The space rock that hit Earth 65 million years ago - which is thought to have killed off the dinosaurs - was probably a fast-moving comet, according to new research.
A group of scientists have been studying the Chicxulub crater in Mexico, which is where the space rock is thought to have hit.
Before now, most scientists thought that it was a large and relatively slow moving asteroid that hit the earth there, leaving the 180 kilometre-wide dent.
The latest study though, suggests the rock was much smaller than previously thought, and this means it must have been travelling much faster to have had such a big impact.
After examining the rocks at the crater, scientists say evidence suggests it was a comet.
The research was revealed at a conference in Texas, where experts from all over the world meet to share their work on planetary science.