One argument against the comet theory is that a comet would have left a large hole in the Earth's surface.

VOA: special.2009.01.20

Nanodiamonds are produced under only a very high temperature and high pressure event, such as a comet striking Earth.

VOA: special.2009.01.20

Scientists at the Goddard Space Flight Center in Greenbelt,Maryland, found the substance glycine in material brought back to Earth from a comet.

VOA: special.2009.09.30

A comet is a huge ball of frozen gas and dust that often releases a long trail of material as it nears the sun.

VOA: special.2009.09.30

But it is the first time an amino acid has been discovered on a comet.

VOA: special.2009.09.30

But scientists think it was either a comet or rock from space.

VOA: special.2009.08.25

Stardust passed through a cloud of material surrounding comet Wild Two in January of two thousand four.

VOA: special.2009.09.30

A specially designed collector gathered dust particles from the comet and stored them on the spacecraft.

VOA: special.2009.09.30