A 54-pound (25-kilogram) rock. Estimated auction price: $2 million to $4 million. Why so expensive? It's the largest piece of ...

The largest fragment of Mars ever discovered on Earth, a 54-pound meteorite named NWA 16788, is set for auction at Sotheby's in New York.

The meteorite is believed to have been blown off the surface of Mars by a massive asteroid strike before travelling 140 million miles to Earth, where it crashed into the Sahara.

Sotheby's is putting some rare items up for auction, including what it calls the largest piece of Mars ever found on earth.