Scientists think that the Earth was dry when it formed billions of years ago and that it likely received water from an external source. Since they sit far enough away from the sun to retain hefty amounts of ice, comets have been a top contender for the delivery vehicle. Some astronomers had suspected that asteroids, which group in a belt between Mars and Jupiter, might boast some ice of their own, but they had not found proof until now.
A paper in this week's Nature details the discovery that one third of the asteroid 24 Themis is covered in frost. Because 24 Themis is close enough to the sun that the frost should have vaporized by now, scientists suspect that the asteroid houses a replenishing supply of ice beneath its rock. If 24 Themis proves an average asteroid rather than an exception, the scientific distinction between comets and asteroids may dissolve in a classification controversy on the level of Pluto's recent demotion from planet to dwarf-planet.
This discovery accompanies Obama's vision for a new NASA, one that scraps a glory-seeking return to the moon to focus on a slow and steady approach to Mars. In his Cape Canaveral speech two weeks ago, Obama declared his goal to send astronauts to an asteroid within 15 years. Asteroids' high-speed trajectories pose various challenges for a manned mission, but a supply of water could prove helpful for drinking and fuel purposes.