Asteroid 1999 RQ36

The greatest asteroid threat known to Earth also may be the source of clues about how life began on our planet. That makes the space rock called 1999 RQ36 an irresistible target for NASA's upcoming robotic mission to retrieve samples.

The $800 million mission, called OSIRIS-REx, aims to inject pure nitrogen into the asteroid and collect the dirt or gravel that gets stirred up. Anything collected by this "inverse vacuum cleaner" approach not only could give scientists a better idea about the odds of the asteroid striking Earth in 2170, currently put at 1-in-1,800, but might contain organic- and water-rich material similar to what possibly seeded early Earth with the ingredients for life.

OSIRIS-REx is slated for launch in 2016 so that it can arrive at its target by 2020. Its samples would arrive on Earth in 2023.

The probe's approach to 1999 RQ36 a space rock a third of a mile (575 meters) wide also could serve as a test run for future asteroid missions.

Besides collecting rock samples, plans for the 400 days spent near 1999 RQ36 include mapping the inside and outside and investigating the chemistry and mineralogy. Scientists also want to measure the so-called Yarkovsky effect, which describes how asteroids gain a gentle push from emitting thermal radiation from their night side.