NASA's OSIRIS-REx mission will be launching today from Cape Canaveral Air Force Station to collect a sample from the asteroid Bennu and bring it back to Earth. But out of over 500,000 known asteroids in the solar system, why did the mission team choose Bennu?

The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) space probe will launch into a two-year space journey to reach Bennu, a huge, dark and roundish asteroid that is included in NASA's list of Potentially Hazardous Asteroids. This means that it is considered as one of the space rocks that could one day hit Earth.

NASA came up with three criteria in choosing the best asteroid for the sampling mission:

1. Proximity

For the OSIRIS-REx mission, the asteroids to be selected should be as close as 1.6 AU (the distance between the Earth and Sun) to 0.8 AU, and should have an Earth-like orbit.

2. Size 

The best candidate for the mission should have a diameter of over 200 meters to enable the spacecraft to land and collect sufficient sample. Smaller asteroids rotate rapidly, causing regolith -- the loose material needed for sampling -- to be ejected from the rock's surface.

3. Composition

The oldest asteroids are rich in carbon and contain organic material that may have potentially been carried to Earth as the earliest forms of life on the planet.

The criteria narrowed down the search to only five asteroid candidates, and from the five space rocks, Bennu was selected. Bennu comes close to the Earth within 0.002 AU every six years and orbits around the Sun every 1.2 years, measures 500 meters in diameter, and has high concentrations of carbon owing to its black surface.

"We selected asteroid Bennu as the final target of OSIRIS-REx because it was so well characterized, with the radar data and telescopic data, that we felt that really reduced risk to the mission profile," Dante Lauretta, principal investigator for the mission, said in a report by Space.

The OSIRIS-REx space probe will launch on board a United Launch Alliance Atlas V 411 rocket and is scheduled to reach Bennu in August 2018. The spacecraft is expected to return to Earth in September 2023 and will fly over Utah to drop the capsule containing the asteroid sample.

According to Lauretta, the mission could provide knowledge about the origin and evolution of life on Earth, as well as some insight on how to prevent a potential asteroid collision.

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