Why this particular asteroid?
OSIRIS-REx's target asteroid, Bennu, was chosen because of its composition, size, and proximity to Earth.
Bennu is a member of a group of asteroids that are considered primitive and relatively rare (classified as "B type"). It is believed to be rich in carbon, which makes it quite dark (it reflects only about 3% of the light that hits it). Scientists also believe that Bennu may contain organic compounds, and could potentially possess clay minerals that can hold water.
Bennu has remained relatively unchanged for the past 4.5 billion years. It is made up of rocks and rubble left over after the planets formed. Like a cosmic time capsule, Bennu may therefore reveal a wealth of information about the early solar system. The mission team hopes to find organic molecules on Bennu like those that may have led to the origin of life on Earth.
Measuring almost 500 metres in diameter, Bennu takes about 1.2 years to orbit the Sun. It moves in an elliptical pattern between Venus and Mars, crossing paths with Earth's orbit every six years, which makes it both an accessible target for a sample-return mission, as well as an asteroid to watch.
Could Bennu strike Earth?
Of the 500 000 or so catalogued asteroids and about 9000 near-Earth objects, Bennu is one of the most potentially hazardous known to humanity, with a 1/2700 risk of colliding with Earth in the year 2182. One of the goals of the OSIRIS-REx mission is to better understand Bennu's orbit and composition, which will help provide more conclusive data on when it might impact Earth.
Where did Bennu's name come from?
Until recently, Bennu was known by its official astronomical catalogue number, Near-Earth Object (NEO 101955) 1999 RQ36. In 2013, Michael Puzio, a nine-year-old student in grade three in North Carolina, USA, won an international student contest to give the asteroid a friendlier name. Michael suggested the name because he imagined the spacecraft's robotic arm and solar panels look like the neck and wings of Bennu, the Egyptian deity often depicted as a grey heron.
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