Know your enemy could be the motto of a mission NASA is leading to the asteroid Bennu. We have visited asteroids before, but Bennu is the first target to pose a threat to us, albeit not for quite a while.
In 2180 Bennu will make a series of close approaches to Earth, and there is a 1 in 1800 chance of a collision. At 493m wide Bennu is not wide enough to destroy civilization, but you wouldn't want to be under it – or even on the same continent really.
The mission, to be launched in 2016 for a 2018 rendezvous will also produce a level of detail we have never seen before from another worldlet. OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security Regolith Explorer) will spend eight months mapping Bennu using a Canadian Laser Altimeter to create 3D maps of Bennu, from which a safe landing site will be chosen, allowing OSIRIS-Rex to touch on and collect a 60 gram sample to return to Earth by 2023.
“The Canadian science team working on OSIRIS-REx iis interested primarily in asteroid geology — how Bennu formed, how it evolved, why it is shaped as it is,” said Alan Hildebrand of the University of Calgary. “Bennu will have a very different geology than has ever been seen up close before.”
The sample will provide a unique opportunity to study the origins of the solar system – NASA describes Bennu as “A pristine, carbonaceous asteroid containing the original material from the solar nebula, from which our Solar System formed.”
Another of NASA's objectives with OSIRIS-Rex is to “Measure the orbit deviation caused by non-gravitational forces.” Pressure of sunlight on the surface can subtly change an asteroid's orbit. Over a period of decades these can add up, to the point where they can determine whether we need to be planning aversion manoeverrs for Bennu in 166 year's time. More importantly, they will help with modeling other asteroids that may pose more immediate dangers.
Bennu was the Egyptian god of rebirth, making the acronym OSIRIS rather appropriate. However, the addition of the Greek "Rex" could outrage pedants.