Bennu – the name comes from that of a heron-like bird in Egyptian mythology that may have provided the origin of the phoenix myth – is on the list of asteroids that offer a credible threat of collision. However, the chance has been estimated as being no worse than 1 in 1800 and the date of impact, should it occur, would not be until about the year 2180. It is only third on the list of ‘asteroids most likely to hit us’ but the other two are not expected to pose a threat any sooner than that of Bennu.
In the meantime, a mission is under way to have a good look at Bennu and see what it is made of. Given that it was formed from material that existed at the very beginning of the Solar System, knowledge of the asteroid’s composition is of considerable interest for understanding how the solar System evolved.
The mission will also be able to study precisely the course that Bennu takes in its orbit. This is clearly of fundamental importance in working out how the asteroid proceeds through space and the extent to which external forces, such as the solar wind, vary the orbit of small bodies such as this (Bennu has a diameter of 493 metres). Predicting future orbits is not easy with current knowledge, but precise information gathered from a close-up look will help to make computer models more accurate when future impact risks are being calculated.
The mission, called OSIRIS-Rex, was launched in September 2016 and is due to reach Bennu in 2018. The spacecraft will make readings of the surface for about eight months and enable a 3-D map to be created. From this map a landing site will be chosen so that a sample of rock can be taken from the surface, after which the craft will return to Earth. It is expected that the mission will conclude in 2023.
© John Welford
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