Free Astronomy Magazine January-February 2019

25 JANUARY-FEBRUARY 2019 SMALL BODIES Bennu. It is no coincidence that NASA has chosen it as a subject to be studied carefully in the two years of planned scientific activity. From a dynamic point of view, researchers are particularly in- terested in non-gravitational forces – the less predictable ones – that move the asteroid. The most relevant of these forces is known as the “Yarkovsky ef- fect,” which consists of a small push that the asteroid gives to itself when, by rotating on its axis, it releases from its night- side hemisphere the heat accu- mulated when that hemisphere was exposed to solar radiation. However small the push may be, any variation of orbital parameters that may unexpectedly result are amplified over the long run by gravitational perturbations produced by the planets and other massive bodies. The more the asteroid’s position is projected into the future, the more the margin of error attributable to gravita- tional perturbations increases – and that A sequence of images taken by the OSIRIS-REx probe that shows Bennu in a com- plete rotation from a distance of about 80 km. The Poly- Cam probe camera obtained 36 frames of 2.2 milliseconds over a period of 4 hours and 18 min- utes. [NASA’s God- dard Space Flight Center/University of Arizona] Alongside, a com- parison of the as- teroids Bennu and Ryugu. [NASA] internal structure by accurately measuring its mass, thus offering us insights into a possible weakness of this type of poten- tially dangerous, Earth-crossing asteroid. We know of almost two thousand small as- teroids that, in the not too distant future (astronomically speaking), could threaten our planet. One of the most worrying is