Free Astronomy Magazine January-February 2019

26 JANUARY-FEBRUARY 2019 SMALL BODIES position becomes even more uncertain if other forces less quantifiable than the gravitational ones are acting on the motion of the asteroid. In this context, the Yarkov- sky effect is decidedly dif- ficult to quantify, as its intensity and efficiency depend on the reflectiv- ity, composition, distribu- tion, and the structure of the surface material. It is evident that only in situ studies of individual types of surface can pro- vide comprehensive in- formation, making such missions as OSIRIS-Rex of great importance. By means of mathematical simulations, as- tronomers have already calculated how the orbit of Bennu will change in the future, taking into account the gravitational per- turbations it will suffer from the Sun, the planets, the Moon, and other asteroids. A rough estimate of the Yarkovsky effect was also included in the calculation. The results of the simulations say that in 2054, 2060, 2080 and 2135, Bennu will pass at a dis- tance of less than 7.5 million km from the Earth. At each close encounter, the aster- oid’s trajectory will be influenced by the T his render shows OSIRIS- REx touching as- teroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM. [NASA’s Goddard Space Flight Center] On the left, in mid-2020, the OSIRIS-REx probe will use its TAGSAM device to lift and collect a sample of loose material from the surface of the as- teroid. That mate- rial will reach Earth in 2023 to be studied. [NASA/Goddard/ University of Arizona] mass of our planet and the mass of the Moon. This makes the position of the aster- oid quite uncertain after its close en- counter in 2060, when Bennu will be about twice the distance of the Moon away within a window of space 30 km wide. Even a minute inaccuracy in the current forecasts could be dangerously amplified in the following encounters, and this window of uncertainty only grows with time. In 2080, this window will be 14,000 km wide, which is all-in-all reassuring; but for the en- counter of 2135, the uncertainty in the po- sition of Bennu reaches 160,000 km and, since this crossing will be closest to Earth in the coming cen- turies, it is inferable that an uncertainty of that magnitude is not reassuring at all, especially if we consider that the Yarkovsky effect for this specific as- teroid could be sig- nificantly different from the estimated one. This is why it is