Free Astronomy Magazine January-February 2019

28 JANUARY-FEBRUARY 2019 SMALL BODIES grams is NASA’s Dou- ble Asteroid Redi- rection Test (DART), a space mission (de- rived from the Aster- oid Impact and De- flection Assessment, now canceled) in the final design and as- sembly phase. Its goal is to crash a spacecraft into the unofficially named “Didymoon,” the small moon of the asteroid 65803 Didy- mos. The DART mis- sion should take place in 2022 or 2024. The spacecraft is, in fact, an impactor with a mass of 500 kg, equipped with navigation instruments only. O n the left, an animation that illustrates how NASA’s DART (Double Asteroid Redirection Test) aims and strikes the smaller ele- ment of the binary asteroid Didymos, to demonstrate how a kinetic im- pact can poten- tially redirect an asteroid as part of the planetary de- fense program. Below, a poster of the DART mission concept. [NASA/JHUAPL] tury, more precisely in 2175 and 2196, Bennu will approach the Earth close enough to have 1-in-2700 chance of hitting it. Although the chances of collision appear rather remote with our current models, the Yarkovsky effect on Bennu could alter the probabilities and increase significantly the chances after the OSIRIS-REx mission results are included in the models. The reduction in the uncertainty of the future positions of Bennu (and other potentially dangerous as- teroids) is crucial for us to know to ignore, or to plan for, possible collisions with our planet. Unlike what happens in science fic- tion movies, we cannot destroy an asteroid by means of nuclear warheads – neither the small asteroids nor, much less, the large ones. The only concrete defense is to ap- propriately modify the orbit of the asteroid many decades or centuries before the date of the possible impact, letting the gravita- tional perturbations of the major bodies act successively in our favor. The earlier one intervenes to change an orbit, the less en- ergy is then required to modify that orbit. Planetary defense programs have been in testing for several years, the purposes of which are to test orbital path deviation techniques for small asteroids. In short, sci- entists plan to give those objects a very pre- cise push, hitting them with spacecraft launched at very high speeds, possibly even strengthening the thrust with the explosion of nuclear warheads. One of these pro-