Free Astronomy Magazine January-February 2019

29 JANUARY-FEBRUARY 2019 SMALL BODIES Didymos is a small asteroid 780 meters in diameter, sep- arated by just 1.1 km from Didymoon, whose diameter does not exceed 160 meters. Even though this binary sys- tem periodically approaches the Earth’s orbit up to a fraction of an astronomical unit, it does not fall into the category of Potentially Haz- ardous Asteroids, and there- fore any intervention on its orbit will not make it more threatening in the future. The effects the DART exper- iment will produce will be small but important. The mission’s designers predict that the spacecraft will hit Didymoon cen- trally at a speed of 6 km/s and will modify the orbital velocity of the little asteroid by about half a mil- limeter per second, which will result in a variation of the or- bital period around Didymos of about ten minutes. This variation will have repercussions on the orbit of the binary system around the Sun (today taking 770 days), with the result being that the orbital velocity will change by about 0.4 mm/s. It is a seem- ingly insignificant value, but in reality exceeds that attrib- utable to the Yar- kovsky effect. If projected over the period of a century, this impact will pro- duce a spatial posi- tioning difference of over one million km! The aftermath of the DART experiment will be followed by ground-based optical and radio telescopes, and it will be possible to prove that the threat posed by potentially dangerous as- teroids can be mitigated by intervening well in advance on their orbits. Here are two comments from researchers directly in- volved in the project. The first one by Tom Statler, program scientist for DART at NASA Headquarters: “A binary asteroid is the per- fect natural laboratory for this test. The fact that Didymoon is in orbit around Didy- mos makes it easier to see the results of the impact” . The second one by Andy Cheng, of the Johns Hopkins Applied Physics Labo- ratory in Laurel, Maryland and DART inves- tigation co-lead: “DART is a critical step in demonstrating we can protect our planet from a future asteroid impact. Since we don’t know that much about their internal structure or composition, we need to per- form this experiment on a real asteroid. With DART, we can show how to protect Earth from an asteroid strike with a kinetic impactor by knocking the hazardous object into a different flight path that would not threaten the planet” . DART will be only a first, shy step towards the realization of international programs of planetary defense, but it is a good start to be able to prevent the only non-terrestrial natural catastrophe that can be avoided with modern technology. F ourteen Arecibo radar images of the near-Earth asteroid Didymos and its moon, taken in November 2003. The photo- metric data indi- cate that Didymos is a binary system and radar images clearly show the secondary body. [National Science Foundation] !