Free Astronomy Magazine November-December 2017

19 NOVEMBER-DECEMBER 2017 SPACE CHRONICLES ity in the binary system. “We de- tected strong indications of the sub- limation of water ice due to the increased solar heating — similar to how the tail of a comet is created,” explains Jessica Agarwal (Max Planck Institute for Solar System Re- search, Germany), the team leader and main author of the research paper. This makes 288P the first known binary asteroid that is also classified as a main-belt comet. Understanding the origin and evo- lution of main-belt comets — aster- oids orbiting between Mars and Jupiter that show comet-like activity — is a crucial element in our under- standing of the formation and evo- lution of the whole Solar System. Among the questions main-belt comets can help to answer is how water came to Earth. Since only a few objects of this type are known, 288P presents itself as an extremely important system for future studies. The various features of 288P — wide separation of the two compo- nents, near-equal component size, high eccentricity and comet-like ac- tivity — also make it unique among the few known wide asteroid bina- ries in the Solar System. The ob- served activity of 288P also reveals information about its past, notes Agarwal: “Surface ice cannot sur- vive in the asteroid belt for the age of the Solar System but can be pro- tected for billions of years by a re- fractory dust mantle, only a few metres thick.” From this, the team concluded that 288P has existed as a binary system for only about 5000 years. Agarwal elaborates on the formation scenario: “The most probable formation scenario of 288P is a breakup due to fast rota- tion. After that, the two fragments may have been moved further apart by sublimation torques.” The fact that 288P is so different from all other known binary aster- oids raises some questions about whether it is not just a coincidence that it presents such unique prop- erties. As finding 288P included a lot of luck, it is likely to remain the only example of its kind for a long time. “We need more theoretical and ob- servational work, as well as more objects similar to 288P, to find an answer to this question, ” con- cluded Agarwal. T he artist’s impression in the back- ground shows the binary asteroid 288P, located in the main asteroid belt between the planets Mars and Jupiter. The object is unique as it is a binary asteroid which also behaves like a comet. The comet-like properties are the result of water sublimation, caused by the heat of the Sun. The orbit of the asteroids is marked by a blue ellipse. [ESA/Hubble, L. Calçada] T his set of Hub- ble Space Tele- scope photos reveals two aster- oids orbiting each other that have comet-like fea- tures. These in- clude a bright halo of material, called a coma, and a long tail of dust. The as- teroid pair, called 2006 VW139/288P, was observed in September 2016 just before the as- teroid made its closest approach to the Sun. The photos revealed ongoing activity in the binary system. [NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research)] !