Free Astronomy Magazine September-October 2018

14 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES T his diagram shows the orbit of the interstellar object `Oumuamua as it passes through the Solar System. It shows the pre- dicted path of `Oumuamua and the new course, taking the new measured velocity of the object into account. `Oumuamua passed the distance of Jupiter’s orbit in early May 2018 and will pass Saturn’s orbit January 2019. It will reach a distance corre- sponding to Uranus’ orbit in August 2020 and of Neptune in late June 2024. In late 2025 `Oumuamua will reach the outer edge of the Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038. In the inset, Marco Micheli. [ESA] vide the small but steady push that is sending `Oumuamua hurtling out of the Solar System faster than ex- pected — as of 1 June 2018 it is trav- eling at roughly 114,000 kilometres per hour. Such outgassing is a be- haviour typical for comets and con- tradicts the previous classification of `Oumuamua as an interstellar aster- oid. “We think this is a tiny, weird comet,” commented Marco Micheli. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.” Usually, when comets are warmed by the Sun they eject dust and gas, which form a cloud of material — called a coma — around them, as well as the char- acteristic tail. However, the research team could not detect any visual ev- idence of outgassing. “We did not see any dust, coma, or tail, which is unusual,” explained co-author Karen Meech of the University of Hawaii, USA. Meech led the dis- covery team’s characterisation of `Oumuamua in 2017. “We think that `Oumuamua may vent unusually large, coarse dust grains.” The team speculated that perhaps the small dust grains adorning the surface of most comets eroded dur- ing `Oumuamua’s journey through interstellar space, with only larger dust grains remaining. Though a cloud of these larger par- ticles would not be bright enough to be detected, it would explain the unexpected change to `Oumua- mua’s speed. Not only is `Oumua- mua’s hypothesised outgassing an unsolved mystery, but also its inter- stellar origin. The team originally performed the new observations on `Oumuamua to exactly determine its path which would have probably allowed it to trace the object back to its parent star system. The new results means it will be more chal- lenging to obtain this information. “The true nature of this enigmatic interstellar nomad may remain a mystery,” concluded team member Olivier Hainaut, an astronomer at ESO. “`Oumuamua’s recently-de- tected gain in speed makes it more difficult to be able to trace the path it took from its extrasolar home star.” !