Free Astronomy Magazine January-February 2018

JANUARY-FEBRUARY 2018 Oumuamua. In record time, several articles were submitted to scientific journals in which the first conclusions were drawn, as briefly summarised below. Oumuamua entered the solar system from above, from a very high point on the plane of the ecliptic (about 123°); on September 2 it dropped below the ecliptic, just inside the orbit of Mercury, and on September 9 it transited to its shortest distance from Sun, 37.6 million km, where its surface probably reached a temperature higher than 550 K, assuming an albedo of 5%. Its travel speed, estimated at 26 km/s, rose to 88 km/s during its passage to the periastron, when the Sun’s gravity dramatically al- tered the object’s trajectory. Indeed, Oumuamua’s course bent sharply, climbing to- wards the ecliptic at a less steep angle than during its arrival, leading it in the di- rection of Earth, which it neared on October 14 at a distance of 24 million km and at a speed of 44 km/s. Four days later, Oumuamua was discovered, and once its distance was known, its ap- proximate diameter, 100-300 meters, could be calculated − depending on the surface reflectivity adopted in the calculation. Accurate photometric observa- tions made with the 4.3-meter Discovery Channel Telescope at the Lowell Observa- tory (Flagstaff, Arizona), allowed the trac- ing of a light curve with a variability of at least 1.2 magnitudes and a rotation period of not less than 3 hours, but probably greater than 5 hours. Assuming that the variability is due to the shape of Oumua- mua, researchers estimated a ratio of 3:1 between its major axis and its minor axis. Different results have come from other re- searchers, led by well-known specialist David Jewitt of UCLA. Five nights using the B elow, Oumuamua variations in brightness during three days in October 2017. The large range of brightness is due to the very elongated shape of this unique object. The differ- ent coloured dots represent measurements through differ- ent filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if Oumuamua were an ellip- soid with a 1:10 aspect ratio. [ESO/K. Meech et al.] A very deep image of Oumuamua created by combining multiple images from ESO’s Very Large Telescope as well as the Gemini South Telescope. The object is marked with a blue circle and appears to be a point source, with no surrounding dust. [ESO/K. Meech et al.]