The small, icy world known as Ultima Thule has been revealed in unprecedented detail by Nasa's New Horizons spacecraft, showing it to be two joined spheres resembling a 'snowman'. The first detailed image from Tuesday's close flyby confirms the double-lobed structure, setting a record for the most distant exploration of a Solar System object.
Ultima Thule orbits the Sun in the Kuiper belt, a region of debris and dwarf planets. Scientists believe the two lobes, named 'Ultima' (the larger) and 'Thule' (the smaller), merged shortly after the Solar System formed 4.6 billion years ago. The collision likely occurred at very low speed, around 2-3 km/h, according to Jeff Moore of Nasa's Ames Research Center.
The object is extremely dark, reflecting only 6-13% of light, similar to potting soil. It has a reddish colour, likely due to irradiation of exotic ices by cosmic rays and X-rays over eons. Principal Investigator Alan Stern highlighted the challenge of targeting the small, dim object, which is about the size of Washington DC and illuminated by a Sun 1,900 times fainter than on Earth.
Less than 1% of the flyby data has been transmitted to Earth; the full dataset will take 20 months to download. The best images so far show features larger than 140 metres across, with higher-resolution pictures expected in February. Ultima's frigid state, near absolute zero, means it is likely perfectly preserved from its formation, offering clues to planetary origins.
