Ultima Thule, floating in the moonlit sky

Early on New Year’s Day, the New Horizons probe, which has already lived up to its name by obtaining the most detailed pictures yet of Pluto, continued its tour of the Kuiper Belt Objects (KBOs) at the fringes of the solar system by visiting Ultima Thule, at 4 billion miles from Earth the most distant object yet visited by one of humanity’s emissaries. Pronounced “Tu-lay” (to the surprise of The Engineer), Ultima Thule orbits the sun at more than a billion miles beyond Pluto, and New Horizons returned a photograph to Earth taken a distance of around 18,000 miles from the object.

Existing images of Ultima Thule suggested that the space rock was shaped approximately like a bowling pin, but New Horizon’s images reveal that this was mistaken. About 21 miles long and 10 miles wide, the KBO is dark red and consists of two roughly spherical lobes joined at a narrow neck. “That bowling pin is gone. It’s a snowman if anything at all,” commented mission principal investigator Alan Stern.

The mission team believe that Ultima Thule is a class of object known as a contact binary, two distinct objects that have been attracted to each other gravitationally and have drifted together. NASA is claiming that the encounter is the first between a spacecraft and a contact binary, but this is probably not the case: the comet 67P/Churyumov–Gerasimenko, memorably visited by the European Space Agency’s Rosetta spacecraft and its robotic lander Philae in 2014, is shaped like a rubber duck and is also believed to be a contact binary. Ultima Thule spins along its axis like a propeller, which explains why it always appears to have the same brightness; elongated bodies tend to tumble, and brighten and fade as their longer side becomes perpendicular to the observer. The steady brightness has puzzled astronomers since the first measurements taken by New Horizons as it approached the Kuiper Belt.

Launched in 2006, New Horizons has reached its distant location thanks to a gravitational slingshot manoeuvre around Jupiter in 2007. The probe is nuclear powered, running off a radioisotope thermoelectric generator fuelled by plutonium-238 oxide (RTGs work by converting the temperature difference between the hot radioactive substance in the frigid external conditions into electricity, and are the power system of choice for probes that cannot utilise solar energy because of their distance from the sun or location in shadow).

The probe carries seven scientific instruments: three optical imagers, two plasma instruments, a dust sensor and a radio science receiver/radiometer; the payload was specified to investigate Pluto and its moons. Now beyond this target, its continuing mission is to explore KBOs, as these are believed to be essentially unchanged in composition and appearance since their formation during the origin of the solar system. The UltimaThule images were taken by an instrument called Long Range Reconnaissance Imager (LORRI), a digital camera built by the Johns Hopkins University Applied Physics Laboratory. New Horizons will continue to send back data, with transmissions taking six hours to reach Earth, for the next 20 months and is on a trajectory that will take it out of the solar system altogether.