To Ultima Thule and Beyond
In the early hours of New Year's Day, billions of miles from any Earthly celebrations, the New Horizons space probe swung by a small and extremely distant lump of ice and rock. It’s known to cataloguers as (486958) 2014 MU69, but the New Horizons team call it 'Ultima Thule' after the ancient expression for a place at the edge of the known world.
The nickname caused those of us gathered in mission control at the Applied Physics Laboratory outside Baltimore some anxiety in pronunciation; the team took its lead from Edgar Allan Poe, buried up the road, who in 'Dream-Land' helpfully rhymes ‘Thule’ with 'newly'.
The poem goes on to describe a ‘wild weird clime’, and Ultima Thule certainly inhabits a dark and eerie place at the distant edge of the Solar System. At noon the light is no brighter than a deep twilight dusk on Earth, and any human visitor would see the Sun reduced to the brightest star among many in a night sky that never sees day.
In 2015, after a voyage of three billion miles, New Horizons flew past Pluto, transforming what had appeared as little more than a point of light into an entire world, with nitrogen glaciers and mountains made of water ice. (There are souvenir posters for sale at mission control purporting to advertise a skiing trip to these crystalline peaks.) On its way out, the spacecraft turned to look back at Pluto, revealing the blue haze of the planet's tenuous atmosphere silhouetted against the dim sunlight.
Studying Pluto tells you about Pluto. Studying Ultima Thule might yield a greater prize in unlocking the secrets of how planets, in general, grow. Undisturbed since the very earliest days of the Solar System, it is a left-over brick, part of the rubble from a construction project which finished more than four billion years ago.
Finding a single brick in the vastness of space is not easy. Because New Horizons was sent first to Pluto, none of the few thousand other objects already known to exist beyond Neptune could be reached without exhausting the craft's reserves of fuel. Astronomers turned to the Hubble Space Telescope and found somewhere they could visit, a small rock visible only as a single point of light moving slowly against the background stars seen towards the centre of the Milky Way.
A few years after discovery, the orbit of Ultima Thule is not well established. With New Horizons dashing past at about eight miles per second, pointing its cameras in the right direction was a difficult task of choreography. Unlocking the secrets of the early Solar System requires thousands of things to go right, each of them taking place on a spacecraft so distant that instructions sent at the speed of light take more than six hours to reach it.
No wonder there were cheers and flag waving at mission control yesterday morning. New Horizons turned briefly towards Earth a few hours after closest approach to communicate with us, sending confirmation that the bounty of images and scientific measurements was safely on board. The last photograph taken before flyby was released, revealing a blurry image of something the rough shape of a skittle.
New Horizons has only a very weak transmitter on board, and it will take up to twenty months for all the information to be received back on Earth. For now, though, that brief signal saying all is well means everything to a celebrating team, some of whom have worked on the mission since the early 1990s. For others, the expected haul of data will provide work for the next decade. Meanwhile, four and a half billion miles away, Ultima Thule continues, undisturbed in the gloom, on its orbit around the Sun.