After the stunning success of New Horizons' voyage past Pluto, NASA sought strange new worlds for it to explore. While the next target, 2014 MU69, was chosen for practicality, it just became a lot more interesting.
Most of the trans-Plutonian objects we know about would require unrealistically large course corrections for New Horizons to visit them. Consequently, MU69 was chosen by default, being in the right place at the right time to get a visit. Once picked, however, mission planners wanted to learn more about their target.
On July 17, they got their chance, as MU69 passed in front of star MU20170617, as seen from a small patch of Earth. Although MU69 is too distant and small for us to learn much about via direct observation (beyond detecting a reddish hue), the way it interferes with light from more distant objects can reveal its shape.
Telescopes in Patagonia, Argentina, observed not one dip in the star’s light but two. Either this means that 2014 MU69 is two asteroids in close orbit or it's an “extreme prolate spheroid”, shaped like a dumbbell or an hourglass. An even more interesting explanation lies in between: The two objects actually touch, known as a contact binary. MU69 is either 30 kilometers (20 miles) long or two objects each about half that size.
“This new finding is simply spectacular. The shape of MU69 is truly provocative, and could mean another first for New Horizons going to a binary object in the Kuiper Belt,” Dr Alan Stern of Southwest Research Institute said in a statement. He added to IFLScience: "It’s exciting and means we are more likely to see an exotic relic of solar system formation during our flyby on 1 Jan 2019."
MU69 is so small and distant its shadow only passed over a small region, which unfortunately lacked large pre-existing telescopes to observe it. Consequently, a 60-member team from the New Horizons mission was deployed to southern Argentina in the middle of winter, taking 24 telescopes with them. Fortunately, they had great local support – the Argentinian Government even closed a nearby highway for two hours to prevent headlights disturbing their view. Despite high winds and biting cold, the New Horizons team were able to make observations with enough telescopes to confirm the double dip.
As challenging as the conditions were, the situation was still better than the blocking out of an even fainter star seven days earlier, when the shadow fell mostly over the Pacific Ocean.
New Horizons’ encounter with MU69 will occur on January 1, 2019, preventing many people at NASA from partying too hard the night before.