New Horizons, the spacecraft that brought you Pluto and its moons, isn't done yet. The craft is still hurling out into the void, and on New Year's Day, 2019, it will fly past a mysterious object out in the frigid realms of the outer solar system known as the Kuiper Belt. The target Kuiper Belt Object (KBO) is known as 2014 MU69 for the year it was discovered. The encounter to start 2019 will be the first time a spacecraft visits an object that was not discovered until after the spacecraft was already launched.
When New Horizons launched in January 2006, the team had some plans for Jupiter observations during a gravity assist flyby. They also knew the craft would still have some gas in the tank after it flew by the primary target for the mission, Pluto and its system of moons. But it wasn't until June 2014, eight and a half years after launch and a year before the Pluto flyby, that the Hubble Space Telescope spotted a small KBO for the first time, subsequently known as 2014 MU69.
The object become the post-Pluto target for New Horizons. Just four and a half years after its discovery, MU69 will receive close examination in a flyby mission, the quickest turnaround from discovery to flyby in human history.
So what is this 2014 MU69 going to be like? Well we don't really know, but astronomers around the world are working to take new observations of the target to gather as much information as possible before the flyby. Originally estimated at around 10 to 30 miles in diameter, recent observations have led researchers to suspect the object is either smaller than that, but particularly bright and reflective, or that it is perhaps even multiple objects.
MU69 recently moved in front of the light from a distant star on June 3, an event known as an occultation, similar in a way to a solar eclipse or a planetary transit, but smaller and quicker. The shadow caused by MU69 as it blocked the background star's light quickly raced across South Africa and Argentina. More than 50 astronomers positioned on both continents set up something of a fence line of telescopes along the predicted path of the shadow, hoping to catch a two-second glimpse of the occultation.
The astronomers were able to capture the change in light, but did not pinpoint the object itself. This tells the team that MU69 might be smaller than originally thought, although it seemed larger in the original Hubble images because it is particularly reflective. Or perhaps MU69 is not only one object.
"These results are telling us something really interesting," New Horizons Principal Investigator Alan Stern of the Southwest Research Institute (SwRI) said in a press release. "The fact that we accomplished the occultation observations from every planned observing site but didn't detect the object itself likely means that either MU69 is highly reflective and smaller than some expected, or it may be a binary or even a swarm of smaller bodies left from the time when the planets in our solar system formed."
In any case, MU69 should be a fascinating little world. If it is a binary or group of objects, their interactions with each other could tell us much about the early solar system, planetary formation, and the process celestial objects go through when they coalesce. If it is one smaller object, then it is very bright, and it could be a gleaming icy chunk of reflective material.
New Horizons, equipped with one of the most advanced telescope-cameras we have beyond Earth orbit, the Long-Range Reconnaissance Imager (LORRI), should capture crystal clear images of MU69 during the flyby.
Two more occultation events are coming up this month, on July 10 and July 17, and you can count on the persistent astronomers to be out there forming another fence line of small telescopes for more observations. On July 10, NASA's 747 with a large 2.5-meter telescope tucked inside, the Stratospheric Observatory for Infrared Astronomy (SOFIA), will also observe MU69 to gather more data and look for debris that could pose a threat to New Horizons as it flies by. During the July 17 event, NASA will point Hubble toward MU69 again for the same reasons.
In 18 months, on the first day of 2019, New Horizons will make closest approach to the mysterious object, and a day or so later MU69 will be revealed in all its small icy glory. Then maybe finally the little celestial object will earn itself a new name.