The encounter with Pluto is over. The void is ahead.
NASA’s New Horizons spacecraft is hurtling through the Kuiper Belt, a cold, dark realm of tiny, icy objects and the occasional dwarf planet such as Pluto. The spacecraft’s instruments are detecting plasma and dust, but its cameras have been turned off. There’s nothing to see and not much to do other than measure the loneliness of deep space.
New Horizons has a bit of an existential, or perhaps astronomical, dilemma: After you pass Pluto, there’s a whole lot of nothing.
The spacecraft can maneuver slightly, but whatever it is going to look at next has to be more or less directly straight ahead. About a month ago, the New Horizons team aimed the spacecraft at an object, roughly 28 miles in diameter and known as 2014 MU69. The spacecraft will pass it on Jan. 1, 2019.
MU69 was discovered last year by the Hubble Space Telescope, which was looking for something that might be in the path of New Horizons. The Hubble found five objects, but MU69 was the most inviting target and requires the least amount of fuel to fly by.
New Horizons may have to surmount a bureaucratic obstacle. As of early September, NASA hadn’t yet approved an “extended mission” for the spacecraft. The New Horizons team has until spring to put together a proposal that lays out what an extended mission would cost. NASA would consult with the broader science community before signing off on the extension.
In some respects, it seems like an easy decision: New Horizons is a healthy spacecraft. It passed through the Pluto system without any nasty collisions, and it has a radioactive power source that can keep it operational for a couple of decades at least. It has half a tank of propellant.
“We don’t have to buy any (new) rocket. We don’t have to fly across 3 billion miles of space. We don’t have to build a spacecraft,” the team leader, planetary scientist Alan Stern, said.
The scientific case for closely studying MU69 is that it’s different from Pluto, much smaller, a “cold, classical” Kuiper Belt Object that is likely to have formed 4.6 billion years ago at the birth of the solar system.
“It is a completely new family of object,” said project scientist Hal Weaver of the Johns Hopkins University Applied Physics Laboratory, the mission headquarters.
On the other hand: This thing isn’t Pluto. It’s small. It will be hard to obtain images as dramatic as the ones New Horizons got when it passed within 7,800 miles of Pluto’s surface.
Project manager Glen Fountain said his team can probably get the spacecraft within 15,000 miles of the target or closer.
But because MU69 was recently discovered, there’s almost no data about its orbit. If you’re Fountain, the questions are: Where is it? And where will it be a day from now? This is known as the ephemeris data. New Horizons can’t be joysticked in real time, because it’s so far away. A one-way message to the spacecraft, which is 3 billion miles from Earth, takes about 4.5 hours even at the speed of light, and MU69 is another billion miles farther.
The location uncertainty was a factor in the Pluto encounter, too.
“We really didn’t know where the object was that well, and we only had one chance,” Fountain said.
New Horizons skimmed Pluto 90 seconds earlier than expected and about 50 miles closer to the surface, said Mission Operations manager Alice Bowman. But that was within the margin of error, a huge success, particularly given a computer glitch 10 days earlier and all the other things that might have derailed the mission.
“I am still amazed and in awe of what we accomplished as a team. In some ways, it’s hard to digest it all,” Bowman wrote in an email. “Maybe it’s because taking all those small, continued steps over these past years became a matter of course, and when those last steps were taken, it was hard to fully comprehend the magnitude of what we’d done.”
After Pluto, a spacecraft learns that space is big and mostly empty.
Mike Brown, the California Institute of Technology astronomer who has discovered a number of dwarf planets in the outer solar system, said that it’s a rarefied realm.
“Each one of them is farther away from the next cool thing than the Earth is from Jupiter. It is a vast, almost completely empty region,” he said.
We now know, extrapolating from the observations of the planet-hunting Kepler Space Telescope, that there are probably tens of billions of exoplanets orbiting stars in our galaxy (not to mention all of the planets in the billions of other galaxies in the known universe). But space is not designed for ease of human travel.
The nearest star to the sun is Proxima Centauri, and it’s about 25 trillion miles away. If New Horizons were racing directly toward Proxima Centauri, it wouldn’t get there for tens of thousands of years.
The more plausible targets for direct robotic exploration are closer to home. One possibility is Eris, a dwarf planet discovered by Brown and colleagues in 2005. It’s a virtual Pluto twin. New Horizons can’t go to Eris, because Eris is on the other side of our solar system; it’s also three times farther from Earth than Pluto is. This might be a better mission for the next century, when Eris’ eccentric orbit will have brought it much closer.
Brown said a better target might be Quaoar, a 700-mile-diameter object that’s not much farther from the sun than Pluto.
“There’s probably a lot of objects out there, and I think it’s probably just a matter of time before we find something similar in size to Pluto or possibly bigger,” said John Grunsfeld, the astronomer-astronaut and Hubble repairman who now is head of science for NASA.
“The idea that there’s a large object out there is rattling around the community,” Brown said.
New Horizons cannot go search for planets. It’s not that kind of probe. Its telescope gives it a narrow gaze — “like looking down a straw,” said Fountain, the project manager.
The team leaders say that New Horizons is carrying a trove of data, including high-resolution imagery, that has yet to return home. The big Pluto download won’t be finished until about the end of October 2016. Transmission rates are slow and will get even slower as the distant spacecraft gets more distant.
