The long wait to send a probe to Pluto, and what we’ve found

BOSTON—Alan Stern, principal investigator of the New Horizons mission to Pluto, started his talk at the American Association for the Advancement of Science meeting by showing off the Hubble Space Telescope's best image of Pluto. It was greeted by laughter, as it took only seconds for the audience to count the dozen pixels that contained actual data. “You may laugh again," Stern said. "We wrote numerous papers based on this image.”

He's now got a lot more data to work with, though he had to be very patient to get it. Not only did it take months to get all the data from New Horizons back to Earth, but it took decades to get the probe approved in the first place. Stern shared that tale with his audience in Boston.

Expanding Horizons

The astronomy community periodically gets together to do what are called Decadal Surveys, which help NASA set priorities for future missions. But as Stern put it, these surveys consider “many more good ideas than there is budget to execute.” So, Pluto missions had appeared in them five times without being approved. But a variety of data on Pluto trickled in even without a visit, and this gradually built the case for sending hardware there.

It might surprise some people to learn that Charon, Pluto's largest moon, wasn't discovered until 1978—not too long before the Voyagers visited Jupiter. Charon's large size relative to Pluto is only shared by one other planet-moon system in the Solar System: our own. Like the Earth's Moon, the relatively small size difference means that Charon was almost certainly formed by a giant impact and thus provides the only other opportunity to study this we'll have access to any time soon.

Later imaging in the infrared showed that its surface was complex, containing carbon monoxide, methane, and nitrogen. All three of these compounds could switch between gas and solid forms under the conditions expected to prevail at Pluto's surface, providing the potential for some complex surface-atmosphere interactions. And imaging of a star as Pluto passed in front from Earth's perspective confirmed that Pluto must have an atmosphere.

Finally, astronomers started identifying a large collection of small, distant objects beyond Pluto. While these eventually got Pluto demoted to dwarf planet status, they converted it from a Solar System oddball into the primary member of an entire class of bodies called Kuiper Belt Objects. Studying Pluto could potentially tell us about the rest of these.

All of this put intellectual heft behind the idea of sending hardware to the distant body, making it a high priority in the Decadal Survey. After five failed proposals, New Horizons was approved. By the time that happened, however, it faced a very tough constraint: there was only one window in the next decade where Jupiter would be well positioned to provide a gravity assist to Pluto. The New Horizons team would have just four years to design and build New Horizons to meet NASA's science objectives.

Despite the limits, Stern says the spacecraft "carries more scientific firepower" into a first reconnaissance than anything we've sent out previously.

Outside-in

Well before New Horizons' closest approach, it was able to provide better images of Pluto than Hubble ever could. And this was essential; Pluto's orbit wasn't known precisely enough to navigate the probe through its flyby. Imaging a few months out allowed the final adjustments to be made.

Stern described the system working from the outside-in. Pluto's four small moons, Hydra, Kerberos, Nix, and Styx, are all irregularly shaped and quite reflective, indicating their surfaces are probably composed of largely clean water ice. Craters on the surface of the two larger ones indicated that they're roughly four billion years old and thus probably formed along with the rest of the Solar System. The moons are arranged in an alternating small-large-small-large order, and Stern says that could be telling us something about their formation, but we don't have any other examples like this, so it could also be random chance.

Charon, Pluto's largest and closest companion, is also heavily cratered and likely formed very early in the Solar System's history as well. All its geography, such as an equatorial belt of ridges, is also very ancient, suggesting that Charon has been inert for a long time. Its surface is mostly water ice with outcrops rich in ammonia, but there are many pits that suggest more volatile materials boiled off and escaped. This would have given Charon a temporary atmosphere at some point in its past.

Now, its only atmosphere comes due to minute volumes of gases lost by Pluto and captured by Charon. This is what gives Charon's poles a darker color (after the gases undergo reactions catalyzed by radiation), and Stern says it's the only example we know other than stars where there's a transfer of atmosphere between two bodies.

The main event

The materials we knew were on Pluto's surface aren't rigid enough to support much in the way of geology, so it wasn't clear what we should expect on the dwarf planet. But for most areas, these materials were just a thin veneer on water ice, which is a very tough material at plutonian temperatures. On the surface, nitrogen is more common in the mid-latitudes and methane at the poles; this, Stern said, can be explained by their volatility at the local temperatures. Pluto's atmosphere turned out to be lower and denser than expected, which limits the rate at which it's lost to space by a factor of about 1,000.

The age of the surface varies, with some heavily cratered regions that look to date to the dwarf planet's formation. There are a few areas with intermediate cratering (the East Tombaugh Reggio), and then there's the Sputnik Planum. It's a nitrogen glacier where there are hundreds of thousands of square kilometers where they can't find a single significant crater. Estimates are that its surface is less than 30 million years old. Visually, it appears that there are currents moving through some parts of the glacier and other sites where smaller glaciers are emptying into it.

During the flyby, “My science team didn’t have a single glaciologist on it,” Stern said. He's now got two.

Stern also suggested that New Horizons' images contain hints of liquids on Pluto's surface, almost certainly nitrogen—he showed an image of what appears to be a frozen lake surrounding a mountain, and others of frozen features that appear to bisect ridges and crater walls. While Pluto is currently too cold for liquid nitrogen, it experiences the same sort of wobbles that drive Earth's glacial cycles, except the tilt of its axis changes by an even larger amount. During one of the dwarf planet's warmer phases, Stern says, calculations suggest that the existence of liquid nitrogen should be possible.

Finally, there is a grab bag of other features that we don't currently understand. Stern showed a picture of a mountain that's similar in scale to Hawaii's Mauna Loa: 100km across and 4,000m high. He said it's recent (based on cratering) and looks like a volcano, but it has not been possible to confirm that yet.

All of New Horizons' Pluto data has now been safely transmitted to Earth, and it has completed the engine firings that are needed to send it to its next target, 2014 MU₆₉. At 44 times the distance from the Earth to the Sun, 2014 MU₆₉ is part of the "cold classical" region of the Kuiper Belt; at 20-40km across, its size is intriguingly in between small planets and comets. So there's a chance New Horizons will have some more big news during its 2019 flyby, or at some other points, before its power supply runs down in the 2030s.

While he's waiting for the next rendezvous, however, Stern won't be bored. He's already taken a position on the science team of a planned Europa mission.