Soon We'll Know What Pluto Looks Like

NASA's New Horizons space probe was launched in 2006 and will make its closest approach to Pluto on July 14, 2015. In less than a month from now we get to see what Pluto looks like. In the words of Principal Investigator Alan Stern, "We're going to turn a point of light into a planet and its moons overnight in the next month."

This is not an exaggeration. Have a look at the very best computer-enhanced Hubble Space Telescope images we had of Pluto before New Horizons. Already, the spacecraft's camera is returning images that are more detailed than any of these, and they are going to get much better. 

"We're going to turn a point of light into a planet and its moons overnight in the next month."
New Horizons Principal Investigator Alan Stern

The Biggest Thing Since Neptune

The last time anything this momentous took place was when Voyager 2 flew past Neptune. When that event took place in the summer of 1989, I was fortunate enough to be working where all the action was taking place at Jet Propulsion Laboratory (JPL). Video monitors hung all around the cafeteria showing a continuous feed of the latest images downloaded from Voyager 2 and the live feed from the daily press conferences.

Today, when you look back at the images captured by Voyager, it's difficult to appreciate what it was like as the images were rolling in. Now that the best images of Neptune and its large moon, Triton, have been stitched together into high resolution photomosaics, it's difficult to remember how each new fuzzy picture that at least for a few hours was the latest "best image so far" would offer new insight and new clarity.

Triton - Pluto's Proxy

For example, Neptune's moon, Triton, is one of the larger moons in the solar system (7th largest -- Earth's moon is 5th largest), and it's unusual in has a retrograde orbit, which means that it orbits around Neptune opposite to the direction Neptune rotates. Many scientists believed, even before the Voyager flyby, that Triton therefore was not formed together with Neptune, and instead that it must have formed in the Kuiper belt (area of the solar system beyond Neptune where Pluto lives along with thousands of other smaller objects) and was later captured by Neptune's gravity and became a moon. 

If this is true, that Triton is a captured Kuiper belt object, then the Voyager 2 flyby offered a chance to see a Kuiper belt object up close, and what we learned from Triton might tell us about other Kuiper belt objects, the largest and most famous of which is Pluto.

What Will Pluto Be Like? 

Before I make any predictions, it might be useful to point out that no matter what I say here, I am likely to be proven slightly, or even spectacularly wrong in a less than a month. Usually people (well, smart people) make their predictions far enough in the future so that when they turn out to be wrong, the original prediction is either long forgotten, or the original predicter is long dead.

So, don't think of these are predictions. Think of them as things to look out for.

The Plutonians will have green skin and three noses.

Just kidding.

Pluto and Charon are different from each other.

The current thinking is that Pluto and Charon formed either by a giant impact that smashed off a large chunk of Pluto's icy outer mantle that later coalesced into Charon, or from two objects smashing together and then settling into orbit around each other. Pluto has a larger proportion of rock than Charon, and Charon consists more of ice.

We know a lot about the differences between Pluto and Charon from some very clever measurements that watched what happened when they occulted (passed in front of and hid) either a star or each other. That's how we've know the sizes of Pluto and Charon and that Pluto has an atmosphere.

If Pluto and Charon really did form from a massive collision, that may have left a lasting imprint on the surface. For example, Miranda, one of the moons of Uranus, looks like it was blasted to pieces, and then pieces melted back together, but you can still see the layers on the surface of what were once the inner layers of the moon.

Many Moons

We already know that there are other smaller moons in orbit around Pluto, not just Charon. Most likely, where there's five moons already that we know about, there are still more to be found.

Rings

Everybody knows that Saturn has big rings, but Jupiter has rings, Uranus has rings, Neptune has rings. Some scientists think Pluto might have rings too. If they are there, they are going to be pretty faint.

Geysers

This was the big surprise about Triton. Not only did there seem to be some evidence of activity on the surface, but the Voyager flyby found several active geysers, or so-called cryovolcanos, shooting columns of dark material up to an altitude of 8 km, with the material then drifting downwind for up to about 100 km. There is evidence that the same sort of conditions could exist on Pluto and Charon, and scientists will be watching closely for evidence of geyser activity.

Is Pluto a Planet?

There is probably nothing that can be discovered on this mission that can settle the debate over whether Pluto should be considered a "planet" or its current designation as a "dwarf planet." That's because the main problem with calling Pluto a planet is that in recent years so many more Pluto-like objects have been discovered in the Kuiper belt. If they were all to be counted as planets, then we'd have dozens of planets, if not more. If Pluto's designation as a planet were retained, but only for Pluto, then that would be arbitrary. Barring some discovery about Pluto that would clearly distinguish it from all of the other dwarf planets, the designation is likely to stay.

What Comes Next?

The several days and weeks around New Horizon's closest approach will bring rapid exciting revelations, but New Horizon is going to collect a lot more data as it flies past, but due to the great distance between Earth and Pluto, it will take a long time to transmit all of that collected data. It will be more than a year before all the data from the flyby will finish downloading. We may find all sorts of interesting things in the pictures, like ice caps, geysers or hazes, but until all of the data is back from the other scientific instruments, scientists may only be able to guess about the details.

In that summer of 1989, I was working at JPL doing a Summer Undergraduate Research Fellowship (SURF) studying the cloud particles of Jupiter's atmosphere by performing analysis on Jupiter images captured by Voyager 1 and Voyager 2 as they flew past Jupiter in 1979. At the time, those images were ten years old, but they were still being mined for research purposes. For many purposes they were the best data available until the Galileo spacecraft went into orbit around Jupiter in 1995 and began to take long term measurements with more sophisticated instruments. The same would happen for Saturn with the Cassini–Huygens spacecraft, which went into orbit around Saturn in 2004, and whose mission is still active.

And Onward...

Even after all the data is downloaded from New Horizons and the spacecraft is put to sleep, the data it will gather will be a resource to study for decades to come.