Pluto is a tiny world, but that’s no reason to think it’s not important. Today, the robot mission New Horizons is as close as we’ve yet gotten to the little iceball. In fact, by the time you read this, the probe will have flown by Pluto and gone on its way again, leaving us with enough scientific data to keep researchers busy for years to come.
Unfortunately we don’t know yet if New Horizons’ flyby was successful, even though it happened today around 7:49 a.m. U.S. Eastern time. That’s because the arrangement of the spaceship doesn’t allow it to simultaneously take vital data and beam it back to Earth. So, while the probe is doing its business, we’ll have to wait anxiously until about 9 p.m. tonight, when we’ll get the signal to report that, hopefully, all is well. (UPDATE: Just before 9 p.m. U.S. Eastern time, New Horizons lead Alan Stern and the science team announced the probe had successfully flown past Pluto. All equipment was functioning just as it should, and the science data has begun its long, slow transmission. With data rates of just around 1 kilobyte per second, the data collected during the flyby will take nearly 16 months to get to us, but we’ll start seeing early images very soon. I witnessed the “phoning home” with more than 100 scientists and science fans in Cleveland, where we cheered and toasted a very successful mission.)
Another complication is that Pluto is a long way from Earth. Radio communications travel at the speed of light, but the signal from New Horizons takes about four hours to reach us. That means the probe is on its own: any command from Earth would also take four hours to get there.
Thankfully, though, nobody is expecting a serious problem. Over the last months and years, astronomers and the New Horizons team have scanned the environment around Pluto to see if they could find any rings (like those we see around big planets and small icy bodies in the Solar System), additional moons, or debris that could wreck the spaceship. So far, the trajectory seems clear, but nobody can breathe easy until New Horizons phones home tonight.
So while we wait, it seems like a good time to look at what we know about the icy world beyond Neptune.
One of the great successes of 19th century astronomy was the prediction of the existence of Neptune by studying the motion of Uranus. In brief, Uranus slowed down and sped up in its motion due to the small gravitational attraction from Neptune. Flushed with that success, some astronomers thought they could see a similar effect in the orbit of Neptune, and went looking for Planet X, a large world even farther out.
But astronomers didn’t find Planet X until 1930, when a young observer named Clyde Tombaugh found Pluto by comparing photographs of the sky taken on successive nights to find anything that moved. In fact, other astronomers had already photographed Pluto, but didn’t spot it in their pictures in part because they were looking for something much bigger than Pluto turned out to be. Ironically, the whole “Planet X” prediction turned out to be in error: Neptune’s orbit didn’t need explanation by another large world farther out, and Pluto is far too small to have that effect anyway.
Pluto was an interesting and enigmatic object for decades after its discovery. Even with powerful telescopes, it was too small to be anything other than a blur. Until this year, the best pictures we had were from the Hubble Space Telescope, and those were just barely good enough to distinguish light and dark patterns.
Pixelated though they were, though, the Hubble images showed us that Pluto’s surface changes in time. That’s probably due to the nitrogen and methane atmosphere freezing onto the surface and sublimating (when a solid melts directly into a gas without becoming liquid) again. From then on, we saw Pluto as a dynamic place, with changes in season far slower but more dramatic than what we see on Earth.
Today we see Pluto’s surface is slightly reddish in color, probably due to chemical compounds known as tholins. Tholins are organic molecules—meaning they contain carbon atoms—produced when ultraviolet light from the Sun hits the methane in Pluto’s atmosphere. These molecules fall to the surface and form a kind of thin powder or sludge, depending on local conditions. “Tholin,” which derives from the Greek word for ink, comes to us from Carl Sagan and his coauthor, Bishun Khare.
In 1978, astronomer James Christy discovered Pluto’s big moon Charon, which is a fascinating world in its own right. Charon’s mass is roughly one-tenth of Pluto’s, making it by far the biggest satellite in proportion to the world it orbits in our Solar System. New Horizons has gotten really good views of Charon too, showing its surface to be dark gray, with craters and possible chasms deeper than the Grand Canyon.
Pluto’s other four moons—Nix, Kerberos, Hydra, and Styx—are much smaller. New Horizons will not see them clearly during its passage, so you can imagine them looking like this Maki Naro comic if you like.
But as I wrote last week, the biggest change in our understanding of Pluto may have come through the discovery of other worlds in the outer Solar System: Eris, Makemake, Haumea, Quaoar, and more. Thanks to new measurements, we know Pluto is the largest of those worlds, but Eris is the most massive. That means Pluto has more ice in its interior, making it less dense than Eris, which must have more rock in its makeup.
Why is that important? Objects like Pluto are fairly pristine leftovers from the early days of the Solar System. Seeing what they are composed of tells us a lot about how the primordial chemical components were spread around. It’s a mystery why two objects in a similar part of the Solar System should have wildly different compositions, the kind of mystery that makes scientists very happy.
This is just a little of what we’ve learned about Pluto and its moons in 85 years. The images that New Horizons is taking today will be as detailed as satellite photos of Earth, though the flight path won’t give us a complete map of Pluto to that level. Scientists will have a lot more to look at and think about in the coming days.