On the Exploration of Pluto

We live in exceptionally exciting times with regards to the Solar System. Right now, there are active spacecraft at Mars, Ceres, 67P-Churyumov-Gerasimenko, Saturn, and soon… Pluto. And three of those are the first time we’ve ever really had a good look at those parts of the Solar System.

Pluto (and Charon) will be the first Kuiper Belt object we examine in detail. I could possibly count Neptune’s backwards-orbiting largest moon Triton as the first imaged KBO (by Voyager 2 in 1989), seeing as it’s likely that it was in fact a captured Kuiper Belt object… but you can make the argument that strange things have happened, and are happening, on Triton because it was captured by and is currently orbiting an ice giant.

It’s hard to remember now (and will be incredibly hard to remember in just a few weeks) that for most of the time we’ve known about Pluto, we really have had no idea what it looked like. When it was first discovered, it was assumed to be the size of the Earth, or something like it. Since then, it’s been dramatically downsized as astronomers have recognized how reflective it (probably) is, and how small it apparently is.

It wasn’t until 1978 that Pluto’s moon was discovered, and with it the realization that Pluto was TINY – the meager light coming back to us was being reflected off TWO objects.
Discovery Image of Charon by James Christie, USNO Flagstaff
That image may look horrible, but Pluto is SMALL. And that was 1978 technology. More recent images from Hubble are a little better, but they still didn’t really tell us what the surface looked like:
Pluto and Charon, with the (then newly discovered) moons Nix and Hydra. Hubble Space Telescope, 2008

What we knew of the surface of Pluto was fragmentary, and basically can be summed up in this 2010 animation built up from multiple images, using super-resolution techniques. Even so, the only surface details visible are hundreds of kilometers across.

That basically means that every representation of Pluto to date has come from an artist’s imagination, and often a mashup of pictures of Jupiter’s moon Ganymede and Neptune’s moon Triton. We did not know what the surface looked like, or any of the potentially crazy things that are happening there.
Charon from Pluto (artist's impression, source unknown)
Pluto and Charon (artist's impression, source unknown)

But now we have New Horizons. And I fully expect Pluto to be weird: Charon is about 11% of Pluto’s mass, which means they orbit around a point in space between them, every 6 days. They are mutually tidally locked, which means that not only does Pluto only ever see one side of Charon, Charon hangs in space above a specific point on Pluto’s surface (and vice versa). If you stood in one spot on Pluto, Charon would hang motionless in the sky as stars wheeled by. And meanwhile, Pluto has at least four other moons – Nyx, Hydra, Styx, and Kerberos, and an atmosphere that’s probably transitory.

So here’s Pluto: Pluto as seen by New Horizons on July 1 (NASA/JPL/SWRI)
Pluto, ironically, looks like it’s got continents, very much like amusingly fake looking scifi planets. Of course, some of that is from contrast sharpening.

And here is Pluto in color, demonstrating that… it’s red! Not red from rust like Mars is, but probably red from hydrocarbons called tholins. This has been known for a while, but it’s interesting to see, particularly because Charon isn’t. That suggests they’re very different worlds, despite being so close to each other.

Of course, these pictures were taken with two weeks to go before close approach. New Horizons is still 8 days away from Pluto, and the pictures will get clearer and sharper as time goes on. And then, for the next few decades, everyone will be looking at these images. (Even if we launched another probe to Pluto now, it would take another nine years to reach the Kuiper Belt.)
But for now, they’re new and about to be revealed for the first time.

Other posts about the edge of the Solar System: Pluto was Never a Planetin which Pluto is revealedon the Most Distant Object in the Solar System, on the Saga of Planet Nine, on the Search for Planet Nine (Part 2)on the Secrets of Pluto


8 Comments Add yours

  1. We’re definitely in for some exciting science. Providing they can reboot New Horizons in time. One of the things that seems most interesting is the diversity – Pluto and Charon are so close by usual moon and primary terms you’d think they should be the same. But they’re clearly not, and that’s going to be a fascinating question to answer.


    1. According to NASA, they’ve found (and presumably come up with a fix for) the problem: a timing error in a set of instructions sent to the craft. Fortunately, nothing critical was supposed to happen in the last two days. Here’s hoping there are no more software glitches until they finish radioing back all the observations.



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