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Get Ready for Pluto

Artist’s conception of New Horizons at Pluto and Charon. Courtesy: JHUAPL/SwRI

As the New Horizons spacecraft enters its final five-week dash to Pluto, the news hits just keep rolling in. Yesterday, I talked about the tumbling moons affected by the orbits of Pluto and Charon. I expect we’ll find more information about them as the spacecraft gets closer, plus some more new images to whet our appetites for all things Pluto. Five weeks from now, it will be Pluto Partay time, for sure!

Have you ever wondered what it is like on the surface there? Of course, we already know that it’s icy and cold. The presence of nitrogen ice on the surface tells us it’s cold enough on Pluto to freeze this gas (which happens at -195.8 C). The mean surface temperature at Pluto is -229 C (44 K), and that’s mighty cold. The atmosphere, while thin, is mostly nitrogen, with contributions from methane and carbon monoxide. As Pluto moves away from the Sun in its 248-year orbit, most of that atmosphere will “freeze out” and fall to the ground as a frost. So, it’s pretty clear things are distinctly chilly at the planet.

The Sun doesn’t do a lot to warm things up. It’s too far away to provide the kind of heat we take for granted here on Earth. And, due to that distance, days aren’t as bright on Pluto as they are here on warm, comfy Earth. Now, New Horizons isn’t going to land on Pluto; it’s a flyby mission. And, it will be a long time before anybody has the chance to send people to Pluto. But, there is a way to experience Pluto’s lighting conditions for yourself. It’s called “Pluto Time”. There’s a time between dawn and dusk when the lighting conditions where you are will be just like it is at noon on Pluto. It’s easy to do; just go to the Pluto Time web page at NASA and follow the directions. In less than a minute, you’ll know when the next “Pluto Time” at your location occurs. You can take a picture of what Pluto Time looks like wherever you are and share it with others, too. I encourage you to check the page out; you’ll be surprised at what you find out about how bright/dim it is at this distant dwarf planet.

Gravitationally Controlled Chaos Reigns at Pluto

The Moons of Pluto, as interpreted by an artist. Shapes are determined by data analysis of light reflected from these moons and seen by Hubble Space Telescope. Courtesy NASA/ESA/STScI

Take two worlds in orbit around a common center of gravity and call them Pluto and Charon. Pluto’s the main one, Charon’s called a “moon”. Their motions around each other create a shifting gravitational field environment. Think of it like two tennis balls whirling around each other in a pond. If you are trying to navigate around them, you have to deal with the waves in the water.

In a similar way, anything ELSE in orbit in the Pluto system is going to be torqued around — literally — by the gravity fields interacting. That’s what’s happening with two of Pluto’s smaller worlds calle Nix and Hydra. They actually wobble quite chaotically in their orbits, and it’s quite likely that the other two moons — Kerberos and Styx — are also orbiting in a chaotic fashion.

These small, potato shaped moons are also tidally locked together, meaning that gravitational influences between them and their larger system buddies make it so if you could view the system from the surface of Nix, its sister moon Styx orbits twice for every three orbits made by Hydra, and so on. Nix, Styx, and Hydra are locked in a resonance, which means that there is a ration bwewteen their orbital periods.

So, the system isn’t completely chaotic. Nothing’s flying off into space, but they ARE tumbling wildly. The motions, as observed by the Hubble Space Telescope, are filling in details about this system and the motions of its members. This is a cool piece of information to get, now that we’re less than six weeks from the New Horizons mission to flyby of Pluto. The hits just keep on coming!

The big question now is: how did those moons get those wild orbits? The best hypothesis is that early in Pluto’s history, it collided with another dwarf-planet-like object. As usual with collisions, lots of debris was generated, and it went spinning into orbit. Gradually pieces coalesced, the resulting small moons fell into the orbits they occupy today. Of course, there are still some details to work out, but since collision and coalescence seem to be major themes in planetary formation, and not just in our own solar system, this seems to be the story that astronomers will follow up on.

One other cool thing about Hubble observations: it appears that the small moon Kerberos is dark — like charcoal. The others are much brighter, so why is this one so shady? Good question, and the answer will have to wait until New Horizons is there to take a direct “look” at the surface. Onboard instruments should uncover the chemical makeup of the surface.

Check out the Hubble Space Telescope web page about these wild moons to get more specifics on their orbits and this latest discovery!

The Spacewriter's Ramblings

High Noon on Pluto: What’s it Like?

Get Ready for Pluto

The Tumbling Potato Moons of Pluto

Gravitationally Controlled Chaos Reigns at Pluto

Exploring Science and the Cosmos