ASTRONOMY INFODUMP (Facebook Post gone too long)

[Whirligig Girl]

ASTRONOMY INFODUMP.

TL;DR This was supposed to be a short post about Ceres and Pluto/Charon, but I digressed too much.

People get excited about the New Horizons because it will be the first close look at a dwarf planet. I bet that's why they ended up changing Dawn's course to intercept Ceres. Ceres is a dwarf planet, and thanks to Dawn, we now have the best images of a dwarf planet ever. And they will get better, and Dawn will be the first to get a close look at a dwarf planet.

In all seriousness though, the Pluto-Charon system that New Horizons will be flying by (four months after Dawn to Ceres) is way more interesting. First of all, there's TWO dwarf planets, Pluto and Charon, orbiting eachother, and the two dwarf planets share five moons! Plus, we already have okay-ish Hubble photos of Ceres, but very very very blurry splotches of Pluto and Charon. I suspect New Horizons might even be able to find more moons of Pluto and Charon.

People think Charon is Pluto's moon, but this is not true. As said, the two orbit one another. http://www.space.com/26791-pluto-moon-video-nasa-new-horizo…

We can see Pluto and Charon's moons that we know of because we have been constantly looking in that vicinity for a really long time, and we finally notice some very very tiny dots moving around Pluto/Charon center of mass. This shows that they're not stars or other kuiperoid-objects*, but in fact moons of Pluto/Charon. Moons far away enough from Pluto/Charon will be moving so slowly that it would be impossible to tell. Pluto/Charon, despite being so small, has a very very large gravitational sphere of influence. This is because it's so far away from the sun, that Pluto can grab onto objects more easily than if it were closer in. But because it has such a low mass, everything orbits really slowly. The further out in an orbit you go, the slower you are. Things near the edge of the Pluto/Charon sphere of influence will be moving barely at all.

*Smaller objects in the kuiper belt, the region of space where all of the dwarf planets (except Ceres) orbit. Ceres orbits in the asteroid belt, and is technically just the biggest asteroid.

Incidentally, Mercury is small enough that if anything else dared orbit as low as it, it would be a dwarf planet. It's orbit is kinda wonky like other dwarf planets, and two moons (Ganymede of Jupiter and Titan of Saturn) are bigger than it. It is, however, so dense that it's surface gravity is only slightly less than Mars, which is 30% larger.

One thing to note is that Pluto's sphere of influence is actually very small, because Charon's gravity will make anything too far out unstable, even capturing it. Charon's Sphere of influence is of course even smaller, being that it is the smaller of the two dwarfs. But when talking about stable moons of the Pluto/Charon system, be talk about their combined sphere of influence. This is an area outside the SOIs of Pluto and Charon that would be the normal SOI of a dwarf planet the size of the two combined. This is also how binary star systems that orbit very closely work. Planets can't be too close to the two stars, or too far away from any one star at a time.

The dynamics of the Pluto/Charon system are interesting on their own, which is why even though they are (and rightfully should be) dwarf planets, that doesn't mean we shouldn't talk about them. We just also have to talk about the others.

I'll end this with a bit about the Plutinan/Charonian seasons. The two dwarfs' seasons are linked, and they happen not based on orientation to the sun, but on their distance to the sun. All areas on Pluto and Charon have equal seasons at any one time. The cool thing about these seasons, is that during the highest parts of the orbit, the thin atmosphere of the two dwarfs (thicker than Mercury and the Moon's atmospheres for Pluto!) actually freeze into snow! We don't have pictures of that, because it hasn't happened since we started observing Pluto and Charon. One concern of the New Horizons team is that the atmosphere might freeze, hiding all the interesting stuff, by the time they get there. Then we'd have to wait another hundred or so years to get a good look.

Okay, I lied about stopping there. People say a Lunar Year is one orbit of the Moon, and this is true. You could say that a year happens once every orbit, as that's what the Earth's year means astronomically. But Pluto and Charon are sort of eachother's moons. Their days are both a little over 6 Earth days long, but this is also how long their orbits are (they are tidally locked). They orbit around eachother, so you could say that their year is only 6 days, making it much much shorter than mercury's 88 day year! Problem with this is that suddenly you have to pretend that the sun doesn't exist at all, right? But then, if we measure Pluto and Charon's year by their barycenter (center of mass)'s orbit, that's like saying we should measure Earth's year the same as the Sun's year, which is it's several-million-year galactic orbit. Technically, the Sun is wobbled a bit by the Earth, after all. But no worry. Because Pluto and Charon are gravitationally special, we can make up rules for binary planets. The year of two binary planets is the time it takes for the Barycenter to move around the Sun. The barycenter is a point moving along a normal gravitational trajectory, and even though Pluto and Charon are orbiting eachother, they're REALLY orbiting the barycenter.

There's also a few more really cool dwarf planets. Eris is a bit bigger, has a single tiny moon, and is really really white. picture: http://knowyourmeme.com/memes/white-girl-problems But Eris isn't really that cool. Haumea is probably the most interesting dwarf planet other than Pluto and Charon. Haumea is spinning way too fast. It's got two known moons as well. Haumea is spinning so quickly, it spins around once every 4 HOURS. That's really quick for a little planet like that. it's spinning so unbelievably fast, that if you stood on the equator, you could run into orbit, but you'd need a proper small rocket stage to get to orbit from the poles. If standing anywhere that's not very near the poles or very near the equator, you'd be pulled by the centrifugal force to the equator. We'd need harpoons there probably. A manned mission to Haumea is one of my dreams, just because of how cool the physics are there! haumea is also very white, probably due to the same impact that made it spin really

fast.

There's also Makemake, which is kinda just as boring as Pluto alone. Probably the same color. Maybe. It's got two known moons.

Speaking if rotation, something people forget when talking about Uranus (which by the way should be pronounced You Rah Nus

And in this video, people from other countries calling it by the proper pronounciation)

And it was NEVER George. Only Georgium Sidis.), is that it's rotating on it's side. This means that it has local seasons (like Earth) that are very very pronounced. Some parts of it's year are averaged nicely, some parts of the year one side is constantly shined upon while the other is freezing. Actually, they're both always freezing. Uranus is the coldest planet, more than Neptune or any of the dwarf planets. Until very recently, (unfortunately, very recently does not include the Voyager flybys) Uranus has been boring and flat all over. Now it's getting cloud bands and storms visible. This is probably a seasonal thing, but it's only been observed close up since Voyagers and then Hubble. Before then it was just a bright blue star with the best telescopes. Uranus has only had about 2.7 years since it was discovered.

There's an interesting thing about the outermost planets from Uranus onwards. Some moons change drastically over very short periods of time. Wanna know what these moons are like? They are "volcanic and changing". The Ice Giants and Dwarf Planets change seasonally, and there can be very drastic changes from season to season. Problem is, they move so slowly that one season the astronomers are babies, the next they're dead. Ever few decades the Ice Giants go and change on us. We thought Uranus was was boring until recently, and now a flyby would be like going to a new planet entirely!

Even things which have been around for a huge amount of time are temporary. Jupiter's Great Red Spot was discovered AFTER the technology to view it would be invented. That storm has been around for only around 400 or so years, after humans were looking at it. It was huge and impressive when the Voyagers flew by, but it's since been shrinking.

For a while there were two red spots in the area, a white "satellite storm" became red, before being reabsorbed again. (Incidentally, when I was a kid and they discovered the little spot, I had some strange attachment to it. Even though it changed the face of Jupiter as we knew it, I hoped it would stay)

The Jovian Red Spot is changing, and it won't last forever. It may only last another hundred years or so before it becomes a lowly white storm again. Jupiter's only distinguishing feature will die. This will happen to Saturn too. It's not even got pretty cloud bands, though the hexagonal storm on the north pole doesn't seem to be leaving us. Saturn's rings are actually in an unstable orbit, and have not existed for most of Saturn's history. Saturn got it's rings when one of it's moons was shattered by going to close. I like to call that moon "Icarus" because it flew too close to Saturn, which would be bright like the Sun. This is also why the rings never formed a new moon (which would definitely be called phoenix), because the gravity would pull such a moon apart very soon. Saturn's rings will die in a few million years.

All of the gas planets have rings, but they're not very bright. Even after Saturn's rings are gone, it will still have a tenuous layer. One interesting ring and moon of Saturn is the Phoebe Ring, and the moon of the same name. They orbit retrograde, so they're probably different rings entirely from the others. Another interesting Saturnian moon is Rhea, which might even have it's own rings. The most interesting moon of Saturn is Titan, which is the only celestial body outside of Earth that has seas. Titan is bigger than Mercury, and it has a thick atmosphere, around five times that of Earth. It rains liquid methane, there are methane clouds, there are methane lakes, and there are water rocks. The ice is so cold it acts like a rock.

Another planet which is slowly changing, but changing faster than geological timescales, but not changing so fast as to be "constantly changing" is a planet that's slowly been getting warmer and warmer. It's atmosphere is changing to have more carbon dioxide and methane, and the warming on the planet is causing local climates to be colder or hotter. This planet is of course Earth. It's change is the weirdest change of all of the planets. The complexity of the mechanisms involved in this sudden change are immense. No geological activites are causing this change. Just the machines on the planet itself. And it's only one type of machine that is causing this change. Ironically, that one type of machine is the only type of machine on the planet which is capable of stopping their change due to more information. I wonder why they don't stop.

There are, to date, seven known celestial bodies with machines on them. Venus, which only have inactive machines, Earth, which has both organic and metallic machines, Luna of Earth, Mars, which has the second most machines of any planet, the Comet 67P/Churyumov–Gerasimenko, the Asteroid 433 Eros, and Titan, which only has one inactive machine. All of the machines originated from Earth, and aside from the organic machines on Earth, only one machine produces the other machines (the ones that change the climate). Those machines are Humans, in case you weren't paying attention. Organic machines being life. Life is interesting in that they are organic, imperfect Von Neumann machines (Also known as Grey Goo, when applied to environment-eating nanobots). Von Neumann Machines are machines which take parts and build machines just like them. That's all organic life forms are, they're just organic molecules that use organic molecules and build more of the same organic molecules. But I digress.

Here have a gif for reading so far.

Edited January 21, 2015 by GregroxMun

[andrewas]

67P/Churyumov–Gerasimenko isn't a celestial body now?

[Kryten]

67P/Churyumov–Gerasimenko isn't a celestial body now?

Or 433 Eros, it would seem.

[Whirligig Girl]

67P/Churyumov–Gerasimenko isn't a celestial body now?

I forgot about it *doh*. Gotta fix that.