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«Astronomy Cast Episode 55: The Asteroid Belt Fraser Cain: I must have got 10 emails, 20 emails, in the last couple weeks saying, “Talk about the ...»

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Astronomy Cast Episode 55:

The Asteroid Belt


Fraser Cain: I must have got 10 emails, 20 emails, in the last couple weeks saying, “Talk about

the asteroid belt. Don’t go straight to Jupiter, what about the asteroid belt.” So we’re

going to talk about the asteroid belt. Your wish is our command.

Now, I am very well versed in this topic, thanks to video games and science fiction movies and television, like, “asteroids,” the video game, where there’s asteroids coming at you from all different directions, or, “The Empire Strikes Back,” where they’re flying the Millennium Falcon through that asteroid belt and the asteroids are bumping into each other and it’s a dangerous place. Or, like, that recent Battlestar Galactica episode where there’s a dogfight in the middle of an asteroid belt against a really tough Cylon, so how well prepared am I for this episode?

Dr. Pamela Gay: You’re not. The real asteroid belt just isn’t that dense. It’s just not that exciting.

Fraser: Alright, then why is it called an asteroid belt?

Pamela: Well, there is a lot of little chunks of stuff that are out and they do form a belt. And if you put a little dot for every one of these asteroids on a diagram that fits on a piece of paper that diagram is going to be almost packed solid with little dots representing asteroids. Well the problem is that real asteroids are often a couple hundred kilometers across at most. And the real solar system is a whole lot bigger than your piece of paper, so the scales just don’t carry over real well.

If you were to go out to the real asteroid belt and just sort of saunter through it nonchalantly you’d have about a one in a billion chance of accidently plowing head first or side first or whatever first into an asteroid. They’re just not that dense out in the asteroid belt.

Fraser: I wonder then, when the mission planners are sending their missions to Jupiter and out o Saturn did they really worry about it?

Pamela: Well, you have to worry about it if you don’t that’s when you’re guaranteed to be that one in a billion who gets clocked. And they also worry about it because they can get secondary science. So you take off from Earth, you’re headed towards Mars, you’re going to go not trough the asteroid belt but you’re plowing along and you might see some of the things that have orbits between Earth and Mars, you’re going from Earth out to Jupiter there you have to go through the asteroid belt and that’s a time to stop, check all of your sensors, make sure they work by getting some science onan asteroid.

So you figure out where the asteroids are not just so that you can avoid them but so that you can actually purposely encounter them and purposely try and learn a little bit of this little bit of mass that’s hanging out between Mars and Jupiter.

Fraser: Alright well I want to talk about the asteroid belt from a controversial point, because wasn’t it almost predicted, like at one point people thought, that there should be another planet in that area?

Pamela: So right after Uranus was found by Herschel everyone got out there and they’re like, “there must be something out here, there’s this mathematical relationship that says there has to be a planet out at the location where we did eventually find the asteroid belt.

Fraser: Is that true? Is there a mathematical relationship between the planets?

Pamela: There is quite coincidentally, and we haven’t been able to find this relationship in any other known solar system. There’s this mathematical relationship, the Bode Relationship. So, Johann Bode suggested that, well, maybe there’s going to be another planet out where we found the asteroid belt, based on the ratios, the distances, between Mercury and Venus, Venus and Earth, Earth and a gap, and the gap and Jupiter and Jupiter and Saturn and Saturn and… we just kept going and the same ratio just kept working and working and working.

We can’t find a reason, scientists have looked for a reason, we’ve looked for this relationship in other places and it just seems to be one of these really strange coincidences that is just out there laughing at us. There might be underlying physics, we can’t find it, so as far as we can tell, this is that time that the monkey decided to type hamlet.

Fraser: Stupid Coincidences

Pamela: But we did find the asteroid belt while looking very hard for a planet that fit this Bode relationship. Now, the first one they found was Ceres, it’s the biggest of all the asteroids, and it’s even really tiny, it’s really tiny compared to the moon. It’s the only one of the asteroids that we call a dwarf planet. Pluto fell into the same sorting bin.

Fraser: I was going to say that. It’s no longer an asteroid, right?

Pamela: Yeah, it got promoted out of the asteroid category and into the dwarf planet category.

Fraser: So everyone’s griping about Pluto, and yet poor Ceres got promoted and no one cares.

See? Astronomers giveth and they taketh away.

Pamela: Exactly Fraser: If you took all of the asteroids in the asteroid belt and mashed them all together into a nice, big planet, would you have another Mars?

Pamela: No. In fact, all the mass of all the main belt asteroids add up to about four percent of the mass of the moon. There’s just not that much stuff out there.

Fraser: That’s pretty small Pamela: Stepping back, the stuff that’s current living in the asteroid belt, we think from theoretical models, that this only represents about a tenth of a percent of the original mass that was located where the asteroid belt is today. That are of the solar system experiences all sorts of crazy gravitational pulls and pushes because of interactions with Jupiter. Jupiter’s sort of a big gravitational bully in the middle of the solar system.

There are all sorts of different places within the asteroid belt where an asteroid can get trapped in gravitational residence with Jupiter and when this happens they get flung to other parts of the solar system. So Jupiter’s out there just constantly pulling stuff out of the asteroid belt and sending it to other parts of the solar system.

So it’s estimated that in the first million years after the stuff in the asteroid belt had formed out of the original solar nebulae that 99.9% of that mass got flung to other parts of the solar system, it collided into planets, it got eating up by Jupiter, flung into the sun, flung in all directions. There was originally enough stuff to make a planet. It couldn’t form a plant because of Jupiter’s gravitational bullying and what’s left is the stuff that’s been able to survive the past 4.6 billion years or so of torture.

Fraser: All right. Now, let’s talk a bit about the kinds of asteroids that we may find, because once again you talk about those science fiction shows that are all big tumbling potatoes.

But I’ve got a chunk of an asteroid on my desk and it’s a hunk of iron. So what’s the difference?

Pamela: Just like planets are made up of different materials, asteroids, which are the stuff that attempted to make a planet, are made up of different materials. We generally group asteroids into three different groups. There’s C type asteroids which are carbonaceous.

These are about 75% of known asteroids. These are basically rocks. Go outside, grab yourself a giant rock, torture it in outer space for awhile, and you have a carbonaceous asteroid.

We also have about 17% of the known asteroids are S type asteroids. These are siliceous asteroids, they are made out of siliceous. There’s silica hanging out in space all over the place.

Fraser: so, like, sand.

Pamela: Yeah, exactly. These are sand, these are glasses, and so that makes up the second largest group of asteroids. And the third group, the 8% rare group, is the L type asteroid, which are metals. These are metallic asteroids.

What’s neat is when we look out at the asteroids we group them based on how much light the reflect, what type of light the reflect, we can do some amount of spectrometry on them based on how they reflect light and we also look at their motions. And we can find families of asteroids that we think are probably the result of something bad happening to a once upon a time larger asteroid where this family of asteroid, all the little piece have similar orbits and they all have similar compositions so they were probably all once one larger, but still very small, object earlier in the solar system.

Fraser: and so I have got a chunk of an L type asteroid on my desk.

Pamela: you have one of the 8% rare asteroid chunks. And that’s kinda cool.

Fraser: Yeah. Ok, so let’s talk a bit about the formation. Where did the asteroid belt come from?

Pamela: Once upon a time when our solar system was young we had this hot star trying to form in the center, periodically blasting things with radiation, not particularly stable, and around it was this disk of material that was slowly working to build up planets. There were different areas within this disk of material. There were areas that were getting blasted so much that all the moisture got blown away, there were areas where water was allowed to exist and it was above the temperature of freezing, then there was a snow line water beyond that formed ice, and in all these different areas of the solar system we end up with different types of objects forming.

We end up with the terrestrial planets forming close in, we end up with the gas giants forming out beyond the snow line, and then out in the very edges of the solar system we end up with the icy Kuiper belt objects, and their more distant kindred that can turn into long period comets forming out even further beyond that. Right smack in the middle of the asteroid belt is where the snow line occurred. So within the main belt you have inside of it, Mercury, Venus, Earth, Mars all happily working to try and form themselves and they’re getting blasted and they’re dry and they have no water on them in this early part of the solar system. And then you have this region of space from about 2AU to about 3.3AU where there’s stuff that would like to be a planet but Jupiter keeps knocking it with its gravity.

We think that Jupiter might have started a little bit further out and as it crept in it sent waves of gravity through the asteroid belt area that helped disperse stuff all over the solar system. And, so, in this belt there’s stuff that is allowed to have water, there’s, in fact, proto-comets in this area, there’s cometary asteroids called main belt comets, there is stuff that would like to be the core of a planet, our metallic asteroids, there’s stuff that would like to be the surface of a planet, our carbonaceous asteroids, but the gravity won’t let them lump together. Every time some of them try to get together, they get torn apart so everywhere else in the solar system, when matter collides it sticks together and gravitationally builds bigger and bigger objects.

In the asteroid belt Jupiter keeps stirring up the pot and every time things start to try and lump together, Jupiter comes along and whacks it apart. It’s a kind of frustrating experience if you’re trying to build a planet, but it led to a kind of neat asteroid belt that periodically sends stuff our way that, as long as it’s not too big, just creates interesting fireworks.

Fraser: It’s interesting though, because I think that a lot of people are under the perception that the asteroid belt is all that remains from some large regular planet that broke up for some reason. Could we talk about how the moon might have been created when a Mars size object crashed into the Earth and so what you’re saying is that more likely the planet never got formed in the first place. Jupiter was just around beating it up non-stop that it never got a chance to form anything like a planet.

Pamela: When we look at the chunks of the asteroids, now we haven’t had a chance to do a lot of really detailed studies, we have sent the NEAR-Shoemaker Space Probe out to Eris and explored Eris in a lot of detail. We’re sending the Dawn space probe out to look at Ceres and Vesta, two of the biggest asteroids. But we’re still working on studying these things. We’re still working to understand them.

From our present day understanding these things don’t show the sign of being a large object that broke together. Instead, these thins appear to be leftover ingredients. It’s like when you’re mixing a pot of materials, and some of the stuff falls, half formed, out onto the counter and you end up with glops of flour and flour mixed with egg and other stuff all over your counter when you’re doing a sloppy job cooking. Well, the asteroid belt is the stuff that got sloshed onto the counter. It’s the leftover bits that’s didn’t get processed, that didn’t get weathered, that didn’t experience any of the reactions that you get when you do form a planet.

Fraser: Although it’s a cluster of tiny objects, there’s a few asteroids of note, so why don’t we go through some of them?

Pamela: Well, the biggest four asteroids are Ceres, Pallas, Juno and Vesta, and these are the first four that were found. What’s kind of cool is we went through thinking that Ceres was a planet, Ceres actually was called a planet for awhile. Until realizing, “wait, no, it’s just part of the belt.” And so we found these other objects, and then the more we studied the more we realized they were different from one another.

When we look at them where they formed effects how they appear to us. And so when we’re looking out we find that Vesta is this big, hot, dry, baked world that formed to the degree that it formed very much like the inner planets did. Now, at the same time Ceres probably formed just enough further out that it actually has water. It wasn’t blasted so much that all of its water got evaporated and sent to somewhere else.

Fraser: So are they on opposite sides of the snow line?

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