Some asteroid things
Published on Mar 9, 2006 at 12:56 pm.
1 Comment.
Filed under asteroids.
I thought that I’d say a few words about asteroids. “Asteroid†is a layman’s term for a large chunk of rocky material orbiting the Sun. The term that professional astronomers use is “Minor Planet.â€Â Smaller bodies are called “meteoroids,†though there is no defined distinction between what makes something a large meteoroid or a small asteroid. I suppose that if it were coming right at you, you’d think most anything were an asteroid. Â
Most asteroids are likely left over from the formation of the Solar System. The great majority seem to have orbits that put them farther from the Sun than Mars, but closer than Jupiter. This region of the Solar System we call the asteroid belt. A popular misconception is that the asteroid belt is chock full of these bodies. You can imagine spacecraft having to weave between them. Well, it isn’t like that at all. Rather, the asteroids are generally so far apart that most would not even be visible to the naked eye even if you were standing on another asteroid. The asteroid belt is mostly empty space. You can plot a spacecraft’s course right through the asteroid belt without even looking to see where know asteroids are located, and the chances are great that you’d never even pass near enough one to see it. You’d have to be extraordinarily unlucky (or lucky, depending on what you wanted) to even encounter one.
When asteroids were first discovered, and for nearly a hundred or so years thereafter, there was speculation that they may have come from a missing planet that had broken up, blown up, or had some such catastrophe that made the planet into a field of asteroids. Well, it isn’t so. There never was a planet there. Most asteroids were likely never any larger than they are now. Most seem to be rubble piles, composed of aggregates of smaller pieces. They never really had a chance to grow into a planet. Jupiter, being so massive, prevented that. Jupiter’s gravity would have kept the asteroids from accumulating into anything larger than they are today. Now a few of the larger asteroids have some characteristics that are more planet-like than being just piles of stuff. Some of the largest ones are spherical due to the effects of their gravity. They also seem to be at least partially differentiated, with the heavier material sinking towards the interior. A few of these may have run into one another, shattering each other into smaller bits, giving us the iron and stony-iron meteorites. Most, though, likely stayed small.
A bit over a century ago, the astronomer Daniel Kirkwood noticed that he didn’t seem to find many asteroids whose orbital periods matched integer ratios of Jupiter’s orbital period. For example, asteroids were not found with orbits having periods 1/2, 1/3, 3/8, 5/8, 3/7, etc of Jupiter’s orbital period. We call these orbits deficient in asteroids the Kirkwood Gaps. He reasoned that somehow Jupiter regularly pulling on these asteroids in the same places in their orbits must move them out of their orbits. A little under two decades ago, Jack Wisdom calculated that asteroids placed in these locations would have orbits that became unstable. In particular, he found that these orbits would become chaotic, with wildly varying eccentricities. The asteroids would then have orbits that carried them far from the classic asteroid belt. Many of these asteroids would then hit the inner planets. Interestingly enough, this would not happen immediately, but a few hundred million years after the formation of the Solar System.Â
Evidence from the Moon and other bodies without erosion to cover up the distant past seems to indicate that the Solar System experienced a period of heavy bombardment early in its history. The textbook explanation for this period of heavy bombardment is that it resulted from the planets sweeping up material left over from the formation of the Solar System. Evidence continues to mount, though, that there was a second, late bombardment period. This late bombardment happened few hundred million years later, while the first heavy bombardment was still winding down. This would seem to fit nicely with Wisdom’s findings.
 An interesting thing, though, about forming the Kirkwood Gaps by Wisdom’s method is that it also would predict that asteroids near those gaps would occasionally go nutty and start flying around with chaotically varying orbits. This means that asteroids that are perfectly well behaved now might in the future be a danger to Earth. By chaotic orbits, I don’t mean random. Rather, chaos, as used here, is a specific mathematical term. Instead of suggesting truly random behavior, it suggests deterministic behavior that is simply beyond our ability to calculate. So, we can’t even tell which asteroids would go nuts, nor when they would do so. Hmm. This might also explain why the period of intense bombardment slackened off, but did not drop to zero. There are still plenty of things out there that pose a potential hazard to Earth. If you look at SpaceWeather.com, you’ll see a listing of recent near misses, and a count of how many potentially hazardous objects are known. A few years ago, when I first had students looking at the site, the number was under 200. Now it is over 750.
Well, we are OK if we find them all, right? No. As I said, some asteroids near the Kirkwood Gaps might become problematic with little advance warning. And there is another problem. Asteroids wander around even without Jupiter’s influence.Â
There is something called the Yarkovsky effect that can also alter the path of an asteroid. A lot of people don’t realize it, but light can push on things. At the subatomic level, there is no real distinction between particles and waves. Everything seems to have both particle and wave properties. So, light, normally thought of in wave terms, can carry momentum, normally something associated with particles. We know that light can push on things. This has been measured. In fact, this also forms a limit to how bright an object such as a massive star or the accretion disk of a black hole can be. If the luminosity is too much, then the light itself simply blows the outer parts of the object off into space. This limit in brightness is called the Eddington Limit, after Arthur Eddington, who described it about a half century ago. Well, now we come to conservation of momentum. If light carries momentum, then the object emitting the light must have momentum in the opposite direction. In other words, if light from object X pushes on object Y, then object X is also pushed in the other direction just as hard. This is an application of Newton’s Third Law of forces: equal and opposite forces. Thus there is a recoil when you emit light. Now, don’t expect your flashlight to jump out of your hand when you turn it on. The recoil is not very much. However, there do exist such powerful lasers that the recoil is noticeable and measurable. So, how does this apply in a discussion about asteroids?
This is where the Yarkovsky Effect comes in. You see, any object whose temperature is higher than absolute zero (ie, everything), emits electromagnetic radiation. The intensity of this radiation is given by the Stefan-Boltzmann Law. This law says that the intensity of light goes as the fourth power of the temperature (measured in Kelvin). Thus, a difference of 1% in temperature yields about a 2% difference in intensity. A difference of 11% in temperature yields more than 50% difference in intensity. Light pressure varies linearly with temperature. So, an asteroid that has a temperature difference on one side as opposed to another would have about a slight force on it from the warmer side.   Asteroids rotate. The side towards the sun heats up, and the side away from the Sun cools off. Thus, as long as the asteroid does not rotate too quickly, an 11% temperature difference between different sides of the asteroid is not unreasonable. As the asteroid rotates, that means that it is warmer on the sunset side than on the sunrise side. Thus, there is a net force that has a component parallel to the orbital motion of the asteroid. This is the Yarkovsky Effect, and it has the effect of speeding up, or slowing down the asteroid in its orbit. This, then, would change the orbit of the asteroid. It would tend to make the asteroid slowly drift either outwards from the Sun, or slowly inwards towards the Sun. If the asteroids orbit is rather eccentric, as most are, then the effect would be more pronounced near perihelion (the point in the orbit at which it is closest to the Sun). Then, the Yarkovsky Effect would not only shift the orbit in or out, but it could also change the eccentricity of the orbit. All of this means that an asteroid safely away from a Kirkwood Gap could even drift into one. Jupiter would then eventually kick it into a chaotic orbit.
 We can be pretty sure that one day, one of these things will be kicked into an orbit that intersects Earth. Astronomers no longer wonder if another large impact will happen on Earth. Rather, we now wonder when it will happen.
So, on that cheerful note, I’ll leave this. I’ll pick up with more asteroid stuff soon.
-Astroprof
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derek on January 29, 2007 at 10:03 am: 1
thank you, your information helped me greatly