How often do asteroids in the asteroid belt hit each other?

Do we know?

These days? Hardly ever. Collisions were frequent during the formation of the belt, but except for a few perturbed by the gravity of the larger outer planets, those left are in stable orbits.

They also average a million miles or so apart. If you were to fly through the “belt” you’d be lucky to see an asteroid. If you did you could be almost certain there wasn’t another anywhere around.

Asteroid belts don’t look like they do in Star Wars. Their orbits have stabilized over the centuries. The only way they will hit each other is is a comet disturbs their orbit.

Since there are asteroids out there with sizes ranging from kilometres to microns, there’s no way to answer the question as posed.


Related question (perhaps meriting its own thread) are the “Star Wars” kind of belts at all
stable or persistent? I’d say not for more than a few hundred years.

The problem with questions about astronomic phenomena is that scale matters a whole lot. Definition matters too.

If you include dust motes, and grain of sand the numbers of “asteroids” goes way up. So does the number of collisions. Of course the ability to detect such impacts doesn’t really exist. If you only include things bigger than a marble, the numbers go way down. The volume of space “occupied” by the asteroid belt is vast, compared to the volume of stuff in it. If you consider only collisions between stuff with names, there are almost none. Asteroids also get hit by stuff other than asteroids. The same (well, similar) pattern of meteor showers that pass through Earth’s orbit pass through the Asteroid Belt. They don’t hit as often, because the targets are smaller. But they do hit.

(Aside: OK, math and astronomy guys: Considering detectable sized objects only, is the aggregate surface area of the Asteroids greater or less than the Earth?)


“It should be possible to explain the laws of physics to a barmaid.” ~ Albert Einstein ~
“You should see the place where Einstein used to drink!” ~ Triskadecamus ~

I had heard that one could fly through the belt, and other than the dust-sized ones- be lucky to see more than one, other than as tiny points.

Less. Much less.

Recent Collisions in the Asteroid Belt, links to this preprint (pdf) Recent origin of the solar system dust bands (2003).

There are probably undetected collisions out there, but this leaves the impression of a belt in which the big rocks hit each other at a rate of less than 1 collision per million years.

You can estimate the rate at which collisions of large bodies with smaller ones occur by looking at the distribution of craters on the surface of the big ones. I can’t look at that literature without throwing my hands up in the air, but there are some actual astronomers around here who might be able to translate it.

I do a “planetary field trip” with students. I don’t have my visuals here but try to picture this:

At a scale of 1 meter = 1,000,000 miles (yeah, I know I’m mixing systems here, but at this scale you can put the moon and earth on a sheet of notebook paper):

[ul][li]The sun is a ball 86.5 cm (almost 3 feet) in diameter.[/li][li]The earth is the size of a chick pea (8 mm) and is 93 meters (1 football field) away from the sun. The moon is a BB (2 mm) orbiting the earth 24 cm away.[/li][li]Mars is 4 mm and is 140 meters from the sun, and Jupiter is a large orange 460 meters away.[/ul][/li]
The total estimated mass of the asteroids is less than the mass of the moon.

So take the BB that is the moon and pulverize it to as many pieces as you can. The largest of the asteroids, 1 Ceres, is about 0.4 mm. There are maybe 10 pieces with half this diameter and the rest will be microscopic.

Now take all of this dust and spread it out in a ring whose inner diameter is just beyond Mars’ orbit (140 meters) and whose outer diameter is approaching Jupiter’s orbit (460 meters).

The space between individual fragments is mind- boggling…

From what I understand, NASA doesn’t even bother to calculate the orbits when it sends crafts through the asteroid belt. The odds of hitting anything are far too low.

I’m not certain that I agree with Exapno on this one… Certainly, their total mass, and hence volume (everything in the Solar System is about the same density) is much less than the Earth’s, but being all in small chunks, they’d have a significantly higher surface-to-volume ratio than the Earth. This is further helped by the fact that they’re all sorts of odd shapes, while the Earth is a sphere. So it’s plausible, at least, that they might have greater surface area than the Earth. Unfortunately, I can’t seem to find a good list of sizes of minor planets to double-check.

Yes, you could be right. I was assuming the answer as if they had been clumped together. I still feel that that interpretation is the spirit of the question, mostly because I’m not sure what the point is of knowing the aggregate surface area of a bunch of pebbles. But that could be very large.

I was skeptical too, but I looked around and I think he’s probably right about the “less” (though maybe not the “much less” and depending somewhat on definitions).

The distributions of masses and diameters in the asteroid belt are still not very well known for asteroids below the resolution limit (they have to be calculated using some guess at their albedo, unless they happen to have satellites), but the asteroid cataloguers seem to be estimating a power-law dependence, N{radius>R} ~ R[sup]-2[/sup], at least for the larger asteroids > ~1km (here’s one cite (PDF)). This gives a logarithmic divergence in the total surface area, so some lower cutoff must be employed, but because the divergence is only logarithmic just about any reasonable cutoff will give a relatively small value.

The PDF above gives n(r)dr ~ (0.810[sup]6[/sup]km[sup]2[/sup])r[sup]-3[/sup]dr for the number density function; using an upper limit r[sub]2[/sub]~400km (slightly smaller than Ceres) gives an integrated volume about 10[sup]-3[/sup]V[sub]earth[/sub], which seems reasonable given a total mass in the belt is ~510[sup]-4[/sup] M[sub]earth[/sub] and a lower asteroid density than that of iron. Then the integrated surface area is 0.02 log(r[sub]2[/sub]/r[sub]1[/sub]), where r[sub]1[/sub] is the necessary lower cutoff. Even assuming an atomic ~1nm lower cutoff only gives a logarithm of ~34 (for an asteroid surface area about 2/3 that of Earth); using >1m as “detectable” gives ~13, and about 1/4 that of Earth.

This is, however, strongly dependent on the exact distribution function n(r) for small r; if it is steep enough at small radius then it is possible, as you say, for the total surface area to be much larger than Earth’s (though possibly not with “detectable” asteroids, depending on what that means).

[This has assumed near-spherical asteroids; the smaller asteroids are likely not spherical, so this estimate is somewhat small. Of course many of the smaller asteroids are likely loosely-bound agglomerations of dust and pebbles, so they would have a much higher “surface area” for chemical or adsorptive activity.]

Not possible. Any asteroids that were that active, relative to each other, would zip off in every different direction within hours. The gravity between the asteroids would be sufficent to deflect each asteroid’s trajectory slightly, but could never keep them all in the same vicinity.

The only possible intact asteroid belt would be one where the asteroids are moving very slowly relative to each other, i.e. in orbit around the same star / planet / whatever.

Only exception I could think of is immediately after a planetary explosion, like Alderaan. So that particular scene, with the Millenium Falcon getting pounded by mini-asteroids, would be the more realistic option.

Actually, I think they do, but not because of any danger of collision. Rather, they check to see if a probe will come close enough to an asteroid to get an image. This happened with the Galileo spacecraft which made a small diversion in it’s trajectory to take pictures of 951 Gaspra. It may never happen again.

The rings of Saturn seem to be pretty stable.

Yeah but the ones in Star Wars et al appear to be a haphazard collection of drifting rocks, as opposed to a part of a ring system.

There is a hypothesis that Saturn’s ring system is not stable over the long term. It consists of particles generated by the destruction of moons/asteroids that wander within the zone in which large objects are torn apart by tidal forces. Interactions between ring particles and moons cause material to be gradually swept into the planet.

Note that all Jovian planets have ring systems, but only Saturn’s is easily visible, implying that Saturn’s rings have been recently (say, within the last 10 ka) “fed” by a moon/captured asteroid, while the other large planets’ rings are in advanced stages of “decay.”

I asked about the total surface area because I was discussing impacts from non asteroids on asteroids. The rate of such impacts would be more closely proportional to the surface area than the mass. By the way, detectable was only added to avoid the inclusion of molecular sized “asteroids” and such. Dust motes, maybe, maybe not. Sand, definitely.

After thinking about it, I decided that cross section area is the critical variable, not surface area, but I think the question is no less difficult to resolve because of that. Thanks for all the input, although I see it is a tougher problem than I first realized.


“What have you done to that cat? It looks half dead!” ~ Mrs. Erwin Schrodinger ~