Astronomers have found over a hundred small asteroids — tens of meters in diameter, so in the decameter range — in the main asteroid belt between Mars and Jupiter. This is a pretty big deal; while we know those rocks are out there they’re too small to be easily seen from Earth.

It’s a truism in nature that when you break something, you tend to get a few big pieces of shrapnel, a bunch of middle-sized pieces, and lots of small pieces. The easiest way to think of this is imagine taking a big ol’ hammer to a rock and whacking it as hard as you can. It may split in half, giving you two biggish pieces, with a larger number of smaller pieces, and a buttload of tiny pieces of shrapnel. You might even get a dust cloud filled with millions of teeny grains of no-longer-rock.

Asteroids should be the same way. They initially formed when the solar system was young. The current thinking is that the material between the orbits of Mars and Jupiter tried to coalesce into bigger objects, but Jupiter’s gravity accelerated them such that impacts were never gentle enough for them to grow in size; instead they slammed into each other and broke apart. Collisions have continued over the past four-and-something billion years, so now what remains are two large asteroids (Vesta and Ceres, and really they were well on their way to becoming planets when they ran out of material to grow, so planetary scientists call them protoplanets), lots of smaller but still big asteroids in the dozens or hundreds of kilometers wide range, and a lot of ones even smaller than that.

There are likely a billion — yes, billion with a b and an illion — asteroids in that part of the solar system larger than 100 meters in diameter. That’s a lot.

But they’re small, which makes them faint — we tend to see them by the light they reflect from the Sun, so their small sizes and large distances makes finding them extremely difficult. Most of the asteroids discovered so far (even some with familiar names, cough cough) are a half-kilometer across or more. There must be many, many billions of them that are even smaller. So how do we find them? Simply taking images of the sky won’t see faint enough objects to spot them.

A team of astronomers decided to try a relatively new method called shift-and-stack [link to journal paper]. It’s a clever idea, and not too hard to understand. Imagine you’re on the side of a racetrack trying to get a good photo of a car as it zips past you. One way to help is to track the camera as quickly as you can in the same direction the car is moving, to reduce the speed blur. Background objects like trees will look like streaks but the car will look sharp.

Astronomers can do the same thing, kinda. They can take a lot of images of some small region of the sky, then after the fact shift the images relative to one another and add them together. We know, for example, how rapidly asteroids in the main belt move across the sky, so the astronomers can shift the images to mimic that motion, then add them up. Stars will appear as streaks or a series of tightly aligned dots, but asteroids moving at the virtually tracked rate will appear as dots. Of course, we don’t know exactly how fast they’re moving, or in what directions, so this method has to be done many many times for each set of images, using intelligent guesses on how to shift them. 

It’s painstaking, but it works! This method has been successfully employed to find new moons around the outer gas giants like Jupiter, Saturn, Uranus, and Neptune! I go into more detail in those links if you’re interested.

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To do this for asteroids, the astronomers turned to JWST. Now, the new(ish) telescope is way, way too overloaded with important observations to take the time to try this method, but here’s the cool thing: all the observations it has ever made are in an archive. Not only that, but even more fun is the team of astronomers had used JWST themselves to observe the nearby red dwarf star TRAPPIST-1 to look at the planets orbiting it, trying to determine their atmospheric characteristics. They had more than ten thousand images at their disposal! Plenty to work with. 

Their method uncovered eight previously known asteroids. But they also found 138 new asteroids, never before detected. Their diameters can be estimated by their measured brightness and known distances, and they are in the tens of meters range, so roughly school bus sized up to a football field.

They figure that JWST can be used to serendipitously detect thousands of such asteroids every year, which is extremely cool. Also extremely useful. We know asteroids in this size range are fragments from collisions of larger asteroids, but there’s precious little observational confirmation of this. By directly measuring how many asteroids there are at different sizes we can learn a lot about how the asteroids collide and what happens to the bits after a collision. If the orbits of these asteroids can be determined, astronomers can check to see if other asteroids have similar orbits; we know that impacts on big ones make smaller ones that then move away, but still follow roughly the same trajectory around the Sun.

A lot of these small asteroids migrate to the inner solar system where they can impact Earth, too. They’re too small to do global damage, but mind you the Chelyabinsk impactor over Russia in 2013 was 19 meters across, at the small size end of the ones just discovered. The more we know about these space rocks, the better!

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