Have you already forgotten about 2024 YR4, the asteroid that, for a short while, looked like it might hit Earth in 2032? I wouldn’t blame you, since these things literally come and go every few months. But YR4 was a little different, since at the peak its chance of hitting Earth was around 2%. That’s not all that high, honestly, but enough to make astronomers look at the rock very carefully.

We were all pretty sure that, as better observations came in, those odds would get longer, and sure enough within a few weeks the orbit was nailed down sufficiently to rule out an Earth impact in 2032^*.

However, at the time when it was a concern astronomers scheduled JWST observations of YR4. While the new observation would only slightly improve the orbital calculation (the asteroid is moving away from us, so perspective means it doesn’t move much in the sky; like trying to see what direction a car is moving when it’s way down the road and headed nearly directly at you) that wasn’t their purpose. Instead, the idea is to measure the size of the asteroid. 

How? Well, first let me show you the image because it’s neat.

The whole image is shown in the left, and as we’re used to seeing by now it has lots of distant galaxies scattered about. Zoomed in, we can see the asteroid. It looks fuzzy, but that’s not “real”; the asteroid is too small and distant to be seen as anything other than a dot. It gets smeared out a bit in the telescope’s optics so it looks like a blob. It was about 115 million kilometers from JWST when this was taken.

So, if it’s too small to see as anything other than a teeny point, how do we get its size?

When we look at an asteroid in visible light (the kind we see), what we’re seeing is sunlight reflected by the rock. Its brightness depends on how far it is from the Sun, how far it is from Earth, how reflective the surface is, and how big it is. We can calculate its distance from its orbit, so we can get the first two numbers. But we don’t really know the last two. Still, we know how reflective asteroids are in general, so we can use that to approximate its size. But that can easily be off by a factor of two either way.

That’s not bad, but when it comes to a potential impactor that’s a big range. The damage it can cause depends on its mass, and that depends on its volume. But volume increases with the radius cubed, so a factor of two too big and we overestimate the mass by 8 times, and if we’re off by a factor of 2 the other way it’s only 1/8^th the mass. A range of 64 times is kinda big.

But we can do better. In the infrared, we don’t see asteroids reflecting sunlight, but instead glowing due to their own heat. That removes that third unknown (the reflectivity). Using well-understood equations, by measuring the brightness we can get a far better measurement of its size. Since JWST sees infrared light, it makes sense to use it in the case of YR4.

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