JWST catches its first accidental asteroid

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It’s a big Universe. Here’s a thing about it.

JWST has discovered its first asteroid, and there will likely to be many more to come.

At first it seemed ironic to me that it would find its first asteroid while making observations of another — but it turns out this wasn’t irony, it was inevitability. Let me explain.

For several months after the observatory arrived at its special L2 point home in space, it was run through a huge shakedown program, testing all its parts and observation modes to make sure everything was copacetic. This meant it took lots of images and spectra of various objects, including stars, galaxies, planets (like Jupiter), and asteroids.

In this case the Mid-Infrared Instrument (or MIRI) was used to take images of the asteroid (10920) 1998 BC1, a rock that’s very roughly 10 kilometers wide orbiting the Sun in the main asteroid belt between Mars and Jupiter. The observations were made to characterize the MIRI filters; that is, to test the performance of the various filters MIRI uses. These data can be compared to what was seen from tests on the ground to make sure everything is behaving as they should be (I’ll note this was part of my job on Hubble when I worked on the STIS camera). 1998 BC1 is a well known asteroid, just the right brightness to be used for these tests.

In the observations, though, was a surprise: a second asteroid, far fainter, seen in observations using all nine filters [link to paper]. The asteroid moved slightly with respect to the background stars and 1998 BC1 between shots, confirming it’s indeed an asteroid.

Processing the data and then using software to analyze its brightness, they found this new and unnamed asteroid is about 100 – 230 meters across, and was between 300 and 375 million kilometers from the Sun at the time of the observations (if it’s actually 100 meters then it’s intrinsically fainter and must be closer in, and if it’s bigger it must be farther out).

Its motion can be used to determine an orbit, though the short overall duration of the observations limits the accuracy of the calculations. They found it’s likely in at most a mildly elliptical orbit that has very low inclination — that is, the orbital tilt is very nearly in the same plane as the major planets including Earth, a plane called the ecliptic.

Astronomers can estimate the number of asteroids of a certain size in the solar system by looking at how many we see over a given size range, and in very rough terms there are likely a billion rocks bigger than 100 meters in size in the main belt. That’s a lot. In fact, using statistical methods based on the brightness of the asteroid and the exposure times used to see it, the scientists estimate that an asteroid in the main belt will be seen in nearly every JWST image taken that looks in the ecliptic! Not only that, most will be ones previously undiscovered.

So like I said, it’s not ironic that this small rock was found when observations of another asteroid were taken. 1998 BC1 orbits the Sun pretty close to the ecliptic plane, so finding another asteroid in the images was extremely likely.

None of this surprises me, to be honest: The same thing happens with Hubble image. It’s not as common as predicted with JWST, but Hubble images are plagued by asteroid tracks. I once found one in some STIS images we took, and determined it was likely a few hundred meters across and in the main belt, but still so faint that, at the time, ground based telescopes wouldn’t have a prayer of seeing it. Too bad, because we didn’t get enough observations to determine an orbit at all. It would’ve been fun to have discovered an actual asteroid that got catalogued!

I expect we’ll see hundreds or more new asteroid discoveries by JWST in this manner. I also don’t think many of them will have enough observations taken to get a decent orbit, and most will be very small and difficult to follow-up (though the scientists mention in their paper that NEO Surveyor may be able to see the new asteroid and many others like it). But that’s OK, just getting measurements of them will help us understand the dynamics and composition of the main belt. In fact they were able to do some rough calculations for 1998 BC1 that matched previous observations, which will help in future observations of unknown asteroids in JWST images.

There are a lot of asteroids we haven’t discovered yet; fewer than a million main belt rocks have been discovered, just 0.1% of the ones bigger than a football field that we know are out there. JWST will find a handful, and NEO Surveyor more, but it’ll be decades — maybe centuries — before we’ve made a real dent in finding the majority of them.

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