A new view of archival Webb telescope data reveals small asteroids in the main asteroid belt are more numerous than we thought.

Astronomers have uncovered an unexpectedly large population of small main-belt asteroids thanks to a new analysis of images from the James Webb Space Telescope. The finding could change calculations of the impact rate on Earth from such bodies, which range from house-size to the size of a stadium.

These are by far the smallest objects ever imaged so far away. Artem Burdanov (MIT) and colleagues applied a computationally intensive shift-and-stack method to Webb's archival images. From the telescope's sensitive infrared detectors, the team was able to accurately determine the sizes of small objects, ranging from about 10 to 500 meters (30 to 1,600 feet) across. An early release of their results appears in Nature.

Larger main-belt asteroids, whose orbits are between those of Mars and Jupiter, tend to remain in relatively stable orbits. However, gravitational interactions more frequently perturb smaller ones, which may enter the inner solar system to become potential impactors. The high number of small objects now found in the main belt — more than an order of magnitude greater than expected — could affect calculations of the frequency of such impacts.

The shift and stack processing approach is used by professional and amateur astronomers alike to provide images of faint, fast-moving objects such as asteroids and comets. In this case, the astronomers used that method on steroids.

Usually, the idea is to move the telescope precisely in the direction and at the rate of motion of an object with a known orbit. Each frame maintains the target at the center, while background stars drift from frame to frame. Thus, even if an individual image doesn't capture the object, dozens or even hundreds of frames can be added together, allowing the object to emerge from the random background noise that changes from frame to frame.

But the process usually depends on knowing the orbit. What if you don't? That's where harnessing a lot of computer power comes in. The team mined an archive of images from the JWST that had been gathered for a different purpose: studying the planets around the nearby star TRAPPIST-1. Because these images involved staring at the same small patch of sky over extended periods as much as 8 hours long, they turned out also to be useful for finding moving objects like asteroids.

To discover asteroids in unknown orbits, Burdanov used essentially a brute-force method, shifting in every possible direction and reasonable speed range for potential moving targets, then stacking those images.

"Since we have quite powerful computational tools — graphical processing units — we can do this search blindly," Burdanov tells Sky & Telescope. "We decided to push the limits to see how faint objects we could find with this telescope," he says.

Burdanov didn't start out working on asteroids. He was already working with the data to study the TRAPPIST-1 exoplanets, it was natural to try looking for other things within that data. This project began in 2020, when he was still a graduate student, and as the COVID-19 pandemic began, he had the time to experiment. "Some people started to make sourdough, and I think doing asteroid work was an exoplanet person's version of doing sourdough," he says.

The initial blind search produced more than 1,000 candidates. Burdanov and co-researchers then narrowed down and confirmed clear evidence for 138 previously unknown asteroids, as well as eight known ones.

JWST is particularly good at finding asteroids, because they're much brighter at infrared wavelengths. The astronomers constrained their diameters to within 10 to 20%, whereas estimates of size based on visible-light observations can vary by more than an order of magnitude. That's because visible light comes from an asteroid's reflection of sunlight, and therefore on how dark its surface is, a quality that can vary by a lot from object to object. The infrared light, on the other hand, results mostly from thermal emission, which won't change much for a given object.

Infrared measurements also provide accurate measurements of objects' shapes, since any variations in brightness would be due to shape rather than variations in surface reflectivity.

Finding so many small asteroids serendipitously, from one set of observations of one star taken for a different purpose, opens the prospect of mining a great deal more from other, similar JWST datasets. The team estimates that there could easily be thousands of such small asteroids discoverable in existing data, which would then enable detailed mapping of these populations.

Team member Richard Binzel (MIT) says, "These asteroid findings fill an important knowledge gap for tracing the source of meteorites and larger potentially hazardous asteroids in Earth's vicinity."
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