Smallest main-belt asteroids ever detected found in JWST data

MIT researchers have identified 138 near-Earth objects using a common astrophotography technique.

Over the past 200 years, astronomers have cataloged over a million asteroids in the solar system’s main belt, down to the size of around a kilometer in diameter. But smaller asteroids were elusive — until recently, when a team of scientists utilized data from the James Webb Space Telescope (JWST) to spot some tiny asteroids as small as 33 feet (10 meters) across — no bigger than a school bus.

Decameter asteroids — referring to asteroids tens of meters in size — originate in the main asteroid belt between Mars and Jupiter. Because smaller asteroids are more susceptible to sunlight and thermal effects altering their trajectories, they are more likely to escape the belt and crash into Earth, on the order of every few years. When they do, they can cause significant damage. For instance, in Chelyabinsk, Russia, in 2013, a roughly 60-foot-wide (18 meters) asteroid exploded in mid-air, causing a shockwave that injured thousands of people and damaged even more buildings. Being able to detect these objects at their origin in the main belt could greatly increase our ability to monitor potential threats.

To detect these asteroids, a team led by MIT planetary scientists Artem Burdanov and Julien de Wit used archival data from JWST and new processing algorithms. In a paper published in Nature on Dec. 9, they report the discovery of 138 asteroids smaller than 328 feet across (100 meters) — the smallest ever detected in the main asteroid belt.

A shift in approach

Since 2016, de Wit and his team have used the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile to learn more about the star TRAPPIST-1 and the numerous exoplanets in its system. In this line of work, objects like asteroids are annoyances that scientists filter out, along with the “noise” from gas and dust in between us and the target.

“For most astronomers, asteroids are sort of seen as the vermin of the sky, in the sense that they just cross your field of view and affect your data,” said de Wit in an MIT news release.

However, de Wit and Burdanov saw an opportunity. Instead of filtering asteroids out, they would try to find them using an image-processing technique called synthetic tracking. This method uses many short exposures of a fixed field of view and combines them while shifting them in various directions. If a faint object happens to be moving across the field of view in the same speed and direction as the shift, stacking the shifted images can reveal it — as if the camera had actually been tracking the object in the first place.

Because an object could be anywhere in the field and moving in any direction, it’s very computationally intensive to test the vast array of possible shifts. To process all of the data, the team wrote software that utilizes off-the-shelf graphics cards, or graphics-processing units (GPUs). (Previous software for synthetic tracking had been written for conventional — and slower — central processing units, or CPUs.)

The team tested the approach using infrared data from several ground-based telescopes, including the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS) project and the Antarctic Search for Transiting ExoPlanets (ASTEP). The team tested the technique on telescopes operating in infrared instead of visible light because main-belt asteroids are dark, but they absorb radiation from the Sun, making them much easier to detect in infrared wavelengths. The technique worked as proof of concept, and the team published their results in two papers in 2023.

Breaking it down

For the new Nature study, the team used 93.5 hours of observations of TRAPPIST-1 from JWST. Because the telescope did not shift its field of view during each of its observing sessions, the data was ideal for the synthetic tracking technique.

The total haul of 138 decameter asteroids was far more than the team anticipated. A few of the asteroids could become near-Earth objects at some point, while one is likely to become a Trojan — an asteroid that circles the Sun ahead of or behind Jupiter in its orbit, outside the main belt.

According to the paper, the researchers plan to eventually use JWST’s observations of 15 to 20 exoplanet host stars to identify hundreds more decameter-sized main belt asteroids.

The team says that the sheer number of decameter asteroids they found is a sign that they are hitting upon a never-before-observed population — the asteroids that result from larger asteroids colliding and fragmenting. “This is a totally new, unexplored space we are entering, thanks to modern technologies,” Burdanov said in the MIT release. “It’s a good example of what we can do as a field when we look at the data differently. Sometimes there’s a big payoff, and this is one of them.”