In this artist’s illustration, NASA’s James Webb Space Telescope reveals a population of small asteroids in the main asteroid belt. Credit: Ella Maru/MIT
Over the past 200 years, astronomers have cataloged more than a million asteroids in the solar system’s main belt, up to about a kilometer in diameter. But smaller asteroids were elusive – until recently, when a team of scientists used data from the James Webb Space Telescope (JWST) to spot some tiny asteroids as small as 10 meters in diameter – 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 that alter their trajectories, they are more likely to escape the belt and crash into Earth, on the order of a few years. When they do, they can cause significant damage. For example, in Chelyabinsk, Russia, in 2013, an asteroid about 18 meters wide exploded in mid-air, causing a shock wave 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 an article published in Nature on December 9 they report the discovery of 138 asteroids smaller than 100 meters in diameter, the smallest ever detected in the main asteroid belt.
A change of 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 type of work, objects like asteroids are nuisances that scientists filter out, along with the “noise” of gas and dust between us and the target.
“For most astronomers, asteroids are seen as parasites on the sky, meaning they pass through the field of view and affect the data,” de Wit said in an MIT press release.
However, de Wit and Burdanov saw an opportunity. Instead of filtering out asteroids, 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 by moving them in various directions. If a faint object moves across the field of view at the same speed and direction as the shift, overlaying the shifted images can reveal it, as if the camera had actually tracked the object in the first place.
Since an object could be anywhere in the field and moving in any direction, it is very computationally demanding to test the wide range of possible displacements. To process all the data, the team wrote software that uses standard graphics cards or graphics processing units (GPUs). (Earlier synthetic tracking software was written for slower conventional 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) project. The team tested the technique on telescopes that operate in infrared rather than visible light because main-belt asteroids are dark, but absorb radiation from the Sun, making them much easier to detect in infrared wavelengths. The technique worked as a proof of concept, and the team published the results in two papers in 2023.
Knocking it down
For the new Nature study, the team used 93.5 hours of TRAPPIST-1 observations from JWST. Because the telescope did not move 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 much more than the team expected. Some asteroids may eventually become near-Earth objects, while one will likely become a Trojan, an asteroid that circles the Sun ahead of or behind Jupiter in its orbit, outside the main belt.
According to the study, the researchers eventually plan to use JWST observations of 15 to 20 exoplanet host stars to identify hundreds of other decade-sized main-belt asteroids.
The team says the huge number of decametric asteroids they found is a sign that they are hitting a population never before observed: asteroids that result from the collision and fragmentation of larger asteroids. “This is a totally new and 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 data differently. Sometimes there is a big profit, and this is one of them.
