James Webb detects a tiny main belt asteroid the size of the Roman Coliseum

The object is probably the smallest observed to date by James Webb and may be an example of an object less than 1 kilometer in length within the main asteroid belt.


European astronomers using NASA ‘s James Webb  Space Telescope have detected an asteroid about the size of the Colosseum in Rome (between 100 and 200 meters in length).

Their project used data from the Mid-InfraRed Instrument (MIRI) calibration, in which the team happened to detect an intercalated asteroid . The object is probably the smallest observed to date by  James Webb and may be an example of an object less than 1 kilometer in length within the main asteroid belt , located between Mars and Jupiter. More observations are needed to better characterize the nature and properties of this object.

“Totally unexpectedly, we have detected a small asteroid in the publicly available MIRI calibration observations,” Thomas Müller, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany, explains in a statement. “The measurements are some of the first by MIRI directed at the ecliptic plane and our work suggests that many new objects will be detected with this instrument.”

a unique look

These Webb observations, published in the journal Astronomy and Astrophysics , were not designed to hunt for new asteroids; in fact, these were calibration images of the main belt asteroid (10920) 1998 BC1, which astronomers discovered in 1998. The observations were made to test the performance of some of MIRI’s filters, but the calibration equipment he considered that they had failed for technical reasons due to the brightness of the objective and a deviated pointing of the telescope.

Despite this, the data on asteroid 10920 helped the team establish and test a new technique for constraining an object’s orbit and estimating its size. The validity of the method was demonstrated for asteroid 10920 using the MIRI observations combined with data from ground-based telescopes and from ESA’s Gaia mission.

In the course of analyzing the MIRI data, the team discovered the smaller asteroid in the same field of view. The team’s results suggest that the object is between 100 and 200 meters across, occupies a very low inclination orbit, and was in the inner region of the main belt at the time of the James Webb observations.

“Our results show that even Webb’s ‘failed’ observations can be scientifically useful, if you have the right mindset and a bit of luck,” explains Müller. “Our detection is in the main asteroid belt , but Webb’s incredible sensitivity made it possible to see this object as small as 100 meters at a distance of more than 100 million kilometers.”