L2 Lagrange Point (WHTM) — A serendipitous discovery demonstrates the capacity of the James Webb Telescope to do science on a small scale.

An asteroid described as “roughly the size of Rome’s Colosseum” was detected by an international team of European astronomers.

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The minuscule space rock, which measures somewhere between 300 to 650 feet in length, was found using data from Webb’s Mid-InfraRed Instrument (MIRI). Oddly enough, this asteroid was discovered while the scientists were studying images of another asteroid, images that weren’t even taken for asteroid hunting.

An illustration of an asteroid. The asteroid roughly the size of Rome’s Colosseum — between 300 to 650 feet (100 to 200 meters) in length — has been detected by an international team of European astronomers using NASA’s James Webb Space Telescope. They used data from the calibration of the MIRI instrument, in which the team serendipitously detected an interloping asteroid. The object is likely the smallest observed to date by Webb and may be an example of an object measuring under 1 kilometer in length within the main asteroid belt, located between Mars and Jupiter. More observations are needed to better characterize this object’s nature and properties.
Credits: ARTWORK: NASA, ESA, CSA, Martin Kornmesser (ESA), Serge Brunier (ESO), N. Bartmann (ESA/Webb)

The Webb telescope observations were taken early on in the mission to test some of the MIRI’s light filters. They were calibration images of a main belt asteroid named (10920) 1998 BC1. (1998 was the year the asteroid was discovered). The calibration team felt the images failed due to the brightness of the target and an offset telescope pointing.

Even so, the European astronomy team was able to use the data to try out a new technique to fine-tune asteroid orbit and size estimates, by combining the Webb imagery with data from ground-based telescopes and the European Space Agency’s Gaia mission, which like the Webb is located at the L2 Lagrange point. But while they were examing the MIRI data on 10920, they found a little something extra. As Thomas Müller, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany explained, “We — completely unexpectedly — detected a small asteroid in publicly available MIRI calibration observations.”

The team believes the asteroid is both the smallest observed to date by Webb and one of the smallest detected in the Solar System’s main asteroid belt, located between Mars and Jupiter. It’s very difficult to find such small asteroids, and even harder to plot their orbits.

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

Bryan Holler, Webb support scientist at the Space Telescope Science Institute in Baltimore, Maryland, notes this discovery opens up a whole new approach to using Webb imagery. “This is a fantastic result which highlights the capabilities of MIRI to serendipitously detect a previously undetectable size of asteroid in the main belt. Repeats of these observations are in the process of being scheduled, and we are fully expecting new asteroid interlopers in those images.”