Nasa’s James Webb telescope accidentally stumbled upon an asteroid roughly the size of Rome’s Colosseum.

The asteroid measuring between 300 to 650 feet (100 to 200 metres) in length, was detected by an international team of European astronomers.

Their project used data from the calibration of the James Webb Space Telescope’s Mid-InfraRed Instrument (MIRI), 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 kilometre in length within the main asteroid belt, located between Mars and Jupiter.

The researchers believe that more observations are needed to better characterize this object’s nature and properties.

‘We — completely unexpectedly — detected a small asteroid in publicly available MIRI calibration observations,’ explained Thomas Müller, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany.

‘The measurements are some of the first MIRI measurements targeting the ecliptic plane and our work suggests that many new objects will be detected with this instrument,’

These Webb observations, published in the journal Astronomy and Astrophysics, were not designed to hunt for new asteroids — in fact, they were calibration images of the main belt asteroid (10920) 1998 BC1, which astronomers discovered in 1998.

The data on asteroid 10920 was used by the team to establish and test a new technique to constrain an object’s orbit and estimate its size. The validity of the method was demonstrated for asteroid 10920 using the MIRI observations combined with data from ground-based telescopes and ESA’s Gaia mission.

In the course of the analysis of the MIRI data, the team found the smaller interloper in the same field of view.

The team’s results suggest the object measures 100–200 meters, occupies a very low-inclination orbit, and was located in the inner main-belt region at the time of the Webb observations.

‘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.’

The detection of this asteroid — which the team suspects to be the smallest observed to date by Webb and one of the smallest detected in the main belt — would have important implications for our understanding of the formation and evolution of the solar system.

Current models predict the occurrence of asteroids down to very small sizes, but small asteroids have been studied in less detail than their larger counterparts owing to the difficulty of observing these objects. Future dedicated Webb observations will allow astronomers to study asteroids smaller than 1 kilometre in size.

However, scientists are waiting on more position data relative to background stars to confirm that the object detected is a newly discovered asteroid.

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