Unsuccessful observations of a specific target with JWST led to something more interesting. A space telescope has detected a previously unknown and extremely small asteroid in the asteroid belt drifting between Mars and Jupiter. The as-yet-unnamed boulder is only 100 to 200 meters (328 to 656 feet) wide and is possibly the smallest object ever retrieved by JWST.

Not only is this a great demonstration of JWST’s abilities, it also hints that these abilities could be used to better classify the millions of pieces of debris lurking in the Main Belt.

“We discovered a completely unexpected small asteroid in the publicly available MIRI calibration observations,” says astronomer Thomas Müller of the Max Planck Institute for Extraterrestrial Physics in Germany.

“These measurements are among the first MIRI measurements to target the ecliptic plane, and our work shows that many new objects will be discovered with this instrument.”

When JWST metaphorically opened its golden eye in July 2022, scientists began testing it, calibrating instruments, and making sure everything was working as it should. One such instrument is the Mid-Infrared Instrument, or MIRI.

MIRI’s calibration target was a much larger asteroid in the Main Belt (10920), designated 1998 BC1, discovered in 1998 and measuring 15.7 kilometers (9.75 miles) in diameter. Unfortunately, the JWST observations weren’t particularly successful: the telescope wasn’t properly oriented, and the images of the target were too bright and blown out.

As for 10920, it wasn’t exactly a fiasco; The images obtained by JWST allowed the researchers to test some methods for constraining the size and orbit of asteroids, along with data from other ground-based and space-based telescopes.

But there was something else. Separate images showed a faint object moving relative to 10920 and the background light source. The team did extensive analysis and found that the fainter object was likely another previously unknown and much smaller asteroid.

The discovery is not yet confirmed, but if confirmed, it will be one of the smallest asteroids ever discovered in the Main Belt. Detecting asteroids of this size is vital for studying the size and frequency distribution of objects in the belt.

You might think finding asteroids in the asteroid belt is a little simple, but it’s a little more complicated than you think. So far, astronomers have definitely identified more than 600,000 Main Belt asteroids, and about 550,000 more tentatively… but it’s estimated there are millions, and most are of small size.

And smaller ones are much more difficult to detect than larger ones. Which really makes someone’s accidental discovery a bit of a blow.

“Our results show that even ‘failed’ Webb observations can be scientifically beneficial if you have the right mindset and a bit of luck,” says Müller.

“Our detection lies in the main asteroid belt, but Webb’s incredible sensitivity made it possible to see this nearly 100-metre object from more than 100 million kilometers away.”

The asteroid belt is a sparsely populated rock ring suspended in the plane of the Solar System, spanning a distance of 2.2 to 3.2 AU (329 million to 478.7 million kilometers or 204.43 million to 297.45 million miles) from the Sun.

However, the average distance between asteroids is estimated to be about 965,600 kilometers. If you’re not looking in the right side of the sky, you may not be able to see anything.

JWST looked at the correct patch of sky in its calibration observations. And researchers believe it may have luckier situations like this one in the future when it observes targets aligned with the solar system plane.

“We predict that MIRI frames with points close to the ecliptic and a short integration time of only a few seconds will always contain a few asteroids,” the researchers write in their paper; most of them will be unknown objects.”