Failed observations of a specific target using the JWST have resulted in something way more interesting.

In the belt of asteroids that drifts between Mars and Jupiter, the space telescope spotted a previously unknown, and exceptionally tiny, asteroid. The yet-unnamed chunk of rock measures just 100 to 200 meters (328 to 656 feet) across and is quite probably the smallest object yet picked up by the JWST.

It’s not only a magnificent demonstration of the JWST’s capabilities, it suggests those capabilities can be harnessed to better categorize the millions of pieces of rubble lurking in the Main Belt.

“We – completely unexpectedly – detected a small asteroid in publicly available MIRI calibration observations,” says astronomer Thomas Müller of 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.”

As the JWST metaphorically opened up its golden honeycomb eye in July 2022, scientists began putting it through its paces, calibrating its instrumentation, making sure everything was operating as it should. One of those instruments is the Mid-Infrared Instrument, or MIRI.

MIRI’s calibration target was a much larger asteroid in the Main Belt called (10920) 1998 BC1, discovered in 1998 and measuring 15.7 kilometers (9.75 miles) across. Unfortunately, the JWST observations were not particularly good ones: the telescope wasn’t quite oriented correctly, and the images of the target were way too bright and blown out.

It wasn’t a total bust as far as 10920 was concerned; the images obtained by the JWST allowed the researchers to test some techniques for constraining the size and orbit of asteroids, combined with data from other ground- and space-based telescopes.

Surprise! The bright object at the bottom is 10920; the new candidate asteroid is the faint dot circled above. (Müller et al., A&A, 2023)

But there was also something else. The individual images showed a faint object that moved with respect to both 10920 and the background light sources. The team made a careful analysis, finding that the faint object was probably another, previously unidentified, and much smaller asteroid.
The discovery is yet to be confirmed, but if it is, it will be one of the smallest asteroids ever discovered in the Main Belt. Detecting asteroids of this size is vitally important for studies of the size-frequency distribution of objects in the belt.

You might think finding asteroids in an asteroid belt is a bit of a no-brainer, but it’s a fair bit trickier than you might think. So far, astronomers have positively identified over 600,000 Main Belt asteroids, and provisionally identified nearly 550,000 more… but there are estimated to be millions and millions more of them, most of them in the small size range.

And the smaller ones are much harder to detect than larger ones. Which makes spotting one accidentally a bit of a coup, really.

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

“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 asteroid belt is a sparsely-populated donut of rocks that hangs out on the plane of the Solar System spanning a distance between 2.2 and 3.2 astronomical units from the Sun – 329 million to 478.7 million kilometers, or 204.43 million to 297.45 million miles.

But the average distance between asteroids is estimated to be around 965,600 kilometers. Unless you’re looking at the right patch of sky, you might not see anything at all.

The JWST, in its calibration observations, was accidentally looking at the right patch of sky. And the researchers believe that, in the future, it may have more such happy accidents as it observes targets that align with the plane of the Solar System.

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

The research has been published in Astronomy & Astrophysics

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