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.
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)
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.
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."
Reference: published in Astronomy & Astrophysics.
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