The newly-discovered asteroid is less than 230 m (755 feet) across, and orbits within the main asteroid belt, which is located between Mars and Jupiter.
An artist’s impression of an irregularly-shaped asteroid. Image credit: N. Bartmann, ESA & Webb / ESO / M. Kornmesser / S. Brunier / N. Risinger, skysurvey.org.
Our Solar System is teeming with asteroids and small rocky bodies.
Astronomers currently know more than 1.1 million of these rocky remnants from the early Solar System.
The ability of the NASA/ESA/CSA James Webb Space Telescope to explore these objects at infrared wavelengths is expected to lead to groundbreaking new science.
But the new Webb observations 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 observations were conducted to test the performance of filters of Webb’s Mid-InfraRed Instrument (MIRI), but the calibration team considered them to have failed for technical reasons due to the brightness of the target and an offset telescope pointing.
Despite this, the data on 1998 BC1 were used by the astronomers to establish and test a new technique to constrain an object’s orbit and to estimate its size.
The validity of the method was demonstrated for 1998 BC1 using the MIRI observations combined with data from ground-based telescopes and ESA’s Gaia star-mapping mission.
“We — completely unexpectedly — detected a small asteroid in publicly available MIRI calibration observations,” said Dr. Thomas Müller, an astronomer at the Max Planck Institute for Extraterrestrial Physics.
“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.”
In the course of the analysis of the MIRI data, Dr. Müller and colleagues found the smaller interloper in the same field of view.
Their results suggest the object measures 100-230 m (328-755 feet), 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,” Dr. Müller said.
“Our detection lies in the main asteroid belt, but Webb’s incredible sensitivity made it possible to see this roughly 100-m object at a distance of more than 100 million km (62 million miles).”
In order to confirm that the object detected is a previously-unknown asteroid, more position data relative to background stars is required from follow-up studies to constrain the object’s orbit.
“This is a fantastic result which highlights the capabilities of MIRI to serendipitously detect a previously undetectable size of asteroid in the main belt,” said Webb support scientist Dr. Bryan Holler, an astronomer at the Space Telescope Science Institute.
“Repeats of these observations are in the process of being scheduled, and we are fully expecting new asteroid interlopers in those images.”
The team’s paper was published in the journal Astronomy & Astrophysics.
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T.G. Müller et al. 2023. Asteroids seen by JWST-MIRI: Radiometric size, distance, and orbit constraints. A&A 670, A53; doi: 10.1051/0004-6361/202245304
