Starship Asterisk*

This artist's impression shows a grey, irregularly-shaped asteroid against a dark background. Image Credit: N. Bartmann (ESA/Webb), ESO/M. Kornmesser and S. Brunier, N. Risinger (skysurvey.org)

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A previously unknown 100–200-metre asteroid — roughly the size of Rome’s Colosseum — has been detected by an international team of European astronomers using the NASA/ESA/CSA James Webb Space Telescope. Their project used data from the calibration of the 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 kilometer in length within the main asteroid belt, located between Mars and Jupiter. More observations are needed to better characterize this object’s nature and properties. ...

The Webb observations which revealed this small asteroid were not originally 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, 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 asteroid 10920 were used by the team 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 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 and previously unknown 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. ...

• Astronomy & Astrophysics 670:A53 (Feb 2023) DOI: 10.1051/0004-6361/202245304