There’s a briefing on our last (known!) mini-moon (“Temporarily-Captured Flyby”, or TCF), 2024 PT5, in Astrophysical Journal Letters for January 10 (volume 978, issue 2):
Bolin, B. Denneau, L. Abron, L-M. The Discovery and Characterization of Minimoon 2024 PT5 L37 ada1d0
ApJL is a side-journal to ApJ. “Letters”, here, are briefs that don’t quite make the level of a research paper, hence, they’re billed as these ‘correspondences’. The nature of transient objects in the sky, and the various ins and outs of the practice of astronomy, apparently draws enough letters to get their own journal title here. Some other journals simply include a few letters at the start/end of a main issue.
Letter or not, this work is worthy, call it what you will. Bolin et al. describe the initial PT5 discovery (using the ATLAS telescope network), and the follow-up observations that such a case warranted. As mini-moons go, this one is not tiny (apparently >5 meters in diameter). It rotates fairly rapidly, and has a rocky composition. We know this, because its spectrum is that of something relatively rocky. We know it’s not a carbonaceous chondrite, but other than that the spectrum isn’t conclusively identifiable. It could be an S-Complex asteroid, it could be a chunk of ejecta from Earth’s moon.
Mini-moons happen all the time; it is just now, that humanity is deploying search telescopes, that we know of them happening. When the Vera Rubin telescope comes online later this year, we will draw the net on them: we should be recognizing mini-moons about once a year, maybe twice in a good year. We’re going to have to get faster on our writing/publishing workflow to keep up with all these objects.