# 16 telescopes in the US timed the (623) Chimaera occultation: ellipse semi-axes 26.2 and 16.0 km

- Published: 2026-11-06 05:21 UTC
- Section: Sky & Space › Astronomy
- Tags: (623) Chimaera, stellar occultation, asteroid size, geometric albedo, Chimaerid family, Emirates Mission to the Asteroid Belt, multi-chord observation, Occult4, amateur telescopes
- URL: https://realtech.media/news/rt-57217ddc05dce7e773246179_16_telescopes_in/

**A preprint reports an October 2025 multi-chord occultation of a V 13.86 star by asteroid (623) Chimaera, a flyby target in the 2030s. The profile fits an ellipse.**

![16 telescopes in the US timed the (623) Chimaera occultation: ellipse semi-axes 26.2 and 16.0 km](https://sgzwpqqelzmajkcubgkl.supabase.co/storage/v1/object/public/rt-media/2026/11/41070444fd44586f.jpg)
*NOIRLab/NSF/AURA/ P. Horálek (Institute of Physics in Opava), T. Slovinský, Wikimedia Commons, CC BY 4.0*

A team of observers recorded a stellar occultation by the asteroid (623) Chimaera on 21 October 2025, according to a paper posted to arXiv · preprint that has not yet been peer reviewed. The asteroid passed in front of UCAC4 626-004193, a star of V magnitude 13.86 in Andromeda. Eighteen telescopes were deployed across the United States, from Arizona to North Carolina, and sixteen of them collected useful data [1].

## Campaign and prediction

The predictions used Gaia DR3 stellar astrometry down to V magnitude 16 and the JPL#108 ephemeris of Chimaera from the Horizons database, built from a 118-year observation period. The authors computed them with the Occult4 software. The search produced one suitable event for a large deployment of telescopes from the University of Virginia in Charlottesville [1].

The stations were spaced by about 4.4 km across the track of the shadow. The shadow was predicted to cross the Atlantic, the southeastern, south-central and southwestern United States, and part of the island of Hawai‘i [1].

## Size, shape and albedo

Combined into one sky-plane plot, the chords give a profile that is well described by an elongated ellipse with a semi-major axis of 26.2 ± 0.8 km and a semi-minor axis of 16.0 ± 1.0 km. The projected area from the ellipse fit is 1316 km², and a limb fit encloses 1312 km² [1].

Photometry in the Gaia G band over a seven-month period around the occultation corresponds to a geometric albedo p_G = 0.059 ± 0.005. Comparison with archival occultation and direct observations gives p_G = 0.051 ± 0.004 and p_V = 0.051 ± 0.004. The authors note that the earlier IRAS-based diameter has unconstrained systematic errors, so better size constraints were needed [1].

The authors also compared the sky-plane projections with 3-D shape models from inversion techniques. They report that the models predict the orientation of Chimaera at the time of each occultation accurately [1].

## Why this asteroid

Chimaera is the largest member of the Chimaerid family, which has 108 identified asteroids, and its spin-axis obliquity is comparable to that of Uranus. It is a target of the Emirates Mission to the Asteroid Belt, which the UAE Space Agency is building and which is expected to launch in 2028. The spacecraft will fly by six asteroids and then enter orbit around the seventh, (269) Justitia. Chimaera is the second largest of the asteroids it will survey [1].

## Sources

1. [arXiv · препринт](https://arxiv.org/abs/2611.08041v1)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-57217ddc05dce7e773246179_16_telescopes_in/