# 12242 Koon
**12242 Koon** (provisional designation 1988 QY) is a Jupiter Trojan asteroid, one of the bodies trapped in a 1:1 mean-motion resonance with Jupiter at its trailing L5 Lagrangian point, about 60 degrees behind the planet in its orbit[1](https://link.springer.com/article/10.1007/s00159-024-00157-w). It was discovered on 18 August 1988 by the husband-and-wife team of C. S. Shoemaker and E. M. Shoemaker at [Palomar Observatory](https://www.edgepedia.org/palomar-observatory)[2](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf).
| Key fact | Detail |
|---|---|
| Designation | Minor planet number 12242, provisional designation 1988 QY[2](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf) |
| Discovery | 18 August 1988, by C. S. Shoemaker and E. M. Shoemaker at Palomar[2](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf) • [3](https://www.dasschaufenster.at/134340/bilder/000001to168313.pdf) |
| Name origin | Koon, eldest son of Antenor in the Trojan War tradition; IAU citation M 41386[2](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf) |
| Upcoming observation | A stellar occultation of a star by Koon is predicted for 23 November 2026, visible from the United States[4](https://www2.boulder.swri.edu/~buie/recon/trojans/12242_Koon_20261123120844.html) |
## Discovery
Koon was found during the Palomar survey for bright Trojan asteroids conducted by Carolyn and Eugene Shoemaker with the 46-cm Schmidt telescope. The survey, begun in 1985, aimed to find all Trojans brighter than H = 10.25 and to obtain precise, multiple-opposition orbits for them; 1988 QY appears in the survey's L5 table with a magnitude of 10.5[5](https://www.lpi.usra.edu/meetings/lpsc1990/pdf/1585.pdf). The Shoemakers' work in 1988 increased the number of known L5 Trojans brighter than that limit by about 60 percent[5](https://www.lpi.usra.edu/meetings/lpsc1990/pdf/1585.pdf).
## Name
The asteroid is named for Koon, described in the IAU naming citation as a notable fighter and the eldest son of Antenor, a Trojan elder. Grieved by the death of his brother Iphidamas at the hands of [Agamemnon](https://www.edgepedia.org/agamemnon), Koon fought and wounded Agamemnon but was in turn beheaded by him. The citation was issued in the Minor Planet Circulars as M 41386[2](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf). The name follows the convention that [Jupiter Trojans](https://www.edgepedia.org/jupiter-trojans) are named for figures from the [Trojan War](https://www.edgepedia.org/trojan-war), with L5 objects taking names from the Trojan side of the conflict[1](https://link.springer.com/article/10.1007/s00159-024-00157-w).
## Orbit and physical properties
As an L5 Trojan, Koon shares Jupiter's roughly 12-year orbit around the Sun while librating around the trailing Lagrangian point. [The 1](https://www.edgepedia.org/the-1):1 mean-motion resonance with Jupiter makes the orbits of Trojan asteroids stable over the lifetime of the [Solar System](https://www.edgepedia.org/solar-system)[6](https://link.springer.com/article/10.1007/s11214-024-01052-7). Koon is listed in a published analysis of the long-term stability and resonant structure of Jupiter's Trojan swarms[8](https://academic.oup.com/mnras/article/372/4/1463/1184958).
A RECON occultation prediction for the 23 November 2026 event gives a cross-track diameter of 37.9 km for deployment planning, with diameter estimates of 39.8 km assuming a 5 percent albedo and 16.2 km assuming a 30 percent albedo; the apparent brightness of the occulting body is magnitude 16.2[4](https://www2.boulder.swri.edu/~buie/recon/trojans/12242_Koon_20261123120844.html). Low albedos are typical of the [Trojan](https://www.edgepedia.org/trojan-celestial-body) population, whose members are believed to be icy bodies that formed in the outer Solar System and were captured during the migration of the giant planets[7](https://www.sciencedirect.com/science/article/abs/pii/S0019103509004771).
## References
1. [Polarimetry of Solar System minor bodies and planets, Astronomy and Astrophysics Review (2024)](https://link.springer.com/article/10.1007/s00159-024-00157-w)
2. [Lutz D. Schmadel, Dictionary of Minor Planet Names, 5th ed.](https://ndl.ethernet.edu.et/bitstream/123456789/32939/1/Lutz%20D.%20Schmadel.pdf)
3. [Numbered minor planets 12001–12500, discovery table](https://www.dasschaufenster.at/134340/bilder/000001to168313.pdf)
4. [RECON event prediction for 12242 Koon, 2026-11-23](https://www2.boulder.swri.edu/~buie/recon/trojans/12242_Koon_20261123120844.html)
5. [C. S. Shoemaker and E. M. Shoemaker, Survey for Bright Trojan Asteroids, LPSC XXI](https://www.lpi.usra.edu/meetings/lpsc1990/pdf/1585.pdf)
6. [Shapes, Rotations, Photometric and Internal Properties of Jupiter Trojans, Space Science Reviews (2024)](https://link.springer.com/article/10.1007/s11214-024-01052-7)
7. [Phase curves of nine Trojan asteroids over a wide range of phase angles, Icarus](https://www.sciencedirect.com/science/article/abs/pii/S0019103509004771)
8. [The resonant structure of Jupiter's Trojan asteroids – I. Long-term stability and diffusion, Monthly Notices of the Royal Astronomical Society](https://academic.oup.com/mnras/article/372/4/1463/1184958)
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*Topic: Encyclopedia › Places and geography › Extraterrestrial places › Small bodies of the Solar System › Jupiter Trojans and other dynamical groups › Trojan-camp Jupiter Trojans*
*Initially written Oct 21, 2026 · Edited: Oct 25, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license