# 12658 Peiraios

**12658 Peiraios** (provisional designation **1973 SL**) is a Jupiter Trojan asteroid, one of the bodies trapped near Jupiter's leading L4 Lagrange point (orbital position where gravitational forces balance, allowing trapped bodies), about 26 km in diameter.
|---|---|
| Designation | Minor planet 12658, provisional designation 1973 SL, named Peiraios<sup>[6](https://asteroidname.com/asteroid/peiraios)</sup> |
| Discovery | 19 September 1973, Palomar Observatory, by C. J. van Houten, I. van Houten-Groeneveld, and T. Gehrels (Palomar-Leiden Survey)<sup>[6](https://asteroidname.com/asteroid/peiraios)</sup> |
| Dynamical class | Jupiter Trojan, L4 camp; JPL defines the class as asteroids trapped in Jupiter's L4/L5 Lagrange points with 4.6 au < a < 5.5 au and e < 0.3<sup>[1](https://ssd-api.jpl.nasa.gov/doc/sbdb_filter.html)</sup><sup> • </sup><sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup>  |
| Orbit | Semimajor axis about 5.168 au, eccentricity about 0.057, inclination about 1.78 degrees<sup>[3](https://iaaras.ru/html/emp2020/emp_2020/elm_2020/20_012001.pdf)</sup> |
| Size | Estimated diameter about 26 km<sup>[6](https://asteroidname.com/asteroid/peiraios)</sup> |
| Rotation | Period 8.151 ± 0.003 hours, lightcurve amplitude 0.562 ± 0.033 magnitudes<sup>[4](https://real.mtak.hu/156658/1/2102.09447.pdf)</sup> |
| Photometry | Absolute magnitude H = 11.2; ATLAS c−o color −0.130 ± 0.091 mag; phase parameter G = 0.454 ± 0.166<sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup> |

## Discovery

The body received the provisional designation 1973 SL and, after its orbit was secured, the permanent number 12658 and the name Peiraios.<sup>[6](https://asteroidname.com/asteroid/peiraios)</sup>

## Orbit and dynamical class

Peiraios belongs to the L4 camp of [Jupiter Trojans](https://www.edgepedia.org/jupiter-trojans), the swarm of asteroids that librates around Jupiter's leading [Lagrange point](https://www.edgepedia.org/lagrange-point), 60 degrees ahead of the planet in its orbit.<sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup> In JPL's Small-Body Database classification scheme, a Jupiter Trojan (code TJN) is an asteroid trapped in Jupiter's L4 or L5 Lagrange points, defined by a semimajor axis between 4.6 and 5.5 au and an eccentricity below 0.3.<sup>[1](https://ssd-api.jpl.nasa.gov/doc/sbdb_filter.html)</sup> Peiraios fits this class comfortably: its osculating semimajor axis is about 5.168 au, with an eccentricity of about 0.057 and an inclination of about 1.78 degrees to the ecliptic.<sup>[3](https://iaaras.ru/html/emp2020/emp_2020/elm_2020/20_012001.pdf)</sup> Its low inclination places it near the core of the Trojan swarm, where number densities are highest.

## Physical properties

**Size and brightness.** The ATLAS survey photometry yields an absolute magnitude H of 11.2, meaning the brightness the body would have at 1 au from both the Sun and the observer at zero phase angle.<sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup> For a body of this size and magnitude, the implied surface is dark, consistent with the Trojan population as a whole: thermal measurements of the largest, well-measured Trojans give a mean V-band albedo of only about 5 to 7 percent, values similar to dark cometary nuclei.<sup>[5](https://link.springer.com/article/10.1007/s11214-024-01060-7)</sup>

**Rotation and shape.** A photometric campaign reported by Kalup and colleagues in 2021 determined a rotation period of 8.151 ± 0.003 hours with a lightcurve amplitude of 0.562 ± 0.033 magnitudes.<sup>[4](https://real.mtak.hu/156658/1/2102.09447.pdf)</sup> An amplitude of this size indicates a noticeably elongated body, since the brightness range over one rotation mainly reflects the changing projected area of an irregular shape.

**Color.** From ATLAS cyan and orange filter observations, Peiraios has a c−o color of −0.130 ± 0.091 magnitudes and a fitted phase-curve parameter G of 0.454 ± 0.166.<sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup> The c−o color is one of the diagnostics used to separate asteroid taxonomic types from the two-filter ATLAS photometry.

## Context among the Trojans

Jupiter's Trojan swarms contain nearly 8,000 known co-orbital asteroids, with the L4 cloud holding roughly 1.8 times as many cataloged objects as L5 as of early 2020.<sup>[2](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)</sup> Peiraios is one of the many L4 members in the 20 to 30 km size range. Like most Trojans, it was long known only from sparse survey photometry; dedicated time-series campaigns such as the one that fixed its 8.15-hour rotation period are what allow individual objects in this population to be characterized.<sup>[4](https://real.mtak.hu/156658/1/2102.09447.pdf)</sup>

## References

1. [SBDB Filter Parameters, JPL Solar System Dynamics](https://ssd-api.jpl.nasa.gov/doc/sbdb_filter.html)
2. [A. McNeill et al. (2021). Comparison of the Physical Properties of the L4 and L5 Trojan Asteroids from ATLAS Data. The Planetary Science Journal 2:6](https://www.researchgate.net/publication/348699143_Comparison_of_the_Physical_Properties_of_the_L4_and_L5_Trojan_Asteroids_from_ATLAS_Data)
3. [Ephemerides of minor planets 2020, Table of elements, Institute of Applied Astronomy RAS](https://iaaras.ru/html/emp2020/emp_2020/elm_2020/20_012001.pdf)
4. [Kalup et al. (2021). Rotation periods and amplitudes of Jupiter Trojans, arXiv:2102.09447](https://real.mtak.hu/156658/1/2102.09447.pdf)
5. [Surface Compositions of Trojan Asteroids, Space Science Reviews (2024)](https://link.springer.com/article/10.1007/s11214-024-01060-7)
6. [(12658) Peiraios – asteroid discovered 1973 | AsteroidName](https://asteroidname.com/asteroid/peiraios)

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*Topic: Encyclopedia › Places and geography › Extraterrestrial places › Small bodies of the Solar System › Jupiter Trojans and other dynamical groups › Greek-camp Jupiter Trojans › Heroes of the Odyssean circle*

*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