15094 Polymele

15094 Polymele /pɒlɪˈmiːliː/ is a primitive Jupiter trojan from the Greek camp, approximately 21 kilometers (13 miles) in diameter. It is a target of the Lucy mission with a close flyby planned to occur in September 2027.[9][13] It was discovered on 17 November 1999, by astronomers with the Catalina Sky Survey at Mount Lemmon Observatory, Arizona, in the United States. The P-type asteroid has a rotation period of 5.9 hours and highly flattened shape.[14][15] It was named after Polymele from Greek mythology, the wife of Menoetius and the mother of Patroclus.[1] In 2022, it was reported to have a natural satellite approximately 5 kilometers (3.1 miles) in diameter.[16]

15094 Polymele
Polymele imaged by the Hubble Space Telescope in 2018
Discovery[1]
Discovered byCatalina Sky Survey
Discovery siteCatalina Station
Discovery date17 November 1999
Designations
(15094) Polymele
Pronunciation/pɒlɪˈmiːliː/[2]
Named after
Polymele
(Greek mythology)[1]
1999 WB2 · 1997 WR57
Jupiter trojan[1][3]
(Greek camp)[4]
Orbital characteristics[3]
Epoch 25 February 2023 (JD 2460000.5)
Uncertainty parameter 0
Earliest precovery date30 November 1951[1]
Aphelion5.682 AU
Perihelion4.679 AU
5.180 AU
Eccentricity0.0968
11.79 yr (4,307 d)
44.314°
0° 5m 0.936s / day
Inclination12.981°
50.319°
4.772°
Jupiter MOID0.2445 AU
TJupiter2.940
Physical characteristics
Dimensions27.0 × 24.4 × 10.4 km[5]
(± 2.0 × 1.6 × 1.6 km)
Mean diameter
21.075±0.136 km[6][7]
Mean density
0.7–1 g/cm3 (assumed)[5]
5.8607±0.0005 h[8]
170.9° (wrt ecliptic)[5]
−80.9°±2.1°[5]
231.8°±4.5°[5]
0.073[8]
0.091±0.017[3]
P[9]
B–V = 0.652±0.065[10]
V–R = 0.477±0.065[10]
V–I = 0.799±0.068[10]
11.60[1][3][7][11][12]
11.691±0.002 (S/R)[8]

    Orbit and classification

    Polymele is a Jupiter trojan asteroid orbiting in the leading Greek camp at Jupiter's L4 Lagrangian point, 60° ahead of the gas giant's orbit (see Trojans in astronomy). It orbits the Sun at a distance of 4.7–5.7 AU once every 11 years and 9 months (4,289 days; semi-major axis of 5.17 AU). Its orbit has an eccentricity of 0.09 and an inclination of 13° with respect to the ecliptic.[3] The asteroid's observation arc begins 48 years prior to its official discovery observation at Mount Lemmon, with a precovery taken at Palomar Observatory in 1951, and published by the Digitized Sky Survey later on.[1]

    Naming

    This minor planet was named after Polymele, the daughter of Peleus from Greek mythology. According to the Latin author Gaius Julius Hyginus (c. 64 BC – AD 17), she is the wife of the Argonaut Menoetius and the mother of Patroclus, who participated in the Trojan War.[1] Polymele is also known as "Philomela"; that name was previously used for the asteroid 196 Philomela. The approved naming citation was published by the Minor Planet Center on 22 February 2016 (M.P.C. 98711).[17]

    Physical characteristics

    Polymele has been characterized as a primitive P-type asteroid by the investigators of the Lucy mission.[9] P-type asteroids are known for their low albedo. It has a V–I color index of 0.799,[10] which is lower than that for most larger Jupiter trojans (see table below).

    Size, shape, and albedo

    According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Polymele measures 21.075 kilometers in diameter and its surface has an albedo of 0.091,[6][7][11] while in 2018, Marc Buie published an albedo of 0.073 and an absolute magnitude of 11.691 in the S- and/or R band.[8] The Collaborative Asteroid Lightcurve Link assumes a standard albedo for a carbonaceous asteroid of 0.057 and calculates a larger diameter of 26.64 kilometers based on an absolute magnitude of 11.6.[12]

    On 27 March 2022, multiple astronomers observed a stellar occultation by Polymele, which revealed an apparently elongated projected shape with projected dimensions of 26.2 km × 12.8 km (16.3 mi × 8.0 mi).[14] Additional occultation observations of Polymele from 2020–2023 revealed that Polymele's true shape is a highly flattened disk with dimensions 27.0 km × 24.4 km × 10.4 km (16.8 mi × 15.2 mi × 6.5 mi), similar to the large flat lobe of the Kuiper belt object 486958 Arrokoth.[15] This unusual flattened shape of Polymele implies that it had retained its original shape from when it formed by accretion.[15]

    Rotation

    In March 2016, a rotational lightcurve of Polymele was obtained from photometric observations by Marc Buie and colleges. Lightcurve analysis gave a rotation period of 5.8607±0.0005 hours with a small brightness amplitude of 0.09±0.03 magnitude (U=2-), which indicates the body is being viewed pole-on.[8] Previously, the Lucy mission team published spin rates of 6.1 and 4 hours, respectively.[9][13]

    The resolved observation of Polymele's shape in multiple occultation events allowed the Lucy team to determine the orientation of Polymele's rotational pole.[5] Polymele's rotational north pole points toward ecliptic latitude –80.9°, which corresponds to an axial tilt of 170.9° with respect to the ecliptic, making Polymele a retrograde rotator.[5]

    Satellite
    Discovery of Polymele's satellite in the 26 March 2022 occultation

    Following observations of an occultation on 26 March 2022, the Lucy mission team reported the discovery of a natural satellite around Polymele. The satellite is a smaller asteroid about 5–6 kilometers (3.1–3.7 miles) in diameter, orbiting nearly in the equatorial plane of Polymele at a distance of 204.4 ± 2.6 km (127.0 ± 1.6 mi).[14][5] Assuming Polymele has a density of 0.7–1 g/cm3, the satellite should have an orbital period between 14.4 and 16.6 days.[5] It will not be assigned a formal name until further observations determine its orbit.[18] The Lucy team refers to the companion by the temporary informal name "Shaun," after Aardman Animations' animated sheep.[16] The satellite was detected again in an occultation on 4 February 2023,[15] in the largest organized occultation expedition in history. Nearly 200 astronomers across two continents participated in the campaign.[19]

    Exploration

    Lucy mission target

    Polymele is planned to be visited by the Lucy spacecraft which launched in 2021. The flyby is scheduled for 15 September 2027, and will approach the asteroid to a distance of 415 km (258 mi) at a relative velocity of 6 km/s (13,000 mph).[9]

    References
    1. "15094 Polymele (1999 WB2)". Minor Planet Center. Retrieved 8 March 2018.
    2. 'Polymela' in Noah Webster (1884) A Practical Dictionary of the English Language
    3. "JPL Small-Body Database Browser: 15094 Polymele (1999 WB2)" (2015-06-10 last obs.). Jet Propulsion Laboratory. Retrieved 26 September 2018.
    4. "List of Jupiter Trojans". Minor Planet Center. 20 June 2016. Retrieved 8 March 2018.
    5. Levison, H. F.; Buie, M. W.; Keeney, B. A.; Mottola, S.; et al. (June 2023). Interpreting the Stellar Occultations of (15094) Polymele – a Lucy Target (PDF). Asteroids, Comets, Meteors Conference 2023. Lunar and Planetary Institute.
    6. Grav, T.; Mainzer, A. K.; Bauer, J. M.; Masiero, J. R.; Nugent, C. R. (November 2012). "WISE/NEOWISE Observations of the Jovian Trojan Population: Taxonomy". The Astrophysical Journal. 759 (1): 10. arXiv:1209.1549. Bibcode:2012ApJ...759...49G. doi:10.1088/0004-637X/759/1/49. S2CID 119101711. (online catalog)
    7. Mainzer, A. K.; Bauer, J. M.; Cutri, R. M.; Grav, T.; Kramer, E. A.; Masiero, J. R.; et al. (June 2016). "NEOWISE Diameters and Albedos V1.0". NASA Planetary Data System: EAR-A-COMPIL-5-NEOWISEDIAM-V1.0. Bibcode:2016PDSS..247.....M. Retrieved 26 September 2018.
    8. Buie, Marc W.; Zangari, Amanda M.; Marchi, Simone; Levison, Harold F.; Mottola, Stefano (June 2018). "Light Curves of Lucy Targets: Leucus and Polymele" (PDF). The Astronomical Journal. 155 (6): 11. Bibcode:2018AJ....155..245B. doi:10.3847/1538-3881/aabd81. Retrieved 26 September 2018.
    9. Levison, H. F.; Olkin, C.; Noll, K. S.; Marchi, S.; Lucy Team (March 2017). "Lucy: Surveying the Diversity of the Trojan Asteroids: The Fossils of Planet Formation" (PDF). 48th Lunar and Planetary Science Conference (1964): 2025. Bibcode:2017LPI....48.2025L. Retrieved 13 April 2017.
    10. Hainaut, O. R.; Boehnhardt, H.; Protopapa, S. (October 2012). "Colours of minor bodies in the outer solar system. II. A statistical analysis revisited". Astronomy and Astrophysics. 546: 20. arXiv:1209.1896. Bibcode:2012A&A...546A.115H. doi:10.1051/0004-6361/201219566. S2CID 54776793.
    11. Mainzer, A.; Grav, T.; Masiero, J.; Hand, E.; Bauer, J.; Tholen, D.; et al. (November 2011). "NEOWISE Studies of Spectrophotometrically Classified Asteroids: Preliminary Results". The Astrophysical Journal. 741 (2): 25. arXiv:1109.6407. Bibcode:2011ApJ...741...90M. doi:10.1088/0004-637X/741/2/90. S2CID 118700974. (catalog)
    12. "LCDB Data for (15094) Polymele". Asteroid Lightcurve Database (LCDB). Retrieved 26 September 2018.
    13. Buie, Marc W.; Zangari, Amanda Marie; Marchi, Simone; Mottola, Stefano; Levison, Harold F. (October 2016). "Ground-based characterization of Leucus and Polymele, two fly-by targets of the Lucy Discovery mission". American Astronomical Society. 48: 208.06. Bibcode:2016DPS....4820806B.
    14. Buie, Marc; Keeney, Brian; Levison, Harold; Olkin, Catherine; et al. (December 2022). Shape and duplicity of Lucy Mission target Polymele from occultation observations. 54th Annual DPS Meeting. Vol. 54. American Astronomical Society. Bibcode:2022DPS....5451203B. 512.03. Retrieved 28 January 2023.
    15. Buie, M. W.; Keeney, B. A.; Levison, H. F.; et al. (June 2023). Occultation results for the (15094) Polymele system – a Lucy Target (PDF). Asteroids, Comets, Meteors Conference 2023. Lunar and Planetary Institute.
    16. Clark, Stephen (14 June 2022). "Ninth asteroid added to Lucy mission; optimism grows on solar array issue". Spaceflight Now. Retrieved 15 June 2022.
    17. "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 8 March 2018.
    18. "NASA's Lucy Team Discovers Moon Around Asteroid Polymele". NASA. 16 August 2022. Retrieved 17 August 2022.
    19. NASA/SwRI. "Spotting a Satellite - Lucy Mission". LUCY Mission Page. Retrieved 1 September 2023.

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