87 Sylvia

Sylvia (minor planet designation: 87 Sylvia) is one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of Cybele group located beyond the main asteroid belt (see minor-planet groups). Sylvia was the first asteroid known to possess more than one moon.

87 Sylvia
Discovery
Discovered byNorman Robert Pogson
Discovery dateMay 16, 1866
Designations
(87) Sylvia
Pronunciation/ˈsɪlviə/[1]
Named after
Rhea Silvia
A909 GA
main belt · (outside core)
Sylvia · Cybele
AdjectivesSylvian /ˈsɪlviən/
Orbital characteristics[2]
Epoch July 01, 2021
(JD 2459396.5, heliocentric)
Aphelion3.81 AU (560 Gm)
Perihelion3.15 AU (480 Gm)
3.48 AU (520 Gm)
Eccentricity0.094
6.5 a (2372 d)
15.94 km/s
213°
0° 9m 6.48s / day
Inclination10.9°
73°
263°
Known satellites2
Physical characteristics[3]
Dimensions(363 × 249 × 191) ±5 km (MPCD) or (374 × 248 × 194) ±5 km (ADAM)
Mean diameter
271±5 km (MPCD) or 274±5 km (ADAM)
Volume(10.5±0.2)×107 km3 (MPCD) or (10.8±0.2)×107 km3 (ADAM)
Mass(14.76±0.06)×1018 kg[4]
(14.6±0.1)×1018 kg[5]
Mean density
1.378±0.045 g/cm3
0.2160 d (5.183641±0.000039 h)
North pole right ascension
14.3°±5°
North pole declination
+83.5°±5°
+64.2°±5°
75.3°±5°
0.0435 [6]
X [7]
6.94

    Discovery and naming

    Sylvia was discovered by N. R. Pogson on May 16, 1866, from Madras (Chennai), India.[8] Antonio Paluzie-Borrell, writing in Paul Herget's The Names of the Minor Planets (1955), mistakenly states that the name honors Sylvie Petiaux-Hugo Flammarion, the first wife of astronomer Camille Flammarion. In fact, in the article announcing the discovery of the asteroid, Pogson explained that he selected the name in reference to Rhea Silvia, mother of Romulus and Remus (MNRAS, 1866).

    Physical characteristics

    Sylvia is very dark in color and probably has a primitive composition, though with some internal differentiation. The discovery of its moons made possible an accurate measurement of the asteroid's mass, density and mass distribution. Its density is low (around 1.4 times the density of water), indicating that the asteroid is porous; best-fit models estimate it had an original composition by volume of 35% rock, 13% ice and 52% internal voids, and that today it consists of a pristine anhydrous outer layer, and a differentiated interior, with meltwater having percolated inward so that the porosity of the rock is filled with ice out to a radius of about 46 km, then ice-free porous rock out to about 104 km.[3]

    Sylvia is a fairly fast rotator, turning about its axis every 5.2 hours, giving it an equatorial rotation velocity of about 65 m/s, almost half the escape velocity.

    Sylvia's shape is flattened and elongated (a/b ≈ 1.45 ; a/c ≈ 1.84) and somewhat irregular. However, its surface has not been imaged well enough for individual features to be resolved.

    Satellite system
    Adaptive Optics observations of (87) Sylvia, showing its two satellites, Remus and Romulus.

    Sylvia has two orbiting satellites. They have been named (87) Sylvia I Romulus and (87) Sylvia II Remus, after Romulus and Remus, the children of the mythological Rhea Silvia.

    Romulus, the first moon, was discovered on February 18, 2001, from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. Remus, the second moon, was discovered over three years later on August 9, 2004, by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France.

    The orbital properties of the satellites are listed in this table.[3][9] The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned. Diameters are estimates based on the assumption that the moons have the same albedo as their primary.

    NameMass [kg]Diameter [km]Semi-major axis [km]Orbital period [days]EccentricityInclination [°]
    Remus (0.8±0.2)×101510+17
    −6    		694.2±0.11.3570±0.00030.005+0.010
    −0.002    		8.7±1.8
    Romulus (1.4±0.4)×101515+10
    −6    		1340.6±0.43.64126±0.000020.000+0.003
    −0.000    		7.4±0.5

    See also

    References
    1. Noah Webster (1884) A Practical Dictionary of the English Language
    2. JPL data Retrieved 2021-09-29
    3. Carry et al. (2021) 'Evidence for differentiation of the most primitive small bodies'. Astronomy & Astrophysics, in press.
    4. F. Marchis; et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia" (PDF). Nature. 436 (7052): 822–4. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362. S2CID 4412813.
    5. Margot and Brown (2001), from Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 2 July 2013. Retrieved 5 December 2008.
    6. Supplemental IRAS Minor Planet Survey Archived 2009-08-17 at the Wayback Machine
    7. PDS spectral class data Archived 2009-08-05 at the Wayback Machine
    8. Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)
    9. Errors were published as 3 sigma. To maintain consistency with the table for Sylvia, they have here been reduced to 1 sigma.

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