 | |
| Discovery | |
|---|---|
| Discovered by | Norman Robert Pogson |
| Discovery date | 16 May 1866 |
| Designations | |
| (87) Sylvia | |
| Pronunciation | /ˈsɪlviə/SIL-vee-ə[1] |
Named after | Rhea Silvia |
| A909 GA | |
| main belt · (outside core) Sylvia · Cybele | |
| Adjectives | Sylvian (/ˈsɪlviən/SIL-vee-ən) |
| Orbital characteristics[2] | |
| Epoch 1 July 2021 (JD 2459396.5, heliocentric) | |
| Aphelion | 3.81 AU (560 Gm) |
| Perihelion | 3.15 AU (480 Gm) |
| 3.48AU (520 Gm) | |
| Eccentricity | 0.094 |
| 6.5a (2372 d) | |
Averageorbital speed | 15.94 km/s[citation needed] |
| 213° | |
| 0° 9m 6.48s / day | |
| Inclination | 10.9° |
| 73° | |
| 263° | |
| Knownsatellites | 2 |
| Physical characteristics[3] | |
| Dimensions | (363 × 249 × 191) ±5 km(MPCD) or(374 × 248 × 194) ±5 km(ADAM) |
| 271±5 km(MPCD) or274±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] |
Meandensity | 1.378±0.045 g/cm3 |
| 0.2160 d (5.183641±0.000039 h) | |
North poleright ascension | 14.3°±5° |
North poledeclination | +83.5°±5° |
| +64.2°±5° | |
| 75.3°±5° | |
| 0.0435[6] | |
| X [7] | |
| 6.94 | |
87 Sylvia is one of thelargest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of theSylvia family and member ofCybele group located beyond themain asteroid belt (seeminor-planet groups). Sylvia was the first asteroid known to possess more than onemoon.
Sylvia was discovered byN. R. Pogson on 16 May 1866, fromMadras (Chennai),India.[8]Antonio Paluzie-Borrell, writing inPaul Herget'sThe Names of the Minor Planets (1955), mistakenly states that the name honorsSylvie Petiaux-Hugo Flammarion, the first wife of astronomerCamille Flammarion. In fact, in the article announcing the discovery of the asteroid, Pogson explained that he selected the name in reference toRhea Silvia, mother ofRomulus and Remus (MNRAS, 1866).
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 andmass 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.
Sylvia has two orbiting satellites. They have been named(87) Sylvia I Romulus and(87) Sylvia II Remus, afterRomulus and Remus, the children of the mythologicalRhea Silvia.
Romulus, the first moon, was discovered on 18 February 2001, from theKeck II telescope byMichael E. Brown andJean-Luc Margot.Remus, the second moon, was discovered over three years later on 9 August 2004, byFranck Marchis ofUC Berkeley, andPascal Descamps,Daniel Hestroffer, andJérôme Berthier of theObservatoire de Paris, France.
The orbital properties of the satellites are listed in this table.[3][a] 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.
| Name | Mass [kg] | Diameter [km] | Semi-major axis [km] | Orbital period [days] | Eccentricity | Inclination [°] |
|---|---|---|---|---|---|---|
| Remus | (0.8±0.2)×1015 | 10+17 −6 | 694.2±0.1 | 1.3570±0.0003 | 0.005+0.010 −0.002 | 8.7±1.8 |
| Romulus | (1.4±0.4)×1015 | 23.1±0.7[9] | 1340.6±0.4 | 3.64126±0.00002 | 0.000+0.003 −0.000 | 7.4±0.5 |
