87 Sylvia

Sylvia (minor planet designation: 87 Sylvia) is the 8th-largest asteroid in the asteroid belt. 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
Adaptive Optics observations of (87) Sylvia, showing its two satellites, Remus and Romulus.
Discovered byNorman Robert Pogson
Discovery dateMay 16, 1866
(87) Sylvia
A909 GA
main belt · (outside core)
Sylvia · Cybele
AdjectivesSylvian /ˈsɪlviən/
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion3.768 AU (563.679 Gm)
Perihelion3.213 AU (480.594 Gm)
3.490 AU (522.137 Gm)
6.52 a (2381.697 d)
15.94 km/s
Known satellites2
Physical characteristics
Dimensions(384 × 262 × 232) ±10 km[2][3]
(385 × 265 × 230) ± 10 km[4]
286 km (mean)
Mass(1.478 ± 0.006) × 1019 kg[2][4]
Mean density
1.2 ± 0.1 g/cm3[2][4]
Equatorial surface gravity
0.0729 m/s2
Equatorial escape velocity
0.1379 km/s
0.2160 d (5.183642 h) [5][6]
0.0435 [7]

Discovery and namingEdit

Sylvia was discovered by N. R. Pogson on May 16, 1866, from Madras (Chennai), India.[9] 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 characteristicsEdit

Sylvia is very dark in color and probably has a very primitive composition. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density. Its density was found to be very low (around 1.2 times the density of water), indicating that the asteroid is porous to very porous; from 25% to as much as 60% of it may be empty space,[4] depending on the details of its composition. However, the mineralogy of the X-type asteroids is not known well enough to constrain this further. Either way, this suggests a loose rubble pile structure. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation velocity of about 230 km/h or 145 mph). The short axis is the rotation axis.[5] Direct images[4] indicate that Sylvia's pole points towards ecliptic coordinates (β, λ) = (+62.6°, 72.4°) with only a 0.5° uncertainty, which gives it an axial tilt of around 29.1°. Sylvia's shape is strongly elongated.

Satellite systemEdit

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.[10] The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned; all planes are aligned within about 1 degree of each other, suggestive of satellite formation in or near the equatorial plane of the primary.

Name Mass [kg] Semi-major axis [km] Orbital period [days] Eccentricity
Remus 7.3×1014 706.5 1.37 0.027
Romulus 9.3×1014 1357 3.65 0.006


  1. ^ Noah Webster (1884) A Practical Dictionary of the English Language
  2. ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 2 July 2013. Retrieved 5 December 2008.
  3. ^ Data sheet compiled by W. R. Johnston
  4. ^ a b c d e 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.
  5. ^ a b M. Kaasalainen; et al. (2002). "Models of Twenty Asteroids from Photometric Data" (PDF). Icarus. 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
  6. ^ PDS lightcurve data Archived 2009-04-09 at the Wayback Machine
  7. ^ Supplemental IRAS Minor Planet Survey Archived 2009-08-17 at the Wayback Machine
  8. ^ PDS spectral class data Archived 2009-08-05 at the Wayback Machine
  9. ^ Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)
  10. ^ Fang, Julia. "Orbits, Masses, and Evolution of Main Belt Triple (87) Sylvia". Astronomical Journal. arXiv:1206.5755. Bibcode:2012AJ....144...70F. doi:10.1088/0004-6256/144/2/70.

External linksEdit