87 Sylvia

outer main-belt asteroid
(Redirected from Romulus (moon))

87 Sylvia is one of the biggest main-belt asteroids. It is a member of the Cybele group. It is beyond the middle of the belt. Sylvia is famous for being the first asteroid known to have more than one moon.

87 Sylvia
ESO - 87 Sylvia (by).jpg
Image showing the positions of Remus and Romulus around 87 Sylvia on 9 different nights. The small image shows the potato form of 87 Sylvia.
Discovered byNorman Robert Pogson
Discovery dateMay 16, 1866
A909 GA
Main belt (Cybele)
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion563.679 Gm (3.768 AU)
Perihelion480.594 Gm (3.213 AU)
522.137 Gm (3.490 AU)
2381.697 d (6.52 a)
15.94 km/s
Physical characteristics
Dimensions385×265×230 ± 10 km[1][2]
Mass1.478±0.006×1019 kg[1]
Mean density
1.2 ± 0.1 g/cm³[1]
0.027 m/s²
0.10 km/s
0.2160 d (5.183642 h)[3][4]
Temperature~151 K
max: 223 K (-52°C)
Spectral type

Discovery and namingEdit

Sylvia was found by N. R. Pogson on May 16, 1866 from Madras (Chennai), India.[7] In the article announcing the discovery of this asteroid (MNRAS, 1866), Pogson said he selected the name in reference to Rhea Silvia, mother of Romulus and Remus.

Physical characteristicsEdit

Sylvia is very dark in color and probably has a primitive composition. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density. Its density is very low (around 1.2 times the density of water), indicating that the asteroid has a lot of empty spaces; From 25% to as much as 60% of it may be empty space.[1] The mineralogy of X-type asteroids is not well known. Sylvia is a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation speed of about 230 km/h or 145 mph).


Sylvia is orbited by two small moons. They have been named Romulus and Remus (the formal names are (87) Sylvia I Romulus and (87) Sylvia II Remus, respectively), after the children of the mythological Rhea Silvia.

It is thought likely that both Sylvia and its moons are rubble piles from a past asteroid collision[1] Other, smaller moons formed in a similar way may also be found.


  1. 1.0 1.1 1.2 1.3 1.4 F. Marchis; et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature. 436 (7052): 822–824. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362. S2CID 4412813.
  2. Data sheet compiled by W. R. Johnston
  3. M. Kaasalainen; et al. (2002). "Models of Twenty Asteroids from Photometric Data" (PDF). Icarus. 159 (2): 369–395. doi:10.1006/icar.2002.6907.
  4. PDS lightcurve data
  5. Supplemental IRAS Minor Planet Survey
  6. PDS spectral class data
  7. Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)

Other websitesEdit