87 Sylvia

**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.

## Discovery and naming

Sylvia was found by N. R. Pogson on May 16, 1866 from Madras (Chennai), India. 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 characteristics

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. 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).

## Moons

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 Other, smaller moons formed in a similar way may also be found.

## Other websites

- Pogson, N. R. (1866), _Minor Planet (87) Sylvia_, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)
- Data on (87) Sylvia from Johnston's archive (maintained by W. R. Johnston)
- _Rubble-Pile Minor Planet Sylvia and Her Twins _ (ESO news release, August 2005) Includes images and artists impressions
- _Adaptive Optics System Reveals New Asteroidal Satellite_ (SpaceDaily.com, March 2001) Includes a discovery image.
- Space.com: First asteroid trio discovered
- IAUC 7588, reporting discovery of S/2001 (87) 1
- IAUC 7590, confirming the discovery
- IAUC 8582, reporting discovery of S/2004 (87) 1 and naming Romulus and Remus
- An animation of (87) Sylvia and its moons (23 MB, DivX)
- Shape model derived from lightcurve (on page 19)

## Infobox (planet)

- **bgcolour**: #FFFFC0
- **name**: 87 Sylvia
- **discoverer**: Norman Robert Pogson
- **image**: ESO_-_87_Sylvia_(by).jpg
- **caption**: 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**: May 16, 1866
- **alt names**: A909 GA
- **mp category**: Main belt (Cybele asteroid, Cybele)
- **epoch**: July 14, 2004 (Julian day, JD 2453200.5)
- **semimajor**: 522.137 Giga, Gmetre, m (3.490 Astronomical unit, AU)
- **perihelion**: 480.594 Gm (3.213 AU)
- **aphelion**: 563.679 Gm (3.768 AU)
- **eccentricity**: 0.080
- **period**: 2381.697 day, d (6.52 Julian year (astronomy), a)
- **inclination**: 10.855degree (angle), °
- **asc node**: 73.342°
- **arg peri**: 266.195°
- **mean anomaly**: 352.763°
- **avg speed**: 15.94 km/second, s
- **dimensions**: 385×265×230 ± 10 km
- **mass**: 1.478±0.006×10<sup>19</sup> kilogram, kg
- **density**: 1.2 ± 0.1 g/cubic centimetre, cm³
- **surface grav**: 0.027 m/s²
- **escape velocity**: 0.10 km/s
- **rotation**: 0.2160 d (5.183642 hour, h)
- **spectral type**: X-type asteroid, X
- **abs magnitude**: 6.94
- **albedo**: 0.0435
- **single temperature**: ~151 kelvin, K
_max:_ 223 K (-52°celsius, C)