87 Sylvia

87 Sylvia (pronounced /ˈsɪlviːə/ sil'-vee-a) is one of the largest main-belt asteroids. It is a member of the Cybele group located beyond the core of the belt (see Minor planet groups). Sylvia is remarkable for being the first asteroid known to possess more than one moon.

Discovery and naming

Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai), India ^[7]. Paul Herget, in his The Names of the Minor Planets (1955), attributes the name as honouring the first wife of astronomer Camille Flammarion, Sylvie Petiaux-Hugo Flammarion (this entry is signed by A. Paluzie-Borrell). However in the article announcing the discovery of this asteroid (MNRAS, 1866), Pogson explained that 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 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^[1], 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^[3]. Direct images^[1] 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 system

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. Romulus, the first moon, was discovered on February 18, 2001 from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. It is about 18 km in diameter and orbits at a distance of 1356±5 km, taking 3.6496±0.0007 days (87.59 h) to complete an orbit of Sylvia. 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. It is 7±2 km in diameter and orbits at a distance of 706±5 km, taking 1.3788±0.0007 days (33.09 h) to complete an orbit of Sylvia. It is thought likely that both Sylvia and its moons are accretions of rubble from a past asteroid collision ^[1] Other, smaller moons formed in a similar way may also be found. From the surface of Sylvia, Romulus and Remus would appear roughly the same size. Romulus, the outermost moon, would be about 0.89° across, slightly bigger than the closer but smaller Remus, which would be about 0.78° across. Because Sylvia is far from spherical, these values may vary by a little more than 10%, depending on where the observer is on Sylvia's surface. Since the two asteroidal moons appear to orbit (as best we can tell) in the same plane, they would occult each other once every 2.2 days. When the season is right, twice during Sylvia's 6.52 year orbital period, they would eclipse the Sun, which, at 0.15° across, is much smaller than when seen from Earth (0.53°). From Remus, the inner moon, Sylvia appears huge, roughly 30°×18° across, while its view of Romulus varies between 1.59 and 0.50° across. From Romulus, Sylvia measures 16°×10° across, while Remus varies between 0.62° and 0.19°. As an aside, some lightcurve analyses in the mid 1990s were used to suggest that Sylvia may be a very close binary of two bodies orbiting a common center of mass with orbital radii of about 200 km − this did not turn out to be the case. ^[8]

External links

• 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)

References

1. ^ ^{a b c d e f} F. Marchis et al (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature 436: 822.
2. ^ Data sheet compiled by W. R. Johnston
3. ^ ^{a b} M. Kaasalainen et al (2002). "Models of Twenty Asteroids from Photometric Data". Icarus 159: 369.
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)
8. ^ V.V. Prokofeva and M.I. Demchik (1995). "Fourier analysis of simultaneous BVR observations of the asteroid 87 Sylvia and its duplicity". Astronomy Lett. 20: 245.