87 Sylvia

From Wikipedia, the free encyclopedia

**87 Sylvia**
Discovery ^A

Discoverer	Norman Robert Pogson
Discovery date	May 16, 1866
Alternate designations ^B	A909 GA
Category	Cybele asteroid
Orbital elements ^C
Epoch July 14, 2004 (JD 2453200.5)
Eccentricity (e)	0.080
Semi-major axis (a)	522.137 G m (3.490 AU)
Perihelion (q)	480.594 Gm (3.213 AU)
Aphelion (Q)	563.679 Gm (3.768 AU)
Orbital period (P)	2381.697 d (6.52 a)
Mean orbital speed	15.94 km/s
Inclination (i)	10.855°
Longitude of the ascending node (Ω)	73.342°
Argument of perihelion (ω)	266.195°
Mean anomaly (M)	352.763°
Physical characteristics ^D
Dimensions	385×265×230 ± 10 km <ref name="Marchis05">F. Marchis et al (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature 436: 822.</ref><ref name="Johnston">Data sheet compiled by W. R. Johnston</ref>
Mass	1.478±0.006×10¹⁹ kg <ref name="Marchis05"/>
Density	1.2 ± 0.1 g/cm³ <ref name="Marchis05"/>
Surface gravity	0.027 m/s²
Escape velocity	0.10 km/s
Rotation period	0.2160 d (5.183642 h) <ref name="Kaasalainen02">M. Kaasalainen et al (2002). "Models of Twenty Asteroids from Photometric Data". Icarus 159: 369.</ref><ref name="PDS lc">PDS lightcurve data</ref>
Spectral class	X <ref name="PDS tax">PDS spectral class data</ref>
Absolute magnitude	6.94
Albedo (geometric)	0.0435 <ref name="IRAS">Supplemental IRAS Minor Planet Survey</ref>
Mean surface temperature	~151 K max: 223 K (-52°C)

87 Sylvia (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 known asteroid to possess more than one moon.

1 Discovery and naming
2 Physical characteristics
3 Moons
4 External links
5 References

[edit] Discovery and naming

Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai), India <ref name="Pogson">Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)</ref>. 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.

[edit] 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 probably very porous; as much as 60% of it may be empty space. 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<ref name="Kaasalainen02"/>. Lightcurve analysis<ref name="Kaasalainen02"/> and direct images<ref name="Marchis05"/> indicate that Sylvia's pole points towards ecliptic coordinates (β, λ) = (+66°, 71°) with a 10° uncertainty, which gives it an axial tilt of around 29°.

[edit] Moons

Sylvia is orbited by two small moons. They have been named Romulus and Remus, 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. Its full designation is (87) Sylvia I Romulus; before receiving its name, it was known as S/2001 (87) 1. It is 18±4 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. It should not be confused with the asteroid 10386 Romulus.

Remus, the second moon, was discovered on images taken starting on August 9, 2004 and announced on August 10, 2005. It was discovered by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France, using the Yepun telescope of the European Southern Observatory (ESO) in Chile. Marchis, the project leader, was waiting for the completion of the image acquisition programme before starting to process the data. Just as he was set to go on vacation in March 2005, Descamps sent him a brief note entitled "87 Sylvia est triple ?" pointing out that he could see two moonlets on several images of Sylvia. The entire team then focused quickly on analysis of the data, wrote a paper, submitted an abstract to the August meeting in Rio de Janeiro and submitted a naming proposal to the IAU.

Remus' full designation is (87) Sylvia II Remus; before receiving its name, it was known as S/2004 (87) 1. 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.

Astronomers believe that these moons were broken off Sylvia by an impact in the past, and that other, smaller moons 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°.

[edit] 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
Rubble-Pile Minor Planet Sylvia and Her Twins (ESO news release)
Adaptive Optics System Reveals New Asteroidal Satellite (SpaceDaily.com)
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)

[edit] References

Minor planets
Previous minor planet	87 Sylvia	Next minor planet

v • d • e</span> Small Solar System bodies
Vulcanoids \| Near-Earth asteroids \| Main belt \| Jupiter Trojans \| Centaurs \| Damocloids \| Comets \| Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.