87 Sylvia

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87 Sylvia
Sylvia and Romulus, which is shown by the arrow
Sylvia and Romulus, which is shown by the arrow
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 Gm (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 [1][2]
Mass 1.478±0.006×1019 kg [1]
Density 1.2 ± 0.1 g/cm³ [1]
Surface gravity 0.027 m/s²
Escape velocity 0.10 km/s
Rotation period 0.2160 d (5.183642 h) [3][4]
Spectral class X [5]
Absolute magnitude 6.94
Albedo (geometric) 0.0435 [6]
Mean surface
temperature		 ~151 K
max: 223 K (-52°C)
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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.

Contents

[edit] 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.

[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 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 well known 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.

[edit] Satellite system

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. 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 independently soon thereafter by 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]

[edit] External links

[edit] 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. ^ PDS spectral class data
  6. ^ Supplemental IRAS Minor Planet Survey
  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.


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