87 Sylvia

87 Sylvia
Adaptive Optics observations of (87) Sylvia, showing its two satellites, Remus and Romulus.
Discovery
Discovered by Norman Robert Pogson
Discovery date May 16, 1866
Designations
MPC designation (87) Sylvia
Pronunciation /ˈsɪlviə/ SIL-vee-ə
A909 GA
main belt · (outer)
Sylvia · Cybele
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion 3.768 AU (563.679 Gm)
Perihelion 3.213 AU (480.594 Gm)
3.490 AU (522.137 Gm)
Eccentricity 0.080
6.52 a (2381.697 d)
15.94 km/s
352.763°
Inclination 10.855°
73.342°
266.195°
Known satellites 2
Physical characteristics
Dimensions (384 × 262 × 232) ±10 km[1][2]
(385 × 265 × 230) ± 10 km[3]
286 km (mean)
Mass (1.478 ± 0.006) × 1019 kg[1][3]
Mean density
 1.2 ± 0.1 g/cm3[1][3]
Equatorial surface gravity
 0.0729 m/s2
Equatorial escape velocity
 0.1379 km/s
0.2160 d (5.183642 h) [4][5]
0.0435 [6]
X [7]
6.94

    87 Sylvia is the 8th-largest asteroid in the asteroid belt. It is the parent body of the Sylvia family and member of Cybele group located beyond the core of the belt (see minor-planet groups). Sylvia is 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.[8] A. Paluzie-Borrell, writing in Paul Erget's The Names of the Minor Planets (1955), mistakenly states that the name honours Sylvie Petiaux-Hugo Flammarion, the first wife of astronomer Camille Flammarion. In fact, in the article announcing the discovery of the asteroid, Pogson explained that he selected the name in reference to Rhea Silvia, mother of Romulus and Remus (MNRAS, 1866).

    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,[3] 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.[4] Direct images[3] 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 has two orbiting satellites. They have been named (87) Sylvia I Romulus and (87) Sylvia II Remus, after Romulus and Remus, 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. 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.

    The orbital properties of the satellites are listed in this table.[9] The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned; all planes are aligned within about 1 degree of each other, suggestive of satellite formation in or near the equatorial plane of the primary.

    NameMass [kg]Semi-major axis [km]Orbital period [days]Eccentricity
    Remus7.3×1014706.51.370.027
    Romulus9.3×101413573.650.006

    References

    1. 1 2 3 Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-05.
    2. ↑ Data sheet compiled by W. R. Johnston
    3. 1 2 3 4 5 F. Marchis; et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature. 436 (7052): 822–4. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362.
    4. 1 2 M. Kaasalainen; et al. (2002). "Models of Twenty Asteroids from Photometric Data" (PDF). Icarus. 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
    5. ↑ PDS lightcurve data Archived 2010-01-17 at WebCite
    6. ↑ Supplemental IRAS Minor Planet Survey Archived 2010-01-17 at WebCite
    7. ↑ PDS spectral class data Archived 2010-01-17 at WebCite
    8. ↑ Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)
    9. ↑ Fang, Julia. "Orbits, Masses, and Evolution of Main Belt Triple (87) Sylvia". Astronomical Journal. arXiv:1206.5755. Bibcode:2012AJ....144...70F. doi:10.1088/0004-6256/144/2/70.
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