87 Sylvia
 | |
| Discovery | |
|---|---|
| Discovered by | Norman Robert Pogson |
| Discovery date | May 16, 1866 |
| Designations | |
| Pronunciation | /ˈsɪlviə/ SIL-vee-ə |
| Alternative names | A909 GA |
| Minor planet category | Outer main belt (Cybele) |
| Orbital characteristics | |
| Epoch July 14, 2004 (JD 2453200.5) | |
| Aphelion | 3.768 AU (563.679 Gm) |
| Perihelion | 3.213 AU (480.594 Gm) |
| Semi-major axis | 3.490 AU (522.137 Gm) |
| Eccentricity | 0.080 |
| Orbital period | 6.52 a (2381.697 d) |
| Average orbital speed | 15.94 km/s |
| Mean anomaly | 352.763° |
| Inclination | 10.855° |
| Longitude of ascending node | 73.342° |
| Argument of perihelion | 266.195° |
| Known satellites | 2 |
| Physical characteristics | |
| Dimensions |
384×262×232 km[1][2] 286 km (mean) 385×265×230 ± 10 km[3] |
| 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 |
| Escape velocity | 0.1379 km/s |
| Rotation period | 0.2160 d (5.183642 h) [4][5] |
| Albedo | 0.0435 [6] |
| Temperature |
~151 K max: 223 K (-52°C) |
| Spectral type | X [7] |
| Absolute magnitude (H) | 6.94 |
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87 Sylvia 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 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.
| Name | Mass [kg] | Semi-major axis [km] | Orbital period [days] | Eccentricity |
|---|---|---|---|---|
| Remus | 7.3×1014 | 706.5 | 1.37 | 0.027 |
| Romulus | 9.3×1014 | 1357 | 3.65 | 0.006 |
References
- ↑ 1.0 1.1 1.2 Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-05.
- ↑ Data sheet compiled by W. R. Johnston
- ↑ 3.0 3.1 3.2 3.3 3.4 F. Marchis et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature 436 (7052): 822. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362.
- ↑ 4.0 4.1 M. Kaasalainen et al. (2002). "Models of Twenty Asteroids from Photometric Data". Icarus 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
- ↑ PDS lightcurve data
- ↑ Supplemental IRAS Minor Planet Survey
- ↑ PDS spectral class data
- ↑ Pogson, N. R. (1866), Minor Planet (87) Sylvia, Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)
- ↑ Fang, Julia. "Orbits, Masses, and Evolution of Main Belt Triple (87) Sylvia". Astronomical Journal. Retrieved 8 November 2012.
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 (on Vimeo)
- Shape model derived from lightcurve (on page 19)
- Instability zones for satellites of asteroids. The example of the (87) Sylvia system (arXiv:1112.5363 / 22 December 2011)
- Orbits, masses, and evolution of main belt triple (87) Sylvia (arXiv:1206.5755 / 25 Jun 2012)
- Occultation of TYC 1856-00745-1 by (87) Sylvia and by its satellite Romulus (E. Frappa, A. Klotz, P. Dubreuil)
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