Sylvia (

minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a leading number (catalog or ...

: 87 Sylvia) is the one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of

Cybele group The Cybele asteroids (also known as the "Cybeles") are a dynamical group of asteroids, named after the asteroid 65 Cybele. Considered by some as the last outpost of an extended asteroid belt, the group consists of just over 2000 members an ...

located beyond the

main asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

(see minor-planet groups). Sylvia was the first asteroid known to possess more than one

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

Discovery and naming

Sylvia was discovered by

N. R. Pogson Norman Robert Pogson, CIE (23 March 1829 – 23 June 1891) was an English astronomer who worked in India at the Madras observatory. He discovered several minor planets and made observations on comets. He introduced a mathematical scale of ste ...

on May 16, 1866, from

Madras (Chennai) Chennai (, ), formerly known as Madras ( the official name until 1996), is the capital city of Tamil Nadu, the southernmost Indian state. The largest city of the state in area and population, Chennai is located on the Coromandel Coast of th ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

.Pogson, N. R. (1866),
Minor Planet (87) Sylvia
',

Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...

, Vol. 26, p. 311 (June 1866)

Antonio Paluzie-Borrell Antonio is a masculine given name of Etruscan origin deriving from the root name Antonius. It is a common name among Romance language-speaking populations as well as the Balkans and Lusophone Africa. It has been among the top 400 most popular male ...

, writing in

Paul Herget Paul Herget (January 30, 1908 – August 27, 1981) was an American astronomer and director of the Cincinnati Observatory, who established the Minor Planet Center after World War II. Career Herget taught astronomy at the University of Cincinna ...

's ''The Names of the Minor Planets'' (1955), mistakenly states that the name honors Sylvie Petiaux-Hugo Flammarion, the first wife of astronomer

Camille Flammarion Nicolas Camille Flammarion FRAS (; 26 February 1842 – 3 June 1925) was a French astronomer and author. He was a prolific author of more than fifty titles, including popular science works about astronomy, several notable early science fiction ...

. In fact, in the article announcing the discovery of the asteroid, Pogson explained that he selected the name in reference to

Rhea Silvia Rhea (or Rea) Silvia (), also known as Ilia (as well as other names) was the mythical mother of the twins Romulus and Remus, who founded the city of Rome. Her story is told in the first book of ''Ab Urbe Condita Libri'' of Livy and in Cassius D ...

, mother of

Romulus and Remus In Roman mythology, Romulus and Remus (, ) are twin brothers whose story tells of the events that led to the founding of the city of Rome and the Roman Kingdom by Romulus, following his fratricide of Remus. The image of a she-wolf suckling the ...

(''MNRAS'', 1866).

Physical characteristics

Sylvia is very dark in color and probably has a primitive composition, though with some internal differentiation. The discovery of its moons made possible an accurate measurement of the asteroid's mass, density and

mass distribution In physics and mechanics, mass distribution is the spatial distribution of mass within a solid body. In principle, it is relevant also for gases or liquids, but on Earth their mass distribution is almost homogeneous. Astronomy In astronomy mass d ...

. Its density is low (around 1.4 times the density of water), indicating that the asteroid is porous; best-fit models estimate it had an original composition by volume of 35% rock, 13% ice and 52% internal voids, and that today it consists of a pristine anhydrous outer layer, and a differentiated interior, with meltwater having percolated inward so that the porosity of the rock is filled with ice out to a radius of about 46 km, then ice-free porous rock out to about 104 km. Sylvia is a fairly fast rotator, turning about its axis every 5.2 hours, giving it an equatorial rotation velocity of about 65 m/s, almost half the escape velocity. Sylvia's shape is flattened and elongated (a/b ≈ 1.45 ; a/c ≈ 1.84) and somewhat irregular. However, its surface has not been imaged well enough for individual features to be resolved.

Satellite system

Sylvia has two orbiting satellites. They have been named (87) Sylvia I Romulus and

(87) Sylvia II Remus Remus is the inner and smaller moon of the main-belt asteroid 87 Sylvia. It follows an almost-circular and close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other Sylvian moon Romulus. Remus was discove ...

, after

, the children of the mythological

Romulus Romulus () was the legendary foundation of Rome, founder and King of Rome, first king of Ancient Rome, Rome. Various traditions attribute the establishment of many of Rome's oldest legal, political, religious, and social institutions to Romulus ...

, the first moon, was discovered on February 18, 2001, from the Keck II telescope by

Michael E. Brown Michael E. Brown (born June 5, 1965) is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), including ...

and

Jean-Luc Margot Jean-Luc Margot (born 1969) is a Belgian-born astronomer and a UCLA professor who specializes in planetary sciences. Career Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries include ( ...

. Remus, the second moon, was discovered over three years later on August 9, 2004, by

Franck Marchis Franck Marchis (born April 6, 1973 in Caen, France), astronomer and planetary scientist, is best known for his discovery and characterization of multiple asteroids, his study of Io volcanism and imaging of exoplanets, planets around other stars ...

UC Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public university, public land-grant university, land-grant research university in Berkeley, California. Established in 1868 as the University of Californi ...

, 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.Errors were published as 3 sigma. To maintain consistency with the table for Sylvia, they have here been reduced to 1 sigma. The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned. Diameters are estimates based on the assumption that the moons have the same albedo as their primary.

References

External links

* Pogson, N. R. (1866),
Minor Planet (87) Sylvia
', Monthly Notices of the Royal Astronomical Society, Vol. 26, p. 311 (June 1866)

(maintained by W. R. Johnston) *
Rubble-Pile Minor Planet Sylvia and Her Twins
' (ESO news release, August 2005) Includes images and artists impressions *

' (SpaceDaily.com, March 2001) Includes a discovery image.

reporting discovery of S/2001 (87) 1

confirming the discovery

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 arXiv (pronounced "archive"—the X represents the Greek letter chi ⟨χ⟩) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not peer review. It consists of ...

:1112.5363 / 22 December 2011)
Orbits, masses, and evolution of main belt triple (87) Sylvia
(

: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) * * {{DEFAULTSORT:Sylvia 000087 000087 Discoveries by N. R. Pogson Named minor planets 000087 000087 000087 18660516