87 Sylvia
Encyclopedia
87 Sylvia is one of the largest main-belt asteroid
s. 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 asteroid known to possess more than one moon
.
, India
. Paul Erget, 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
.
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, depending on the details of its composition. However, the mineralogy of the X-type asteroid
s 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. Direct images 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.
Sylvia is orbited by two small moons
. They have been named Romulus and Remus
((87) Sylvia I Romulus
and (87) Sylvia II 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 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. 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°.
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s. It is a member of the Cybele group
Cybele asteroid
Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele...
located beyond the core of the belt (see minor-planet groups). Sylvia is remarkable for being the first asteroid known to possess more than one moon
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
.
Discovery and naming
Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai)Chennai
Chennai , formerly known as Madras or Madarasapatinam , is the capital city of the Indian state of Tamil Nadu, located on the Coromandel Coast off the Bay of Bengal. Chennai is the fourth most populous metropolitan area and the sixth most populous city in India...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. Paul Erget, in his The Names of the Minor Planets (1955), attributes the name as honouring the first wife of astronomer Camille Flammarion
Camille Flammarion
Nicolas Camille Flammarion 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 novels, and several works about Spiritism and related topics. He also published the magazine...
, 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
Rhea Silvia
Rhea Silvia , and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome...
, mother of Romulus and Remus
Romulus and Remus
Romulus and Remus are Rome's twin founders in its traditional foundation myth, although the former is sometimes said to be the sole founder...
.
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 massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
. 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, depending on the details of its composition. However, the mineralogy of the X-type asteroid
X-type asteroid
The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.-Tholen classification:In the Tholen classification the X-group contains the types:* E-type* M-type, the largest grouping* P-type...
s is not known well enough to constrain this further. Either way, this suggests a loose rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...
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. Direct images indicate that Sylvia's pole points towards ecliptic coordinates
Ecliptic coordinate system
The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the celestial sphere over the course of a year. It is also the intersection of the Earth's orbital plane and the celestial...
(β, λ) = (+62.6°, 72.4°) with only a 0.5° uncertainty, which gives it an axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
of around 29.1°. Sylvia's shape is strongly elongated.
Satellite system
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
. They have been named Romulus and Remus
Romulus and Remus
Romulus and Remus are Rome's twin founders in its traditional foundation myth, although the former is sometimes said to be the sole founder...
((87) Sylvia I Romulus
Romulus (moon)
Romulus is the outer and larger moon of the main-belt asteroid 87 Sylvia, not to be confused with the directly Sun-orbiting asteroid 10386 Romulus. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Remus.Romulus was...
and (87) Sylvia II Remus
Remus (moon)
Remus is the inner and smaller moon of the main-belt asteroid 87 Sylvia. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Romulus....
), after the children of the mythological Rhea Silvia
Rhea Silvia
Rhea Silvia , and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome...
.
Romulus
Romulus (moon)
Romulus is the outer and larger moon of the main-belt asteroid 87 Sylvia, not to be confused with the directly Sun-orbiting asteroid 10386 Romulus. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Remus.Romulus was...
, the first moon, was discovered on February 18, 2001 from the Keck II telescope by Michael E. Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....
and Jean-Luc Margot
Jean-Luc Margot
Jean-Luc Margot is a Belgian-born astronomer and a Professor at UCLA. He specializes in planetary sciences. He was awarded the H. C. Urey Prize by the American Astronomical Society in 2004....
. 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
Remus (moon)
Remus is the inner and smaller moon of the main-belt asteroid 87 Sylvia. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Romulus....
, the second moon, was discovered over three years later on August 9, 2004 by Franck Marchis
Franck Marchis
Franck Marchis , astronomer and planetary scientist, is best known for his discovery and characterization of multiple asteroids and its study of Io volcanism.The asteroid 6639 Marchis was named in his honor on April 4, 2007....
of UC Berkeley, and 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. 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°.
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)