87 Sylvia (sil'-vee-a) is one of the largest main belt asteroids. It orbits beyond most of the main belt asteroids, so it is classed as one of the Cybeles (see Minor planet groups). Sylvia is remarkable for being the first known asteroid to possess more than one moon. Norman Robert Pogson (March 23, 1829 – June 23, 1891) was a British astronomer. ... May 16 is the 136th day of the year in the Gregorian Calendar (137th in leap years). ... 1866 is a common year starting on Monday. ... The provisional designation of comets and asteroids are similar to each other: they both follow a pattern set in 1925 by the Minor Planet Center of the IAU. Historical designations At first, astronomers strove to assign symbols to the minor planets: 1 Ceres a stylized sickle 2 Pallas a lozenge... Minor planets, or planetoids are minor bodies of the solar system orbiting the sun that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a diameter of about 4880 km). ... The asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found. ... 65 Cybele (sib-a-lee) is one of the largest asteroids in the main belt. ... In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... July 14 is the 195th day (196th in leap years) of the year in the Gregorian Calendar, with 170 days remaining. ... 2004 is a leap year starting on Thursday of the Gregorian calendar. ... The Julian day or Julian day number (JDN) is the number of SI days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC in the proleptic Julian calendar 1. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... Giga (symbol: G) is a prefix in the SI system of units denoting 109, or 1 000 000 000. ... The metre, or meter, is the basic unit of length in the International System of Units (SI: Système International dUnités). ... The astronomical unit (AU or au or a. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... A day is any of several different units of time. ... A Julian year is the length of an average year in the Julian calendar, 365. ... The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body. ... The second (symbol s) is a unit for time, and one of seven SI base units. ... Inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planets equator or the ecliptic), stated in degrees. ... A degree (or in full a degree of arc), usually symbolized °, is a measurement of plane angle, representing 1/360 of a full rotation. ... The Longitude of the ascending node () is one of the orbital elements used to specify the orbit of an object in space. ... The argument of the perihelion is one of the orbital elements describing the orbit of a planet. ... In the study of orbital dynamics the mean anomaly is a measure of time, specific to the orbiting body p, which is a multiple of 2π radians at and only at periapsis. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... A cubic centimetre (cm3) is an SI derived unit of volume, equal to the volume of a cube with side length of 1 centi metre. ... It has been suggested that Law of universal gravitation be merged into this article or section. ... In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a... In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standardized distance away. ... The albedo is a measure of reflectivity of a surface or body. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The title of this article is incorrect due to technical limitations. ... The asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... 65 Cybele (sib-a-lee) is one of the largest asteroids in the main belt. ... Minor planets, or planetoids are minor bodies of the solar system orbiting the sun that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a diameter of about 4880 km). ... 243 Ida and its moon Dactyl An asteroid moon is an asteroid that orbits another asteroid. ...

Discovery and naming

Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai), India. 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), but in the article (MNRAS, 1866) announcing the discovery of this asteroid, Pogson explained that he selected the name in reference to Rhea Silvia, mother of Romulus and Remus. Norman Robert Pogson (March 23, 1829 – June 23, 1891) was a British astronomer. ... May 16 is the 136th day of the year in the Gregorian Calendar (137th in leap years). ... 1866 is a common year starting on Monday. ... Chennai (சென்னை in Tamil), formerly known as Madras, is the capital of the state of Tamil Nadu and is Indias fourth largest city. ... 1955 is a common year starting on Saturday of the Gregorian calendar. ... Camille Flammarion (February 26, 1842 – June 3, 1925) was a French astronomer. ... 1866 is a common year starting on Monday. ... Rhea Sylvia (also written as Rea Silvia), and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome. ... Romulus and Remus, (771 BC¹-717 BC Romulus, 771 BC-753 BC Remus), the traditional founders of Rome, appeared in Roman mythology as the twin sons of the priestess Rhea Silvia, fathered by the god of war Mars. ...

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 (in the 1.2 to 1.6 range), indicating that the asteroid is probably very porous; as much as 60% of it may be empty space. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation velocity of about 160 km/h or 98 mph). Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ...

Moons

Sylvia is orbited by two small moons. They have been named Romulus and Remus, after the children of the mythological Rhea Silvia. 243 Ida and its moon Dactyl An asteroid moon is an asteroid that orbits another asteroid. ...

Romulus, the first moon, was discovered on February 18, 2001 from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. Its full designation is (87) Sylvia I Romulus; before receiving its name, it was known as S/2001 (87) 1. It is 18±4 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. It should not be confused with the asteroid 10386 Romulus. February 18 is the 49th day of the year in the Gregorian Calendar. ... 2001: A Space Odyssey. ... The Mauna Kea Observatory, an institute of the University of Hawaii, is considered one of the most important land-based observatories in the world for its isolated, unobstructed views of space without interference from man-made light sources. ... Dr. Michael (Mike) E. Brown has been an associate professor of planetary astronomy at the California Institute of Technology (Caltech) since 2002. ... Jean-Luc Margot is a Belgian astronomer and an Assistant Professor at Cornell University. ... A kilometre (American spelling: kilometer, symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ...

Remus, the second moon, was discovered on images taken starting on August 9, 2004 and announced on August 10, 2005. It was discovered by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France, using the Yepun telescope of the European Southern Observatory (ESO) in Chile. Marchis, the project leader, was waiting for the completion of the image acquisition programme before starting to process the data. Just as he was set to go on vacation in March 2005, Descamps sent him a brief note entitled "87 Sylvia est triple ?" pointing out that he could see two moonlets on several images of Sylvia. The entire team then focused quickly on analysis of the data, wrote a paper, submitted an abstract to the August meeting in Rio de Janeiro and submitted a naming proposal to the IAU. August 9 is the 221st day of the year in the Gregorian Calendar (222nd in leap years), with 144 days remaining. ... 2004 is a leap year starting on Thursday of the Gregorian calendar. ... August 10 is the 222nd day of the year (223rd in leap years) in the Gregorian Calendar. ... 2005 is a common year starting on Saturday of the Gregorian calendar and is the current year. ... The University of California, Berkeley (also known as Cal, UC Berkeley, UCB, or simply Berkeley) is a prestigious, public, coeducational university situated in the foothills of Berkeley, California to the east of San Francisco Bay, overlooking the Golden Gate and its bridge. ... The Paris Observatory (in French, Observatoire de Paris or Observatoire de Paris-Meudon) is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. ... The European Southern Observatory (ESO) is an international astronomical organisation, composed and supported by ten countries from the European Union plus Switzerland and was created in 1962. ...

Remus' full designation is (87) Sylvia II Remus; before receiving its name, it was known as S/2004 (87) 1. 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.

Astronomers believe that these moons were broken off Sylvia by an impact in the past, and that other, smaller moons 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
• Rubble-Pile Minor Planet Sylvia and Her Twins (ESO news release)
• Adaptive Optics System Reveals New Asteroidal Satellite (SpaceDaily.com)
• 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

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