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Antiope (
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 ...
: 90 Antiope) is a
double asteroid A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple ast ...
in the outer
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 ...
. It was discovered on October 1, 1866, by
Robert Luther Karl Theodor Robert Luther (April 16, 1822, Świdnica – February 15, 1900 Düsseldorf), normally published as Robert Luther, was a German astronomer. While working at the Bilk Observatory in Düsseldorf, Germany, he searched for asteroids and ...
. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At average diameters of about 88 km and 84 km, both components are among the 500 largest
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s. Antiope is a member of the
Themis family The Themis family (''adj. Themistian''; ) is a Asteroid family, family of carbonaceous asteroids located in the Kirkwood gap, outer portion of the asteroid belt, at a mean distance of 3.13 Astronomical Unit, AU from the Sun. It is one of the Asteroi ...
of asteroids that share similar
orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
.


Naming

The asteroid's proper name comes from
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
, but it is disputed whether this is Antiope the Amazon or Antiope the mother of Amphion and Zethus. Since the discovery of Antiope's binary nature, the name "Antiope" technically refers to the slightly larger of the two components, with the smaller component bearing the
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
S/2000 (90) 1. However, the name "Antiope" is also used to refer to the binary system as a whole.


Properties

The most remarkable feature of Antiope is that it consists of two components of almost equal size (the difference in mass is less than 2.5%), making it a truly "double" asteroid. Its binary nature was discovered on 10 August 2000 by a group of astronomers using
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
at the
Keck Telescope The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state In the United States, a state is a constituent political entit ...
on
Mauna Kea Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii and second-highest peak of an island on Earth. The peak is ...
. Before this,
IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...
observations had suggested that the asteroid was 120 km in diameter.


Orbital

Antiope orbits in the outer third of the core region of the
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 ...
, and is a member of the
Themis family The Themis family (''adj. Themistian''; ) is a Asteroid family, family of carbonaceous asteroids located in the Kirkwood gap, outer portion of the asteroid belt, at a mean distance of 3.13 Astronomical Unit, AU from the Sun. It is one of the Asteroi ...
. Since each component is about 86±1 km across, with their centers separated by only about 171 kilometers, the gap separating the two halves is about the same as the diameter of each component. As a result, the two bodies orbit around the common
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
which lies in the space between them. The orbital period is approximately 16.50 hours, and the eccentricity below 0.006. Every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth. Using
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
's third law, the mass and density of the components can be derived from the orbital period and component sizes. The axis of the mutual orbit of the two components points towards
ecliptic coordinates The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodi ...
(β, λ) = (200°, 38°)Descamps et al., 2007, Icarus article published in April 2007
/ref> with 2 degrees uncertainty. This is tilted about 63° to the circumsolar orbit of the system.


Physical

Antiope itself has an average diameter of about 88 km, while its twin, S/2000 (90) 1, has an average diameter of 84 km. Like most bodies in this region, the components of the Antiope system are of the dark C spectral type, indicating a
carbonaceous Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electrons available to form covalent bond, covalent chemical bonds. It belongs to gro ...
composition. The low
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
() of its components (see below) suggests a significant
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
(>30%), indicating rubble-pile asteroids composed of debris that accumulated in the aftermath of a previous asteroid collision, possibly the one that formed the Themis family. Complementary observations using
adaptive optic Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
observations on 8–10 m class telescopes and mutual events photometric lightcurve over several months have served as input quantities for a derivation of a whole set of other physical parameters (shapes of the components, surface scattering, bulk density, and internal properties). The shape model is consistent with slightly non-spherical components, having a size ratio of 0.95 (with an average radius of 42.9 km), and exhibiting equilibrium figures for homogeneous rotating bodies. A comparison with grazing occultation event lightcurves taken in 2003 suggests that the real shape of the components do not depart much from Roche equilibrium figures (by more than 10%). Observations from the VLT-UT4 telescope equipped with an
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
system in 2007 and
lightcurve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
data analysis suggest that one of the components appears to have a 68 km bowl-shaped impact crater that may be the result of a violent collision of
proto Proto or PROTO may refer to: Language * Proto-, an English prefix meaning "first" Media * ''Proto'' (magazine), an American science magazine * Radio Proto in Cyprus Music * ''Proto'' (Holly Herndon album), 2019 * ''Proto'' (Leo O'Kelly ...
-Antiope into two equisized bodies. The impactor is calculated to have been more than 17 km in diameter. The crater can not be resolved using the W.M. Keck II telescope. The two parts of the Antiope have very similar spectra. This implies they may have a common origin, such as being formed from the breakup of a larger rubble-pile asteroid, but other formation scenarios cannot be ruled out.


Occultations

There have been 9 occultations observed since 1988, many of which are multichord occultations. The best is the July 19, 2011 event observed from 57 stations spread out along the western USA coast where 46 stations recorded positive occultations and 11 stations observed misses. However many of the misses were important to clearly separate the two components of 90 Antiope. Alas, many planned stations were clouded. Many stations were so-called Mighty-Mini or Mighty-Maxi consisting of a binocular objective (homemade using binoculars + hacksaw + plumbing fittings) as well as a video camera and Video Time Inserter (VTI), and were pre-pointed and left to run unattended, thereby allowing one observer to deploy many stations. The crater mentioned above was confirmed by this occultation.


References


External links


''Discovery of Companions to Asteroids 762 Pulcova and 90 Antiope''
SWrI Press Release.

datasheet, johnstonsarchive.net

Robert Johnston, johnstonsarchive.net
online data on the Antiope system
maintained by F. Marchis; includes images and simulated occultation movies.

The Impossible Siblings
UC-Berkeley Press-Release published on May 29, 2007
Binary asteroid revealed as twin rubble piles
Antiope, a true binary asteroid
The
Planetary Society The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, and ...
weblog, E. Lakdawalla, 11 April 2007.
An Occultation by the double asteroid (90) Antiope seen in California
(Franck Marchis) * * * {{DEFAULTSORT:000090 Themis asteroids Beagle asteroids Antiope Antiope Binary asteroids Articles containing video clips C-type asteroids (Tholen) C-type asteroids (SMASS) Objects observed by stellar occultation 18661001