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90 Antiope (pronounced /ænˈtaɪəpi/ an-tye'-ə-pee) is an asteroid discovered on October 1, 1866 by Robert Luther. The 90th asteroid to be discovered, it is named after Antiope from Greek mythology, though it is disputed as to whether this is Antiope the Amazon or Antiope the mother of Amphion and Zethus.
Antiope orbits in the outer third of the core region of the main belt, and is a member of the Themis family of asteroids. Like most bodies in this region, it is of the dark C spectral type, indicating a carbonaceous composition. The low density (1.3±0.2 g/cm³) of its components (see below) suggests a significant porosity (>30%), indicating a rubble pile asteroid composed of debris that accumulated in the aftermath of a previous asteroid collision (possibly the one that formed the Themis family).
One observed stellar occultation by Antiope has been reported, on June 11, 1980.
[edit] Double asteroid
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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%[1]), making it a truly "double" asteroid. Its binary nature was discovered on 10 August 2000 by a group of astronomers using adaptive optics at the Keck Telescope on Mauna Kea.[2] The "secondary" is designated S/2000 (90) 1.
Each component is about 86±1 km across, with their centers separated by only about 170 kilometers. This means that the gap separating the two halves is a mere 60 km, or so. The two bodies orbit around the common center of mass which lies in the space between them. The orbital period is approximately 16.50 hours, the eccentricity below 0.03 (best estimate 0.01 ± 0.02).[3] Every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth.[4] Using Kepler'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 (β, λ) = (200°, 38°)[5] with 2 degrees uncertainty.[5] This is tilted about 63° to the circumsolar orbit of the system.
The complementary observations using adaptive optive observations on 8-10m 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 a 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%).
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