90 Antiope

90 Antiope ([æn ˈtaɪ o pe]) 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 the namesake 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 carbonate composition. The low density (1.25±0.05 g/cm3)[reference] of its components (see below) suggests a high 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.

Adaptive optics image of Antiope.

[edit] Double asteroid

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 110±16 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.

There is some uncertainty regarding the orbital/rotation periods, and whether the rotation of the components is synchronous or not. Of the most recent analyses, one set of adaptive optics observations over two months in 2004 gave an orbital period of 16.527 ± 0.001 h^[3][5]. However, an analysis of several years of lightcurve observations suggests that the orbital and rotation periods are slightly mismatched (orbital: 16.5051±0.0002 h, rotational: 16.5047±0.0002 h)^[4], also significantly disagreeing with the 16.527±0.001 value.