90 Antiope

90 Antiope is an asteroid found on October 1, 1866 by Robert Luther. The 90th asteroid to be found, 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 farther 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 that it is made of carbonaceous chondrite.

One seen stellar occultation by Antiope has been reported, on June 11, 1980.

Double asteroid

The most remarkable feature of Antiope is that it has two asteroids of almost equal size (the difference in mass is less than 2.5%), making it a truly "double" asteroid. Its binary nature was found on 10 August, 2000 by a group of astronomers using adaptive optics at the Keck Telescope on Mauna Kea. The "secondary" is designated S/2000 (90) 1.

Each asteroid is about 86±1 km across, with their centers separated by only about 170 kilometers. This means that the space separating the two halves is only 60 km, or so. The two bodies orbit around the same center of mass which lies in the space between them. The orbital period is about 16.50 hours, the eccentricity below 0.03 (best estimate 0.01 ± 0.02). Every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth. Using Kepler's third law, the mass and density of the asteroids can be derived from the orbital period and asteroid sizes.

The axis of the mutual orbit of the two asteroids points towards ecliptic coordinates (β, λ) = (200°, 38°) with 2 degrees uncertainty. This is tilted about 63° to the circumsolar orbit of the system.

S/2000 (90) 1

Discovery
Discovered by	W. J. Merline, L. M. Close, J. C. Shelton, C. Dumas, F. Menard, C. R. Chapman, and D. C. Slater
Discovery date	August 10, 2000
Designations
Minor planet category	Main belt (Themis family)
Orbital characteristics
Semi-major axis	171 ± 1 km
Eccentricity	0.01 ± 0.02
Orbital period	0.687713 ± 0.00004 d (16.5051 ± 0.0001 h)
Average orbital speed	18.0 m/s
Satellite of	Binary with 90 Antiope
Physical characteristics
Dimensions	86±1 km
Mass	~ 8.1−8.5 ×1017 kg
Equatorial escape velocity	variable; ~ 35−40 m/s
Rotation period	0.687 d (16.50 h)
Absolute magnitude (H)	9.02

90 Antiope

Antiope Douplet by VLT
Discovery [1]
Discovered by	Robert Luther
Discovery date	October 1, 1866
Designations
Alternative designations	1952 BK2 [2]
Minor planet category	Main belt  (Themis family)
Orbital characteristics [3]
Epoch August 18, 2005 (JD 2453600.5)
Aphelion	545.753 Gm 3.648 AU
Perihelion	398.502 Gm 2.664 AU
Semi-major axis	472.128 Gm 3.156 AU
Eccentricity	0.156
Orbital period	2047.856 d (5.61 a)
Average orbital speed	16.66 km/s
Mean anomaly	348.378°
Inclination	2.220°
Longitude of ascending node	70.235°
Argument of perihelion	242.480°
Physical characteristics
Dimensions	86±1 km (each component)
Mass	8.3×1017 kg (whole system) ~ 4.1−4.2 ×1017 kg (components)
Mean density	1.25 ± 0.05 g/cm³ (each)
Equatorial surface gravity	variable; ~ 0.03−0.04 m/s²
Equatorial escape velocity	variable; ~ 35−40 m/s
Rotation period	0.687 d (16.50 h).
Geometric albedo	0.060
Surface temp. min mean max Kelvin ~158 K 244 K Celsius -115 C -29°
Spectral type	C
Absolute magnitude (H)	8.27 (together) 9.02 (each component)

Other websites

• Discovery of Companions to Asteroids 762 Pulcova and 90 Antiope SWrI Press Release.
• Data on (90) Antiope from Johnston's archive (maintained by W. R. Johnston)
• online data on the Antiope system maintained by F. Marchis; includes images and simulated occultation movies.
• ESO Press-Release published on May 29, 2007 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 weblog, E. Lakdawalla, 11 Apr, 2007.