90 Antiope facts for kids

90 Antiope
	Antiope Douplet by VLT
Discovery and designation [1]
Discovered by	Robert Luther
Discovery date	October 1, 1866
Names
Other names	1952 BK2 [2]
Category	Main belt; (Themis family)
Orbit [3]
	Reference date August 18, 2005; (JD 2453600.5)
Longest distance from the Sun	545.753 Gm; 3.648 AU
Shortest distance from the Sun	398.502 Gm; 2.664 AU
Longest distance from the center of its orbital path; ("semi-major axis")	472.128 Gm; 3.156 AU
How long it takes to complete an orbit	2047.856 d (5.61 a)
Average speed	16.66 km/s
Mean anomaly	348.378°
Angle above the reference plane; ("inclination")	2.220°
Size and other qualities
Measurements	86±1 km; (each component)
Mass	8.3×1017 kg; (whole system); ~ 4.1−4.2 ×1017 kg (components)
Average density	1.25 ± 0.05 g/cm³ (each)
Surface gravity	variable; ~ 0.03−0.04 m/s²
Escape velocity	variable; ~ 35−40 m/s
Rotation period	0.687 d (16.50 h).
How much light it reflects	0.060
Spectral type	C
True brightness; ("absolute magnitude")	8.27 (together); 9.02 (each component)

S/2000 (90) 1
Discovery and designation
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
Names
Category	Main belt (Themis family)
Orbit
Longest distance from the center of its orbital path; ("semi-major axis")	171 ± 1 km
How long it takes to complete an orbit	0.687713 ± 0.00004 d (16.5051 ± 0.0001 h)
Average speed	18.0 m/s
What it orbits	Binary with 90 Antiope
Size and other qualities
Measurements	86±1 km
Mass	~ 8.1−8.5 ×1017 kg
Escape velocity	variable; ~ 35−40 m/s
Rotation period	0.687 d (16.50 h)
True brightness; ("absolute magnitude")	9.02

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.