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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	10 August 2000
Designations
Minor planet category	Main belt (Themis family)
Orbital characteristics
Semi-major axis	171 ± 1 km
Eccentricity	<0.006
Orbital period (sidereal)	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	89.4×82.8×79.6 km
Mean diameter	83.8 ± 1.0 km
Mass	~ 8.1−8.5 ×1017 kg
Equatorial escape velocity	variable; ~ 35−40 m/s
Synodic rotation period	0.687 d (16.50 h) (synchronous)
Absolute magnitude (H)	9.02

Discovery [1]
Discovered by	Robert Luther
Discovery date	1 October 1866
Designations
MPC designation	(90) Antiope
Pronunciation	/ænˈtaɪəpiː/
Alternative designations	1952 BK2
Minor planet category	Main belt; (Themis family)
Adjectives	Antiopean
Orbital characteristics
	Epoch 23 July 2010; (JD 2455400.5)
Aphelion	545.94 Gm; 3.6494 AU
Perihelion	398.02 Gm; 2.6606 AU
Semi-major axis	471.19 Gm; 3.1550 AU
Eccentricity	0.15670
Orbital period (sidereal)	2046.9 d (5.60 yr)
Average orbital speed	16.66 km/s
Mean anomaly	304.12°
Inclination	2.2195°
Longitude of ascending node	70.21°
Argument of perihelion	242.96°
Physical characteristics
Dimensions	93.0×87.0×83.6 km; 87.8 ± 1.0 km
Mass	8.3×1017 kg; (whole system); ~ 4.1−4.2 ×1017 kg (components)
Mean density	1.25 ± 0.05 g/cm3 (each)
Synodic rotation period	0.687 d (16.50 h) (synchronous)
Geometric albedo	0.060
Spectral type	C
Absolute magnitude (H)	8.27 (together); 9.02 (each component)

90 Antiope is a double asteroid in the outer asteroid belt. It was discovered on 1 October 1866, by Robert Luther. 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 asteroids. Antiope is a member of the Themis family of asteroids that share similar orbital elements.^[11]

Naming

The asteroid's proper name comes from Greek mythology, 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 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%^[12]), 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.^[9] Before this, IRAS observations had suggested that the asteroid was 120 km in diameter.^[2]

Orbital

Antiope orbits in the outer third of the core region of the asteroid belt, and is a member of the Themis family.^[13]

Since each component is about 86±1 km across, with their centers separated by only about 171 kilometers,^[4] 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 which lies in the space between them. The orbital period is approximately 16.50 hours, and the eccentricity below 0.006.^[4] Every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth.^[6] Using Kepler's third law^{[broken anchor]}, 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.

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 composition. The low density (1.3±0.2 g/cm³) of its components (see below) suggests a significant porosity (>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.^{[citation needed]}

Complementary observations using adaptive optic 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%).^{[citation needed]}

Observations from the VLT-UT4 telescope equipped with an adaptive optics system in 2007 and lightcurve 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 that broke proto-Antiope into two equisized bodies.^[14] The impactor is calculated to have been more than 17 km in diameter.^[15] 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.^[16]

Occultations

There have been 9 occultations observed since 1988,^[17] many of which are multichord occultations.

The best is the 19 July 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. Many planned stations were unfortunately clouded. Many stations were so-called Mighty-Mini or Mighty-Maxi, consisting of a binocular objective (homemade using binoculars + hacksaw + plumbing fittings) with 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.^[18]^[19]

The 2011 occultation as seen in this video from one of the stations lasted about 20 seconds.

References

^ Noah Webster (1884) A Practical Dictionary of the English Language
^ ^a ^b ^c ^d ^e "JPL Small-Body Database Browser: 90 Antiope" (2010-06-13 last obs). Archived from the original on 21 December 2015. Retrieved 8 July 2010.
^ ^a ^b ^c ^d Wm. Robert Johnston (23 November 2008). "(90) Antiope and S/2000 (90) 1". Johnston's Archive. Retrieved 23 January 2008.
^ ^a ^b ^c ^d 90 Antiope A & B Archived 2008-08-28 at the Wayback Machine, online data sheet, F. Marchis
^ ^a ^b ^c ^d Descamps et al., 2007, Icarus article published in April 2007
^ ^a ^b ^c "T. Michałowski, et al. (2004). "Eclipsing binary asteroid 90 Antiope". Astronomy & Astrophysics. 423 (3): 1159. Bibcode:2004A&A...423.1159M. doi:10.1051/0004-6361:20040449.
^ Supplemental IRAS Minor Planet Survey Archived 2009-08-17 at the Wayback Machine
^ PDS spectral class data Archived 2009-08-05 at the Wayback Machine
^ ^a ^b IAUC 7503
^ "90 Antiope: Raw Keck Image". SWrI Press Release. August 2000. Retrieved 20 October 2009.
^ Moore, Patrick; Rees, Robin, eds. (2011), Patrick Moore's Data Book of Astronomy (2nd ed.), Cambridge University Press, p. 165, ISBN 9781139495226.
^ F. Marchis; F. Descamps; P. Hestroffer; Berthier, J. & I. de Pater (2004). "Fine Analysis of 121 Hermione, 45 Eugenia, and 90 Antiope Binary Asteroid Systems With AO Observations". Bulletin of the American Astronomical Society. 36: 1180. Bibcode:2004DPS....36.4602M.
^ Florczak, M.; et al. (February 1999). "A spectroscopic study of the THEMIS family". Astronomy and Astrophysics Supplement. 134 (3): 463–471. Bibcode:1999A&AS..134..463F. doi:10.1051/aas:1999150.
^ Marchis, Franck; Enriquez, J. E.; Emery, J. P.; Berthier, J.; Descamps, P. (2009). The Origin of the Double Main Belt Asteroid (90) Antiope by Component-Resolved Spectroscopy. DPS meeting #41. American Astronomical Society. Bibcode:2009DPS....41.5610M.
^ Descamps, P.; Marchis; Michalowski; Berthier; Pollock; Wiggins; Birlan; Colas; et al. (2009). "A giant crater on 90 Antiope?". Icarus. 203 (1): 102–111. arXiv:0905.0631. Bibcode:2009Icar..203..102D. doi:10.1016/j.icarus.2009.04.022. S2CID 119300811.
^ Marchis, F.; Enriquez, J.E.; Emery, J.P.; Berthier, J.; Descamps, P.; Vachier, F. (2011). "The origin of (90) Antiope from component-resolved near-infrared spectroscopy". Icarus. 213 (1): 252–264. arXiv:1102.3458. Bibcode:2011Icar..213..252M. doi:10.1016/j.icarus.2011.02.011. S2CID 119279769.
^ "Asteroid Data Sets". sbn.psi.edu. Retrieved 28 April 2018.
^ Antiope Occultation Yields Double Bonanza Archived 12 November 2013 at the Wayback Machine. Sky & Telescope
^ Franck Marchis (21 July 2011). "An Occultation by the double asteroid (90) Antiope seen in California". NASA blog (Cosmic Diary). Retrieved 28 January 2012.