90 Antiope

Antiope (minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 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]

Discovery 
Discovered byRobert Luther
Discovery dateOctober 1, 1866
Designations
(90) Antiope
Pronunciation/ænˈtaɪəpiː/[1]
1952 BK2[2]
Main belt[2]
(Themis family)
AdjectivesAntiopean
Orbital characteristics[2]
Epoch July 23, 2010
(JD 2455400.5)
Aphelion545.94 Gm
3.6494 AU
Perihelion398.02 Gm
2.6606 AU
471.19 Gm
3.1550 AU
Eccentricity0.15670
2046.9 d (5.60 yr)
16.66 km/s
304.12°
Inclination2.2195°
70.21°
242.96°
Physical characteristics
Dimensions93.0×87.0×83.6 km[3]
87.8 ± 1.0 km[3]
Mass8.3×1017 kg
(whole system)[4]
~ 4.1−4.2 ×1017 kg (components)
Mean density
1.25 ± 0.05 g/cm3 (each)[5]
0.687 d (16.50 h)[6] (synchronous)
0.060[7]
C[8]
8.27 (together)[2]
9.02 (each component)
    S/2000 (90) 1
    Discovery[9]
    Discovered byW. J. Merline, L. M. Close,
    J. C. Shelton, C. Dumas,
    F. Menard, C. R. Chapman,
    and D. C. Slater
    Discovery dateAugust 10, 2000[10]
    Designations
    Main belt (Themis family)
    Orbital characteristics[4]
    171 ± 1 km
    Eccentricity<0.006
    0.687713 ± 0.00004 d (16.5051 ± 0.0001 h)
    18.0 m/s
    Satellite ofBinary with 90 Antiope
    Physical characteristics
    Dimensions89.4×82.8×79.6 km[3]
    Mean diameter
    		83.8 ± 1.0 km[3]
    Mass~ 8.1−8.5 ×1017 kg[5]
    Equatorial escape velocity
    		variable; ~ 35−40 m/s
    0.687 d (16.50 h)[6] (synchronous)
    9.02

      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, 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.

      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%).

      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 July 19, 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]

      90 Antiope occulted the star LQ Aquarii on July 19, 2011, in western USA. 46 stations observed a positive, 11 stations observed a miss and others were clouded out. Many stations were pre-pointed and left unattended.

      References
      1. Noah Webster (1884) A Practical Dictionary of the English Language
      2. "JPL Small-Body Database Browser: 90 Antiope" (2010-06-13 last obs). Archived from the original on 21 December 2015. Retrieved 8 July 2010.
      3. Wm. Robert Johnston (23 November 2008). "(90) Antiope and S/2000 (90) 1". Johnston's Archive. Retrieved 23 January 2008.
      4. 90 Antiope A & B Archived 2008-08-28 at the Wayback Machine, online data sheet, F. Marchis
      5. Descamps et al., 2007, Icarus article published in April 2007
      6. "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.
      7. Supplemental IRAS Minor Planet Survey Archived 2009-08-17 at the Wayback Machine
      8. PDS spectral class data Archived 2009-08-05 at the Wayback Machine
      9. IAUC 7503
      10. "90 Antiope: Raw Keck Image". SWrI Press Release. August 2000. Retrieved 20 October 2009.
      11. Moore, Patrick; Rees, Robin, eds. (2011), Patrick Moore's Data Book of Astronomy (2nd ed.), Cambridge University Press, p. 165, ISBN 9781139495226.
      12. 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.
      13. Florczak, M.; et al. (February 1999). "A spectroscopic study of the THEMIS family". Astronomy and Astrophysics Supplement. 134: 463–471. Bibcode:1999A&AS..134..463F. doi:10.1051/aas:1999150.
      14. 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.
      15. 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.
      16. 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.
      17. "Asteroid Data Sets". sbn.psi.edu. Retrieved 28 April 2018.
      18. Antiope Occultation Yields Double Bonanza Archived 12 November 2013 at the Wayback Machine. Sky & Telescope
      19. Franck Marchis (21 July 2011). "An Occultation by the double asteroid (90) Antiope seen in California". NASA blog (Cosmic Diary). Retrieved 28 January 2012.

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