90 Antiope

90 Antiope
Adaptive-optics image of Antiope
Discovery 
Discovered by Robert Luther
Discovery date October 1, 1866
Designations
MPC designation (90) Antiope
Pronunciation /ænˈtaɪəpi/ ann-TY-ə-pee
1952 BK2[1]
Main belt[1]
(Themis family)
Orbital characteristics[1]
Epoch July 23, 2010
(JD 2455400.5)
Aphelion 545.94 Gm
3.6494 AU
Perihelion 398.02 Gm
2.6606 AU
471.19 Gm
3.1550 AU
Eccentricity 0.15670
2046.9 d (5.60 yr)
16.66 km/s
304.12°
Inclination 2.2195°
70.21°
242.96°
Physical characteristics
Dimensions 93.0×87.0×83.6 km[2]
87.8 ± 1.0 km[2]
Mass 8.3×1017 kg
(whole system)[3]
~ 4.1−4.2 ×1017 kg (components)
Mean density
 1.25 ± 0.05 g/cm³ (each)[4]
0.687 d (16.50 h)[5] (synchronous)
0.060[6]
C[7]
8.27 (together)[1]
9.02 (each component)
    S/2000 (90) 1
    Discovery[8]
    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[9]
    Designations
    Main belt (Themis family)
    Orbital characteristics[3]
    171 ± 1 km
    Eccentricity <0.006
    0.687713 ± 0.00004 d (16.5051 ± 0.0001 h)
    18.0 m/s
    Satellite of Binary with 90 Antiope
    Physical characteristics
    Dimensions 89.4×82.8×79.6 km[2]
    Mean diameter
    		 83.8 ± 1.0 km[2]
    Mass ~ 8.1−8.5 ×1017 kg[4]
    Equatorial escape velocity
    		 variable; ~ 35−40 m/s
    0.687 d (16.50 h)[5] (synchronous)
    9.02

      90 Antiope (/ænˈtaɪəpi/ ann-TY-ə-pee) 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.[10]

      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%[11]), 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.[8] Before this, IRAS observations had suggested that the asteroid was 120 km in diameter.[1]

      Orbital

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

      Since each component is about 86±1 km across, with their centers separated by only about 171 kilometers,[3] 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.[3] Every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth.[5] 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°)[4] with 2 degrees uncertainty.[4] 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 of proto-Antiope into two equisized bodies.[12] The impactor is calculated to have been more than 17 km in diameter.[13] The crater can not be resolved using the W.M. Keck II telescope.

      Occultations

      There have been 9 occultations observed since 1988,[14] 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. Alas, many planned stations were clouded. Many stations were so-called Mighty-Mini or Mighty-Maxi consisting of a binocular objective (homemade using binoculars + hacksaw + plumbing fittings) as well as 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.[15][16]

      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. 1 2 3 4 5 "JPL Small-Body Database Browser: 90 Antiope" (2010-06-13 last obs). Retrieved 2010-07-08.
      2. 1 2 3 4 Wm. Robert Johnston (2008-11-23). "(90) Antiope and S/2000 (90) 1". Johnston's Archive. Retrieved 2008-01-23.
      3. 1 2 3 4 90 Antiope A & B Archived 2008-08-28 at the Wayback Machine., online data sheet, F. Marchis
      4. 1 2 3 4 Descamps et al., 2007, Icarus article published in April 2007
      5. 1 2 3 "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.
      6. ↑ Supplemental IRAS Minor Planet Survey Archived 2010-01-17 at WebCite
      7. ↑ PDS spectral class data Archived 2010-01-17 at WebCite
      8. 1 2 IAUC 7503
      9. ↑ "90 Antiope: Raw Keck Image". SWrI Press Release. August 2000. Retrieved 2009-10-20.
      10. ↑ Moore, Patrick; Rees, Robin, eds. (2011), Patrick Moore's Data Book of Astronomy (2nd ed.), Cambridge University Press, p. 165.
      11. ↑ 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.
      12. ↑ 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.
      13. ↑ 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.
      14. ↑ "Asteroid Data Sets". sbn.psi.edu. Retrieved 2018-04-28.
      15. ↑ Antiope Occultation Yields Double Bonanza. Sky & Telescope
      16. ↑ Franck Marchis (July 21, 2011). "An Occultation by the double asteroid (90) Antiope seen in California". NASA blog (Cosmic Diary). Retrieved 2012-01-28.

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