90 Antiope

Lore & Background

The asteroid's proper name comes from Greek mythology, but it is unclear whether it refers to Antiope the Amazon or Antiope the mother of Amphion and Zethus. Since the discovery of its binary nature, the name 'Antiope' technically refers to the slightly larger component, while the smaller is sometimes unofficially designated S/2000 (90) 1; however, 'Antiope' is also used for the binary system as a whole.

Antiope orbits in the outer third of the core region of the asteroid belt and is a member of the Themis family. The two components are separated by about 171 km, with the gap between them roughly equal to the diameter of each component. They orbit a common center of mass with an orbital period of approximately 16.50 hours and an eccentricity below 0.006. The axis of their mutual orbit points towards ecliptic coordinates (β, λ) = (200°, 38°) with 2 degrees uncertainty, tilted about 63° to the system's circumsolar orbit.

Both components are of the dark C spectral type, indicating a carbonaceous composition. Their low density (1.3±0.2 g/cm3) suggests significant porosity (>30%), implying they are rubble-pile asteroids formed from debris after a previous collision, possibly the one that created the Themis family. Observations from the VLT-UT4 telescope in 2007 and lightcurve data suggest one component has a 68 km bowl-shaped impact crater, possibly from a collision that broke proto-Antiope into two equal bodies; the impactor is calculated to have been more than 17 km in diameter.

Reader's Guide

90 Antiope is significant as one of the few known truly double asteroids, where both components are nearly equal in size (mass difference less than 2.5%). Its binary nature was discovered on 10 August 2000 using adaptive optics at the Keck Telescope, revealing a system where the two bodies orbit a common center of mass lying between them. The system's physical parameters—including component shapes, surface scattering, bulk density, and internal properties—have been derived from adaptive optic observations and mutual event photometric lightcurves. The shape model is consistent with slightly non-spherical components in equilibrium figures for homogeneous rotating bodies, and comparisons with grazing occultation events in 2003 suggest the shapes do not depart much from Roche equilibrium figures (by more than 10%). The similar spectra of both components imply a common origin, possibly from the breakup of a larger rubble-pile asteroid, though other formation scenarios cannot be ruled out. The system has been observed in 9 occultations since 1988, with the best event on 19 July 2011 recorded from 57 stations along the western USA coast, which confirmed the presence of the impact crater.

Did You Know?

• The two components have average diameters of about 88 km and 84 km, with a mass difference of less than 2.5%.
• One component may have a 68 km bowl-shaped impact crater from a collision with an impactor more than 17 km in diameter.