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Space Topics: Asteroids and CometsBinary Main Belt Asteroid 90 Antiope
Asteroid 90 Antiope is a truly binary body, with two components almost equal in size that spin together in a very tight circular orbit. Detailed study has shown that they are almost exactly the shape that is predicted for rotating, tidally locked, fluid bodies, according to a physical model first proposed by Edouard Roche in 1849. Basic FactsAntiope's two components, 91 and 86 kilometers (57 and 53 miles) in diameter, circle each other every 16.5 hours at a distance of only 171 kilometers (106 miles). The distance between two objects in space is measured with respect to the objects' centers of mass. Therefore, hidden in those numbers is the fact that the surfaces of the two bodies are separated by a distance that is less than their diameters! The two bodies will eventually have separate names; the larger of the two will retain the name Antiope, while the smaller will be given its own name. Size: Component A: 93.0 x 87.0 x 82 kilometers; Component
B: 89.4 x 82.8 x 78 kilometers (ratio 0.95 ± 0.01) How Do Scientists Know the Shape of Antiope?Antiope and its companion are small bodies, too small to be resolved by even the largest telescopes with the most advanced Adaptive Optics systems. This set of photos was taken by astronomers Pascal Descamps and Franck Marchis with the Adaptive Optics-equipped Very Large Telescope in 2004. The sharp vision of the Very Large Telescope permitted the astronomers to determine the orbital period and the distance separating the two bodies to very high accuracy. The orbital period of 16.5051 days is accurate to less than one part in 100,000! Knowledge of the shape of the orbit also permitted the determination of the system's total mass.
While the Very Large Telescope can resolve the two components of Antiope, each component is still a point source of light; their sizes and shapes cannot be determined from images alone. So how did Descamps and Marchis figure out the sizes and shapes? It actually took the help of large numbers of amateurs with small telescopes. Once they determined the orbit, the astronomers realized that from May to November of 2005, the two components would transit each other as seen from Earth.
These mutual events presented the opportunity to measure the shapes of the two bodies with high precision, simply by studying their light curves -- how the brightness of the pair of objects changed as one occulted light reflected from the other. It was not necessary to resolve the objects as disks; by precisely timing the changes in brightness during months of mutual events, the astronomers could determine the diameters of the objects. The astronomers then hypothesized that these two bodies, locked in such a close orbit, might have the slightly squashed ellipsoidal shapes that could be predicted by assuming that they were fluid. This doesn't mean that they assumed the bodies were liquid; rather, they assumed that the bodies were rubble piles with no internal strength, so that their shapes should mostly be dictated by minimizing the local force of gravity across their surfaces. The light curves predicted by their model matched the observed light curves, so the astronomers concluded that this fluid hypothesis was correct. It is actually very surprising for such small objects to have the shapes that are predicted based on the fluid assumption. Even if they were made entirely of small grains of material that were not at all stuck to each other, frictional forces -- the same forces that allow a pile of sand to hold up against gravity instead of flowing flat like water -- should theoretically keep these bodies from the theoretical fluid ellipsoid shapes. How Did the Double Asteroid Form?Explaining how the double asteroid (90) Antiope formed is a challenge. The most common explanation of how objects acquire moons involves either the gravitational capture of a small object by a much larger object or the collision of two bodies, after which some of the collisional debris ends up in orbit around the larger body. It's likely that Antiope formed as a result of a collision: it's a member of the Themis family of asteroids, which are thought to have originated when a large asteroid experienced a catastrophic impact that broke it into multiple pieces that now share a similar orbit. Other asteroids in the Themis family include (24) Themis, (62) Erato, (104) Klymene, and (171) Ophelia. However, while a collision likely broke Antiope off from the parent body of the other Themis asteorids, neither the capture or collision explanations can produce two same-sized bodies orbiting each other. The explanation for similar-sized binary Trojan asteroids, like Patroclus and Menoetius, involves a gravitational encounter with a large planet; but Antiope and its companion are in the Main Belt of asteroids, which have not had close encounters with any of the planets. Descamps and Marchis suggest that Antiope might originally have started as a single, fast-rotating body. A close encounter with another asteroid or an oblique impact -- perhaps with another collisional fragment from the Themis parent body -- could have imparted even more spin to the proto-Antiope; if it spun fast enough, centrifugal force could have overcome the force of gravity, and one body would have split into two. The notion that the two bodies may have begun as one is supported by the fact that they appear to have similar physical properties. Resources90
Antiope A & B information from Franck Marchis' website, including
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