Sometimes, an observation is too preposterous to seem possible.Thirty years ago today, amateur astronomers across the northwestern U.S. and western Canada planted their gear in the hope of watching the asteroid 216 Kleopatra pass directly in front of a star, briefly blacking it out. Back then, when asteroid occultations were hard to predict accurately, they were rare events. But this one had more riding on it: Kleopatra's brightness was known to vary wildly as it rotated, leading to speculation that it was very elongated or perhaps a contact binary (two asteroids touching and joined as one).
As occultation czar David Dunham would later report in Sky & Telescope, nine different observing stations visually timed blinkouts of the 9th-magnitude star SAO 128066. Collectively these timings traced chords across an irregular, elongated silhouette measuring roughly 60 by 80 miles (95 by 130 km). "But there was an unexpected secondary occultation too!" Dunham wrote. "Gerald Rattley and Bill Cooke, observing from sites 2,000 feet apart near Loma Prieta, California, independently and nearly simultaneously saw the star disappear for about one second."
Since Rattley and Cooke were far from the track where other positive sightings occurred, Dunham concluded that the two had seen the star disappear behind a satellite of Kleopatra, "perhaps 8 km in diameter, moving with it in space about 475 km [295 miles] east as seen from our earthly perspective."
Remarkably, despite Rattley and Cooke's independent sightings, the evidence for a binary satellite wasn't considered compelling. The skepticism of Brian Marsden (then director of the IAU's Minor Planet Center) echoed the feeling of many dynamicists of that era. "Thirty years ago was still in the dark ages for asteroidal satellites," he recalls. "I wasn't resistant to the notion, but the observational evidence at the time was extremely shoddy, and I was therefore not prepared at that time to accept that asteroidal satellites existed."
Fast forward to August 28, 1993, when the Jupiter-bound Galileo spacecraft paid a quick hi-and-bye visit to asteroid 243 Gaspra. Because the main antenna on Galileo was a useless tangle (it had failed to deploy properly), results from the encounter took many tedious months to transmit home via a smaller antenna. During one of these glacial playbacks, in February 1994, mission scientists learned Ida was not the solitary interplanetary wanderer they had imagined. It had company: a small, round moon, later named Dactyl.Fast forward again to last Thursday. Addressing a meeting of planetary scientists in Pasadena, California, Franck Marchis (University of California, Berkeley) ticks through a list of known triple asteroids. There are five on his list, including Kleopatra. The others are 45 Eugenia (discovered as such in 2007), 87 Sylvia (2001), 93 Minerva (2009), and 3749 Balam (2008).
These days, finding an asteroid pair is common. According to a list maintained by W. Robert Johnston, the growing collection of minor binary planets includes 72 main-belt asteroids, 49 Earth- and Mars-crossers, and roughly 30% of all finds in the main part of the Kuiper Belt.
Of the five triples, Marchis led the observing teams that found all but Sylvia's. He and colleagues Pascal Descamps and Jérome Berthier (Paris Observatory) identified them two years ago during a dedicated search aided by the incredible adaptive-optics-enhanced resolving power of the 10-meter Keck II telescope.(Interestingly, Kleopatra's family did not come to light when pinged by Arecibo Observatory's powerful radar in November 1999. However, the observing team, led by the late Steven Ostro, concluded that much of Kleopatra's interior is likely an unconsolidated pile of rubble.)
Marchis suspects that only Sylvia has gained its satellites due to a collision. The others, most likely, were do-it-yourself affairs that resulted when the parent body gradually spun up (due to weak effects of solar radiation) to the point that it finally flung parts of itself away. Balam is an odd duck: it has only one real moonlet, but the central body itself is twinned.
During his presentation, Marchis tipped his hat to amateur occultation observers, whose careful timings (now often done by videorecording) provide the "icing on the cake" for refining the sizes and orbits of these systems.
As for their 1980 sighting? Sometimes an observation is too preposterous to seem likely — and it turns out to be right.