Seeing Double, 30 Years Later

Sometimes, an observation is too preposterous to seem possible.

Kleopatra occultation plot
A reconstruction of the stellar occultation by asteroid 216 Kleopatra on October 10, 1980. Kleopatra happened to be turned nearly end-on to us at the time. Eleven observers saw the central object occult the star, while two farther away (marked by pink cross) saw a 0.9-second disappearance by what is know known to be one of the asteroid's two companions (yellow orbits).
Franck Marchis
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.

Sylvia and its moons
Artist's impression of the triple asteroid 87 Sylvia, discovered in 2005. Its two moons are named Romulus (lower left) and Remus.
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.

Kleopatra and its moons
A view of the triple asteroid 216 Kleopatra, acquired in 2008 with an adaptive-optics system on the Keck II telescope. The two moonlets (arrowed) are an estimated 3 and 5 km across.
(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.

7 thoughts on “Seeing Double, 30 Years Later

  1. Sam Storch

    Thanks for the reminder of the 30th anniversary of those observations. Later on, on January 19, 1991, I was part of an effort to see another occultation of a star by Kleopatra; the results of that effort appearing in Sky & Telescope during 1992. The IOTA Observers Manual also shows the collected results and also shows some Keck images done in 1999 (See Fig. 6.14, page 142).

    Now, we learn even more about the peanut-shaped object. The feeling of elation that the puzzle pieces keep coming together is a reminder that the same light “rains down on us” as into the great glass eyes on hilltops and in orbit around our planet.

    Little did I know that an observation taken on a cold Long Island winter night long ago could still be so satisfying!

    Seize the night!

  2. Enrico the Great

    Mr. Storch’s comment shows his appreciation for the science and wonder of Astronomy. Real Astronomy. Keep on observing.
    P.S., was the putative satellite of the asteroud Herculina ever confirmed????

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