Mini Moons Why Do Asteroids A surprisingly large fraction of small bodies <strong>com</strong>e in binaries and triplets. 28 <strong>December</strong> <strong>2012</strong> sky & telescope LYNETTE COOK / W. M. KECK OBSERVATORY
in Pairs? Michael Shepard FRANCK MARCHIS ET AL. / NATURE Come In 1977, johns hopkins University astronomer David Dunham organized a group of amateurs and professionals to observe a star occulting the asteroid 6 Hebe. The asteroid’s shadow passed through central Mexico, where three observers witnessed the star briefl y fl icker out. But experienced Texas amateur Paul Maley visually observed a 0.5-second occultation at the same time — some 500 miles north of the other observers. Was this evidence for an asteroid satellite? Unfortunately, researchers had no way to confi rm Maley’s sighting. Over the next 15 years observers found hints of binary asteroids in other occultations, odd light curves, and radar echoes, but nothing could be confi rmed. All of the speculation became moot on August 28, 1993. On that day, NASA’s Galileo spacecraft fl ew by asteroid 243 Ida en route to Jupiter and made a profound discovery — a tiny moon later named Dactyl (last month’s issue, page 28). Up to then, astronomers had discovered more than 10,000 asteroids using ground-based telescopes. None were known to have <strong>com</strong>panions and only a few were suspected to have one. Now, after two asteroid fl ybys (Galileo fl ew past 951 Gaspra in 1991), one was discovered to have a moon. Which of these statistics refl ected the true nature of asteroids? Were binaries a few in 10,000, or one in two? Opening the Floodgates Within months of Dactyl’s discovery, planetary astronomers began to report more evidence of asteroid satellites. Most were orbiting near-Earth asteroids (NEAs), tiny objects only a few kilometers wide that cross the orbits of the inner planets. The <strong>com</strong>panions were suggested in high-quality light curves that could only be explained if there were two objects orbiting each other. At least one was spotted when one member eclipsed its partner, just like an eclipsing binary star. By 2000 two new techniques for binary discoveries became mainstream and important: adaptive optics (AO) and radar. Large telescopes fi tted with AO to beat atmospheric distortion could now resolve asteroid <strong>com</strong>panions (the Hubble Space <strong>Telescope</strong> could also ac<strong>com</strong>plish this feat). Using the 3.6-meter Canada-France-Hawaii <strong>Telescope</strong>, William Merline (Southwest Research Institute) and his colleagues discovered the fi rst asteroid satellite with AO in 1999 when they found that 45 Eugenia was orbited by a small moon now known as Petit-Prince. Likewise, the newly upgraded Arecibo radio telescope in Puerto Rico could use radar echoes to produce high-resolution asteroid images. The fi rst radar binary discovered was the near-Earth asteroid (185851) 2000 DP107. In 2005 planetary scientists received another pleasant surprise when Franck Marchis (University of California, Berkeley) and his colleagues found a triple asteroid, the large main-belt asteroid 87 Sylvia. The primary body had two small <strong>com</strong>panions — later named Romulus and Remus for the mythical founders of Rome (Sylvia was their mother). And in 2008, astronomers using Arecibo discovered the fi rst triple near-Earth asteroid, 2001 SN263. The discovery fl oodgates had opened. This brief history lesson would be in<strong>com</strong>plete without mentioning the other main reservoir of binary “asteroids” — the Kuiper Belt. Technically, the fi rst discovery in this region was that of Pluto’s moon Charon in 1978. Because BINARY ASTEROID Opposite page: This artist rendition depicts the binary Trojan asteroid 617 Patroclus, which trails Jupiter by 60° in the planet’s orbit around the Sun. Near left: Using the laser-guide-star adaptive optics system on the 10-meter Keck II telescope, astronomers have taken images showing that Patroclus actually consists of two bodies nearly equal in size, making it a true binary-asteroid system. The larger object (Patroclus) is about 122 kilometers (76 miles) across and the smaller body (now named Menoetius) is 112 km across. They orbit a <strong>com</strong>mon center of mass every 4.3 days at an average distance of 680 km. The two binary members might have originated from a single body that was tidally ripped apart during a close pass of Jupiter billions of years ago. <strong>Sky</strong>and<strong>Telescope</strong>.<strong>com</strong> <strong>December</strong> <strong>2012</strong> 29
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