This spring, a comet with plenty of history and quite a bit of personality visits our skies. Schwassmann-Wachmann 3 was discovered photographically three-quarters of a century ago by two German professional astronomers. It brightened to sixth magnitude by the time it passed close to Earth in 1930, and the orbit had a short, 5-year period with a perihelion very close to the Earth’s distance from the Sun. Unfortunately, due to a combination of poor geometry, intrinsic faintness, and gravitational perturbations by Jupiter, several subsequent returns went unseen.

Recovered in 1979—a month late, missed in 1985, SW-3 was finally re-recovered in 1989-90. In 1995, amateurs observed a 7-magnitude outburst as the comet split into multiple nuclei. Following soon after SL-9’s “string of pearls comet” encounter with Jupiter, astronomers have kept a close watch on SW-3, and several fragments were well observed in 2000-2001. This year, what’s left will come closer than any time since its discovery, and SW-3 is living up to, even exceeding, expectations. Through February it rushed northward, passing Arcturus in late March. The comet continues to disintegrate, and we have a front-row seat for this apparition. 
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Now crossing Corona Borealis, SW-3 is fast approaching the May 12th, 0.08 a.u., flyby of Earth, among the dozen or so closest approaches ever recorded. It will be cruising at 6° per day through summer’s constellations of Lyra, eastward into Cygnus to Vulpecula and across the Milky Way, and then into Pegasus. Tracking through such a rich region, SW-3 glides by prominent deep-sky objects, including M13 on May 1st, and M57 on May 8th, passing within several arcminutes of the Ring Nebula at 11 pm EDT on the evening of the 7th (see below). A day later, it passes M56, zips through the vicinity of the showpiece double star Alberio and the Dumbell Nebula M27 on the 10th and 11th, and toward a rendezvous with its third globular cluster M15 around May 15th. (These predictions refer to fragment C; other fragments have notably different paths.) Conveniently high in the east by late evening, moonlight will however interfere after First Quarter on the 5th. No doubt, many telescopes at the Star Gaze XII will be pointed at SW-3.

With this rapid movement, you’ll need an accurate chart such as those in the May’s Sky & Telescope (pp. 60-65) and Skyhound Observing Pages for April. Updates and observing reports are also available at the the Sky & Telescope site, BAA Comet Section,  Seiichi Yoshida's comet page, several active threads at the Cloudy Nights site, and even the Comet Chasing Yahoo! group. Fragment C is at about ninth magnitude, visible in large binoculars, and sporting a nice tail. Fragment B has apparently disintegrated further but is still visible. Although many other fragments have been reported—36 at last count—all are much fainter. But stay tuned, and check these web links or the club’s Yahoo site. 

The conjunction with the Ring Nebula (right, and a faint spiral galaxy IC 1296) should be a spectacular sight in any telescope or large binoculars. Will SW-3’s motion be perceptible against field stars? How many fragments will still be visible in amateur telescopes? Will this be our last chance to see this unique comet? If it continues to disintegrate on this close passage to Sun, little may be left for 2011. On the other hand, some have predicted that the disrupted dust will eventually intersect our orbit. With any luck, the crumbles of comet SW-3 may reappear as new meteor shower!

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 April 2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.