Five Asteroids Meet

Chart showing five close asteroids
By the time of their most compact grouping on November 9th, five asteroids brighter than magnitude 16 will easily fit inside a 1° field. Only Melpomene (magnitude 11.7) matches the dimmest stars on this chart; the other asteroids are fainter.
Sky & Telescope illustration; data courtesy the Minor Planet Center.
How often do five asteroids appear close enough on the sky to be imaged together with a small amateur telescope and CCD camera? According to Aldo Vitagliano (University of Naples), this is a very rare event, if asteroids smaller than 50 kilometers or fainter than magnitude 16.0 are excluded. “I searched for all such groupings with a maximum separation of 1°,” he writes. “Ignoring those taking place at an elongation of less than 25° from the Sun, I found just six groupings during the entire 21st century — and the best among them occurs in November of 2003!”

On November 9th near 8h Universal Time, minor planets Melpomene (magnitude 11.7), Hersilia (14.2), Marianna (14.8), Beagle (15.6), and Francette (15.7) have their minimum angular spread of 0.749°. But they actually satisfy the 1° criterion from November 3rd through 14th. As shown above, the five asteroids are moving toward the southeast at rates of 0.6" to 0.9" per minute. A set of CCD images spanning an hour or so, when stacked, should clearly show the five asteroids’ dotted trails.

The next quintuple grouping occurs on May 21, 2006, and involves Una, Fidelio, Andromacha, Tekmessa, and Libya. But the next after that, as defined above, does not take place until the year 2073.

By relaxing the conditions slightly, Vitagliano obtained a much longer list of asteroid get-togethers. When he took into account more than 2,000 asteroids that are believed to be larger than 20 km, he found 50 such groupings in the coming century, and one involves six asteroids. On June 15, 2024, Bondia, Ruth, Sirona, Lucia, Vanadis, and Eulalia will meet in a 1° patch in Gemini.

For this search Vitagliano used SOLEX (version 8.5), the remarkable computer program he has been developing since 1995. To learn more or to download this free program, visit his Web site.

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