An odd feature that for more than 40 years was thought to be structurally associated with a peculiar galaxy has been identified as the superposed trail of a main-belt asteroid that happened to be crossing the galaxy as it was being photographed in 1964. An amateur astronomer who imaged the galaxy in 2008 first noted the modern lack of the 'spike.'

The mystery begins in 1966, when astronomer Halton C. Arp published a compilation of 338 photographs of highly asymmetrical galaxies and galaxy groups taken at Palomar Mountain in Southern California.

Entitled "The Atlas of Peculiar Galaxies," the book was one of the first serious inquiries into the nature of what was, at the time, a small class of disturbed galactic systems that were neither spiral nor elliptical in form.

One of these galaxies, number 192 (also known as NGC 3303), has a distinctive spike appearing to jut from the galaxy's bulbous northwest side. Based on its appearance in the Arp Atlas, Peter Nilson described the feature as a "sharp jet" in his 1973 Uppsala Galaxy Catalogue.

Forty years after the Atlas was published, two astronomy writers, Jeff Kanipe and Dennis Webb, published their commemorative book "The Arp Atlas of Peculiar Galaxies: A Chronicle and Observer's Guide" (Willmann-Bell, 2006). The book recounts Arp's research leading to the compilation of the Atlas and reproduces all of the original Atlas images, along with modern images made by amateur astronomers. Ironically, the authors ranked the Arp 192 spike/jet as a "challenge" for advanced amateurs to photograph.

In August 2009, Kanipe and Webb learned that an amateur astronomer in Minnesota, Rick Johnson, had made a deep image of Arp 192, one he thought should have revealed the spike's presence. Johnson reported, however, that the spike was not visible.

Suspecting that Johnson's exposure probably had not gone deep enough, Kanipe consulted recent images of Arp 192, the best being from the Sloan Digital Sky Survey. He inverted the image to a negative format, converted it to gray scale, and then contrast-enhanced it so the resulting image approximated the density of the one in the Atlas. No vestige of the spike was apparent. It had truly vanished.

"This was an exciting turn of events, and not a little disturbing," says Kanipe. "Galactic structures do change, but they change over periods of several hundred million years. The human race hasn't existed long enough to observe dramatic structural changes in any galaxy, but here was a feature that had evaporated in 40 years, and nobody noticed."

Possible explanations for the spike's conspicuous absence included it having been a photographic artifact, a plate defect, or an asteroid trail. The orbits of thousands of minor planets have been so well determined that by calculating "backward" in time one can ascertain an asteroid's position on any day in the past.

Archivists at the Carnegie Observatories headquarters in Pasadena, California, confirmed to Kanipe that the original plate was taken on 19 February 1964. Brian Marsden, director emeritus of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, subsequently confirmed that minor planet (84447) 2002 TU240 would have been located near the sky coordinates for Arp 192 on that date in 1964. With that, the mystery was solved.

The asteroid was discovered on 6 October 2002 by the Near Earth Asteroid Telescope on Haleakala, Maui, Hawaii. The Arp 192 Atlas image is the earliest known prediscovery image of this object. Just after Thanksgiving 2009, Kanipe learned from Marsden that minor planet (84447) 2002 TU240, now bore a different name: 84447 Jeffkanipe.

The extraordinary coincidence of an asteroid trail being captured as it happened to transit a small peculiar galaxy understandably led to the erroneous conclusion that the spike was an intrinsic feature. Thereafter, it became part of the astronomical literature; if not for the question posed by an amateur astronomer, it might still be.