One-in-a-trillion comet

Discovering a comet in a spectrograph's slit is the astronomical equivalent of a hole in one.Matthew Kenworthy
By | Published: November 3, 2004 | Last updated on May 18, 2023
November 3, 2004
These days, automated search programs discover most new comets. The rest go to keen-eyed amateurs, whose photographic memories and wide-angle binocular views lend themselves to spotting these cosmic vagabonds. But even astronomers can win the lottery sometimes — or at least catch a comet by pure chance.

On September 22, Sandhya Rao and Dave Turnshek, both of the University of Pittsburgh, were taking spectra of a bright star just after twilight at the MMT Observatory’s 6.5-meter telescope in southern Arizona. This, they thought, would help set up the telescope in preparation for their scheduled observations of faint quasars. Suddenly, as their exposure of BD+303639 came to an end, a new spectrum appeared.

Cometary movement
Time-lapsed spectra show the movement of the probable comet along the spectrograph’s slit.
MMT Observatory’s
The spectrograph has an effective field of view only 2″ wide by 2′ long. Because that’s some 100,000 times smaller in area than a typical pair of binoculars, they didn’t expect the object to reappear on the next exposure. Much to their surprise, another exposure confirmed the mysterious object’s existence; it had perceptibly moved closer to the star’s position.

A quick check revealed that both new spectra were identical, and that the team was seeing a new object moving at a rate of 2.2′ per hour along the spectrograph’s slit. Further exposures confirmed the motion of this interloper, but its spectrum remained a mystery. A back-of-the-envelope calculation showed the object’s apparent motion limited it to the solar system, but what could it be?

A comet or an asteroid seemed to be the most likely possibilities, and an analysis of the object’s spectrum identified it as a previously undiscovered 16th-magnitude comet. With time pressing, the astronomers moved onto their next target star, but despite calls to other telescopes in the hopes of capturing a confirming image, the object has not been observed again to date.

Matthew Kenworthy is an instrument scientist at the University of Arizona’s Steward Observatory, in Tucson, Arizona.
Matthew Kenworthy is an instrument scientist at the University of Arizona’s Steward Observatory, in Tucson, Arizona.