Astronomers are celebrating the return of a comet last seen during the seventeenth century. Ikeya–Zhang, named after the Japanese and Chinese amateur astronomers who spotted it on 1 February of this year, is the brightest comet seen from Earth since Hale–Bopp's visit in 1997. Although comets with longer periods may exist, for the time being Ikeya–Zhang has set a new record: its return after 341 years gives it the longest period of any comet so far observed making successive orbits of the Sun.
The historical records of periodicity cannot be interpreted with certainty but they are persuasive. After observing the comet for about 20 days, Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, calculated that the period of the comet is 341 years. This, and the shape of its trajectory, match it to a comet first seen by Polish astronomer Johannes Hevelius in 1661. Hevelius's comet was one of those later used by Edmond Halley — of Halley's comet fame — in his treatise on comet periodicity. Chinese astronomical records also report a comet in the sky in the years 1320 and 979, which would tally with Ikeya–Zhang's 341-year period, but these sightings may not be reliable.
Comets are essentially dirty snowballs of dust and ice, and are thought to be the remnants of the galactic debris that formed the planets of our Solar System. Measurements of the composition and temperature of the dust and ionized gas trailing behind Ikeya–Zhang are in progress, and will be of particular interest to astronomers. Because of its lengthy periodicity, Ikeya–Zhang makes far fewer trips to the warmer climes closer to the Sun than shorter-period comets such as Halley's (which returns at 76-year intervals), and may have quite a different make-up as a result.
The comet is now around 70 million kilometres from Earth and at its closest approach, on 29 April, it will pass within 60 million kilometres. Since it was spotted a couple of months ago, Ikeya–Zhang has put on quite a display for stargazers. Initially, its tail appeared long and wispy, but now it is short and stubby, so that the comet resembles a shuttlecock. Last week, it seemed to graze the bright 'star' of the giant spiral galaxy, Andromeda. But it wasn't such a close shave — Andromeda is 2.5 million light years away.
Seen from Earth, Ikeya–Zhang is moving over the North Pole and should be visible as a faint smudge to observers in the Northern Hemisphere throughout the night. The best time for viewing is before sunrise, when the comet is about 15 degrees above the horizon (the width of a hand held at arm's length roughly marks out 15 degrees). Although it is visible to the naked eye, its brightness varies, and a pair of binoculars or a small telescope will enhance the show. After 29 April, it will gradually fade from view — and won't return until 2343.
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Clarke, T. The wanderer returns. Nature 416, 597 (2002). https://doi.org/10.1038/416597a