Comet 161P/Hartley–IRAS slipped past Earth at its closest distance in more than two decades on Thursday, cruising roughly 75.5 million kilometers (about 0.505 AU) from our planet as it heads toward the inner solar system ahead of a late-November perihelion. The flyby marks the nearest approach of this particular comet since astronomers began tracking it in 1983, though at its current brightness it remains well out of reach of the naked eye.
The comet is a Halley-type object, meaning it orbits the sun on a steep, retrograde path roughly once every 21.5 years, swinging from a perihelion of 1.265 astronomical units out to an aphelion of more than 14 AU. It was discovered independently and almost simultaneously in November 1983: Australian-based astronomer Malcolm Hartley spotted it on a photographic plate at Siding Spring Observatory, while the newly launched Infrared Astronomical Satellite picked it up from orbit days later, an overlap reflected in its hybrid name. According to JPL's Small-Body Database, the comet is currently shining around magnitude 12, bright enough for backyard telescopes and serious binoculars but invisible without optical aid.
Sky-watchers tracking the comet through October will find it crossing the faint constellations Aquarius, Equuleus and Delphinus before moving into Aquila and Sagitta in November. Observers in the Southern Hemisphere had the better vantage point in the weeks leading up to opposition on September 27; the view now favors the Northern Hemisphere as the comet climbs toward its November 27 perihelion, when it should brighten slightly to around magnitude 11.
Why a faint, decades-old comet still matters
Comets like 161P are of more than passing interest to planetary scientists because each close pass offers a chance to refine measurements of its orbit and composition. Spectroscopic observations taken during a previous apparition detected hydroxide, cyanogen and other gases typical of an active, outgassing nucleus, evidence that the comet continues to shed material each time it nears the sun. Because that outgassing produces small, non-gravitational nudges to the comet's path, repeated tracking, much of it contributed by amateur observers logging positions with backyard telescopes, helps keep orbital forecasts accurate decades into the future, a record maintained in observation databases such as the Paris Observatory's comet-tracking archive.
Halley-type comets, which make up a small minority of the roughly 4,000 comets catalogued to date, are thought to originate largely in the Oort Cloud or trans-Neptunian region before gravitational encounters send them into the shorter, steeply tilted orbits that make close Earth passes like this one possible. The catalogued orbital history of 161P/Hartley–IRAS shows a comet that has made the trip past Earth several times since its discovery without ever posing any impact risk; its orbit keeps it comfortably clear of Earth's path.
With this week's flyby now past, the comet will continue brightening only modestly on its way to perihelion next month before fading back into the outer solar system. Its next similarly close approach to Earth is not expected until the mid-2040s, giving professional and amateur astronomers a long wait before this particular visitor swings back into range.
