It has been 20 years since Halley's comet last passed through the inner solar system. This famous comet takes roughly 76 years to circle the Sun. Perhaps you remember seeing the comet for yourself through binoculars or a telescope, or maybe you participated in a tour that took you to some exotic locale south of the equator where you could get a naked-eye view of the comet climbing high into the sky.
Or maybe, because of light pollution or the comet's low altitude above the horizon you didn't see Halley at all. If you missed out on the 1986 event, you have two options:
- Wait and hope that you'll be around when the comet makes its next return during the summer of 2061.
- Step outside before dawn during the next week or so and try to catch a view of some "cosmic litter" that has been left behind in space by Halley's comet.
Halley's comet has left a legacy that is visible to us in the form of two annual meteor showers, one of which will soon be reaching peak activity: the Orionid meteors.
This will be an excellent year to look for them, since the Moon will be New on Oct. 22, which is right around the time that these meteors will be near their peak, ensuring that skies will be dark.
Comets are the leftovers of the day of creation, the odd bits and pieces of simple gases-methane, ammonia, carbon dioxide and water vapor-that went unused when the Sun and its attendant planets came into their present form. Meteoroids that are released into space out of this cometary debris are the remnants of a comet's nucleus. All comets eventually disintegrate into meteor swarms and Halley's is well into that process at this time.
These tiny particles-mostly ranging in size from dust to sand grains-remain along the original comet's orbit, creating a "river of rubble" in space. In the case of Halley's comet, which has likely circled the Sun many hundreds, if not thousands of times, its dirty trail of debris has been distributed more or less uniformly all along its entire orbit. When these tiny bits of comet collide with Earth, friction with our atmosphere raises them to white heat and produces the effect popularly referred to as "shooting stars."
The orbit of Halley's comet closely approaches the Earth's orbit at two places. One point is in the early part of May, producing a meteor display known as the Eta Aquarids. The other point comes in the middle to latter part of October, producing the Orionids.
When to watch
The best time to watch begins from 1 or 2 a.m. local daylight time until around dawn, when the shower's radiant (in Orion's upraised club, just north of the bright red star, Betelegeuse) is highest above the horizon. The higher the radiant, the more meteors appear all over the sky. The Orionids are one of just a handful of known meteor showers that can be observed equally well from both the Northern and Southern Hemispheres.
The Orionids are one of the better annual displays, producing about 15 to 20 meteors per hour at their peak. Add the 5 to 10 sporadic meteors that always are plunging into our atmosphere and you get a maximum of about 20 to 30 meteors per hour for a dark sky location.
Most of these meteors are relatively faint, however, so any light pollution will cut the total way down.
The shower may be quite active for several days before or after its broad maximum, which may last from the 20th through the 24th. Step outside before sunrise on any of these mornings and if you catch sight of a meteor, there's about a 75 percent chance that it likely originated from the nucleus of Halley's comet.
A few Orionids may appear as early as Oct. 9 and as late as Nov. 7. In 1993 and again in 1998, maximum activity unexpectedly came early, on the night of Oct. 17-18. "The activity is not constant from year to year," notes Russian meteor expert, Dr. P.B. Babadzhanov. "The time of maximum shifts significantly and there are secondary maxima."
"They are easily identified . . . from their speed," write David Levy and Stephen Edberg in Observe: Meteors, an Astronomical League manual. "At 66 kilometers (41 miles) per second, they appear as fast streaks, faster by a hair than their sisters, the Eta Aquarids of May. And like the Eta Aquarids, the brightest of family tend to leave long-lasting trains. Fireballs are possible three days after maximum."
Recent studies have shown that about half of all Orionids that are seen leave trails that lasted longer than other meteors of equivalent brightness. This is undoubtedly connected in some way to the makeup of Halley's comet. So it is, that the shooting stars that we have come to call Orionids are really an encounter with the traces of a famous visitor from the depths of space and from the dawn of creation.
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- Astrophotography 101
- Sky Calendar & Moon Phases
- 10 Steps to Rewarding Stargazing
- All About Meteor Showers
- Comet Images
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.