Jason Jenkins photographed this 45-minute composite shot of the Orionid meteor shower on October 20, 2012. You can also see Jupiter at far left and the Pleiades near the center of the frame. Image credit: Jason Jenkins via Flickr // CC BY-SA 2.0
Look up late tonight, October 20, and you will be treated to a veritable fusillade of meteors hurled by the phantom limbs of Halley's comet. The Orionid meteor shower has been active in the night sky for the past couple of days, and it will continue blasting streaks of light for a few days yet. Tonight, however, is the big night, when the shower peaks and thus puts on the best show. If the sky is clear and the light pollution in your area low, you might catch up to 20 meteors per hour. These numbers might have been better if not for some particularly bright moonlight—the very same moonlight that made last weekend's super hunter's moon so spectacular.
If you don't want to stay up all night, another way to see the best of the Orionid meteor shower is to wake before dawn tomorrow, October 21, when the Earth is still bundled in the blanket of night and the waking world has yet to stir. It's just you, a dark sky, and the serene thrill of the shooting star.
Halley's Comet crossing the Milky Way, photographed from the Kuiper Airborne Observatory, C141 aircraft, in April 1986. Image credit: NASA
The Orionids are a parting gift from the comet Halley, which visits the Earth every 75 to 76 years. As the comet goes about its orbit, it leaves behind a trail of dust- and sand-sized particles. When the Earth passes through that debris field, those particles slam into our atmosphere at tens of thousands of miles per hour, generating terrific streaks of light as they burn away. So it is with every meteor shower, regardless of origin.
Halley's Comet might be the most famous object of its kind, and remains one of the best studied. On its last visit, in 1986, nations of the world even sent spacecraft to observe it up close. Though NASA opted to sit that one out, Bob Farquhar, one of the agency's mission designers, committed a kind of act of space piracy when he sent the ISEE-3 space weather satellite—which had been launched for an entirely different mission—on a wildly complicated trajectory that not only allowed the U.S. to encounter the comet, but to make first contact. When a comet inspires spacecraft theft, you know it's important.
And yet for all the centuries that we've been studying it, the finer points of comet Halley's orbit remain shrouded in mystery. The problem of calculating its precise orbit is that its internal processes, coupled with the influence of planets and smaller celestial bodies, throw the math off very quickly. The upshot is that the timescale over which the comet's orbit can be predicted accurately is extremely short.
Earlier this year, however, astronomers from the Netherlands and Scotland conducted the most comprehensive set of calculations ever attempted of comet Halley, and managed to stretch things out a bit, bringing the predictability of the comet to about 300 years. They determined also that the comet's orbit was most disturbed of late not by Jupiter (whose dominance in the solar system has long made it the most obvious candidate), but rather, by Venus. Don't cry for Jupiter, however. The solar system's largest planet will have its way in the 6th millennium CE, when comet Halley will pass extremely close by, and Jupiter's influence will seize dominance.
WHAT IF IT'S RAINY?
If you want to see the Orionids but live in an area of extreme light pollution, or if the weather overhead is simply not cooperative, you have at least one option. Slooh will be broadcasting the event all through the night on October 20 through the early hours of October 21. If you are fortunate, however, and the sky is clear and the light on the ground dim to nonexistent, find a nice patch of ground before dawn on the 21st, lay out a blanket, let your eyes adjust to the darkness, and wait. No telescopes or binoculars are needed. You'll have a front row view of the sky as it falls.