Orionids: Diving into Comet Halley's Tail
As we approach another famous meteor shower on Oct. 21, where do those dazzling 'shooting stars' come from?
With the recent news of the discovery of Comet ISON which is set to grace our skies in November 2013, my mind drifted back to childhood memories and views of Comet Halley.
The comet last visited us in 1986 when I was a keen 13-year-old amateur astronomer. I even remember trying to observe it through a poor excuse for a telescope but, although the view wasn't perfect, I was happy I saw it.
Unlike ISON, which current predictions suggest is going to put on an incredible show, Halley was pretty unimpressive in '86. Currently, the 15-kilometer (9-mile) wide Halley is heading out into the dark depths of the solar system and is somewhere around the orbit of Neptune. It will be another 12 years before it reaches its most distant point when it will start its 38 year journey back toward the sun.
As Halley chugs around the solar system it sheds material along its path. Its almost like the breadcrumbs left behind as a trail by Hansel in the children's story Hansel and Gretel. The orbit of Halley is left traced out by debris and each year, around the latter half of October, the Earth ploughs through the debris sweeping up anything in its path. This all builds to a (hopefully) impressive crescendo when the Earth passes through the densest part of the shower before dawn on Oct. 21.
The exact moment of time when we reach this point is hard to predict accurately, but what is certain is that over the period of time around 21st, the Earth will be bombarded by tiny fragments of Comet Halley. On Earth, we would see this as short flashes of light as 'shooting stars' that streak across the sky. The reality of course is that they are tiny pieces of rock burning up high in the atmosphere -- known as meteors.
The number of meteors seen at peak is affected by many things, not least of which are the altitude of the point in the sky the meteors seem to come from (the radiant), the phase of the moon and of course the density of the of particles in the stream of debris.
Meteors from this shower can generally be seen a week either side of the maximum which sometimes peaks with around 50 meteors per hour. This year's display is forecast to be less impressive with around 20 meteors per hour at maximum but it often surprises with bright fireballs, so it will still be worth watching. The October meteor shower is known as the 'Orionids' based on the location of the radiant that lies in the constellation of Orion. Halley is responsible for a second meteor shower in May, known as the 'Eta Aquariids.'
The tips for observing meteors are simple. You will see more when you are on the forward facing side of the Earth which translates to the hours before dawn at your location. Wrap up warm, get a comfy chair, lay back and watch. Don't look in the direction of the radiant which is near the club of Orion, look some distance away and keep your eyes open. With luck, you might just catch glimpses of pieces of rock -- pieces of perhaps the most famous comet in history.
The 9-mile wide icy nucleus of Comet Halley asseen by the European Giotto spacecraft in 1986. ESA