The Orionid Meteor Shower in 2009

Back in 1986, Halley’s Comet returned after a 76 year journey through the outer solar system. Not predicted to return again until 2061, you might think you missed your chance to see Halley. Well, yes and no. It will be a long time before anyone sees the comet again, but twice a year small pieces of Halley are visible as they burn up in the Earth’s atmosphere. Both the Eta Aquariid meteor shower in early May and the Orionids of mid-October were created by small dust particles released over thousands of years by Comet Halley.

Comet Halley is one of the most active and brightest known comets. Though there are brighter comets than Halley, it is the brightest comet which returns after a reasonable length of time (~76 years). As a result, it has been observed at 30 returns dating back to 239 BC. It was likely observed even earlier but those records have not survived. Halley travels in an orbit that takes it from as far as 35 AU (just beyond the orbit of Neptune) from the Sun to as close as 0.59 AU (just inside the orbit of Venus) from the Sun. It’s orbit is inclined 162 degrees from the Earth’s orbit. This means the comet orbits the Sun in almost the exact opposite direction than the Earth and the other planets which is common for comets  with orbital periods greater than ~30 years. Two plots of Halley’s orbit are posted below.

What exactly are the Orionids?

Comets are mountain sized rocks which contain a large amount of ices (water, carbon monoxide, methane, and many others). Every time a comet passes close to the Sun, these ices sublimate (go directly from a solid to a gas). As the gases build up inside the comet, they eventually erupt into space much like geysers on Earth. These geysers of gas (on comets they are called “jets”) carry lots of dust with them. It is all of this escaping gas and dust that gives comets their “fuzzy” appearance. Over time the dust particles spread out and line the entire orbit of the comet. As a result, even though the comet may be far away (right now Halley is beyond the orbit of Neptune) dust can be found at any point in Halley’s orbit. When the Earth passes close to the orbit of Halley we can see some of this dust as it burns up in the atmosphere as meteors.

Over time the orbit of Halley changes. Computer simulations of the past movements of Halley and its dust suggest that most of this year’s Orionid meteors were released by Halley over 2000-3000 years ago. Enhanced activity may be possible due to an excess of particles released by Halley during its returns in 1400 BC and 11 BC.

So what can we expect this year and when should you look?

This Orionid meteor shower is usually active from Oct 3 to Nov 11 with a broad peak between Oct 18 and 24. During their peak, rates can be as high as 20-70 meteors per hour. In 2007 rates of 70 meteors per hour were observed but “only” 39 per hour were seen last year. How active this year’s Orionids will be is unknown though the 1400 BC and 11 BC streams should keep active at a level no less than last year’s.

The Orionids appear to come from an area in northern Orion. This area, called the radiant, rises around 10pm local time. It is best to wait till the radiant is high in the sky before looking for meteors (say 1am). The radiant is highest around 3:30am which is the best time to look. Meteors can appear anywhere in the sky so you don’t have to look at the radiant.

If you live in a major city with only a few bright stars visible, you will not see too many, if any, meteors. So it is always best to observe from a dark site. The International Meteor Organization has a near real-time graph of the activity level of the Orionids at this page. Note that the rates shown have been compensated for bright skies (meaning this is the rate you would have seem if you were observing from a dark rural location). The actual rate of meteors will be lower (perhaps much lower) if you are at a site with a bright sky.

Additional information on the Orionids can be found at the American Meteor Society.