The Orionid Meteor Shower
October 18, 2008 1 Comment
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 Aquarid 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 many of this year’s Orionid meteors were released by Halley between 1265 BC and 910 BC (for some points of reference, the Trojan War took place around 1200 BC and King David ruled around 1000 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. Last year rates reached 70 meteors per hour and similar circumstances are predicted for this year with the best time being the morning of Oct 19 (for the US). However, a bright quarter Moon will wash out many of the fainter meteors resulting in smaller rates. Due to the bright Moon, rates may only be 1/3 as high as last year.
The Orionids appear to come from an area in northern Orion (marked by a yellow X in an O on the sky chart below). 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. As you can see on the sky chart, the Moon is almost on top of the radiant. Meteors can appear anywhere in the sky so you don’t have to look at the radiant. In fact, with the Moon nearby, it is best to look without having the Moon in your line of sight.
If you live in a major city with only a few bright stars visible, you will not see too many, if any, meteors. It is always best to observe from a dark site. But due to the Moon, even dark sites may only see 10-20 meteors per hour. 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 and moonlight (meaning this is the rate you would have seem if there was no Moon and you were observing from a dark rural location). The actual rate of meteors will be lower (perhaps much lower) due to the Moon and if you are at a site with bright skies.
Additional information on the Orionids can be found at the American Meteor Society.