There are two annual meteor showers associated with Halley's Comet, easily the most famous of all comets. The first is the Eta Aquariids shower which occurs in May and the second and more prolific is the Orionids.
The Orionids or Orionid meteor shower takes place every year in late October and usually lasts for about a week although occasionally longer. Sometimes, Orionid meteors may peak at rates of up to 70 per hour but normally the shower is not so active. This year's peak is predicted to be a respectable 25 meteors an hour on the night of the 21st October 2011. Also, at this time the Moon is past 3rd quarter and therefore won't significantly interfere.
Radiant
The radiant of the Orionids is located in the NE part of the constellation Orion, not far from the Gemini border. Since Orion straddles the celestial equator, the Orionids are one of the few annual showers that are good for both northern and southern hemisphere observers.
When observing meteors showers it is best to wrap up warm (especially in winter), sit comfortably, preferably in a deck chair and look in the general region of the sky around the radiant. It is best not to look directly at the radiant itself as meteors can appear many degrees from it. For the Orionids, all meteor trails will trace back to the shower's radiant in the diagram shown below.
Orionids Radiant and Star Chart - pdf format
What to expect
The Orionid meteoroids are fast. They hit the atmosphere at very high speeds of 235,000 km/hour (145,000 miles/hour). At this velocity they will streak through the sky. Although no major burst of activity is expected the shower is usually reliable and should put on a nice show.
It is also worth noting that these particles are leftovers from Halley's Comet apparitions every 76 years. Although Halley is well pass Jupiter at the moment and will not return close to Earth until 2061, remember that every Orionid you observe is actually a small part of the famous comet streaking through and subsequently burning up in the Earth's atmosphere.
Halley's Comet Data Table
| Comet name | 1P/Halley |
|---|---|
| Discoverer | Prehistoric. Edmond Halley first recognised the periodicity. |
| Aphelion (AU) | 35.1 |
| Perihelion (AU) | 0.586 |
| Semi-major axis (AU) | 17.8 |
| Eccentricity | 0.967 |
| Orbital period (years) | 75.3 |
| Inclination (degrees) | 162.3 |
| Last perihelion | 9th February 1986 |
| Next perihelion | 28th July 2061 |
Orionids Data Table 2011
| Meteor shower name | Orionids |
|---|---|
| Radiant Constellation | Orion |
| Dates | 2nd October -> 7th November |
| Peak Date | 21st October |
| RA (J2000) | 6hr 20m |
| DEC (J2000) | +16d |
| Speed (km/s) | 65 |
| ZHR | 20 to 70 |