The Orionid Meteor Shower – dust from Halley’s Comet

One of the year’s annual and reliable displays of “shooting stars” is produced by the Orionid meteor shower. It can be seen in October and is moderately strong.

An Orionid meteor flies through Orion in this image taken on 23 October, 2017. Image credit: Paul Sutherland

The meteors are known as the Orionids for short, and are so named because they appear to come from the direction of the constellation of Orion. This point, the radiant, is actually between the familiar patterns of Orion and neighbouring constellation, Gemini. The meteors themselves can appear in any part of the sky, so observers do not usually look at the radiant.

The shower is produced by a stream of dust particles shed by Halley’s Comet (1P/Halley) along its orbit and spread by the gravitational pull of the planets, notably Jupiter.

October is the second time we encounter Halley debris during the year. The other crossing produces the η Aquarids which can be seen in late April and May.

Activity from the Orionids is generally regarded as occurring from October 14 to 31, with a peak from the 21st to the 23rd. During this peak period, an observer might see a rate of 20 to 25 meteors an hour under ideal conditions. Occasionally, there has been enhanced activity from the Orionid meteor shower and they have reached rates two or three times higher.

This rate predicted is called the Zenithal Hourly Rate (ZHR) and is what one might expect to see if, for example, the radiant was directly overhead and the sky was completely dark and clear. In practice, such conditions are rarely met, and so the rate seen is lower.

The radiant, or direction from which the Orionid meteors appear to come, around the date of maximum. They will appear in any part of the sky however. Image credit: Skymania.com

Nevertheless, the Orionids are a shower well worth watching when conditions are good and moonlight does not cause problems. In 2017, the Moon will be near its New phase and so it will be a very good year to look for the meteors.

Incidentally, research by NASA scientists in recent years has shown that meteor showers continue beyond the limits usually quoted. For example, the International Meteor Organization (IMO) now gives the period of activity as being from September 23 to November 27. However, rates during those extended periods are negligible.

The Orionid meteors are swift when they appear, entering the atmosphere at a speed of around 67 km (41 miles) per second. No meteors can be seen until the shower’s radiant has risen above the horizon, and from mid-northern latitudes, this happens mid-evening. For other parts of the world, check the rising time for your location by using a planisphere or star chart like that on our night sky guide.

Related: How to observe meteors

Related: How to photograph a meteor shower


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