The Orionid Meteors are Here!

This could be a particularly good year for the Orionid meteor shower, which runs from roughly October 17th to 25th with one or more peaks around the morning of the 21st. The Moon is out of the sky during the good meteor-watching hours from midnight to dawn. Moreover, in years past the Orionids have shown a 12-year cycle in their strength — and based on this we should be seeing a bumper crop of Orionids in 2009, with peak rates of up to 30 meteors visible per hour by a single person under ideal observing conditions.

The seething nucleus of Halley’s Comet, as photographed on May 8, 1910, by the 60-inch telescope at Mount Wilson Observatory in California.

Courtesy Carnegie Institution of Washington.

The Orionids have an illustrious parentage. Like the Eta Aquarids of May, they are bits of debris shed long ago by Halley's Comet. The two showers are essentially one and the same; Earth intersects a single, broad stream of meteoroids at two places in its orbit on opposite sides of the Sun.

Like the Eta Aquarids, the Orionids tend to be faint and swift — only the Leonids hit Earth's atmosphere faster — and they often leave briefly glowing trains. The shower is actually a complex of several components with different maxima spread over several days. These radiants (the points in the sky from which the meteors appear to radiate) are grouped near Orion's Club, as shown on the accompanying chart.

The Orionid meteor shower has several radiants in northern Orion near the feet of Gemini. They all drift about 1° eastward per day (as indicated by dashed arrows) due to Earth’s orbital motion.

S&T: Gregg Dinderman

For observers around 40° north latitude, these radiants rise high in the eastern sky (at least 45° up) by about 2 a.m. daylight saving time. So that's about when the meteor activity gets pretty good. The first light of dawn begins stealing into the east about four hours later.

Halley's Comet last came through the inner solar system in 1985–86, and at that time its nucleus shed a layer of dirty ice about 20 feet (6 meters) thick on average. This has been happening every 76 years for many millennia. During that time the dirt bits have spread all around Halley's elongated orbit and a fair distance from it sideways, which is why some of the particles now intersect Earth even though the comet's orbit does not. (The orbits of Halley and Earth are separated by 22 million km, or 15% of the average Earth-Sun distance, at their closest point.) No one knows how long it took the Orionid meteoroids to drift so far off track — one estimate is 4,000 to 10,000 years — but it's clear that as shower meteoroids go, the Orionids are old.

They've been seen for a long time too. The first known Orionid shower was recorded by the Chinese in AD 288, when "stars fell like rain." The shower has been well observed ever since astronomers first recognized its radiant in 1864.