Moonlight Meteor Shower Spawned By Halley's Comet

A junior version of the famous Perseid meteor shower thought to have originated from the remains of Halley's Comet will hit its peak over the next week, but the light of the moon may intrude on the sky show.

This upcoming meteor display is known as the Orionids because the meteors seem to fan out from a region to the north of the Orion constellation's second brightest star, ruddy Betelgeuse.

The annual event peaks before sunrise on Thursday (Oct. 21) but several viewing opportunities arise before then for skywatchers in North America. [Where to look to see the Orionids]

The shooting stars are created by small bits of space dust — most no larger than sand grains — thought to be left over from the famed Halley's Comet, which orbits the sun once every 76 years.  

Currently, Orion appears ahead of us in our journey around the sun, and has not completely risen above the eastern horizon until after 11 p.m. local daylight time.  

The constellation is at its best several hours later. At around 5 a.m. – Orion will be highest in the sky toward the south – Orionids typically produce around 20 to 30 meteors per hour under a clear, dark sky.    

But skywatchers beware: You will be facing a major obstacle in your attempt to observe this year’s Orionid performance. As bad luck would have it, the moon will turn full on Oct. 23. Bright moonlight outshines fainter meteors, seriously reducing the number anyone can see.

The gradual build up to the full moon will hamper – if not outright prevent – dark-sky observing during the Orionid meteor shower's peak on Oct. 21.

The Orionids are actually already underway, having been active only in a very weak and scattered form since about Oct. 2. But a noticeable upswing in activity is expected to begin around Oct. 17, leading up to their peak night. 

"Orionid meteors are normally dim and not well seen from urban locations,"  notes meteor expert, Robert Lunsford, adding that "it is highly suggested that you find a safe rural location to see the best Orionid activity."

Damage control for 2010

With all this as a background, perhaps the best times to look this year will be during the predawn hours several mornings before the night of full moon. That’s when the constellation Orion (from where the meteors get their name) will stand high in the northeast sky. 

In fact, three "windows" of dark skies will be available between moonset and the first light of dawn on the mornings of Oct. 18, 19 and 20.

Generally speaking, there will be about 150 minutes of completely dark skies available on the morning of the 18th.This shrinks to about 100 minutes on the 19th, and to about 50 minutes by the morning of the 20th.

This skywatching table shows prime Orionid meteor shower viewing times for some select U.S. cities.

In the table, all times are a.m. and are local daylight times.  "Dawn" is the time when morning (astronomical) twilight begins.  A "Window" is the number of minutes between the time of moonset and the start of twilight.

For example:  When will the sky be dark and moonless for Orionid viewing on the morning of Oct. 20 from Houston? 

Answer:  There will be a 50-minute period of dark skies beginning at moonset (5:16 a.m.) and continuing until dawn breaks (6:06 a.m.).

Perhaps up to a dozen forerunners of the main Orionid display might appear to steak by within an hour’s watch on these mornings, particularly on the 20th, the morning before the peak. It might even be worthwhile to try on Thursday morning, Oct. 21, although for most places, the moon will not set until just after the first light of dawn. 

Halley's legacy

In studying the orbits of many meteor swarms, astronomers have found that they correspond closely to the orbits of known comets.

The Orionids are thought to result from the orbit of Halley's Comet, as some of the dust that has been shed by this famous object intersect earth’s orbit around the sun during October.

There are actually two points along Halley’s path, where it comes relatively near to our orbit. Another one of these points occurs in early May causing a meteor display from the constellation Aquarius, the Water Carrier.

The tiny particles that are responsible for the Orionid and Aquarid meteors are – like Halley itself – moving through space in a direction opposite to that the earth.  This results in meteors that ram through our atmosphere very swiftly at 41 miles (66 km) per second. Of all the meteor displays, only the November Leonids move faster.      

Orionid postmortem

After the peak, activity will begin to slowly descend, although most of the meteors will be squelched by the light of the moon. Rates drop back to around five per hour around Oct. 26. The last stragglers usually appear sometime around Nov. 7.

It is indeed unfortunate that the Moon will likely obliterate most of the Orionids in the nights following the peak, but the viewing odds will be much better before the break of dawn on those mornings leading up to the peak. Almost certainly, you should sight at least a few of these offspring of Halley's Comet as they streak across the sky.

In the absence of moonlight a single observer might see at least a couple of dozen meteors per hour on the morning of the peak, a number that sadly can not be hoped to be approached in 2010. In fact, it appears that this year, fans of the Orionids will be uttering the same lament that the old Dodger fans in Brooklyn used to:  "Wait till next year!"