Orionid meteors appear every year when Earth travels through an area of space littered with debris from Halley’s Comet. Credit: NASA/JPL
Perhaps the best known of all comets is Halley (correctly pronounced so that it rhymes with rally, not daily) named after the 17th-18th century astronomer Edmond Halley. This familiar object is not only popular but also very reliable, visiting the inner solar system every 76 years. Unfortunately, this is a long time to wait; Halley last came into view in 1985/1986 and won’t be back again until 2061. That means another 37 years of waiting, but thanks to the convenient interplay between Earth and Halley debris floating in space, sky watchers have another way to see at least small parts of this “dirty snowball”, in the form of the Orionid Meteor Shower.
Every time Halley nears the Sun in its orbit, the comet warms up and discharges some of its gases and dust. Over time, these sand-sized grains of debris spread throughout the comet’s orbit. When the orbits of Earth and the debris intersect, many of the particles come in contact with Earth’s atmosphere and burn up, creating a shower of meteors. If they don’t burn up completely, the leftover fragments that hit Earth are known as meteorites.
The debris from Halley’s Comet meets up with Earth’s atmosphere twice a year, once in October to produce the Orionid Meteor Shower and again in May for the Eta Aquarid Meteor Shower. The Orionids are some of the fastest moving of any meteor showers. The fragments of Halley travel at speeds of about 66 kilometers per second (41 miles per second or 148,000 miles per hour). At this rate, one could cover the distance from Flagstaff to Phoenix in less than four seconds.
A few meteors are visible on any given night when dust and other solar system junk sporadically enter Earth’s atmosphere and burn up. Meteor showers, besides being associated with a specific cloud of cometary remnants, are also more predictable and generally consist of a greater number of meteors.
Meteors are often called shooting stars or falling stars but have nothing to do with stars. These misnomers go back to the days when the true nature of meteors was misunderstood, and they were believed to be the remnants of stars. In fact, the term meteor has been used to describe many objects in the sky, including not only “shooting stars”, but also clouds, raindrops (the term hydrometeor is still sometimes used to describe a raindrop) and more. A holdover of this concept is our modern term “meteorology,” which is not the study of “shooting stars” but rather the study of the weather.
Viewing meteor showers is easy. Most observers lie on a blanket or sit back on a chair and look up at the sky. Although the meteors may be seen at any time of night, the majority are visible after midnight, when Earth is turning into the path of the oncoming debris. While the Orionid Meteor Shower runs from about October 3 through November 12, activity will peak on the night of October 21 and the morning of October 22. While 20 or so meteors would normally be visible per hour during the peak, this year’s shower will be largely washed out by light from the gibbous Moon. In any event, sky watchers should still be able to see at least a few of these Halley leftovers.