All About the Orionids Meteor Shower

The meteors of the Orionid Meteor shower appear to radiate from the direction of the constellation Orion. They are, in reality, bits of dust from a comet vaporizing in Earth's upper atmosphere. Carolyn Collins Petersen

Every year, Earth passes through a stream of particles left behind by Comet Halley. The comet, which is making its way through the outer solar system right now, constantly scatters particles as it moves through space. Those particles eventually rain down through Earth's atmosphere as the Orionids meteor shower. This happens in October, but you can learn more about it in advance lets you be ready for the next time Earth passes through the trail of the comet.

How It Works

Each time Comet Halley swings by the Sun, solar heating (which affects all comets that come near the Sun) evaporates about six meters of ice and rock from the nucleus. Comet debris particles are usually no bigger than grains of sand, and much less dense. Although they are very small, these tiny 'meteoroids' make brilliant shooting stars when they strike Earth's atmosphere because they travel at tremendous speeds. The Orionids meteor shower happens each year when Earth passes through the debris stream of Comet Halley, and meteoroids hit the atmosphere at incredibly high speed.

Studying a Comet Up Close

In 1985, five spacecraft from Russia, Japan, and the European Space Agency were sent to rendezvous with Halley's comet. ESA's Giotto probe captured close-up color pictures of Halley's nucleus showing jets of solar-heated debris spewing into space. In fact, just 14 seconds prior to its closest approach, Giotto was hit by a small piece of the comet which altered the spacecraft's spin and permanently damaged the camera. Most of the instruments were unharmed, however, and Giotto was able to make many scientific measurements as it passed within 600 kilometers of the nucleus.

Some of the most important measurements came from Giotto's 'mass spectrometers', which allowed scientists to analyze the composition of the ejected gas and dust. It's widely believed that comets were formed in the primordial Solar Nebula at about the same time as the sun. If that's true, then comets and the Sun would be made of essentially the same thing—namely light elements such as hydrogen, carbon and oxygen. Objects like Earth and the asteroids tend to be rich in heavier elements like silicon, magnesium, and iron. True to expectations, Giotto found that light elements on comet Halley had the same relative abundances as the Sun. That's one reason why the tiny meteoroids from Halley are so light. A typical debris particle is about the same size as a grain of sand, but it is much less dense, weighing only 0.01 gram.

More recently, the Rosetta spacecraft (also sent by ESA) studied duckie-shaped Comet 67P/Churyumov-Gerasimenko. It measured the comet, sniffed its atmosphere, and sent a landing probe to gather first-hand information about the comet's surface. 

How to View the Orionids

The best time to view the Orionid meteors is after midnight when Earth's rotation aligns our line of sight with the direction of Earth's motion around the Sun. To find the Orionids, go outside and face south-southeast. The radiant, shown on the image here, is near two of the sky's most familiar landmarks: the constellation Orion and the bright star Sirius. At midnight the radiant will be rising in the southeast, and by a.m. Orion will be high in the sky when you face due south. The higher in the sky the radiant is, the better your chances are of seeing a good number of Orionid meteors.

Experienced meteor observers suggest the following viewing strategy: dress warmly, since October nights are likely to be cold. Spread a thick blanket or  sleeping bag over a flat spot of ground. Or, use a reclining chair and wrap yourself in the blanket. Lie down, look up and somewhat toward the south. Meteors can appear in any part of the sky, although their trails will tend to point back toward the radiant.

Edited and updated by Carolyn Collins Petersen.