The Orionid meteor shower may be no match for the August Perseids, but it's special for different reasons: the material responsible for the annual shower comes directly from Halley's Comet, and the meteors it produces zing across the sky in the blink of an eye.
Every October the Earth crosses the trail of dust and pebbles called meteoroids lost by the Comet during its repeated trips to the inner solar system. Heated by the sun, some of Halley's dirty ice (the principal ingredient of comets) boils away and releases the trapped debris. That material settles and spreads across the comet's orbit in a great band that travels like a conveyor belt around the sun.
Twice each year, first in May and then October, Earth's orbital path intersects that of Halley's Comet. Our first encounter produces the Eta Aquarid meteor shower, the second the Orionids.
(The Orionid meteor stream animation is courtesy of Peter Jenniskens and Ian Webster.)
We enter the debris trail starting around Oct. 2 and part ways in early November. Sparse at first, the number of meteors ticks up until reaching a peak of 15-20 per hour on Wednesday morning Oct. 21, the time of shower maximum. In case you're plagued by clouds, numbers will still be moderately good the following night, Oct. 21-22. To get a feel for how the meteoroids stream past Earth, play with the interactive animation above by clicking, dragging and scrolling with your mouse.
Earth literally slams into the dust the same way you'd drive into a rain storm, but instead of water droplets pinging your car's windshield, dust particles strike the upper atmosphere at an altitude around 60 miles (100 km). Each mote moves so swiftly — 147,000 miles an hour (67 km/sec) — that friction with the air heats it to glow. At the same time, the particle's rapid passage through the air excites or ionizes molecules along its path, which respond by giving off light. The combined light of the burning particle and ionized air makes the familiar streak of light called a meteor.
Because the Earth is zooming through Halley's old dust, meteors appear to radiate from a point in the distance called the radiantexactly the same way raindrops or snowflakes radiate from a point ahead of your car when driving through a storm. The radiant gets its name from the constellations in which it appears, in this case Orion.
All Orionids stream from a point in Orion's upraised arm to the upper left of the bright red star Betelgeuse. If you can draw a meteor's track backward to this point, it's a genuine Orionid. If originates from another part of the sky, we call it a sporadic meteor.
Shower members are white, swift and occasionally flare as fireballs, meteors as bright as Venus or brighter. You can start observing the Orionids as early as midnight, when the radiant climbs to 20 degrees (two fists) in the eastern sky. The higher Orion climbs, the more meteors you'll see. That's why the shower will be best from 1 a.m. till dawn, when Orion stands up in the southern sky.
No moon will spoil the show this year. Just find a spot away from the neighbor's yard lights and relax in a reclining chair under a blanket or sleeping bag. There's no preferred direction, since meteors will appear anywhere in the sky, but I like facing south for this shower.
The reality of light pollution means you'll probably see fewer than 20 per hour, but give the shower an hour. Seeing even a few will leave you satisfied. And it's cool to think that every time a meteor streaks across the starry sky, you'll only 60-some miles from Halley's Comet.
In other news, tonight at 5:12 p.m. CDT, NASA will make its first attempt to fetch a sample of the asteroid Bennu with the OSIRIR-REx space probe. The craft will descend to the asteroid and touch the surface for just 16 seconds to retrieve dust and small rocks before returning to orbit. You can watch collection activities live at NASA's website starting at 4 p.m. Central Time (5 p.m. Eastern) and read more details about the mission in my recent article.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.