See what cosmic dust can do! Head outside this weekend for the peak of the Orionid meteor shower and an eyeful of zodiacal light.
It's all about the dust. Something that most people consider a nuisance or even a danger plays a crucial role in so many universal processes. Without it, there'd be no planets and no us, since dust is required to build these things. Dust provides the nuclei upon which water vapor condenses to form rain and clouds, and by extension, rainbows. No dust, no rainbows.
Comet tails? Dust. Meteor showers? Dust again. This week, the wasteful ways of Comet Halley will literally come to light as particles shed by the comet from coma and tail tear through the upper atmosphere. Earth's orbit intersects that of the famous comet twice a year, first in early May to bring the Eta Aquariid meteor shower, then again in the third week of October to fire up the Orionids.
The shower's expected to peak on Sunday morning, October 22nd. For a couple days before and after, rates will be around 10 per hour, but at maximum, we can expect up to 25 dusty darts per hour to shoot from Orion's upraised club under dark skies. No worries about the Moon, either, which sets in the evening sky long before the radiant rises.
Orionids are swift, striking the atmosphere at 238,000 km/hour, a combination of the stream's speed and Earth's orbital velocity of 108,000 km/hour — the nightside of the planet faces directly into the shower in the early morning hours so we get bonked head on. Orionid meteors can zip across the sky so fast, I've done double-takes wondering if what I just saw was a meteor or not.
Meteors start as meteoroids, solid particles in space in orbit about the Sun. They range in size from grains of sand to small pebbles and generally weight less than a gram or two. But what they lack in mass, they make up for in speed.
Much of the kinetic energy possessed by a moving meteoroid is converted into heat and light when it strikes the air between 80 and 120 km above our heads. Air molecules slam into and excite atoms in the particle, while the particle's extreme speed excites molecules in the surrounding air. The clash pumps the electrons in both materials into higher energy states, but only briefly. A moment later they return to their previous "relaxed" states, launching packets of light (photons) of different colors in the process. We see that this all as a bright streak or "shooting star." A blue-green meteor betrays the dust's excited magnesium, an orange one, sodium.
Most meteor showers produce occasional meteors that leave a wake or train that can last for many seconds. They're caused by free electrons sprung loose from their comfy atomic homes to wend their way back to their parent ions for a luminous reunion. Every meteor is another example of how the finger of the cosmos touches Earth. Let there be light.
Use the scroll and left button on your mouse to explore this interactive graphic of the Orionid meteor shower.
Peter Jenniskens / Ian Webster
While the radiant, the point in the sky from which the Orionids appear to stream, is up by midnight, you'll get the best view of the shower when Orion stands tall in the southeastern sky between 2:00 and 5:30 a.m. Meridian crossing occurs at 5 a.m. local time. Plan to spend an hour or more with the shower to see a good assortment of meteors. I don a warm coat and lay out on sleeping bag on the deck or driveway in a state of relaxed awareness, ready for whatever might come.
As always, sporadic meteors are part of the mix. These "strays" pepper the sky at the rate of 4–8 meteors per hour toward dawn and are easily parsed from shower meteors by following their trails backwards — if they don't point to the radiant, they're pretenders.
If you have a camera and tripod, see if you can capture an Orionid or two with a time exposure. I use a 20-mm focal length lens set to f/2.8 (wide-open aperture), ISO 1600, and 30-second exposures. You can use your finger to press the shutter button or purchase an inexpensive intervalometer on eBay or Amazon, or at your local camera store, that will automatically take pictures at set intervals, thereby freeing you up to relax and watch meteors. Point the camera off to one side of the radiant, or for something more scenic, include the shower's namesake constellation in the photo. One bit of advice: regularly check your front lens element. In cool, damp conditions it can fog up in as little as a half hour. A quick blast from a hair dryer will take care of the problem.
If you plan to watch the shower, stay up a little longer into early twilight for yet another manifestation of the beauty of dust — the zodiacal light. The light has two seasons, dusk in spring and dawn in autumn. If you have a dark eastern sky and face that direction about two hours before sunrise, you'll notice a big cone of diffuse light, broad at the base and tapering along its length. The soft, glowing nature of the light resembles that of the Milky Way. But while the Milky Way’s appearance results from the combined light of billions of distant suns, the zodiacal light originates from sunlight scattered off quadrillions (at least!) of tiny, dust-mote sized comet grains and bits of asteroid debris.
The dust nearest the Sun is lit brightest, hence the bright and broad base of the cone. The farther up and away you look from the Sun, the less intense the scattered light and the fainter the cone becomes. Though visible well before dawn, I've found the zodiacal light most impressive at the very start of twilight or about 1 hour 45 minutes before sunup.
Like you, I'm hoping for clear skies this weekend. My wish, as always, is to see more of what dust can do before I'm forced to bite it.