Our planet periodically collides with other objects, sometimes-as we were made aware this year by two blockbuster movies-with catastrophic consequences. Recently, I was asked how long we have to wait before we get hit. My answer: It depends on how big an object you're waiting for. Collisions with one-gram meteors occur continually, especially this month and next. Meetings with larger bodies are much more rare.
That jibes with a motif in the universe: smaller things are far more prevalent than bigger ones. For example, there are only 50 billion galaxies in the known universe but some 1080 electrons (a 1 followed by 80 zeros). Thus, apple-seed-size meteors appear overhead about every ten minutes. One-pounders, which light up the sky and cast shadows, materialize every year or two. A house-size meteor might visit just once in a millennium.
To get our terms straight, meteor is generic for anything that enters Earth's atmosphere. Asteroid fragments that find their way here are meteors, as is the skimpy, icy material from a comet and chunks from the moon or Mars. Hurl your aunt Bertha into space, and her reentry through the atmosphere will make her a meteor, too.
So the composition of meteoroids (those same objects in space, before they streak through our sky) is not discernible from their appearance. Mostly, they look different because of their size (bigger equals brighter) and their speed. The velocity range-from a lazy 6 miles per second up to a sizzling 45 mps-is obvious even to the casual observer and depends on whether the object is striking Earth head-on or catching up to us from behind.
For no apparent reason, our planet collides with meteoroids more often in the last five months of the year than it does in the first seven. Not a single major shower occurs between the Quadrantids of January 3 and the famous Perseids of August 11 (which were ruined this year by a bright moon); September is quiet as well. Then, starting in October, our world meets four separate streams of meteoroidal material. Here's a brief guide.
October 8, nightfall to midnight: Draconids. These slow (12 mps) "shooting stars" sometimes put on a rich, active display when their parent comet revisits the inner solar system. That happens this fall. It can be a bust-or real fireworks.
October 21ñ22, best after midnight: Orionids. Very fast (40 mps) meteors associated with Halley's comet; they usually produce one shooting star every three minutes or so. Although it's not a profuse display, half the meteors leave glowing trails. The moon's absence ensures optimal viewing conditions.
November 17, all night, but especially sunset to 9 p.m.: Leonids. In recent years a Leonid has been spotted every two to three minutes. But the return of the parent comet, Tempel-Tuttle, next February has astronomers hoping for a repeat of the famous meteor storm of 1966, when a stunning 100 meteors per second overwhelmed the heavens. More than half of these extremely fast (44 mps) streakers display persistent trails.
December 14, sunset onward: Geminids. Since they're debris from an asteroid, these medium-speed (20 mps) meteors are denser than most. The abundant one-a-minute frequency will probably make this the finest shower of 1998-unless the Leonids or Draconids produce a storm.
Few if any of these meteors will make it to the ground and become meteorites. This is not Armageddon material. But for sheer pleasure, bring a child or companion outdoors and let one of these autumn showers make its own deep impact.