Orion’s Arm Pit

Why do the Orionids (meteor shower) always come from Orion’s arm pit, and only just before dawn?

And what about the Leonids next month and the Geminids in December — how do they all know to come in those same hours, and why do each of those meteor showers seem to come from a different place such as the Orion constellation or Leo or Gemini, but the Leonids always from Leo, the Geminids from Gemini and the Orionids always from Orion?

How does all that work?

As you probably expect, there’s a reasonable explanation but, until I asked a professional astronomer friend who also shoots Bullseye, I didn’t know.

But in the time it took us to walk down and score our Rapid Fire targets one Tuesday evening, he explained.

Aha!

Earth’s path through the debris field. I’m in Ink Blob #1 and see the impacts. You, in Ink Blob #2, are sleeping in a hotel in Hawaii and can’t see what I see in the paper-thin atmosphere above my head. (Click for bigger.)

I drew a picture the next day and, coupling that with a little Googling for some hard numbers, I think I get it.

First, meteors are the uncontained trash of the universe cast off by comets. Comets are frozen balls of ice and small bits of rock and, when warmed by the sun, they litter the road behind them.

It’s like Saturday night with the teens all driving around in their cars filled with friends as they circulate from one drive-in to the next. Soda straw wrappers, empty coke cups, crumpled white bags and the occasional, used unmentionable fly out the windows and mark the paths.

Similarly, comets have been scattering their trash along the paths they’ve followed around our sun, but for a lot longer than Burger King has been flame-broiling hamburgers. All along these multi-billion year old paths you’ll find little bits of rock and ice still trailing along behind the comet, but running way, way back, literally all the way around the sun.

When the Earth passes through one of those trash-filled paths, we see shooting stars as we slam into the little flecks, heat them up and torch them.

We do that because we’re moving around the sun at such an enormous velocity — we have to go more than 67,000 MPH to get all the way around the sun in a year — and so when we plow into any of that litter, friction with our atmosphere heats it white hot. It burns up.

Just for comparison, the now-retired space shuttles re-entered the Earth’s atmosphere at about 17,000 MPH and Apollo 11, when it returned from way “up there” on the moon, was moving right along at more than 24,000 MPH when it hit the atmosphere. Both of those vehicles were designed to withstand the heat, but only if they came in at the correct angle to avoid too much atmosphere too fast.

Those little flecks of cosmic debris that we’re hitting at 67,000 MPH don’t have a chance.

They’re torched!

Okay, you might say, since we follow, more or less, the same path each time around the sun, I can see why we cross those old comet paths each year at about the same time, but why do these regular meteor showers always happen before dawn and always seem to come from the same constellation?

Good question. There are a couple of facts that help explain this.

First, it’s probably obvious that the burning meteors are usually not bright enough to be seen in daylight hours, nor are you looking up at the bright sky very much. Even if they happened during the day — a few do, but they’re rare and they won’t be the recurring “meteor shower” type — repeating showers of burning meteors are, therefore, most easily seen when the sky is dark.

Next, if you compare the thickness of our atmosphere (about 100 miles) to the diameter of our planet (8000 miles), you’ll see that the atmosphere is comparatively thin. Shooting stars happen even closer, about 30 to 80 miles up. That lower altitude means they have to be almost directly overhead to be seen.

That is, if you’re standing on the Earth — say in “Ink Blob #1” in the diagram — you’ll see meteors directly overhead burning up in the atmosphere but, as you move farther and farther away, the Earth’s curvature starts getting in the way. At some point, you won’t be able to see something “over there” that’s 30 miles up. And someone that’s way over in “Ink Blob #2” won’t be able to see the atmosphere above your head at all — it’s too far around.

So, if you connect “Earth slams through the debris field of dirt from a comet” with the fact that the atmosphere is very thin, that explains why meteors almost always seem to emanate (come from) a point directly overhead.

And the only time “directly overhead” is on the front of the bus (Earth) as it plows through that debris field is at dawn.

And, at dawn on that day we pass through that debris field each year, way off in the direction that’s directly overhead will be the same constellation each year.

Thusly, the Orionids appear

  1. at the same time every year — when we pass through the debris trail from Halley’s comet,
  2. just before dawn — because that’s when we are on the part of Earth plowing through that debris,
  3. directly overhead — because the atmosphere is relatively thin and we simply aren’t able to see “meteors in the atmosphere” much further around the curve, and
  4. the constellation of Orion is, on that day, what Earth happens to be aimed toward as we curve our way around the sun.

Voila!

(Thanks, Chris, I get it.)

But meteor showers do not account for all the shooting stars we see. Sometimes instead of us running into them, they run into us or, worse, we have a head-on collision!

And sometimes they’re not made of paper straws and napkins. Sometimes they’re great big chunks of iron and rock.

Big!

A strike by one of those is what some scientists say killed off the dinosaurs. That was an “extinction event.”

On the other hand, if we’re lucky and the “strike” has the meteorite merely skimming through the air instead of hitting the ground, there will be reports spanning hundreds of miles as it streaks along overhead. Lots and lots of people can see it.

Fortunately, almost all of these burn up completely because even though our wrapper of atmosphere isn’t that thick, when something going that fast hits it, the air feels really dense, generates immense heat, and most meteorites simply melt and drizzle away in tiny droplets that cool and eventually “rain down” as dust.

Rarely, however, one may explode from the incredible heat. When that happens and it’s high up, we may see a spray of flaming meteors shooting out from the explosion.

But if it’s low when it explodes, that could be bad. Real bad.

The Tunguska Event in 1908 is widely believed to be a low altitude explosion of a big chunk of rock or iron and it unleashed the energy equivalent of 10-15 megatons of TNT, on the order of the largest nuclear weapons on Earth. At the estimated altitude of 3-6 miles (15,000-30,000 feet, airplane territory), it could easily have obliterated an entire city. Fortunately for you and me, it took place in a remote spot in Russia.

So, thank you, I’ll stick with the “grain of rice” variety that’s common with the meteor showers that recur each year, each one when we pass through its finely ground-up debris field.

Maybe a little trash isn’t that bad after all.

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