Monday, February 1, 2016

Where Do Killer Meteorites Come From?

I was browsing through social media when I came across someone asking an intriguing question:
How do meteors leave the asteroid belt?

This question brought to mind a science story I knew – about the asteroid that killed the dinosaurs.

I'll get to that, but first, a sub-question: do meteors even come from the asteroid belt?

Yes and no. Any small floating rock in space (a meteoroid) might end up entering Earth’s atmosphere, where it will either burn up completely (a meteor) or survive its trip and crash into the ground (a meteorite). These meteoroids can come from many sources. Most famously, our planet is treated every few months to a meteor shower, but those meteors don’t come from the asteroid belt, they come from comets.

Comets circle the sun just like the Earth does, but instead of a nice, mostly-circular path, comets swing very close to the sun, then shoot way out, sometimes into the deepest reaches of the solar system. These oblong orbits can intersect the rounder orbits of planets, and since comets are constantly disintegrating, crossing the orbit of a comet means plowing through a trail of tiny pieces of ice and rock. These particles burn up in the atmosphere and voila! A meteor shower! Earth crosses the paths of several comets on its journey around the sun, hence our several annual meteor showers.

But back to the asteroid belt. The belt is home to a swarm of meteoroids (which are small) and asteroids (which are large), ranging in size from millimeters to kilometers. But these asteroids tend to be quite comfortable in their orbits around the sun, so what can cause an asteroid to leave its happy orbit and go hurtling away? Well, the less dramatic option is gravitational interactions over time (often with Jupiter), which can slowly force an asteroid’s orbit to change. The other option is crashing into another asteroid.

The craters all over asteroids like 951 Gaspra are evidence of
collisions with smaller meteoroids.
Image: "951 Gaspra" from NASA.
There exists a group of asteroids collectively called the Baptistina family, a name first presented to me as “Baptistina: The Asteroid that Killed the Dinosaurs!” Asteroid families are thought to be formed when a big asteroid crashes and breaks into lots of smaller asteroids with similar features and orbits. The date of the Baptistina collision was originally estimated at about 160 million years ago, ideal timing for a big fragment to have careened off to a place where Jupiter’s gravity could shift its orbit enough to send it out of the belt and into the inner solar system, where it could have reached Earth just in time for the end-Mesozoic extinction at 66 million years ago!

This idea was short-lived, though. A more recent analysis changed the estimated collision date of Baptistina to about 80 million years ago, which astronomers generally think doesn’t leave enough time for it to also have produced the dino-killer. Oh, well.

So yes, asteroids can leave the belt. In fact, most of the meteorites found on Earth (large and small) are thought to have come from the asteroid belt. Many may have been thrown off-course by collisions.

So a new question arises: What causes asteroids collide with each other?

There are many millions of asteroids in the asteroid belt, which sounds crowded, but remember that the asteroid belt is huge. Really huge. So huge, in fact, that the millions of asteroids are separated from each other by, on average, hundreds of thousands of kilometers. Asteroids aren’t colliding with each other every day – they spend most of their time nowhere near each other.

Images like this make the asteroid belt seem densely populated, but
space is big, and asteroids are comparatively tiny. There's lots of space between them.
Image from Wikimedia Commons.
So what makes an asteroid close that distance and body slam another? Well, asteroid orbits vary in a lot of ways: 1) distance from the sun; 2) the shape of the orbit (round vs. oblong); 3) the “tilt” of the orbital path; and 4) the velocity of the asteroid at different points of its orbit, just to name a few. What’s more, over time, an asteroid’s orbit can change due to gravitational “nudges” from other bodies in the solar system. Given enough time, these variations eventually can bring two asteroids dangerously close. And if a collision breaks a big asteroid into 100 smaller asteroids, some might find stable orbits in the belt; some might end up being nudged completely out of the belt; and some might go on to participate in more asteroid collisions.

For a long time, astronomers figured these collisions were so rare they didn’t expect to catch one in the act. They were wrong. In 2010, the Hubble Space Telescope and the Swift Gamma-Ray Burst Mission caught sight of two asteroids that had suddenly grown comet-like “tails” of debris – the wreckage from recent collisions. It seems these collisions aren't as uncommon as previously suspected, although astronomers still think major collisions - the kind that could produce killer asteroids - are pretty rare.

Actually, one of those colliding asteroids that was spotted - named P/2010 A2 - is part of the Flora asteroid family, which has also been suggested as a possible origin for the dino-killing asteroid.

There’s actually another source of Earth-bound meteoroids that I haven’t mentioned. Let’s say two asteroids collide, and one of its fragments is nudged out of the belt, falls into the inner solar system, and smashes into Mars. This collision is so powerful, it throws a bunch of Mars rocks into space, where one of them hurtles around for a few millions years before crashing into Earth. Boom. Meteorite from Mars. The same thing could happen with the Moon. Or any other rocky body in the solar system.

And in fact, it does. Hundreds of small meteorites on Earth have been identified as Martian or Lunar meteorites. This is the part where most people would be tempted to go off on a tangent about Panspermia, but I'll leave that for another post.

So to answer the big question: Where do meteors and meteorites come from?
They come from the asteroid belt, also from comets, and even sometimes from other planets or moons. The biggest ones, though, probably did come from the asteroid belt. The asteroid that took out the dinosaurs and most of Earth's Mesozoic life vaporized long ago when it impacted, but its family members are probably still floating around out there - and perhaps, so is the impactor that sent it our way...

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