Meet the asteroids

Are they gonna hurt us?

Will they obliterate the earth?

How are asteroids found?

Did asteroids deliver life?

What are they made of?

What about comets?

Asteroids in orbit

Like, chaotic
Why are most asteroids stuck in orbit beyond Mars, while a few manage to escape and cross Earth's orbit? Remember, orbiting in the asteroid belt is a mellow place to be (if you don't need to breath or anything ...). The forces are balanced, and you just keep circling the sun as regular as time, tides and taxes.


Until about 20 years ago, scientists couldn't account for "Earth-crossing" asteroids, or for meteorites, either. They couldn't figure out what would nudge asteroids from their sleepy orbits.

Then astronomer Jack Wisdom of the Massachusetts Institute of Technology showed how gravity could do the trick. He calculated that some asteroids would be pulled repeatedly by Jupiter's gravity until their orbits became so eccentric -- egg-shaped -- that they would start crossing Earth's orbit, possibly becoming collision hazards.

What seems to happen is that when an asteroid and a planet repeat a certain configuration, the forces "don't average out," says Alessandro Morbidelli, an astronomer at the National Council for Scientific Research in Nice, France, but instead conspire to rip the rock from its circular orbit.

For a rough approximation, imagine that you were pacing the kitchen, and every time you neared the fridge, you nibbled some cheesecake. Pretty soon, you'd have taken a lot of nibbles. Just as your waistline might reflect all these "just-one-more-bite" bites, the gravitational pull of Jupiter adds up to send a few asteroids our way.

A temporary asteroid
But one small problem remained. Once asteroids do become Earth-crossers, their days are numbered, since they become prey to the sun's huge gravitation. "Near-Earth asteroids are transient -- they're unstable," says Morbidelli. "They are scattered by Earth, and as their orbits grow more elliptical, their end state is to collide with the sun."


According to Morbidelli, the maximum lifetime for Earth-crossing asteroids is 10 million years, but most only last two to five million years.

If asteroids are so, well, temporary, then calculations based on Jupiter's gravity cannot explain the number that appears on astronomers' plates. As Morbidelli says, the existing mechanisms "were so efficient at funneling asteroids out toward the sun that we needed to find new sources."

Brother can you spare a planet?
The original computer simulations ignored the inner planets -- Mercury, Venus, Earth and Mars. But when Morbidelli and his colleagues added the inner planets, more asteroids suddenly began zooming into our neighborhood (see "Dynamical Lifetimes of Objects..." in the bibliography).

Oddly enough, tiny Mars -- with only 14 percent of Jupiter's gravity -- played a major role in explaining the Earth-crossing asteroids, although as Morbidelli acknowledges, "It may be astounding that Mars is so effective in stimulating chaos in the belt, because it is not massive."

Did somebody say "chaos"? The Why Files is interested... Essentially "chaos" means that small perturbations -- astronomese for "disturbances" -- can cause large changes in orbits. Indeed, the improved simulation produced an inner asteroid belt that "is almost entirely chaotic," Morbidelli says.

When Morbidelli projected the motion of 400 real asteroids, 25 percent of them reached inside Mars' orbit in the next 100 million years. Indeed, he estimates that about 50 asteroids larger than five kilometers across -- capable of packing a wondrous wallop -- could cross Mars' orbit within the next 25 million years. And most of them would eventually become Earth-crossers too, he says.

Translation: we'll need to hunt Earth-killer asteroids for at least the next million centuries...

Need to hunt up more reading on dangerous orbiting rocks?

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