Asteroids

When you think of asteroids, you probably think of the Millennium Falcon screaming through a densely packed collection of tumbling rocks, swerving every which way to avoid them while fleeing from a Star Destroyer.  At least, you do if you're a nerd like me.  But that's the wrong image.

Why it's true that there are billions upon billions of these rocks, ranging anywhere in size from a few grains of sand to several hundred kilometers across, they're also spread out over millions upon millions of miles.  So unless the rock you're standing on has orbital companions (and several asteroids do have "moons"), chances are slim you'd see many at all anywhere near the one you're standing on.  In fact, the total mass of the asteroid belt is roughly 4% that of the moon.

Some of you might also picture the belt as the remains of a destroyed planet, torn apart by some ancient cataclysm.  This is a cool mental image, but it's also wrong.  The asteroid belt never was a planet - the push and pull of Jupiter's gravity never allowed it to happen, try as it might.

And it did try.  Back in the early days of our solar system, the main belt had a lot more mass - computer models suggest it had roughly the mass of the Earth.  So what happened to all that rock and ice?  Once again,  blame Jupiter.

As discussed in our previous chapter, the inward migration of the gas giant planets, especially Jupiter, sent a good chunk of rocky and icy debris left over in the belt on a collision course with the Earth and the other inner planets, as evidenced by the numerous craters on the inner rocky bodies.  Some of it was also captured in a "Trojan" orbit - following Jupiter's orbit around the sun.  And some of it was flung from the inner solar system to further regions, or sucked into the mass of Jupiter; a few lucky ones managed to remain as moonlets of the gas giants.

The rocky bodies that smashed into Earth added to our planet's mass; the icy ones gave us a sizeable amount of water.  The same was true of Venus and Mars; but on Venus, it was cooked away by high heat and solar radiation; on Mars it remains (mostly) as ice, locked up in permafrost and polar ice caps.

Of course, not all of the asteroids sent inward hit the planets during formation.  Several lasted millions or even billions of years before colliding with anything - and several more remain to this day.  Several of these will never hit an inner planet; there's a lot more space outside of their orbits than within.

Several did hit, however, and some of them caused planet-wide catastrophes, like the one that caused the mass extinction some 65 million years ago.  But these impacts are important for our species even being around.

Imagine a world where the dinosaurs (except, thankfully, for those beautiful birds) hadn't been wiped out by that impact.  Chances are, they would have died off eventually - at least the really big ones - but they would have been around, evolving and changing with the Earth, for millions more years.  Seeing as they were the dominant life form on the planet, this would not have allowed the mammals to take over the way they did (at least not 65 million years ago.  They would have been, more than likely, killed off by ice ages, changes in vegetation patterns, changes in weather brought on by continental drift and volcanic activety, etc).  The death of the dominant life forms allowed mammals to fill the vacant niches and lead to the wide variety of mammals around today, which includes us.

As unlikely as it seems, this impact wasn't the most important to our evolution.  Long before the age of the dinosaurs there was a much, much, MUCH bigger impact that had much bigger consequences for life on our planet.  And you'll read about it in the next chapter.