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Debris: friend or foe? Both! (NASA)

Debris From Space Helped Shape Who We Are

by Adam Frank
Feb 19, 2013

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The Geminid meteor shower makes a December 2012 appearance over the ESO's Paranal Observatory in Chile. But you don't have to be a professional astronomer to enjoy similar sights. All you have to do is go out in the countryside and look up.

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It's the light that really takes your breath away. It's a bright flare, with stark shadows trailing behind it across the Siberian sky. You can get mesmerized by the videos, watching the white-hot incandescence of ancient stone blasting through the atmosphere. Then it's gone, a brief meeting of heaven and Earth, leaving behind a bone-shaking ripple of sonic booms and broken windows.

The 17-meter wide chunk of asteroid that became a meteor streaking over Chelyabinsk, Russia, on Friday caused a worldwide sensation. Its not every day that space drops a 500-kiloton explosion into the skies of a populous, camera-laden city. It was a collective moment of amazement that touched the entire planet. But before the images fade into yesterday's news, we might take just a moment to pause and reflect.

We might let this encounter wash over us, let it lift us out of the moment's wow-ness and into a contemplative space. With nothing more than a clear night, a nice patch of grass and and some spare time, any one of us can lay on our backs and wait for the meteors.

Shooting stars are always there, waiting for us to notice. Last Friday's eye-popping, ear-splitting meteor over Russia was an invitation to look up. It was a call from the heavens to gaze into the skies and come to terms with life's cosmic context.

The message, you see, is that it is all about debris.

The solar system was a long-term construction project. It began as a vast tumbling cloud of interstellar gas and dust that stretched across light years — about 10 trillion kilometers, if you can wrap your mind around that number. Collapsing under its own gravity, this cloud folded inward upon itself. Some material fell directly into the cloud's center, forming a bright natal sun. But like a spinning Olympic skater pulling her arms inward, the parts of the cloud that didn't fall directly on to the new star spun ever faster as they collapsed.

This rapidly spinning gas eventually formed a dense disk surrounding the new sun. Over the next few million years the dust in this swirling frisbee collided and formed tiny pebbles. These sun-orbiting pebbles continued clumping, slowly forming meter-sized rocks in the process. The upward cascade of collisions continued: rocks collided, forming boulders; boulders collided, forming mountains; mountains merged to form the embryos of planets.

After a few hundred million years, the cascade of collisions had largely ended and the sun was surrounded by its modern-day family of planets. Eight worlds ranging from the tiny iron ball that is Mercury to the swirling psychedelic clouds of gaseous Jupiter.

The construction debris, however, never went away. Like piles of waste left over after a building is erected, our home in space remained populated by unused "bricks" tumbling slowly through the interstices of the planetary orbits. Between Mars and Jupiter a belt of asteroids circles the sun, testament to a planet that might have been. Beyond distant Neptune another extended belt of kilometer-sized muddy ice balls float through space. Some of these Kuiper belt objects are now large enough to be called dwarf planets (like poor, orphaned Pluto). Some are small enough to become comets when gravity pulls them into the solar system's bosom. At the solar system's outer reaches — 50,000 times farther than the Earth's orbit from the sun — sits the Oort cloud. It's a region of cometary material patiently waiting for a passing star to knock bits of the cloud into a million-year journey toward the sun.

Rock and ice, pebble and mountain, the solar system's narrative would be incomplete without the story of these lonely interlopers. More importantly, our own story would be incomplete without these actors. The trajectory of life on our world, and perhaps on others, has been both both hostage to, and beneficiary of, these stony intruders.

Planets are gravity wells in the overall architecture of the solar system. From sand-sized bits of rock blown off the surface of comets to stadium-sized asteroids, when debris flies too close to the gravitational pull of a planet their orbits shift. Large enough shifts lead to impacts as planets and debris collide in a blinding flash of light. And every once in a long while, terrible carnage ensues.

These impacts represent a critical chapter in the early story of our solar system. Life on Earth may have started more than once before the planet was sterilized in the cataclysmic impacts of the so-called Late Heavy Bombardment, some four billion years ago.

On the other hand, our planet's blue-green oceans are likely a blessing delivered to our world through collisions with comets and asteroids rich in water. There are even those who argue that Earth's life itself may have formed elsewhere (like Mars), arriving here as a hitchhiker on a stone blown off some other world.

Over billions of years, the density of debris in the solar system slowly dropped. That did not however end the importance of these free-floating building blocks. Impacts have continued across the solar system, perhaps contributing to the thinning of Mars' once-dense atmosphere. And on Earth, of course, the 135-million-year reign of the dinosaurs ended in an instant, a blinding flash as a 10-kilometer wide asteroid hit our planet.

This was the 5th of the six great extinction events in Earth's history. It eliminated 75 percent of all species from the book of life. While it is unclear if any of the other great extinctions came from space, it is clear that there have been many impacts across the eons powerful enough to serve as localized catastrophes.

Meteor Crater is a 1.2-kilometer wide, 170-meter deep crater in the Arizona desert formed 50,000 years ago by the impact of an iron-rich chunk of asteroid. It remains today as a stark reminder of the power of cosmic interlopers, even relatively small ones.

Which brings us back to the night and that patch of grass you're laying on, looking up at the stars. We rarely grant ourselves the freedom to do something as simple — as empty — as staring into the night sky for pleasure. Given the flood of street lights and city illumination, you may even have to work for such freedom, driving out of town to find a space dark enough that the stars might make an appearance for you.

Once you're there, it's time to surrender to the night. It's time to just watch and wait. It may take an hour, or it may take 10 minutes, but it sooner or later it will come. There's a flash, a bright streak across the dark sky. In that split second a gasp of surprise bursts out of your throat, unbidden.

You have seen a shooting star. You have caught a glimpse of a meteor. It's an ancient ritual now given new meaning because, unlike the thousand generations that have come before, you know the truth. You can greet that billion-year-old stone knowing exactly what it is and where it came from.

"Brother or sister," you might whisper into the night, "welcome; your long journey has ended."


You can keep up with more of what Adam Frank is thinking on Facebook and on Twitter: @AdamFrank4

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