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Between the Planets
Natural History, Oct, 2001 by Neil deGrasse Tyson
The emptiness of interplanetary space has been much exaggerated.
From a distance, our solar system looks empty. If you enclosed it within a sphere large enough to contain the orbit of Neptune, then the volume occupied by the Sun, the planets, their moons, and all asteroids would take up little more than one-trillionth of the space. When viewed close-up, however, the space between the planets contains all manner of pebbles, chunky rocks, ice balls, dust, streams of charged particles, and far-flung probes. The space is also permeated by monstrous gravitational and magnetic fields.
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Space is so not-empty that Earth, during its eighteen-mile-per-second orbital journey, plows through hundreds of tons of interplanetary debris per day--much of it no larger than a grain of sand. Nearly all of it burns in Earth's upper atmosphere, slamming into the air with so much energy that it vaporizes on contact. The larger, golf-ball-sized pieces of debris heat fast but unevenly and often shatter into smaller pieces before they vaporize. Still larger pieces get their surfaces singed but otherwise make it all the way to the ground intact. You'd think that by now, after 4.6 billion trips around the Sun, Earth would have vacuumed up all possible debris in its orbital path.
But things were once much worse. For half a billion years after the formation of the Sun and its planets, so much junk rained down on Earth that the energy from the impacts sustained a heated atmosphere and a molten surface.
One hunk of junk in particular was quite substantial--it's what led to the formation of the Moon. The unexpected paucity of iron and other high-mass elements in the Moon (deduced from lunar samples returned by Apollo astronauts) indicates that the Moon most likely burst forth from Earth's iron-poor crust and mantle when our planet had a glancing collision with a wayward, Mars-sized protoplanet. The orbiting flotsam resulting from this encounter coalesced to form our lovely, low-density satellite. Apart from this newsworthy event, the period of heavy bombardment that Earth endured during its infancy was not unique in the solar system: all the planets and other large bodies sustained similar damage, with the airless, uneroded Moon and Mercury preserving much of the cratered record from this period.
Not only is the solar system littered with the detritus of its formation, interplanetary space also contains rocks of all sizes that were thrust from Mars, the Moon, and probably Earth as the ground recoiled from high-energy impacts. Computer studies of meteor strikes demonstrate conclusively that surface rocks near ground zero can get thrown upward with enough speed to escape a celestial body's gravitational tether. Meteorites originating from Mars turn up so often on Earth that scientists have concluded that as much as a thousand pounds of Martian rocks may rain down on us each year. Perhaps the same amount reaches Earth from the Moon. Indeed, we didn't have to go there to retrieve Moon rocks, although we didn't know this during the Apollo program. Plenty come to us, even if they are not of our choosing.
If Mars ever harbored life--billions of years ago, when its surface was wet with liquid water--then unsuspecting bacteria stowed away in the nooks and crannies (especially the crannies) of the ejected rocks could have traveled to Earth for free. We already know that some varieties of bacteria can survive long periods of hibernation as well as the high doses of ionizing radiation to which traveling microorganisms would have been exposed en route to Earth. So the existence of space-borne bacteria is neither a crazy idea nor pure science fiction. The concept even has an important-sounding name: panspermia. If Mars spawned life before Earth did, and if simple life traveled from Mars on an ejected rock and seeded Earth, then we may all be descendants of Martians. Awareness of this fact may also obviate environmentalists' fears about astronauts sneezing on the red planet's surface, spreading their germs on the alien landscape.
Most of the solar system's asteroids live and work in the main asteroid belt, a somewhat flat zone between the orbits of Mars and Jupiter. Often drawn by artists as a region of cluttered, floating rocks in the plane of the solar system, the asteroid belt's total mass is less than 5 percent that of the Moon, which is itself not much more than 1 percent the mass of Earth. Sounds insignificant. But accumulated perturbations of the asteroids' orbits continually feed a deadly subset of objects, perhaps a few thousand, whose eccentric paths intersect Earth's orbit. A back-of-the-envelope calculation demonstrates that most of them will hit our planet within the coming 100 million years. Those larger than about a mile across will bang into Earth with enough energy to destabilize our ecosystem and put most land species at risk of extinction. That would be bad.
Asteroids are not the only space objects that pose a risk to life on Earth. The Kuiper belt is a circular, comet-strewn swath of real estate that begins just beyond the orbit of Neptune, includes Pluto, and extends perhaps as far again from Neptune as Neptune is from the Sun. The Dutch-born American astronomer Gerard Kuiper advanced the idea that frozen leftovers from the formation of the solar system reside in the cold reaches of space beyond Neptune's orbit. Without a massive planet to fall upon, most of these comets will orbit the Sun for billions more years. Like their counterparts in the asteroid belt, a subset of Kuiper belt objects travel on eccentric paths that cross the orbits of other planets. Pluto and its ensemble of siblings called Plutinos cross Neptune's path around the Sun. Other Kuiper belt objects plunge all the way down to the inner solar system, crossing planetary orbits with abandon. One of these is Halley, the most famous comet of them all.
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