Origins by Neil DeGrasse Tyson

Author:Neil DeGrasse Tyson
Language: eng
Format: epub
Publisher: W. W. Norton & Company
Published: 2012-07-05T00:00:00+00:00

CHAPTER 12

Between the Planets

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 Sun, together with all its planets and their moons, would occupy little more than one trillionth of all the space in that sphere. This result, however, assumes that interplanetary space is essentially empty. Viewed close up, however, the spaces between the planets turn out to contain all manner of chunky rocks, pebbles, ice balls, dust, streams of charged particles, and far-flung man-made probes. Interplanetary space is also permeated by immensely powerful gravitational and magnetic fields, invisible but nonetheless quite capable of affecting the objects in our neighborhood. These small objects and cosmic force fields present a serious ongoing threat to anyone who attempts to travel from place to place in the solar system. The largest of these objects likewise pose a threat to life on Earth, if they happen—as they certainly do on rare occasion—to collide with our planet at speeds of many miles per second.

Local regions of space are so not-empty that Earth, during its 30-kilometer-per-second orbital journey around the Sun, plows into hundreds of tons of interplanetary debris per day—most of it no larger than a grain of sand. Nearly all of this matter burns in Earth’s upper atmosphere, slamming into the air with so much energy that the incoming particles vaporize. Our frail species evolved beneath this protective blanket of air. Larger, golf-ball-size pieces of debris heat rapidly but unevenly, and often shatter into many smaller pieces before they vaporize. Still larger pieces singe their surfaces but otherwise make their way, at least in part, down to the ground. You might think that by now, after 4.6 billion trips around the Sun, Earth would have “vacuumed” up all possible debris in its orbital path. We have made progress in this direction: things were once much worse. During the first half billion years after the formation of the Sun and its planets, so much junk rained down on Earth that the impact energy generated a strongly heated atmosphere and a sterilized surface.

In particular, one hunk of space junk was so substantial that it led to the formation of the Moon. The unexpected paucity of iron and other high-mass elements in the Moon, deduced from the lunar samples that the Apollo astronauts brought to Earth, indicates that the Moon most likely consists of matter spewn from Earth’s relatively iron-poor crust and mantle by a glancing collision with a wayward, Mars-sized protoplanet. Some of the orbiting flotsam from this encounter coalesced to form our lovely, low-density satellite. Apart from this newsworthy event about 4.5 billion years ago, the period of heavy bombardment that Earth endured during its infancy was similar to that experienced by all the planets and other large objects in the solar system. They each sustained similar damage, with the airless, erosionless Moon and Mercury still preserving most of the craters produced during this period.

In addition to the flotsam left

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