Spring 2015

It Came from Outer Space

Earth’s meteoric past, present, and future

Maria Golia

The solar system has filled up a lot since the dawn of the science of meteoritics in the late eighteenth century; Jupiter, which had four known moons, now has fifty.1 Four asteroids were known by 1807; as of 2013, over six hundred thousand were catalogued—some little more than flying rocks, the biggest a third the size of the moon—but by some estimates they number in the millions.2 Typically depicted as orbiting the sun within a thickly peppered belt between Mars and Jupiter some two hundred million kilometers from Earth, asteroids are in fact at immense distances from one another, though they are not as far apart as the planets. Agoraphobics are advised not to contemplate what the solar system looks like drawn to scale. Reduce Earth to the size of a pea and Jupiter would be three hundred meters away, Pluto two kilometers away “and about the size of a bacterium.” The closest star, Proxima Centauri, would be sixteen thousand kilometers away from the pea-sized Earth.3 Having calculated the possible number of planets that may exist beyond our solar system, Carl Sagan noted that the emptiness between them is so great “that if you were randomly inserted into the universe” the chances of landing on or near one would be less than one in a billion trillion.4 Advances in astronomy suggest that stars and galaxies comprise only four to five percent of a universe composed largely of invisible “dark matter” and the “dark energies” that drive its expansion.5 Science may sketch the contours of this reality, but the intellect, even aided by intuition, can neither grasp nor convey the spaciousness of space.

That Earth should regularly encounter asteroidal, cometary, and planetary debris may seem extraordinary, but our planet whizzes through the quasi-void around the sun at a speed of thirty kilometers per second, in constant collision with such objects. An impact event with the energy of the Hiroshima nuclear bomb occurs around once a year, while a megaton event is expected at least once a century. No one would be around to calculate these probabilities, however, were it not for Earth’s superbly protective atmosphere; most bodies break up when they encounter it and are fragmented and dispersed by aerodynamic stresses before they reach its lower realms. More discrete is the microscopic powdering of interplanetary and interstellar dust that Earth receives annually by the tens of thousands of tons. This dust contains enough micrometeorites (a.k.a. magnetic spherules, average size 0.2 millimeters) for at least one to fall on each meter of the planet’s surface per year. Every mundane stretch of highway hosts some tiny relic of the early solar system.6

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