For quite the longest time, astronomers thought of the galaxy as a kingdom of independent principalities. Each star held sway in its own little area, mostly cut off from all the others. The Milky Way at large determined the grand course of cosmic history, but the sun ran the day-to-day affairs of the solar system. Gradually, though, it has dawned on researchers that the sun's sovereignty is not so inviolable after all. Observations have shown that 98 percent of the gas within the solar system is not of the solar system--it is foreign material that slipped through the sun's Maginot Line. One of every 100 meteoroids entering Earth's atmosphere on an average night is an interstellar intruder.
"When I was an astronomy grad student in Berkeley in the late '60s, interstellar matter was what you observed towards other stars," says Priscilla C. Frisch of the University of Chicago, a pioneer in this subfield of astronomy. "No one dreamed that it was inside of the solar system today." Telescopes have cobbled together a map of our neighborhood; deep space probes have sampled trespassing dust and gas; and radar facilities have tracked interstellar meteors, which distinguish themselves by their unusually high speeds.
This article was originally published with the title Interstellar Pelting.