If this were some 1950s sci-fi thriller, the Doomsday Cloud would loom dark and ominous in the evening sky. Each night, more and more stars would wink out along its edges. The cloud would sweep past Jupiter, swallowing it whole, and race on toward Earth. There’d be an inky darkness at noon. And so on.
I don’t like the doomsday business, the killer cloud, all that stuff, says Priscilla Frisch, a University of Chicago astronomer who has spent decades studying the wispy matter that lies between stars. And no, nothing is going to blot out the sun. But recent observations and numerical simulations suggest that eventually (in a few millennia, maybe) the solar system will plow into a cloud of gas and dust a thousand times denser than the space we travel through now. This soupy cloud will reduce the sun’s sphere of influence until most of the outer planets are sitting naked in interstellar space. Dust and gas will penetrate as far as Earth’s orbit and might begin eating away at the oxygen in our upper atmosphere. The solar wind, now greatly compressed, will no longer provide adequate protection from the high- speed electrons and ions ripping through space. These cosmic rays will tear straight into the atmosphere, to the detriment of the delicate molecules of life.
It isn’t exactly Million Dollar Movie material, but it is one bad cloud.
For worrywarts, comets and asteroids have been the bugbears of choice in recent years. An asteroid, after all, is thought to have wiped out the dinosaurs, and the statistical threat of another strike has led some scientists to propose a $50 billion asteroid defense program. Galactic hazards are almost certain not to occur in our lifetime, but if one did, it would make a stray asteroid look like child’s play. A giant interstellar cloud would mess up the entire solar system for decades. Until recently, worry about such events rested on mere speculation. In the last couple of years, however, astronomers have found real hazards associated with actual objects in the sky. For example, according to the latest calculations, the sun will soon be getting a visit from a stellar neighbor—a visit that might send a rain of comets hurtling toward Earth. (Don’t panic. Astronomers have a much different sense of time than the rest of us. When they say soon, they’re talking tens of thousands of years from now.) And the edge of a possibly disastrous cloud of interstellar gas is less than 4 light-years away. Of course, it might take 10,000 non-light-years to get here, but in cosmic terms that’s a mere heartbeat.
