How Long Will We Be Around?
BY FREDERIC GOLDEN

Remember the young Woody Allen's cosmic angst in the film Annie Hall? Having just read that the universe is expanding, the boy is so worried that he can't do his homework. "Someday it will break apart," he tells his shrink, "and that will be the end of everything." But, his mother snaps, "you're here in Brooklyn! Brooklyn is not expanding!"

Maybe not, but Woody was on to something. Long before the universe finally fades out, his nightmare will come true—though not as he imagined. Now about halfway through its estimated 10 billion-year lifetime, our sun is slowly brightening. In about 1 billion years, its energy output will have increased at least 10%, turning Earth into a Venusian hothouse where plants wither, carbon dioxide levels plummet and the oceans boil off.

It's doubtful Woody's descendants—or anyone else's—will hang around for the cookout. One possible escape route: an exodus to Mars, which is farther from the sun and hence cooler. But it would take a lot of engineering to turn Mars' frozen, Gobi-like surface into a livable habitat. (Among proposals that have been floated: heating the planet with artificial greenhouse gases, deploying huge orbiting mirrors to catch sunlight and sprinkling heat-absorbing soot on the Martian icecaps.) Eventually, says Mars promoter Robert Zubrin, visitors wouldn't need spacesuits anymore.

But terraforming (as scientists call it) a neighboring world is not the only exit strategy. Donald Korycansky of the University of California at Santa Cruz and his collaborators suggest that Earth could be edged out of harm's way with a gravitational slingshot, a trick long used to boost the speed of planetary probes. Earth would be the spacecraft, grabbing orbital energy from a passing asteroid. That would increase Earth's speed and enlarge its orbit. Repeated every few thousand years, Korycansky & Co. reckon, such flybys could stretch Earth's habitable lifetime by billions of years.

Even so, we would still have to quit the sun's neighborhood before it begins its final death throes. With its nuclear fuel exhausted, the fiery orb will collapse upon itself like a giant soufflé, only to see its internal furnace briefly restoked in several last gasps. These will swell the sun's outer layers so they engulf all the inner planets, including Earth, turning it into what astrophysicist Neil de Grasse Tyson calls "a red-hot charred ember." The sun's red-giant phase will be brief, however. Shedding its heat and gases, it will become a cold, compact cadaver no bigger than Earth, a white dwarf lost in space.

Let's assume that humans—or some higher life-form—are still around to witness these cataclysms—an iffy supposition since we've been on Earth for only a fraction of the dinosaurs' time (2.5 million vs. 160 million years) yet already have the power to destroy ourselves. If we're still around, we will have to seek out homes on other planets orbiting other warming stars. That will take some giant leaps. Even the speediest galactic ark would have to travel hundreds of years, during which multiple generations would live and die on board, before reaching even a nearby star like Proxima Centauri, 4.3 light-years away. And in the end, not even such a sanctuary would save our descendants, or any other life-forms still inhabiting the universe, from its last, dying gasp.