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While Prather and others explore the environmental impact of airborne bacteria, Christner is trying to understand this mysterious high-altitude ecosystem as a whole. Working not far from his base at LSU, he is setting out to learn how much is alive in the sky and how far up it lives.
On a cold autumn morning, Christner guides his pickup down a two-lane highway in central Louisiana, speeding past pine thickets, swamps, and dirt roads that don't show up on his truck's navigation system. "I'm not sure where the hell we're going," he admits.
Christner's movements are at the mercy of the shifting winds thousands of feet overhead. He and five other vehicles are following a weather balloon that was launched several minutes ago. "It's at 1,500 metres and still ascending — we're about three kilometers ahead of it on the road," says a voice over the radio.
The balloon's 5 kg payload, including a GPS transponder, is held together with flimsy string and Styrofoam—designed, per FAA regulations, to disintegrate if it encounters the turbines of a passenger plane. Between 3,000 and 9,000 metres, a door will spring open. The device will collect drifting cells in a coat of gooey grease, just as the front grill of Christner's truck is collecting insects. At 10,700 metres the germ catcher will cut its string, drift down on a parachute, and hopefully land where it can be found.
The device has previously come to rest in swamps and rice paddies and tangled itself 18 metres up in a longleaf pine. Today the scientists find it in a dense forest, 1.1 km past a locked gate adorned by two deer skulls with metal stakes driven through their foreheads.
Christner's Ph.D. student Noelle Bryan will grow the bacteria the balloon collects in order to identify them. Later she'll look to see if they carry the gene for ice nucleation. "My guess," Christner says, "is there are probably a whole slew of organisms out there that have this capability that we simply haven't identified."
The balloon launches are also aimed at discovering how high in the atmosphere life can survive. This past September Christner's team made its highest launch yet, to 37,200 metres over New Mexico. A video camera on board recorded the balloon's rise into the stratosphere — a place that hovers at the edge of space, with Earth's blue curving surface below and black, starry sky above.
Conditions in the stratosphere resemble those on the surface of Mars. Levels of damaging ultraviolet radiation are 1,000 times as high as those at sea level, threatening to chisel a cell's DNA into haiku-length snippets. And the atmospheric pressure is only half a per cent of what it is at sea level, threatening to shrivel cells into freeze-dried corpses. Christner sees his studies in the stratosphere as a test run for looking for life in other worlds. "What we are doing," he says, "is not so different from what will happen in the future when we send missions to Mars to take samples to bring them back to Earth."