The Year in Science: Earth 1997

Wee Comets Again

By Tim Appenzeller|Thursday, January 01, 1998
Could Lou Frank be right after all? Twelve years ago, the University of Iowa space physicist startled his colleagues with an explanation for tiny dark spots in satellite images of the upper atmosphere. The spots, said Frank, marked places where house-size snowballs, flying in from space 20 times a minute, were expiring in clouds of water vapor underneath the satellite, briefly blocking its view of the atmosphere’s ultraviolet glow. A steady drizzle of comets throughout Earth’s history, he added, would have been enough to fill the oceans. Nonsense, said most space scientists: the spots were just bad pixels in the satellite’s camera.

Now Frank has flown a far more sensitive camera aboard a new spacecraft called Polar. To the surprise of nearly everyone but Frank, the camera seemed to see the spots again, hundreds of miles above Earth. This time, though, each spot filled not just one pixel but ten or more; the spots appeared in successive frames, and they moved from frame to frame, just like objects streaking in from space. The camera also picked up long, glowing trails, apparently made of sunlight-shattered water molecules. Last spring Frank declared victory. It was a shock to a lot of people that it turned out to be right, he says.

Within weeks another satellite had come to his support: it had detected about 50 percent more moisture than expected in a layer of atmosphere about 50 miles up. The evidence was enough to convince some former unbelievers—but far from all—that Frank’s spots are real. I don’t know how else to get that water up there unless you just put it in by hand, says Thomas Donahue of the University of Michigan. Says Robert Meier of the Naval Research Laboratory, I’m certainly willing to acknowledge that he’s observing something.

But is it tiny comets? All the old objections to the idea remain. On the moon, seismometers left behind by the Apollo astronauts should have picked up a drumroll of comet impacts, but they haven’t heard a thing. And why the hell isn’t Mars inundated with water? asks Donahue. Alex Dessler, a space physicist at the University of Arizona in Tucson, argues that the vaporizing comets, if real, would be obvious to anyone looking up. They would be as bright as Venus, he says. In some cases, as bright as the full moon.

Frank has all his old answers—his comets, unlike ordinary ones, might be so fluffy they hit the moon with nary a tremor; on Mars the comet water is hiding under polar ice caps; and amateur astronomers are dismissing the brief flashes of light they see as insignificant. Dessler and others think it’s more likely that Frank’s spots are inconsequential. I’m firmly convinced there are no small comets, he says. Once again we’re being shown an instrument artifact—cosmic-ray tracks in the detectors or random fluctuations in the electronics.

Dessler has a long history as a critic. But the scientist responsible for a second ultraviolet camera aboard the Polar spacecraft, space physicist George Parks of the University of Washington in Seattle, is also skeptical about Frank’s spots—because his camera has seen spots, too. Both instruments show dark pixels, but we come to different conclusions, Parks explains. I can account for all the dark spots that I see through instrument artifacts.

Yet how could the spots be mere noise, Frank asks, if they cover multiple pixels, move like comets, wobble as the spacecraft wobbles, and show up in both cameras? Frank says he wants to stop debating whether tiny comets exist and move on. Along with Donahue, Meier, and others, he hopes to persuade NASA to fund a small satellite specially outfitted to study them. What we need to do is go up to 600 miles and watch a small comet break up over San Francisco, he says. We need to go up there and greet them.
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