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NASA and European Astronomers Say

THE WOODLANDS, Texas — Grains of dust from the edge of the solar system could be finding their way to Earth. And NASA may already have a handful of them, researchers report. 

An estimated 40,000 tons of space dust settle in Earth’s atmosphere every year. The U.S. space agency has been flying balloon and aircraft missions since the 1970s to collect samples of the dust. Astronomers had long thought the dust mostly comes from smashups involving comets and asteroids — especially from ones inside the orbit of Jupiter.

In fact, some of the dust grains may come from the Kuiper (KY-pur) Belt. That’s a distant region beyond Neptune of icy objects. Like the planets, they too orbit the sun. Lindsay Keller is a planetary scientist at the Johnson Space Center in Houston, Texas. He reported the findings, here, on March 21 at the Lunar and Planetary Science Conference.

Studying those particles could reveal what distant, mysterious objects in the Kuiper Belt are made of, and perhaps how they formed.

“We’re not going to get a mission out to a Kuiper Belt object to actually collect [dust] samples anytime soon,” Keller said. “But we have samples of these things in the stratospheric dust collections here at NASA.”

One way to find a dust grain’s home is to probe the particle for microscopic tracks. These tracks are typically left as heavy charged particles from solar flares punched through a dust grain. The more tracks a grain has, the longer it has wandered in space — and the more likely it originated far from Earth, Keller explains.

Probing the dust’s likely source

To gauge precisely how long a dust grain had been traveling, Keller first needed to know how many tracks a grain would typically pick up each year. Measuring that rate required a sample with a known age and known density of tracks. Moon rocks brought back on the Apollo missions would work. But the last track-rate estimate had been done in 1975. And it had been done with less precise tools than are available today.

So Keller and planetary scientist George Flynn reexamined that same Apollo rock with a modern electron microscope. (Flynn works at the State University of New York in Plattsburgh.) The two found that the rate at which rocks pick up flare tracks was just one-twentieth of what the previous study had estimated. That means it takes longer for dust flakes to pick up tracks than astronomers had assumed.