On a blustery overcast day in December, Scott Peterson spent more than two hours kneeling on the roof of an industrial building in Hamel, sifting through small piles of dirt with a brush and a strong magnet.

He was looking for stardust.

Peterson is a 36-year-old stay-at-home dad with a toddler, a combat veteran from the wars in Iraq and Afghanistan, and a hunter of alien invaders.

More precisely, he's an amateur scientist from New Hope who is one of the few people in the world to find micrometeorites — minuscule motes from outer space — that have landed in the middle of a city.

In the past few months, Peterson has found tiny space rocks on roofs of Twin Cities area buildings ranging from North Hennepin Community College in Brooklyn Park to Kramarczuk's deli in Minneapolis. His finds are among the first verified micrometeorites discovered by a citizen scientist in the United States.

"There have been thousands of others who have tried before him and not managed to break the code. Scott has, and now he is finding new, exciting extraterrestrial visitors all the time," said Jon Larsen.

Larsen is Peterson's unlikely mentor as a micrometeorite sleuth.

A noted "gypsy jazz" guitarist from Norway, Larsen recently pioneered a field of citizen science: hunting for micrometeorites in urban settings.

He's created an initiative called Project Stardust that has been recognized by scientific journals, and he has written a new book, "In Search of Stardust: Amazing Micrometeorites and Their Terrestrial Imposters," published, coincidentially, by Minneapolis-based Voyageur Press. Micrometeorites are specks of cosmic dust wandering through the solar system, including debris from comets and asteroids. They are grains of extraterrestrial material possibly older than the sun.

After traveling up to billions of miles of space, some of that space dust falls on the Earth, maybe 200 tons a day, according to one estimate.

Half a teaspoon in all

Micrometeorites were first detected on the ocean floor in the 1870s in a British science expedition conducted by the HMS Challenger. Later, micrometeorites were collected in the Antarctic, in remote deserts and on glaciers and by high-altitude planes and balloons.

Peterson got interested in meteorites when he began studying physics and astronomy at Minneapolis Community and Technical College. That was after he got out of the Army in 2005. He had enlisted in June 2001 straight out of Menomonie High School in western Wisconsin.

He was in basic training when the 9/11 attacks happened, making him among the first infantrymen sent into Afghanistan and Iraq.

He had searched for micrometeorites three or four years with no luck. "I was doing it wrong," he said. "If I did see one, I wouldn't have known it."

The problem is that micrometeorites are tiny, typically less than a half-millimeter wide. Particles from space are easily lost in all the other dust and grit already on the Earth generated by humans and nature.

Until recently, the idea that extraterrestrial flecks might be found in less pristine environments, on urban rooftops or gutters by amateurs or school kids doing science projects, has been treated by most researchers as an urban myth.

That changed when Larsen, a rock collector as a kid, started hunting for micrometeorites in 2010 after being intrigued by a shiny speck that landed on an outdoor table in Norway.

He began taking dust samples on roofs, parking lots and roads, nearly 1,000 searches in total, in nearly 50 countries. He learned to identify and eliminate all the naturally occurring and man-made particles in an urban environment, such as road dust crystals, roof tile granules and dust from power tools.

And in February 2015, "I found what was thought of as impossible — a cosmic dust particle," he said.

Since then, Larsen has found 1,171 micrometeorites; his discoveries were published last February in the journal of the Geological Society of America.

More than a thousand micrometeorites sounds like a lot, but the particles are so small that the total amount of cosmic dust retrieved on Earth amounts to about half a teaspoon.

Up on the rooftops

The dust is worth the fuss, according to scientists. Micrometeorites can be studied for insights about the creation of the solar system and may even give clues to the origin of life on Earth.

Larsen's "In Search of Stardust," which was published in August, is sort of a cross between a field guide and a coffee table photo book for the urban micrometeorite hunter.

It also was a godsend for Peterson.

"Without his book, I'd still be on my roof, looking in parking lots and driveways," Peterson said.

Armed with a better idea of what he was looking for, Peterson got permission to get on the roof of the community college he's attending. That's where, in October, he found his first micrometeorite.

Since then, Peterson's stardust trek has taken him to the roofs of a car wash in Wayzata, a park building in St. Louis Park and a food distribution company in New Hope.

He said he pores over Google satellite maps to find places that would be good collectors of micrometeorites. The perfect roof, according to Peterson, is flat, smooth and vinyl-covered, and preferably old, so it has had a lot of time to be exposed to a potential extraterrestrial touchdown.

He said he's e-mailed requests to about 50 or 60 places with good roofs.

"I say 'I know this is an odd request,' " he said.

Most of the time he gets turned down. But every once in a while, Peterson persuades someone to say yes. (Peterson said drop him a note at mnmicrometeorites@gmail.com if you want your roof inspected for tiny space invaders.)

"Anytime anyone wants to get up on the roof, I'm immediately skeptical," said Nick Kramarczuk, general manager of Kramarczuk's. But he said Peterson eventually convinced him that it was for science.

"He seemed like he knew what he was talking about," he said.

On the roof of Loram Maintenance of Way in Hamel, Peterson examined small piles of dirt that had accumulated in corners or washed up near the roof drains.

He collected specks that stuck to a powerful hockey-puck-sized magnet he had brought along. After about 2½ hours of work, he had about a cupful of magnetic particles.

"It's a lot. There's got to be something in there," he said.

He took the dust home and rinsed it off in a plastic Sweet Martha's Cookie bucket, dried it in his oven on sheets of aluminum foil and sifted the material through special screens, leaving behind particles 0.17 to 0.4 millimeters in width, the likeliest size of a micrometeorite.

Contributing to science

Then he spends hours at a microscope, examining what looks like finely ground black pepper, using tweezers to discard all the magnetic bits that aren't from outer space.

A few of the specks that were left looked like the ones in Larsen's book. He's e-mailed images of his finds to Larsen, who has confirmed that they are, in fact, micrometeorites. Altogether, Peterson believes he's found 36 particles from space.

Peterson, who is studying to be an engineer, said he hopes that his work will contribute to science. He would like to publish his finds in a scientific journal.

His wife, Kelly, once was puzzled by his fascination with what looked like bits of dirt.

"I think it's really cool now," she said. "I know it's not dirt. It's stardust."

Peterson has gotten Larsen to send him a few of his Norwegian micrometeorites so he can make comparisons. He's also e-mailed NASA to see if he could get a sample of lunar dirt collected from one of the Apollo missions so he could search that for micrometeorites. NASA said no.

But the University of Minnesota recently allowed him time to examine his finds on its electron microprobe, a high-tech instrument that can determine the chemical composition of tiny things.

As microprobe lab manager Anette von der Handt adjusted the controls of the machine, one of Peterson's particles swam into view on a screen. A little speck smaller than a grain of sand was magnified to an image the size of a grapefruit.

It looked like a barren little planetoid. It was easy to imagine that the rough grooves on the sphere were the scars of a long, harsh journey through space that led to a plunge through the Earth's atmosphere at up to 50 times the speed of a rifle bullet, ultimately hitting Minnesota.

"This is awesome," Peterson said. "They've come from millions and billions of miles away. They're millions and billions of years old. They landed on this roof. And I get to see them."

Richard Chin • 612-673-1775