© John Sebastian Russo / The Chronicle 2010
The Lick Observatory in San Jose, with the Fremont Peak Observatory and a ground site in San Joaquin County's Lodi, monitors the sky for meteoroids.

A stream of dusty fragments from a comet born in the outermost reaches of the solar system has hit the Earth on a path that leads astronomers to conclude the comet itself could be "potentially hazardous" if it crashes into the planet.

The comet's location is unknown, making it difficult to say when it will approach Earth, but "the orbits of the dust trail tells us that the comet is on a path that could eventually hit us," said Peter Jenniskens, an astronomer at the SETI Institute and the NASA Ames Research Center in Mountain View.

"It's very unlikely," he conceded Wednesday. "Such impacts are extremely rare in Earth's history."

The trail of dust grains, known as meteoroids, were shed by the comet long ago as it passed the sun and Earth on a long orbit that could have taken thousands of years to complete, Jenniskens said.

The comet was born billions of years ago and trillions of miles away in the cold comet nursery called the Oort Cloud, and streams of the comet's dusty progeny have returned to Earth once or twice every 60 years or so when their orbits come under the influence of Saturn and Jupiter, Jenniskens said.

Eyes on the sky

Sixty specialized cameras that operate at UC's Lick Observatory, the Fremont Peak Observatory and a ground-based site, formerly in Mountain View but now in Lodi under a project called Cameras for Allsky Meteor Surveillance, constantly monitor the night sky for meteoroids.

Jenniskens said he was scanning the orbits of the dust stream's fragments from images snapped at Fremont Peak and in Mountain View on Feb. 4 when he noted a tightly linked cluster of six objects streaming at nearly 80,000 mph in a shower seven hours long.

"I couldn't believe my eyes at first," he said. But once he had had determined the identical orbits of the fragments he teamed with Finnish astronomer Esko Lyytinen to predict that the dust trail will return in 2016, again in 2023, and once again in 2076.

Cold, crowded place

The Oort Cloud where the comet was born billions of year ago is known as the deeply cold birthplace of all the long-period comets that watchers on Earth can rarely see because they take as long as 10,000 years to orbit the sun.

The cloud lies at the outer edge of the sun's influence and may hold as many as a trillion or more icy bodies, all waiting to be turned into comets by the gravity tug of some unknown wandering star or by some other unknown cosmic object.

The dusty comet trail discovered by Jenniskens and his Fremont Peak colleagues appears to come from the direction of the yellow giant star called Eta Draconis in the constellation Draco, and the International Astronomical Union has named the new-found stream of fragments the "February Eta Draconids."

Jenniskens' scientific report will be published by the Journal of the International Meteor Organizations. Its title is: "Discovery of the February Eta Draconids: the dust trail of a potentially hazardous long-period comet."