SACRAMENTO (AP/CBS) — A group of scientists took to the skies in a slow-moving airship Thursday in search of meteorites that rained over California’s gold country last month.

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It’s the latest hunt for extraterrestrial fragments from the April 22 explosion that was witnessed over a swath of Northern California and Nevada.

Treasure hunters have swarmed the Sierra foothills over the past two weeks, snatching up pieces of meteorites. Most of the recovered space rocks have been tiny, with the largest weighing in at 19 grams, or the weight of one AA battery.

Researchers from NASA and the nonprofit SETI Institute in Mountain View were on the lookout for larger fragments. After a brief weather delay, they took off from a Sacramento airfield aboard an airship outfitted with sensors and cameras.

For Peter Jenniskens this is what he’s been waiting for because meteors rarely rain down like this.

“Not in California, this type of impact, this strong, happens over the whole earth maybe once a year,” said Jenniskens.

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From the air, scientists scoured the terrain for places where sizable meteorites might have scattered. The survey took them over the Colma and Lotus areas in El Dorado County where James W. Marshall first discovered gold in California in 1848. Once they pinpoint possible impact sites, they plan to follow up with a search party.

“Only small pieces have been found. There has to be big pieces out there,” SETI Institute meteor astronomer Peter Jenniskens said before the trip. “We’re just hoping to collect more meteorites for research.”

Days after the approximately 154,300-pound meteor plunged through Earth’s atmosphere with a loud sonic boom that was heard from Sacramento to Las Vegas, Jenniskens organized a team and found a 4-gram meteorite in the parking lot of a park.

NASA estimated the minivan-sized meteor released energy equal to one-third the explosive power of the atomic bomb dropped on Hiroshima during World War II. An event this size occurs once a year usually over unpopulated areas.

Initial inspection of rock fragments indicates this type of meteorite is among the oldest, dating to the solar system’s birth 4 billion to 5 billion years ago.

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(Copyright 2012 The Associated Press.)