Two NASA probes are running a gauntlet of tests and checks in preparation for their mission to watch some of the Sun's largest explosions in three dimensions.

Engineers are ensuring the space worthiness of NASA's twin Stereo spacecraft for their upcoming mission to study coronal mass ejections, enormous solar eruptions of high-energy particles that can interfere with satellites and pose a danger to orbiting astronauts when directed at Earth.

Once launched, the two Stereo probes will take up Sun-watching positions ahead and behind Earth to record the first real-time 3-D images of the nearest star.

"From the space weather standpoint, this will be very important," Michael Kaiser, Stereo project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. -- where the probes are currently being tested -- said in a telephone interview. "The events on the Sun that you're very interested in are the ones coming right at you. We'll be viewing them from the side."

Keeping tabs on coronal mass ejections and the radiation spewed from the Sun will become even more important in the future, when astronauts leave the relative protection of the Earth's magnetic field on long-duration spaceflights, Kaiser added.

By launching two spacecraft instead of one, researchers hope to shed new light on how massive solar eruptions form and propagate throughout the solar system. Previous 3-D images of the Sun taken by the Solar and Heliospheric Observatory (Soho) were based on sequential rather than near-simultaneous observations, meaning there could have been changes in between them, Kaiser said.

Both Stereo spacecraft are set to launch atop a Boeing-built Delta 2 rocket this coming spring from Florida's Cape Canaveral Air Force Station.

The Stereo probes -- short for Solar Terrestrial Relations Observatory -- will carry four instrument suites to study coronal mass ejections as they blast outward from the Sun into Earth's orbit and beyond.

"In terms of technology, Stereo is sort of an odd mission," said Andrew Driesman, spacecraft systems engineer with Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., which built the probes. "We tried to get two spacecraft for the price of one."

The mission has an estimated NASA cost of about $460 million, as well as $60 million in support from European partners, NASA officials said.

Each probe carries a set of coronagraphs and imagers similar to those aboard Soho, which has spent nearly 10 years observing the Sun. The Stereo craft will relay real-time observations of the Sun to Earth, where researchers expect to combine the data to build 3-D views of the star, as well as its coronal-mass-ejection and solar-wind activity. A trio of antennae on each spacecraft will record radio-signal bursts from the energetic solar events.

"They're set up to take observations within half a second of each other," Kaiser said. "This is kind of a poor man's formation flying."

The probes are not identical; they have subtle differences due to their different orbital destinations. Their launch positions and final destinations gave them their tentative names, mission team members said.

"Right now they're [Stereo] A and B for 'Ahead' and 'Behind,'" Driesman said, adding that the A probe will sit atop the Delta 2 launch stack, with the B spacecraft positioned below it.

Stereo A is slated to fly just inside and slightly ahead of Earth's orbit, completing one full circuit in about 347 days. Because it will fly closer to the Sun than its sister craft, Stereo A's coronagraphs required larger occulting disks used to blot out the Sun's body during corona observations.

Stereo A also needed more thermal protection than its twin, which is slated to trail the Earth in an orbit just a bit farther from the Sun and complete one circuit in 387 days, NASA officials said. "They end up on orbits slowly moving in opposite directions," Driesman said.

Each spacecraft will move farther from Earth during the two-year mission, though the gradual separation should not hinder Stereo's science goals.

"Eventually, you run into a point where they're both on opposite sides of the Sun," Kaiser said, adding that it should occur well after the primary mission is completed. "The mission could probably go on for five or six years [depending on funding]."

Data from other solar probes like Soho and the Advanced Composition Explorer can be compared with the Stereo mission's findings to build a more comprehensive picture of the Sun's behavior, researchers said.

"It's nice to have that one [location] right in the middle," Kaiser said. "That third vantage point from Soho -- that helps."

Kaiser added that the Stereo mission is a reunion of sorts for solar astronomers, many of whom are either working together or collaborated on past Sun-watching missions.

Once they are launched, it should take the Stereo probes about three months to take up their respective positions.

Both spacecraft will swing past the Moon, using its gravity to fling them toward their final orbits -- Stereo A will have to do so twice.

"We're sort of hooked to the lunar cycle," Dreisman said.

Plans call for l