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 hunt of coronal mass ejections (CMEs), 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 "3D" images of our nearest star.

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

Keeping tabs on CMEs 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 the STEREO mission will shed new light on how massive CMEs form and propagate throughout the Solar System. Previous 3D observations of the Sun taken by the Solar and Heliospheric Observatory (SOHO) used images caught one at a time, during which time conditions may have changed, Kaiser said.

Both STEREO spacecraft are currently set to launch spaceward atop a Boeing-built Delta 2 rocket in Spring 2006 in a flight to be staged from Florida's Cape Canaveral Air Force Station.

Scanning for space explosions

NASA's STEREO probes - short for Solar TErrestrial RElations Observatory - carry four instrument suites to study CMEs as they blast outward from the Sun out past the Earth's orbit.

"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 (APL), which built the probes. "We tried to get two spacecraft for the price of one."

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

Each of the Sun-watching probes carries a set of coronagraphs and imagers similar to those aboard SOHO, which has spent nearly 10 years observing our parent star. Both STEREO will relay real-time observations of the Sun to Earth, where researchers expect to combine the to build three-dimensional views of the star, as well as its CME and solar wind activity. A trio of antennas on each spacecraft will also record the radio signal bursts from the energetic solar events.

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

But the probes are not completely identical, and carry subtle differences due to their different orbital destinations. Each probe's launch position and final destination gave the spacecraft their tentative names, mission team members said.

"Right now they're [STEREO] A and B for 'Ahead' and 'Behind,'" Driesman told SPACE.com, adding that the A probe will also sit atop the Delta 2 launch stack, while the B spacecraft will be positioned below it.

STEREO A is slated to fly just inside Earth's orbit but ahead of the planet, completing one full orbit in about 347 days. Because it will fly closer to the Sun, the star will appear larger to the probe's coronagraphs and required larger occulting disks used to blot out the Sun's body during corona observations.

Additional care to thermal protection was needed on the STEREO A than its companion, which is slated to trail the Earth in an orbit just a bit farther from the Sun and complete one orbit in 387 days, NASA officials said

"They end up on orbits slowly moving in opposite directions," Driesman said.

According to their flight profile, each spacecraft will move further from Earth during the STEREO mission's two-year mission, though the gradual separation should not hinder its 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. "The mission could probably go on for five or six years [depending on funding]."

Dynamic duo, with company

Despite their joint mission, the two STEREO probes won't be the sole observers of the Sun during their spaceflight.

Spacecraft such as the successful SOHO, and other missions such as Wind and Advanced Composition Explorer (ACE), have provided bonus observations from relatively close to Earth when compared with the planned STEREO flight. Data and observations from those spacecraft can be added to STEREO'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 Sun-focused researchers, many of whom are either working together or collaborated in the past on the other Sun-watching missions.

Rounding the Moon

It should take the two STEREO probes about three months to take up their respective sun-watching positions.

Both spacecraft will swing past the Moon, using its gravity to fling them toward their final orbits, though STEREO A will have to fly past the grey satellite twice in order to accelerate past Earth to its intended station, NASA officials said.

"We're sort of hooked to the lunar cycle," Dreisman said, adding that while the STEREO mission plans to launch sometimes between April and June 2006, there is actually more flexibility to make the space shot. "We have a launch window [almost] every month of the year."

While most months have a period of about 14 days - each with a 15-minute launch window - to loft the two STEREO probes, a December liftoff would require an extended coast phase that could prove too long for the probes' batteries, he said.

But before STEREO A and B can leave Earth, engineers must be sure they're fit to fly. Over the next few months, the probes will be locked away in vacuum chambers, subjected to the intense vibrations and noise they will experience at launch and witness the extreme temperatures they must endure in order to successfully perform their mission.

"It's been such a long road here, we've been looking forward to this," Kaiser said of the testing phase. "I think [the mission] is going to open up a whole new world for us."

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