Scientists at NASA’s Goddard Space Flight Center are close to wrapping up testing on twin spacecraft that will launch this summer on a two-year mission that will provide the first-ever 3-D views of solar events known as coronal mass ejections.

The Solar Terrestrial Relations Observatory (Stereo) spacecraft will use different orbital perspectives to take stereoscopic images of the Sun’s surface. Scientists are particularly interested in detailed images of coronal mass ejections — gigantic bubbles of gas threaded with magnetic-field lines that are periodically thrown off the surface of the Sun, potentially creating problems on Earth.

The international mission, part of NASA’s Sun-Earth Connections theme, is about two years behind the schedule laid out in 1999, when NASA announced the selection of the satellites’ instrument payload. The mission’s total price tag, including launch, development and operations, is about $478 million, compared to NASA’s $420 million estimate when development began in earnest back in 2002, NASA officials said.

Part of the reason for the cost growth — which at 14 percent is relatively modest in the realm of space development — was a delay due to a component problem that affected numerous U.S. government satellite programs, said Vicki Elsbernd, NASA program executive for the Stereo mission.

The Stereo satellites were built by the Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md. They are scheduled to be shipped to NASA’s Kennedy Space Center in Florida some time between April 28 and the first week of May, said NASA spokeswoman Rachel Weintraub. There they will undergo additional testing before being placed one-atop-the-other on a Boeing Delta2 rocket to meet a launch window that runs from July 22 through Aug. 6, Nick Chrissotimos, project manager for Stereo, said during a press briefing here at Goddard Space Flight Center March 28.

The two-week window is due to the fact that the spacecraft will rely on a gravity assist from the Moon to get them into orbit about the Sun. There are two weeks during each month when the Earth and Moon are in the proper alignment, Chrissotimos said.

The Stereo spacecraft will fly in the same orbital track as the Earth, with one just ahead of Earth and the other behind. That will provide the stereoscopic views necessary to take the 3-D images.

“We’re really excited about this mission,” Chrissotimos said. “We’re going to bring back some very cool science.”

That science is focused primarily on coronal mass ejections, which can bombard the Earth with charged particles and plasma that can disrupt and even damage sensitive electronic and communications systems, including satellites. They also can be dangerous to astronauts in space, explained Terry Kucera, a Stereo project scientist at Goddard.

Through Stereo, scientists are hoping to get a better understanding of what causes coronal mass ejections, how fast they move and, eventually, when they are going to occur. The 3-D view is key, particularly because it is difficult to measure certain properties of coronal mass ejections when viewing them from a “head-on” perspective, Kucera said.

Previous missions such as the NASA-European Space Agency Solar and Heliospheric Observatory and the Transition Region and Coronal Explorer have studied coronal mass ejections, but not in stereo , Chrissotimos said.

Coronal mass ejections are often — but not always — associated with solar flares, massive eruptions on the Sun’s surface whose frequency ebbs and flows on 11-year cycles. While scientists generally understand that coronal mass ejections are caused by the Sun’s magnetic fields, they have yet to determine what position those fields need to be in to produce the events , Kucera explained.

Instruments on the two spacecraft will collect a variety of data, looking at solar wind, radio waves, ultraviolet images and other phenomena , Kucera said.

The Naval Research Laboratory in Washington built Stereo’s group of imaging instruments. The Paris-Meudon-Nancay O bservatory in France built a radio burst tracker, while the University of California, Berkeley and the University of New Hampshire each designed plasma-measuring instruments.

Stereo’s key 3-D observation time will be for two years, after which the spacecraft will move too far apart from each other to provide the proper view , Chrissotimos said.