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The newspaper of The Johns Hopkins University January 29, 2007 | Vol. 36 No. 19
Twin APL-built Spacecraft Swing Past Moon

STEREO mission on course as it prepares for first 3-D studies of the sun

By Kristi Marren
Applied Physics Laboratory

NASA's twin STEREO spacecraft, built and operated by the Johns Hopkins Applied Physics Laboratory, completed a series of complex maneuvers on Jan. 21 to position the spacecraft in their mission orbits. The spacecraft, whose acronym comes from Solar TErrestrial RElations Observatory, will be in position to produce the first 3-D images of the sun by April.

Spacecraft trajectories and lunar swingby maneuvers were created by mission design engineers at APL. "STEREO is the first mission to use the moon's gravity to redirect multiple spacecraft, launched aboard a single rocket, to their respective orbits," said Ron Denissen, APL STEREO project manager.

During the initial weeks following launch, mission operations personnel at APL guided both spacecraft through a series of four highly elliptical phasing orbits around Earth to position them for their lunar gravitational assists that propelled them into their respective mission orbits.

On Dec. 15, STEREO's "A" observatory flew past the moon at a distance of approximately 4,550 miles above its surface, using lunar gravity to redirect the spacecraft away from Earth and into its orbit "ahead" of Earth.

The "B" observatory passed approximately 7,300 miles above the lunar surface, where gravity is slightly weaker. Although the "B" observatory's orbit was slightly boosted, the spacecraft didn't undergo its full lunar gravitational assist until Jan. 21, when it re-encountered the moon. The spacecraft then came within approximately 5,468 miles of the surface, swinging past the lunar body in the opposite direction of the "A" spacecraft and into an orbit "behind" Earth.

The two observatories will orbit the sun from this perspective, separating from each other by approximately 45 degrees per year. Just as the slight offset between your eyes provides you with depth perception, this mirror-image-like positioning of the spacecraft will allow them to take 3-D images and particle measurements of the sun.

During post-launch instrument checkouts, scientists got a close-up view of some intense solar activity from our nearest star, the sun, when the "A" observatory sent back its first images in early December.

When the cover to the "A" observatory's SECCHI Extreme Ultraviolet Imager telescope was removed on Dec. 4, it captured images of a very powerful active region on the sun known as AR903, which produced a series of intense flares last month. SECCHI, for Sun-Earth Connection Coronal and Heliospheric Investigation, was built by the Naval Research Laboratory in Washington, D.C., and is the imaging instrument suite aboard both observatories.

A few days later, during an unusually active solar period, the "A" observatory captured images of a coronal mass ejection with one of SECCHI's two white-light coronagraphs.

Coronal mass ejections, among the largest explosions in the solar system, are giant clouds of plasma shot into space from the sun's atmosphere and can equal the force of a billion megaton nuclear bombs. When they collide with Earth at speeds approaching 1 million mph, CMEs can produce spectacular auroras and trigger severe magnetic storms. The energetic particles associated with these storms can cause electrical power outages, disrupt and/or damage communications satellites and be hazardous to astronauts.

Each STEREO observatory is carrying more than a dozen instruments, which are housed in a platform designed and built by APL. STEREO's data, when combined with that from observatories on the ground or in space, will allow scientists to track in 3-D the buildup and liftoff of magnetic energy from the sun and the trajectory of Earth-bound coronal mass ejections.

STEREO, the third mission in NASA's Solar Terrestrial Probes Program, is sponsored by NASA's Science Mission Directorate, Washington, D.C. NASA Goddard's Solar Terrestrial Probes Program Office, in Greenbelt, Md., manages the mission, instruments and science center. APL designed and built the spacecraft and is operating them for NASA during the mission.

For more about the mission, go to


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