Twin Spacecraft Swing Past Moon, Preparing For 3-D Solar Studies

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," says 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, 2006, STEREO's "A" observatory flew past the moon at a distance of approximately 4,550 miles (7,340 kilometers) 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 (11,776 kilometers) 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 January 21 when it re-encountered the moon. The spacecraft then came within approximately 5,468 miles (8,818 kilometers) 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.

FIRST IMAGES 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, 2006, it captured images of a very powerful active region on the sun known as AR903 that produced a series of intense flares last month. SECCHI (Sun-Earth Connection Coronal and Heliospheric Investigation), built by the Naval Research Laboratory (NRL) in Washington, D.C., 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 are giant clouds of plasma shot into space from the sun's atmosphere. One of the largest explosions in the solar system, they can equal the force of a billion megaton nuclear bombs. When they collide with Earth at speeds approaching one 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 are often hazardous to astronauts.

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

STEREO's instruments were built by numerous organizations worldwide with a principal investigator, or PI, leading each instrument team. The instruments and PIs are as follows: SECCHI -- Russell Howard, NRL; In situ Measurements of PArticles and CME Transients (IMPACT) -- Janet Luhmann, University of California, Berkeley; PLAsma and SupraThermal Ion Composition (PLASTIC) -- Antoinette Galvin, University of New Hampshire; and STEREO/WAVES (S/WAVES) -- Jean-Louis Bougeret, Paris Observatory, Meudon.

STEREO is the third mission in NASA's Solar Terrestrial Probes Program. STEREO 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 information about STEREO or to download additional images, visit stereo.jhuapl.edu or http://stereo.gsfc.nasa.gov/gallery/gallery.shtml.