NASA RELEASING FIRST VIEWS OF THE ENTIRE SUN ON SUPER SUN-DAY

To view the image with supporting visuals and information, visit:
http://www.nasa.gov/stereo

WASHINGTON -- NASA will score big on super SUN-day at 11 a.m. EST, Sunday, Feb. 6, with the release online of the first complete view of the sun's entire surface and atmosphere.

Seeing the whole sun front and back simultaneously will enable significant advances in space weather forecasting for Earth, and improve planning for future robotic or crewed spacecraft missions throughout the solar system.

These views are the result of observations by NASA's two Solar Terrestrial Relations Observatory (STEREO) spacecraft. The duo are on diametrically opposite sides of the sun, 180 degrees apart. One is ahead of Earth in its orbit, the other trailing behind.

Launched in October 2006, STEREO traces the flow of energy and matter from the sun to Earth. It also provides unique and revolutionary views of the sun-Earth system. The mission observed the sun in 3-D for the first time in 2007. In 2009, the twin spacecraft revealed the 3-D structure of coronal mass ejections which are violent eruptions
of matter from the sun that can disrupt communications, navigation, satellites and power grids on Earth.

STEREO is the third mission in NASA's Solar Terrestrial Probes program within the agency's Science Mission Directorate in Washington. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the mission, instruments and science center.

The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., designed and built the spacecraft and is responsible for mission operations.

The STEREO imaging and particle detecting instruments were designed and built by scientific institutions in the U.S., UK, France, Germany, Belgium, Netherlands and Switzerland.

For information about NASA and other agency programs, visit:
http://www.nasa.gov

How STEREO Views the Entire Sun

Starting in February 2011, and continuing on for the next eight years, mankind will have its first ever 360 degree view of a star — our own Sun. By combining images from NASA's Solar Terrestrial Relations Observatory (STEREO) Ahead and Behind spacecraft, together with images from NASA's Solar Dynamic Observatory (SDO) satellite, a complete map of the solar globe can be formed. Previous to the STEREO mission, astronomers could only see the side of the Sun facing Earth, and had little knowledge of what happened to solar features after they rotated out of view. Would active regions grow larger, and affect the space weather environment when they rotated back again two weeks later, or would they decay away? What about new active regions forming on the far side of the Sun, waiting to surprise us? With STEREO's 360 degree view of the entire Sun, that will no longer happen.

Rotating solar globe combining images from the STEREO-Ahead, STEREO-Behind, and SDO, taken on 4 January 2011 in the Helium II emission line at 304 Angstroms. The small black wedge on the far side of the Sun will be filled in starting February 2011. http://stereo.gsfc.nasa.gov/360blog/

STEREO'S ORBIT

STEREO is able to accomplish this feat because of the unique orbits of its two spacecraft. Each spacecraft is in its own orbit about the Sun (a heliocentric orbit) with orbital parameters which differ just slightly from those of Earth. It's these slight differences which make all the difference. The STEREO-Ahead spacecraft has an orbit which is a little bit closer to the Sun than Earth, and therefore orbits a little bit faster. STEREO-Behind, on the other hand, has an orbit just slightly outside Earth's, and is thus a little bit slower. The end result is that each spacecraft seems to slowly drift in opposite directions away from Earth by about 22 degrees per year, as illustrated below.

Although, as seen from Earth, the two spacecraft seem to be going in opposite directions, they're really going in the same direction, just at different speeds. This Quicktime movie shows the STEREO orbits as they would be seen by a hypothetical observer above the solar system. The green dot represents Earth, and the red "A" and blue "B" represent the STEREO "Ahead" and "Behind" spacecraft respectively. The yellow dot represents the Sun. The orbits of Mercury, Venus, and Mars are also shown.

Note that the orbits of the two STEREO spacecraft differ not only in their orbital distances, but also by how much that distance varies over the orbit. This property is described by a parameter known as the orbital eccentricity, denoted with the symbol e. A perfectly circular orbit would have e=0. STEREO-Ahead's orbit is very close to circular, with e=0.006, while that of STEREO-Behind is more eccentric (e=0.042). The eccentricity of Earth's orbit falls somewhere in between (e=0.017).

STEREO ORBITAL INSERTION

Getting the STEREO spacecraft into orbit around the Sun was not simple. It involved using the Moon's gravity to "slingshot" the spacecraft in their proper orbits. Both spacecraft were originally launched together on a single Delta II rocket on 26 October 2006. Immediately after launch they are placed into highly elliptical orbits which range from just a few hundred kilometers above Earth's surface out to a little beyond the distance of the Moon. Over the next few weeks the two spacecraft slowly separated from each other, and the Mission Operations carefully adjusted the orbits of each to line them up for when both flew by the Moon a few minutes apart on 15 December 2006. The Moon's gravity grabbed both spacecraft, and flung STEREO-Ahead completely away from Earth into its orbit about the Sun. STEREO-Behind was also flung out, but not completely, and came back to swing by the Moon again on 21 January 2007, when it was then completely flung away in the opposite direction into its own orbit about the Sun. These motions are demonstrated in this Quicktime movie where the green and grey dots represent Earth and the Moon respectively, and the red "A" and blue "B" represent the STEREO "Ahead" and "Behind" spacecraft as before.

For information about NASA and other agency programs, visit: http://www.nasa.gov