This stunning image of the Sun, produced by combining images from NASA’s twin Sun-orbiting STEREO spacecrafts, shows the far side of the Sun. The dark line across the middle of image represents a gap in the spacecrafts’ data. As each of the two STEREO spacecrafts can image only one hemisphere of the Sun, and, at the time the images were taken, they were not exactly on opposite sides of the Sun, the view is incomplete, and there is a dark line across the middle of image, representing a lack of data.
Credit: NASA

On 6 February 2011, NASA's twin STEREO spacecrafts, currently orbiting the Sun, moved into planned position, on opposite sides of the Sun. This is an orbital configuration that enables the two spacecrafts to acquire 3-D views of the Sun, and the spacecrafts are now relaying numerous images of the entire mother star.

The two STEREO (Solar TErrestrial RElations Observatory) spacecrafts are nearly identical. They are placed into Sun-centered orbits, to monitor spectacular solar phenomena, occurring in the Sun’s extremely hot tenuous atmosphere, that may affect Earth and our lives in various ways. Each STEREO probe photographs a hemisphere of the Sun, and sends the images to Earth. Scientists combine the two views, to create 3-D pictures of the Sun.

The STEREOs carry onboard telescopes that observe the Sun in specific wavelengths of ultraviolet radiation, particularly to record solar flares, tremendous explosions, occurring in the Sun’s atmosphere.
"For the first time ever, we can watch solar activity in its full 3-dimensional glory," said Angelos Vourlidas, a researcher of the STEREO science team. "This is a big moment in solar physics," he added.

An artist's rendition of STEREO spacecrafts, observing the Sun from directly opposite locations in space
Credit: NASA

Solar flares and other solar eruptions strongly affect the space environment in the vicinity of Earth. They may fling toward Earth prodigious amounts of particles, particularly protons. When these particles reach Earth, they interact with its atmosphere and magnetosphere. This electromagnetic effect may disrupt telecommunications, cause power blackouts, or damage equipment aboard artificial satellites. It could even be harmful to astronauts aboard space vehicles. Therefore, scientists are so concerned about monitoring and predicting solar activity.

When a solar eruption takes place, its cloud of particles, plowing through the inner Solar System, reaches Earth’s orbit 3-4 days later. Thanks to STEREO’s data, scientists now can watch violent events, evolving on the far side of the Sun, hidden from Earth-based observers, and predict possible influence on Earth, several days in advance.

The STEREO spacecrafts were launched into space on 26 October 2006, aboard a Delta II rocket, from NASA’s Kennedy Space Center. Interestingly, they flew by the Moon, to benefit from the influence of the lunar gravity for adjusting their trajectories. The STEREOs are technically known as the ahead (A) and the behind (B) spacecrafts, respectively, as A moves ahead, while B lags behind Earth. The orbital periods of the probes A and B are, respectively, 346 days and 388 days.  

References

NASA
www.nasa.gov/

Further Reading
STEREO Mission Homepage
www.nasa.gov/stereo

Aymen Mohamed Ibrahem
Senior Astronomy Specialist