The entire sun — both front and back — can be seen simultaneously for the first time in new images released by NASA.
The photos of the sun's whole surface and atmosphere were taken by twin spacecraft called the Solar Terrestrial Relations Observatory (STEREO) and released Sunday by the U.S. space agency. That was the day the two probes, which are each designed to photograph half the sun at once using special telescopes, were on exactly opposite sides of the sun.
The probes were launched in 2006 to monitor the movement of energy and matter from the sun to the Earth, such as "coronal mass ejections" — violent eruptions on the sun's surface that can disrupt communications, navigation, satellites and power grids when they reach the Earth.
That allows more accurate forecasts of "space weather" for airlines, utilities, satellite operators and other groups that might be affected.
The spacecraft orbit the sun slightly closer and slightly farther away from the Earth respectively, moving at different speeds so that their relative positions change gradually over time. They are equipped with special telescopes that monitor certain wavelengths of ultraviolet light coming from the sun, keeping an eye on different types of solar explosions such as flares, tsunamis and magnetic filaments. Their images combine to create a 3D model of the sun.
STEREO allows researchers to keep an eye on what is happening on the part of the sun facing away from the Earth, including the formation sunspots — cooler, darker areas of the sun that are linked to violent solar outbursts.
When the sun rotates, such areas could hurl energy and matter toward the Earth — previously, with little warning.
"Not anymore," said Bill Murtagh, senior forecaster at the National Oceanographic and Atmospheric Administration in a statement.
"Thanks to STEREO, we know they're coming."
The two probes are expected to provide views of the back of the sun for the next eight years.