NASA's STEREO mission has lived up to its name, placing two spacecraft in position to observe both sides of the sun simultaneously. Most solar missions are in mono, so to speak—they rely on a single observatory, from which only one hemisphere of the sun is visible at any given time.
But STEREO comprises twin spacecraft in orbit around the sun, one racing ahead of Earth and one trailing ever farther behind. The mission gathers data for the study of the solar wind and of dramatic solar blasts known as coronal mass ejections. The two STEREO craft launched in 2006 and went their separate ways. On February 6, the separation between the twin orbiters placed them directly opposite each other, with the sun in the middle. For the first time, the entire sun was within STEREO's grasp.
Of course, that privileged view will not last forever—as STEREO A (ahead) streaks onward from Earth's perspective and STEREO B (behind) continues to lag, the orbiters are inching away from opposition and will see their fields of view begin to overlap once again. In fact, the craft are slowly closing in on a rendezvous in 2015, when they will cross paths on the far side of the sun.
Orbital diagram of the STEREO A (red) and STEREO B (blue) spacecraft on February 6 and combined STEREO image: NASA