One of the STEREO spacecraft spotted this coronal mass ejection on January 21, 2009, when it was a bulbous "cloud" with a bright core (left). The ejection continued expanding the next day (middle). Ninety-three million miles later, the coronal mass ejection was detected by the other STEREO spacecraft--thanks to STEREO's Heliospheric Imager (HI).
A nested set of cameras, HI can view half the sky, allowing STEREO to follow an ejection all the way from the sun to Earth.
The pictured ejection had no adverse effect on Earth, but solar storms are known to disrupt GPS, power grids, and cellular signals as well as bombard astronauts with dangerous radiation. STEREO's new 3-D imaging technology should help avert such outcomes by allowing scientists to warn of dangerous space weather a full day before it reaches Earth.