First Complete Image of the Solar Far Side of the Sun
With 492 votes, the winner is "Twin Solar Spacecraft Take First Complete Image of Far Side of Sun" referring to an image captured by NASA's two Solar Terrestrial Relations Observatory (STEREO) spacecraft. The STEREO satellites are currently in orbit around the sun and separated such that each has a view of the opposite sides of our closest star. Together the two observatories captured the first simultaneous images of the entire sun in June of 2011.
The STEREO spacecraft reached opposition (180° separation) on February 6 but part of the sun was inaccessible to their combined view until June 1. This image represents the first day when the entire far side could be seen.
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The image is aligned so that solar north is directly up. The seam between the two images is inclined because the plane of Earth's -- and STEREO's -- orbit, known as the "ecliptic", is inclined with respect to the sun's axis of rotation. The data was collected by STEREO's Extreme Ultraviolet Imagers in the SECCHI instrument suites.
STEREO was built and is operated for NASA by the Applied Physical Laboratory of the Johns Hopkins University; the spacecraft were launched on October 25, 2006 aboard a Delta II. The SECCHI instrument suite is a collaboration led by the Naval Research Laboratory, and the EUVI instruments were built by the Lockheed Martin Solar and Astrophysics Laboratory.