Traveling at the speed of light
In February, NASA's STEREO probes, two observatories that were launched in 2006 to survey the sun, reached opposite ends of the sun and thus, were able to give scientists (and the rest of us!) a never-before-seen view of the far side of the star at the center of our solar system. The composite image above was captured on June 1, 2011, and according to NASA, "is the first complete image of the solar far side, the half of the sun invisible from Earth." Click here for more on the STEREO probes.
While it's not a "discovery, " per se, it's a milestone that will no doubt lead to many new findings about the smallest and innermost planet in our solar system. In March, after a 6 1/2-year, 4.9 billion mile journey, NASA's Messenger spacecraft reached Mercury's orbit. Messenger, the first spacecraft to orbit Mercury, orbits the planet every 12 hours. In November, NASA announced that the spacecraft's mission, which was supposed to end on March 17, 2012, would be extended for an additional year.
In July, NASA said that its Hubble Space Telescope discovered an eight to 21-mile-wide moon circling the dwarf planet.
In March, NASA's Swift satellite saw an unusually long explosion of gamma-rays. After studying the X-rays, scientists found that they came from a black hole that had become reenergized when it devoured a star. From NASA: Astronomers soon realized the source, known as Swift J1644+57, was the result of a truly extraordinary event - the awakening of a distant galaxy's dormant black hole as it shredded and consumed a star. The galaxy is so far away, it took the light from the event approximately 3.9 billion years to reach Earth.