2011: The year in space (44 images)

This undated NASA artist rendering released on December 21, 2011 shows the Grail spacecrafts mapping the lunar gravity field. The two probes are scheduled to enter the Moon's orbit over the New Year's holiday. UPI/NASA/JPL

This undated NASA composite image shows the central region of the spiral galaxy NGC 4151. X-rays (blue) from the Chandra X-ray Observatory are combined with optical data (yellow) showing positively charged hydrogen (H II) from observations with the 1-meter Jacobus Kapteyn Telescope, December 31, 2011. UPI/NASA

This undated NASA image taken by the WISE, Wide-field Infrared Survey Explorer, shows Barnard 3, or IRAS Ring G159.6-18.5, that is awash in bright green and red dust clouds. Interstellar clouds like these are stellar nurseries, where baby stars are being born, December 31, 2011. UPI/NASA

This undated NASA the south polar region of Saturn's largest moon, Titan, and shows a depression within the moon's orange and blue haze layers near the south pole, captured by the NASA's Cassini spacecraft, December 31, 2011. UPI/NASA