This ominous, dark shape sprawling across the face of the Sun is a coronal hole - a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. NASA / Goddard / SDO AIA Team
Saturn's northern hemisphere is presently a serene blue, more befitting of Uranus or Neptune, as seen in this natural color image from Cassini. Light rays here travel a much longer path through the relatively cloud-free upper atmosphere. Along this path, shorter wavelength blue light rays are scattered effectively by gases in the atmosphere, and it is this scattered light that gives the region its blue appearance. Why the upper atmosphere in the northern hemisphere is so cloud-free is not known, but may be related to colder temperatures brought on by the ring shadows cast there. NASA/JPL/Space Science Institute
NASA's Spitzer, Hubble, and Chandra space observatories teamed up to create this multi-wavelength, false-colored view of the M82 galaxy. The lively portrait celebrates Hubble's "sweet sixteen" birthday. X-ray data recorded by Chandra appears in blue; infrared light recorded by Spitzer appears in red; Hubble's observations of hydrogen emission appear in orange, and the bluest visible light appears in yellow-green. NASA/JPL-Caltech/STS cI/CXC/UofA/ESA/AURA /JHU
Astronauts headed to Mars and back would face risks, like malnutrition, caused by the roughly three-year trip. Smithsonian Magazine/AP
An expended Saturn IVB stage was being used as a target for simulated docking maneuvers over Sonora, Mexico, during Apollo 7's second revolution around Earth on Oct. 11, 1968. NASA
This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera. The images were taken at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach. NASA/JPL
An artist's imagination of hydrocarbon pools, icy and rocky terrain on the surface of Saturn's largest moon Titan is seen in this illustration. Steven Hobbs/NASA
Voyager 2 obtained this high-resolution color image of Neptune's large satellite Triton during its close flyby on Aug. 25, 1989. Approximately a dozen individual images were combined to produce this comprehensive view of the Neptune-facing hemisphere of Triton. Fine detail is provided by high-resolution, clear-filter images, with color information added from lower-resolution frames. NASA
This artist's concept shows what the very early universe might have looked like, just after its first stars began bursting onto the scene. Scientists theorize that the universe arose around 13.7 billion years ago in an explosion known as the Big Bang. Almost instantaneously afterward, matter began clumping together due to quantum fluctuations. Gravity kicked in next, causing those clumps to grow into larger clouds of invisible hydrogen gas (colored blue here). Eventually, around 200 to 400 million years after the Big Bang, the gas ignited and stars were born. NASA/JPL-Caltech/R. Hurt (SSC)
This astronaut photograph shows the Calabria region of southern Italy - the toe of Italy's 'boot' - outlined by the Ionian and Tyrrhenian Seas to the southeast and northwest, respectively. The water appears almost mirror-like due to sunglint. This phenomenon is caused by sunlight reflecting off the water surface directly back towards the observer aboard the International Space Station. NASA
Halley's Comet was photographed May 13, 1910, by a wide-angle camera at Lowell Observatory, Flagstaff, Ariz. A streak across the comet near the coma is a meteor trail, and not a scratch on the negative. Streaks at the bottom right are the city lights of Flagstaff. Bright spot above the city lights is the planet Venus. NASA
Dimitri Bontinck, a Belgian ex-soldier who retrieved his own son from the Islamic State, goes back to the Turkish-Syrian border to help another anguished father. But luring away jihadis is hard work.
ByDominique Soguel, Correspondent
Europe’s anguish over the rise of homegrown jihadis – young Muslims and Muslim converts who flock to the banner of foreign extremist groups, especially the Islamic State – was on private display here this past week.