To Fly Free in Space
Image Credit: NASA, STS-41B

Explanation: What would it be like to fly free in space? About 100 meters from the cargo bay of a space shuttle, Bruce McCandless II was living the dream -- floating farther out than anyone had ever been before. Guided by a Manned Maneuvering Unit (MMU), astronaut McCandless, pictured, was floating free in space. During Space Shuttle mission 41-B in 1984, McCandless and fellow NASA astronaut Robert Stewart were the first to experience such an "untethered space walk". The MMU worked by shooting jets of nitrogen and was used to help deploy and retrieve satellites. With a mass over 140 kilograms, an MMU is heavy on Earth, but, like everything, is weightless when drifting in orbit. The MMU was later replaced with the SAFER backpack propulsion unit.

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NGC 147 and NGC 185
Image Credit & Copyright: Chuck Ayoub

Explanation: Dwarf galaxies NGC 147 (left) and NGC 185 stand side by side in this deep telescopic portrait. The two are not-often-imaged satellite galaxies of M31, the great spiral Andromeda Galaxy, some 2.5 million light-years away. Their separation on the sky, less than one degree across a pretty field of view toward the constellation Cassiopeia, translates to only about 35 thousand light-years at Andromeda's distance, but Andromeda itself is found well outside this frame. Brighter and more famous satellite galaxies of Andromeda, M32 and M110, are seen much closer to the great spiral. NGC 147 and NGC 185 have been identified as binary galaxies, forming a gravitationally stable binary system. But recently discovered faint dwarf galaxy Cassiopeia II also seems to be part of their system, forming a gravitationally bound group within Andromeda's intriguing population of small satellite galaxies.

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In Green Company: Aurora over Norway
Image Credit & Copyright: Max Rive

Explanation: Raise your arms if you see an aurora. With those instructions, two nights went by with, well, clouds -- mostly. On the third night of returning to same peaks, though, the sky not only cleared up but lit up with a spectacular auroral display. Arms went high in the air, patience and experience paid off, and the creative featured image was captured as a composite from three separate exposures. The setting is a summit of the Austnesfjorden fjord close to the town of Svolvear on the Lofoten islands in northern Norway. The year was 2014. This year, our Sun is just passing solar maximum, the peak in its 11-year surface activity cycle. As expected, some spectacular aurora have recently resulted.

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Spiral Galaxy NGC 1512: Wide Field
Image Credit & Copyright: Daniel Stern

Explanation: Most galaxies don't have any rings -- why does this galaxy have three? To begin, a ring that's near NGC 1512's center -- and so hard to see here -- is the nuclear ring which glows brightly with recently formed stars. Next out is a ring of stars and dust appearing both red and blue, called, counter-intuitively, the inner ring. This inner ring connects ends of a diffuse central bar of stars that runs horizontally across the galaxy. Farthest out in this wide field image is a ragged structure that might be considered an outer ring. This outer ring appears spiral-like and is dotted with clusters of bright blue stars. All these ring structures are thought to be affected by NGC 1512's own gravitational asymmetries in a drawn-out process called secular evolution. The featured image was captured last month from a telescope at Deep Sky Chile in Chile.

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Supernova Remnant Cassiopeia A
Image Credit: NASA, ESA, CSA, STScI; D. Milisavljevic (Purdue University), T. Temim (Princeton University), I. De Looze (University of Gent)

Explanation: Massive stars in our Milky Way Galaxy live spectacular lives. Collapsing from vast cosmic clouds, their nuclear furnaces ignite and create heavy elements in their cores. After only a few million years for the most massive stars, the enriched material is blasted back into interstellar space where star formation can begin anew. The expanding debris cloud known as Cassiopeia A is an example of this final phase of the stellar life cycle. Light from the supernova explosion that created this remnant would have been first seen in planet Earth's sky about 350 years ago, although it took that light 11,000 years to reach us. This sharp NIRCam image from the James Webb Space Telescope shows the still-hot filaments and knots in the supernova remnant. The whitish, smoke-like outer shell of the expanding blast wave is about 20 light-years across. A series of light echoes from the massive star's cataclysmic explosion are also identified in Webb's detailed images of the surrounding interstellar medium.

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NGC 2442: Galaxy in Volans
Image Credit & Copyright: Mike Selby

Explanation: Distorted galaxy NGC 2442 can be found in the southern constellation of the flying fish, (Piscis) Volans. Located about 50 million light-years away, the galaxy's two spiral arms extending from a pronounced central bar give it a hook-shaped appearance in this deep and colorful image, with foreground stars scattered across the telescopic field of view. The image also reveals the distant galaxy's obscuring dust lanes, young blue star clusters and reddish star forming regions surrounding a core of yellowish light from an older population of stars. But the star forming regions seem more concentrated along the drawn-out (upper right) spiral arm. The distorted structure is likely the result of an ancient close encounter with a smaller galaxy that lies off top left of the frame. This telescopic field of view spans over 200,000 light-years at the estimated distance of NGC 2442.

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Phobos: Doomed Moon of Mars
Image Credit: NASA, LPL (U. Arizona), MRO, HiRISE

Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of our Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the robotic Mars Reconnaissance Orbiter, which can image objects as small as 10 meters. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In perhaps 50 million years, Phobos is expected to disintegrate into a ring of debris.

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