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Explanation: This close-up from the Mars Reconnaissance Orbiter's HiRISE camera shows weathered craters and windblown deposits in southern Acidalia Planitia. A striking shade of blue in standard HiRISE image colors, to the human eye the area would probably look grey or a little reddish. But human eyes have not gazed across this terrain, unless you count the eyes of NASA astronauts in the scifi novel The Martian by Andy Weir. The novel chronicles the adventures of Mark Watney, an astronaut stranded at the fictional Mars mission Ares 3 landing site corresponding to the coordinates of this cropped HiRISE frame. For...

Explanation: In this sharp snapshot, the Solar System's largest moon Ganymede poses next to Jupiter, the largest planet. Captured on March 10 with a small telescope from our fair planet Earth, the scene also includes Jupiter's Great Red Spot, the Solar System's largest storm. In fact, Ganymede is about 5,260 kilometers in diameter. That beats out all three of its other fellow Galilean satellites, along with Saturn's Moon Titan at 5,150 kilometers and Earth's own Moon at 3,480 kilometers. Though its been shrinking lately, the Great Red Spot's diameter is still around 16,500 kilometers. Jupiter, the Solar System's ruling gas...

Explanation: Now at Ceres, Dawn's camera recorded this closer view of the dwarf planet's northern hemisphere and one of its mysterious bright spots on May 4. A sunlit portrait of a small, dark world about 950 kilometers in diameter, the image is part of a planned sequence taken from the solar-powered spacecraft's 15-day long RC3 mapping orbit at a distance of 13,600 kilometers (8,400 miles). The animated sequence shows Ceres' rotation, its north pole at the top of the frame. Imaged by Hubble in 2004 and then by Dawn as it approached Ceres in 2015, the bright spot itself is...

Explanation: Sculpted by stellar winds and radiation, a magnificent interstellar dust cloud by chance has assumed this recognizable shape. Fittingly named the Horsehead Nebula, it is some 1,500 light-years distant, embedded in the vast Orion cloud complex. About five light-years "tall", the dark cloud is cataloged as Barnard 33 and is visible only because its obscuring dust is silhouetted against the glowing red emission nebula IC 434. Stars are forming within the dark cloud. Contrasting blue reflection nebula NGC 2023, surrounding a hot, young star, is at the lower left. The gorgeous featured image combines both narrowband and broadband images....

Explanation: How different does sunset appear from Mars than from Earth? For comparison, two images of our common star were taken at sunset, one from Earth and one from Mars. These images were scaled to have same angular width and featured here side-by-side. A quick inspection will reveal that the Sun appears slightly smaller from Mars than from Earth. This makes sense since Mars is 50% further from the Sun than Earth. More striking, perhaps, is that the Martian sunset is noticeably bluer near the Sun than the typically orange colors near the setting Sun from Earth. The reason for...

Explanation: What if you could stand at the top of a volcano and peer out across the universe? It the timing is right, you might see an amazing panorama like the one featured here. In this case, the volcano is the Hawaii's Mauna Kea, and the time was a clear night last summer In the foreground of this south-facing panorama lies a rugged landscape dotted with rocks and hardy plants. Slightly above and further out, a white blanket of clouds spreads horizontally to the horizon, seemingly dividing heaven and Earth. City lights illuminate the clouds and sky on the far...

Explanation: The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star's life occurs as its outer layers are ejected - its core becoming a cooling, fading white dwarf. In 1995, astronomers used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the hourglass. The unprecedented sharpness of the HST images has revealed surprising details of...

Explanation: This popular group is famous as the Leo Triplet - a gathering of three magnificent galaxies in one field of view. Crowd pleasers when imaged with even modest telescopes, they can be introduced individually as NGC 3628 (left), M66 (bottom right), and M65 (top). All three are large spiral galaxies but they tend to look dissimilar because their galactic disks are tilted at different angles to our line of sight. NGC 3628 is seen edge-on, with obscuring dust lanes cutting across the plane of the galaxy, while the disks of M66 and M65 are both inclined enough to show...

Explanation: In only about 12,000 years Vega will be the North Star, the closest bright star to our fair planet's North Celestial Pole. By then, when you fix your camera to a tripod long exposures of the night sky will show the concentric arcs of star trails centered on a point near Vega as Earth rotates on its axis. Of course, presently the bright star conveniently near the North Celestial Pole is Polaris, but that will change as the Earth's axis of rotation precesses, like the wobble of a spinning top with a precession period of about 26,000 years. If...

Explanation: Did you ever want to just look through the eyepiece of a large telescope in space? If you could, you would see a sharp view that was diffraction limited. Unaffected by atmospheric blurring that ultimately plagues earthbound observers, the angular resolution of your diffraction limited view would be determined only by the wavelength of light and diameter of the telescope lens or mirror; the larger the diameter, the sharper the image. Still, in this working earth-based snapshot a new active adaptive optics system (MagAO) is being used to cancel out the atmospheric blurring in a visual observation of famous...

Explanation: Have you ever seen the Summer Triangle? The bright stars Vega, Deneb, and Altair form a large triangle on the sky that can be seen rising in the early northern early spring during the morning and rising in the northern fall during the evening. During summer months, the triangle can be found nearly overhead near midnight. Featured here, the Summer Triangle asterism was captured last month from Gunma, Japan. In the foreground, sporting a triangular shape of its own, is a flowering 500 year old cherry tree, standing about 15 meters tall. The triangular shape of the asterism is...

Explanation: What's that under the surface of Mercury? The robotic MESSENGER spacecraft that had been orbiting planet Mercury for the past four years had been transmitting its data back to Earth with radio waves of very precise energy. The planet's gravity, however, slightly changed this energy when measured on Earth, which enabled the reconstruction of a gravity map of unprecedented precision. Here gravitational anomalies are shown in false-color, superposed on an image of the planet's cratered surface. Red hues indicate areas of slightly higher gravity, which in turn indicates areas that must have unusually dense matter under the surface. The...

Explanation: Sometimes the sky lights up unexpectedly. A trip to northern Norway to photograph auroras was not going as well as hoped. It was now past midnight in Steinsvik, Troms, in northern Norway, and the date was 2014 February 8. Despite recent activity on the Sun, the skies were disappointing. Therefore, the astrophotographer began packing up to go. His brother began searching for a missing lens cap. When the sky suddenly exploded with spectacular aurora. Reacting quickly, a sequence detailing dramatic green curtains was captured, with the bright Moon near the image center, and the lens-cap seeking brother on the...

Explanation: How can the Moon rise through a mountain? It cannot -- what was photographed here is a moonrise through the shadow of a large volcano. The volcano is Mauna Kea, Hawai'i, USA, a frequent spot for spectacular photographs since it is arguably the premier observing location on planet Earth. The Sun has just set in the opposite direction, behind the camera. Additionally, the Moon has just passed full phase -- were it precisely at full phase it would rise, possibly eclipsed, at the very peak of the shadow. The Moon is actually rising in the triangular shadow cone of...

Explanation: Follow the handle of the Big Dipper away from the dipper's bowl until you get to the handle's last bright star. Then, just slide your telescope a little south and west and you might find this stunning pair of interacting galaxies, the 51st entry in Charles Messier famous catalog. Perhaps the original spiral nebula, the large galaxy with well defined spiral structure is also cataloged as NGC 5194. Its spiral arms and dust lanes clearly sweep in front of its companion galaxy (right), NGC 5195. The pair are about 31 million light-years distant and officially lie within the angular...

Explanation: The first to orbit Mercury, the MESSENGER spacecraft came to rest on this region of Mercury's surface yesterday. Constructed from MESSENGER image and laser altimeter data, the scene looks north over the northeastern rim of the broad, lava filled Shakespeare basin. The large, 48 kilometer (30 mile) wide crater Janacek is near the upper left edge. Terrain height is color coded with red regions about 3 kilometers above blue ones. MESSENGER'S final orbit was predicted to end near the center, with the spacecraft impacting the surface at nearly 4 kilometers per second (over 87,000 miles per hour) and creating...

Explanation: A long solar filament stretches across the relatively calm surface of the Sun in this telescopic snap shot from April 27. The negative or inverted narrowband image was made in the light of ionized hydrogen atoms. Seen at the upper left, the magnificent curtain of magnetized plasma towers above surface and actually reaches beyond the Sun's edge. How long is the solar filament? About as long as the distance from Earth to Moon, illustrated by the scale insert at the left. Tracking toward the right across the solar disk a day later the long filament erupted, lifting away from...

Explanation: What's happening to Comet 67P/Churyumov-Gerasimenko? As the 3-km wide comet moves closer to the Sun, heat causes the nucleus to expel gas and dust. The Rosetta spacecraft arrived at the comet's craggily double nucleus last July and now is co-orbiting the Sun with the giant dark iceberg. Recent analysis of data beamed back to Earth from the robotic Rosetta spacecraft has shown that water being expelled by 67P has a significant difference with water on Earth, indicating that Earth's water could not have originated from ancient collisions with comets like 67P. Additionally, neither Rosetta nor its Philae lander detected...

Explanation: It is one of the more massive galaxies known. A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way and captured by this...

Explanation: What's that in front of the Moon? It's the International Space Station. Using precise timing, the Earth-orbiting space platform was photographed in front of a partially lit Moon last year. The featured image was taken from Madrid, Spain with an exposure time of only 1/1000 of a second. In contrast, the duration of the transit of the ISS across the entire Moon was about half a second. The sun-glinting station can be seen just to the dark side of the day / night line known as the terminator. Numerous circular craters are visible on the distant Moon, as well...