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The Expedition 24 crew on the International Space Station photographed this image of polar mesospheric clouds illuminated by an orbital sunrise. Polar mesospheric, or noctilucent ("night shining"), clouds usually are seen at twilight, following the setting of the sun below the horizon and darkening of Earth's surface. Occasionally the station's orbital track becomes nearly parallel to Earth's day/night terminator for a time, allowing the clouds to be visible to the crew at times other than the usual twilight because of the station's altitude.

This photograph shows polar mesospheric clouds illuminated by the rising, rather than setting, sun at center right. Low clouds on the horizon appear yellow and orange, while higher clouds and aerosols are illuminated a brilliant white. Polar mesospheric clouds appear as light blue ribbons extending across the top of the image. The station was located over the Greek island of Kos in the Aegean Sea (near the southwestern coastline of Turkey) when the image was taken at approximately midnight local time. The orbital complex was tracking northeastward, nearly parallel to the terminator, making it possible to observe an apparent "sunrise" located almost due north. A similar unusual alignment of the ISS orbit track, terminator position and seasonal position of Earth's orbit around the sun allowed for this striking imagery of over the Southern Hemisphere.

Image Credit: NASA

Spiral galaxy NGC 4921 presently is estimated to be 320 million light years distant. This image, taken by the Hubble Space Telescope, is being used to identify key stellar distance markers known as Cepheid variable stars. The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.

Image Credit: NASA, ESA, K. Cook (LLNL)

The north polar layered deposits are layers of dusty ice up to 2 miles thick and approximately 620 miles in diameter. We can see the layers exposed on the walls of troughs and scarps cut into the deposits, such as the trough wall imaged here.

The bright region at the top is the flat surface above the trough wall; it is higher than the terrain underneath. The wall exposing these layers has a vertical relief of about 1970 feet.

It is thought that the north polar layered deposits likely formed recently (i.e., millions of years ago) as rhythmic variations in Mars' orbit changed the distribution of water ice around the planet. As ice moved to and from the polar region in response to a changing climate, layers of ice and dust built up at the poles. By studying the history of these deposits, we hope to understand how the Martian climate has changed, similar to how scientists on Earth study ice cores from the North and South Poles.

Three things are immediately apparent about the layers exposed on this trough face. First, individual layers have different surface textures, which some scientists believe could reflect changing physical properties (such as dust content or ice grain size) of the underlying layer. Second, there are several unconformities, or places where one layer is interrupted and overlain by another layer. These unconformities are due to periods where layers were eroded or removed, followed by times when new layers were deposited. Mapping the locations of unconformities can tell us how the deposit shrank and grew over time, and tell us where large changes in climate occurred, causing water ice to be removed from the polar regions. Finally, the dark and bright streaks are due to recent winds blowing surface frost around, and can tell us about wind patterns in the current polar climate.

This was imaged by the HiRISE camera onboard the Mars Reconnaissance Orbiter. HiRISE is the most powerful camera of its kind ever sent to another planet. Its high resolution allows us to see Mars like never before and could help other missions choose a safe spot to land for future exploration.

Credit: NASA/JPL/University of Arizona

Hundreds of fires burned across western Russia on August 2, 2010, but it is the smoke that conveys the magnitude of the disaster in this true-color image from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Dense gray-brown smoke extends across the width of this image, a distance of about 1,700 kilometers (1,000 miles). The smoke clearly continues both east and west beyond the edge of the image, and is visible in both previous and successive orbits of the Terra satellite. The smoke is so thick that it is not possible to see the ground beneath it.

Image Credit: NASA/MODIS Rapid Response

Perfectly circular, powerful Hurricane Celia spaned hundreds of miles over the Pacific Ocean in this image from June 24, 2010. Rough-textured clouds surround the storm’s distinct eye. Farther from the center of the storm, spiral arms appear thinner and smoother.

The Moderate Resolution Imaging Spectroradiometer, or MODIS, on NASA’s Aqua satellite captured this true-color image of Hurricane Celia at 1:55 p.m. Pacific Daylight Time on June 24, 2010. Just five minutes later, the U.S. National Hurricane Center classified Celia as a Category 4 hurricane with sustained winds of 135 miles per hour.

Image Credit: NASA

Recently, technicians at NASA's Marshall Space Flight Center in Huntsville, Ala., completed a series of cryogenic tests on six James Webb Space Telescope beryllium mirror segments at the center's X-ray & Cryogenic Facility. During testing, the mirrors were subjected to extreme temperatures dipping to -415 degrees Fahrenheit, permitting engineers to measure in extreme detail how the shape of the mirror changes as it cools.

The Webb telescope has 18 mirrors, each of which will be tested twice in the Center's X-ray & Cryogenic Facility to ensure that the mirror will maintain its shape in a space environment -- once with bare polished beryllium and then again after a thin coating of gold is applied.

The cryogenic test gauges how each mirror changes temperature and shape over a range of operational temperatures in space. This helps predict how well the telescope will image infrared sources.

The mirrors are designed to stay cold to allow scientists to observe the infrared light they reflect using a telescope and instruments optimized to detect this light. Warm objects give off infrared light, or heat. If the Webb telescope mirror is too warm, the faint infrared light from distant galaxies may be lost in the infrared glow of the mirror itself. Thus, the Webb telescope's mirrors need to operate in a deep cold or cryogenic state, at around -379 degree Fahrenheit.

Image Credit: NASA

Dreamy, Young Stars

The Orion Nebula is a 'happening' place where stars are born and this colony of hot, young stars is stirring up the cosmic scene in this image from NASA's Spitzer Space Telescope. The young stars dip and peak in brightness; shifting cold and hot spots on the stars' surfaces cause brightness levels to change. In addition, surrounding disks of lumpy planet-forming material can obstruct starlight. Spitzer is keeping tabs on the young stars, providing data on their changing ways. The hottest stars in the region are the Trapezium cluster.

This image was taken after Spitzer's liquid coolant ran dry in May 2009, marking the beginning of its "warm" mission.

Image Credit: NASA/JPL-Caltech

This observation from NASA's Mars Reconnaissance Orbiter shows the floor of a large impact crater in the southern highlands, north of the giant Hellas impact basin. Most of the crater floor is dark, with abundant small ripples of wind-blown material. However, a pit in the floor of the crater has exposed light-toned, fractured rock.

The light-toned material appears fractured at several different scales. These fractures, called joints, result from stresses on the rock after its formation. Joints are similar to faults, but have undergone virtually no displacement. With careful analysis, joints can provide insight into the forces that have affected a rock, and thus yielding clues into its geologic history. The fractures appear dark, which may be due to dark, wind-blown sand, precipitation of different minerals along the fracture, or both.

Image Credit: NASA/JPL-Caltech/University of Arizona