Astronomy Picture of the Day: December 2012, Chapter I.

Northern Mercury
Credit: NASA / JHU Applied Physics Lab / Carnegie Inst. Washington

Explanation: Innermost planet Mercury would probably not be a good location for an interplanetary winter olympics. But new results based on data from the Mercury orbiting MESSENGER spacecraft indicate that it does have substantial water ice in permanently shadowed regions within craters near its north pole. The possibility of ice on Mercury has been entertained for years, inspired by the discovery of radar bright, hence highly reflective, regions near the north pole. Highlighted in yellow in this map based on projected MESSENGER images, radar bright regions are seen to correspond with floors and walls of north polar impact craters. Farther from the pole the regions are concentrated on the north facing crater walls. MESSENGER's neutron spectroscopy and thermal models for the craters indicate material in these regions has a hydrogen content consistent with nearly pure water ice and is trapped in an area with temperatures that remain below 100 kelvins (-280 deg.F, -173 deg.C). In circumstances similar to permanent shadows in craters of the Moon, debris from comet impacts is thought to be the source of ice on Mercury.

The Gegenschein Over Chile
Image Credit & Copyright: Yuri Beletsky (ESO)

Explanation: Is the night sky darkest in the direction opposite the Sun? No. In fact, a rarely discernable faint glow known as the gegenschein (German for "counter glow") can be seen 180 degrees around from the Sun in an extremely dark sky. The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets. Pictured above from 2008 October is one of the more spectacular pictures of the gegenschein yet taken. Here a deep exposure of an extremely dark sky over Paranal Observatory in Chile shows the gegenschein so clearly that even a surrounding glow is visible. In the foreground are several of the European Southern Observatory's Very Large Telescopes, while notable background objects include the Andromeda galaxy toward the lower left and the Pleiades star cluster just above the horizon. The gegenschein is distinguished from zodiacal light near the Sun by the high angle of reflection. During the day, a phenomenon similar to the gegenschein called the glory can be seen in reflecting air or clouds opposite the Sun from an airplane.

A Quadruple Lunar Halo Over Spain
Image Credit & Copyright: Dani Caxete

Explanation: Sometimes falling ice crystals make the atmosphere into a giant lens causing arcs and halos to appear around the Sun or Moon. This past Saturday night was just such a time near Madrid, Spain, where a winter sky displayed not only a bright Moon but as many as four rare lunar halos. The brightest object, near the top of the above image, is the Moon. Light from the Moon refracts through tumbling hexagonal ice crystals into a 22 degree halo seen surrounding the Moon. Elongating the 22 degree arc horizontally is a circumscribed halo caused by column ice crystals. More rare, some moonlight refracts through more distant tumbling ice crystals to form a (third) rainbow-like arc 46 degrees from the Moon and appearing here just above a picturesque winter landscape. Furthermore, part of a whole 46 degree circular halo is also visible, so that an extremely rare -- especially for the Moon -- quadruple halo was actually imaged. The snow-capped trees in the foreground line the road Puerto de Navacerrada in the Sierra de Guadarrama mountain range near Madrid. Far in the background is a famous winter skyscape that includes Sirius, the belt of Orion, and Betelgeuse all visible between the inner and outer arcs. Halos and arcs typically last for minutes to hours, so if you do see one there should be time to invite family, friends or neighbors to share your unusual lensed vista of the sky.

In the Center of Saturn's North Polar Vortex
Image Credit: NASA/JPL-Caltech/Space Science Institute

Explanation: What's happening at the north pole of Saturn? A vortex of strange and complex swirling clouds. The center of this vortex was imaged in unprecedented detail last week by the robotic Cassini spacecraft orbiting Saturn. These clouds lie at the center of the unusual hexagonal cloud system that surrounds the north pole of Saturn. The sun rose on Saturn's north pole just a few years ago, with Cassini taking only infrared images of the shadowed region previously. The above image is raw and unprocessed and is being prepared for release in 2013. Several similar images of the region have recently been condensed into a movie. Planetary scientists are sure to continue to study this most unusual cloud formation for quite some time.

Plasma Jets from Radio Galaxy Hercules A
Image Credit: NASA, ESA, S. Baum & C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF),
and the Hubble Heritage Team (STScI/AURA)

Explanation: Why does this galaxy emit such spectacular jets? No one is sure, but it is likely related to an active supermassive black hole at its center. The galaxy at the image center, Hercules A, appears to be a relatively normal elliptical galaxy in visible light. When imaged in radio waves, however, tremendous plasma jets over one million light years long appear. Detailed analyses indicate that the central galaxy, also known as 3C 348, is actually over 1,000 times more massive than our Milky Way Galaxy, and the central black hole is nearly 1,000 times more massive than the black hole at our Milky Way's center. Pictured above is a visible light image obtained by the Earth-orbiting Hubble Space Telescope superposed with a radio image taken by the recently upgraded Very Large Array (VLA) of radio telescopes in New Mexico, USA. The physics that creates the jets remains a topic of research with a likely energy source being infalling matter swirling toward the central black hole.

47 Tuc Near the Small Magellanic Cloud
Image Credit & Copyright: Ivan Eder

 

Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with around 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud (SMC) in the constellation of the Toucan. Of course, the SMC is some 210,000 light-years distant, a satellite galaxy of our Milky Way and not physically close to 47 Tuc. Stars on the outskirts of the SMC are seen at the upper left of this broad southern skyscape. Toward the lower right with about the same apparent diameter as a Full Moon, dense cluster 47 Tuc is made up of several million stars in a volume only about 120 light-years across. Away from the bright cluster core, the red giants of 47 Tuc are easy to pick out as yellowish tinted stars. Globular cluster 47 Tuc is also home to exotic x-ray binary star systems.

Earth at Night
Image Credit : NASA, NOAA NGDC, Suomi-NPP, Earth Observatory,
Data and Processing: Chris Elvidge and Robert Simmon

Explanation: This remarkably complete view of Earth at night is a composite of cloud-free, nighttime images. The images were collected during April and October 2012 by the Suomi-NPP satellite from polar orbit about 824 kilometers (512 miles) above the surface using its Visible Infrared Imaging Radiometer Suite (VIIRS). VIIRS offers greatly improved resolution and sensitivity compared to past global nightlight detecting instrumentation on DMSP satellites. It also has advantages compared to cameras on the International Space Station. While the space station passes over the same point on Earth every two or three days, Suomi-NPP passes over the same point twice a day at about 1:30am and 1:30pm local time. Easy to recognize here, city lights identify major population centers, tracking the effects of human activity and influence across the globe. That makes nighttime images of our fair planetamong the most interesting and important views from space.

Baku Moonrise
Image Credit & Copyright: Tunç Tezel (TWAN)

Explanation: A Full Moon rises in this waterfront scene. Its colorful, watery reflection is joined by harbor lights and a windowed skyscraper's echo of the western horizon just after sunset. The tantalizing image is a composite of frames recorded at 2 minute intervals on November 28 from the Caspian Sea port city of Baku, Azerbaijan. Still, this Full Moon was not really as big or as bright as others, though it might be hard to tell. In fact, November 28's Full Moon was near apogee, making it the smallest Full Moon of 2012. As it rose over the Baku boardwalk (along with much of the eastern hemisphere), it was also in the Earth's lighter or penumbral shadow. The subtle effect of the penumbral lunar eclipse is just discernible as the slightly darker left side of the lunar disk. Opposite the Sun in planet Earth's sky, the Full Moon was also joined by bright planet Jupiter, only a few days from its own opposition.

The Astronaut Who Captured a Satellite
Image Credit: STS-51A, NASA

Explanation: In 1984, high above the Earth's surface, an astronaut captured a satellite. It was the second satellite captured that mission. Pictured above, astronaut Dale A. Gardner flies free using the Manned Maneuvering Unit and begins to attach a control device dubbed the Stinger to the rotating Westar 6 satellite. Communications satellite Westar 6 had suffered a rocket malfunction that left it unable to reach its intended high geosynchronous orbit. Both the previously caught Palapa B-2 satellite and the Westar 6 satellite were guided into the cargo bay of the Space Shuttle Discovery and returned to Earth. Westar 6 was subsequently refurbished and sold.

2008 Timelapse from Joe Cali on Vimeo.

Time-Lapse: A Total Solar Eclipse
Video Credit & Copyright: Colin Legg

Explanation: Have you ever experienced a total eclipse of the Sun? The above time-lapse movie depicts such an eclipse in dramatic detail as visible from Australia last month. As the video begins, a slight dimming of the Sun and the surrounding Earth is barely perceptible. Suddenly, as the Moon moves to cover nearly the entire Sun, darkness sweeps in from the left -- the fully blocked part of the Sun. At totality, only the bright solar corona extends past the edges of the Moon, and darkness surrounds you. Distant horizons are still bright, though, as they are not in the darkest part of the shadow. At mid-totality the darkness dips to the horizon below the eclipsed Sun, created by the shadow cone -- a corridor of shadow that traces back to the Moon. As the total solar eclipse ends -- usually after a few minutes -- the process reverses and Moon's shadow moves off to the other side. Solar eclipses can frequently be experienced at gatherings organized along the narrow eclipse path as well as specialized cruises and plane flights.

 
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