Vesta, Before and After Dawn

These two images compare topographic maps of the giant asteroid Vesta as discerned by NASA's Hubble Space Telescope (top) and as seen by NASA's Dawn spacecraft (bottom). Hubble has been in an orbit around Earth, while Dawn orbited Vesta from 2011 to 2012. Although the absolute scale ranges are slightly different in Dawn data, Vesta's relative topography is remarkably consistent between the two data sets. The relative topography in Hubble data varies from 7.5 miles (12 kilometers) below to 7.5 miles (12 kilometers) above a reference ellipsoid shape of 180 by 174 by 142 miles (289 by 280 by 229 kilometers). The relative topography in Dawn data varies from 14 miles (22 kilometers) below to 12 miles (19 kilometers) above a reference ellipsoid shape of 177 by 177 by 142 miles (285 by 285 by 229 kilometers).

These two maps of the giant asteroid Vesta show patterns of brightness from NASA's Hubble Space Telescope (top) and NASA's Dawn spacecraft (bottom). Hubble's view is from an orbit around Earth. Dawn went into orbit around Vesta from 2011 to 2012. Scientists have been able to correlate several bright and dark features originally identified in Hubble images with features imaged at high resolution by the Dawn spacecraft's framing camera.

Image credits: (top) NASA/ESA/Cornell and NASA/JPL-Caltech/UCLA/MPS/DLR/IDA; (bottom) NASA/ESA/PSI/MIT and NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Note: For more information, see PIA17467: Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta, Take a Virtual Tour of Vesta With New High-Resolution Images and Dawn Reality-Checks Telescope Studies of Asteroids.

Spitzer's Hunt for Exoplanets

Over its ten years in space, NASA's Spitzer Space Telescope has evolved into a premier tool for studying exoplanets. The engineers and scientists behind Spitzer did not have this goal in mind when they designed the observatory back in the 1990s. But thanks to its extraordinary stability, and a series of engineering reworks after launch, Spitzer now has observational powers far beyond its original limits and expectations.

This artist's concept shows Spitzer surrounded by examples of exoplanets the telescope has examined.

Illustration credit: NASA/JPL-Caltech

Note: For more information, see How Engineers Revamped Spitzer to Probe Exoplanets.

Neutron Star IGR J18245-2452

IGR J18245-2452: A binary system that includes a pulsar located about 18,000 light years from Earth.

These two images from NASA's Chandra X-ray Observatory show a large change in X-ray brightness of a rapidly rotating neutron star, or pulsar, between 2006 and 2013. The neutron star - the extremely dense remnant left behind by a supernova - is in a tight orbit around a low mass star. New observations with both X-ray and radio telescopes of IGR J18245-2452 provide important information about the evolution of pulsars in binary systems.

Scale: Each panel is 1.2 arcmin across (About 6 light years).

Image credit: X-ray: NASA/CXC/ICE/A.Papitto et al

Note: For more information, see IGR J18245-2452: Neutron Star Undergoes Wild Behavior Changes.

NGC 6334 in Submillimeter Wavelengths

This image of the star formation region NGC 6334 is one of the first scientific images from the ArTeMiS instrument on APEX. The picture shows the glow detected at a wavelength of 0.35 millimeters coming from dense clouds of interstellar dust grains. The new observations from ArTeMiS show up in orange and have been superimposed on a view of the same region taken in near-infrared light by ESO’s VISTA telescope at Paranal.

Image credit: ArTeMiS team/Ph. André, M. Hennemann, V. Revéret et al./ESO/J. Emerson/VISTA Acknowledgment: Cambridge Astronomical Survey Unit

Note: For more information, see The Cool Glow of Star Formation.

An Ordinary Pulsar Evolving Into a Millisecond Pulsar

This animation shows an artist's impression of the evolutionary process that is believed to turn pulsars into millisecond pulsars.

The emission mechanism of pulsars transforms kinetic rotational energy into radiation: as this energy is radiated over time, the rotation is slowed down. Whilst pulsars spin rapidly at birth, they tend to rotate more slowly – with periods of up to a few seconds – as they age.

The mysterious millisecond pulsars – old but extremely quickly rotating pulsars with periods of a few thousandths of a second – are explained through a theoretical model known as the 'recycling' scenario. If a pulsar is part of a binary system and is accreting matter from a stellar companion via an accretion disc, then it may also gain angular momentum. This process can 'rejuvenate' old pulsars, boosting their rotation and making their periods as short as a few milliseconds.

Using data from INTEGRAL and XMM-Newton, astronomers have discovered IGR J18245-2452, a millisecond pulsar that within only a few weeks switched from being accretion-powered and X-ray bright to rotation-powered and bright in radio waves. As the evolutionary link between these two categories of sources, this millisecond pulsar brings conclusive evidence to the 'recycling' scenario.

The animation shows a pulsar in a binary system, with a low-mass, red star as a companion. The two objects orbit around their mutual center of gravity; for clarity, this motion is not shown in the animation.

At the beginning of the animation, the pulsar spins very fast, then its rotation gradually slows down. At this stage, the pulsar's emission is entirely supported by its rotation and results in two narrow beams of radio waves (shown in purple). The slowing down process may last several millions of years.

Eventually, the gravitational pull of the pulsar – which is a very dense object – starts drawing matter from the companion star. As the pulsar accretes matter via an accretion disc, it gains angular momentum and its rotation becomes extremely rapid again.

During the accretion process, the high density of accreted matter inhibits the acceleration of particles that cause radio emission, so the pulsar is not visible in radio waves but only in X-rays (shown as wide, white beams). When the accretion rate decreases, the magnetosphere expands and pushes matter away from the pulsar: as a consequence, the X-ray emission becomes weaker and weaker, while the radio emission intensifies.

Over a period of at least several hundreds of millions of years, the pulsar keeps swinging back and forth between the two states several times, emitting alternately X-rays and radio waves. When the accretion process stops, the pulsar becomes a purely rotation-powered, radio-emitting millisecond pulsar, while its companion star has evolved into a white dwarf.

Video credit: ESA; text credit: ESA

Note: For more information, see Volatile Pulsar Reveals Millisecond Missing Link.

M60-UCD1

M60-UCD1: A galaxy about 54 million light years from Earth.

The densest galaxy in the nearby Universe may have been found. The galaxy, known as M60-UCD1, is located near a massive elliptical galaxy NGC 4649, also called M60. This composite image of M60 and the region around it presents X-rays from Chandra (pink) and optical data from Hubble (red, green, and blue). The Chandra image shows hot gas and double stars containing black holes and neutron stars, and the HST image reveals stars in M60 and neighboring galaxies including M60-UCD1. The inset is a close-up view of M60-UCD1 in an HST image. The density of stars in M60-UDC1 is about 15,000 times greater than found in Earth’s neighborhood in the Milky Way, meaning that the stars are about 25 times closer.

Scale: Image is 3.2 arcmin on a side (about 50,000 light years).

Image credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI

Note: For more information, see M60-UCD1: NASA's Hubble and Chandra Find Evidence for Densest Nearby Galaxy.

Comets Tempel 1 and Hartley 2

NASA's Deep Impact spacecraft made history flying past two of the solar system's icy nomads. On Independence Day, 2005, Deep Impact flew past comet Tempel 1. On November 4, 2010, it flew past the bowling-pin-shaped comet Hartley 2.

Launched on a clear winter day in January 2005, NASA's Deep Impact spacecraft spanned 268 million miles (431 million kilometers) of deep space in 172 days, then reached out and touched comet Tempel 1. The collision between the coffee table-sized impactor and city-sized comet occurred on July 4, 2005, at 1:52 a.m. EDT. This hyper-speed collision between spaceborne iceberg and copper-fortified, rocket-powered probe was the first of its kind. It was a boon to not only comet science, but to the study of the evolution of our solar system.

The mission of Deep Impact was supposed to conclude within weeks of this July 4 cometary smackdown. Then, NASA approved a mission extension, re-enlisting the Deep Impact spacecraft for two distinct celestial targets of opportunity. EPOXI, as the mission was renamed, was a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI).

The Deep Impact spacecraft, history's most traveled deep-space comet hunter, provided many significant results for the science community. Here are the top five, according to the mission's principal investigator, Michael A'Hearn of the University of Maryland, College Park.

Studies of imagery showed that that the luminous flash created within a fraction of a second after Deep Impact's impactor was atomized by comet Tempel 1 was much fainter than expected. Comparison with experimental impacts at the Vertical Gun Range at NASA Ames Research Center in Moffett Field, California, showed that such a faint flash was consistent only with a surface layer (depth a few times the diameter of the impactor) that was more than 75 percent empty space. This surprisingly high porosity was in contrast with theories that predicted comets were armored with a stronger, solid crust that impeded outgassing.

Observations of comet Tempel 1 by Deep Impact's spectrometer instrument showed that water was arising primarily at longitudes near noon and peaking near the equator, whereas most of the carbon dioxide was arising from far southern latitudes, not too far from comet Tempel 1's south pole. This could be due to seasonal effects (southern hemisphere just going into winter darkness) or due to differences in the chemical composition in different parts of the nucleus. During the mission extension, the EPOXI observations of comet Hartley 2 showed that the comet's smooth waist was emitting pure water, while the small end was emitting excess carbon dioxide, regardless of time of day. This was a clear sign that chemical diversity was the important factor in a comet's chemical makeup.

For many years we have known that a handful of comets (fewer than 10 percent) produced more water vapor than should be possible by sublimation of nucleus of water ice, in which the sizes of the nuclei are known. The flyby of comet Hartley 2 showed a large number of icy grains in the coma are driven out of the nucleus by the outgassing of carbon dioxide. These icy grains are plausibly the source of much of the water coming from the comet.

Observations of Hartley 2 by the Deep Impact spacecraft showed the importance of carbon-dioxide ice relative to carbon-monoxide ice in comets, and led to reexamination of all previous observations of these two ices in comets. The relative abundances in short-period and long-period comets imply that the short-period comets formed under warmer conditions than did the long-period comets. Thus, the short-period comets must have formed closer to the sun than their longer-period brethren. This is contrary to popular belief in the astronomical community (for many decades) that the short-period comets formed in the Kuiper belt beyond Neptune, while the long-period comets formed in the vicinity of the giant planets. The new model fits well with measurements by other astronomers of heavy water in Hartley 2, and with the newest dynamical studies of planetary migration.

The excavation of a crater on Tempel 1 was the trigger that allowed the proposal for the Stardust NExT mission to succeed. In addition to searching for the crater formed by Deep Impact, a key goal of that Stardust-NExT mission was to measure changes in the surface of the comet over an orbital period. This second set of measurements of Tempel 1 surface features showed that much of the evolution was in discrete, large areas, i.e., there was not a small, uniform erosion of the all parts of the surface, but rather large changes in a few places. Thus, comets evolve in a manner analogous to erosion - most erosion takes place in discrete events (floods that make large, local changes) rather than as a slow, continuous process.

Image credit: NASA/JPL-Caltech/UM

Note: For more information, see NASA's Deep Space Comet Hunter Mission Comes to an End.

HD 184738 aka Campbell's Hydrogen Star

This new image, snapped by NASA/ESA Hubble Space Telescope, shows the star HD 184738, also known as Campbell’s hydrogen star. It is surrounded by plumes of reddish gas — the fiery red and orange hues are caused by glowing gases, including hydrogen and nitrogen.

HD 184738 is at the center of a small planetary nebula. The star itself is known as a [WC] star, a rare class resembling their much more massive counterparts — Wolf-Rayet stars. These stars are named after two French astronomers, Charles Wolf and Georges Rayet, who first identified them in the mid-nineteenth century.

Wolf-Rayet stars are hot stars, perhaps 20 times more massive than the Sun, that are rapidly blowing away material and losing mass. [WC] stars are rather different: they are low-mass Sun-like stars at the end of their lives. While these stars have recently ejected much of their original mass, the hot stellar core is still losing mass at a high rate, creating a hot wind. It is these winds that cause them to resemble Wolf-Rayet stars.

However, astronomers can look more closely at the composition of these winds to tell the stars apart; [WC] stars are identified by the carbon and oxygen in their winds. Some true Wolf-Rayet stars are rich in nitrogen instead, but this is very rare among their low-mass counterparts.

HD 184738 is also very bright in the infrared part of the spectrum, and is surrounded by dust very similar to the material that the Earth formed from. The origin of this dust is uncertain.

Photo credit: ESA

Coma Cluster

Coma Cluster: A collection of thousands of galaxies about 320 million light years from Earth.

Enormous arms of hot gas have been revealed in the Coma galaxy cluster in data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton. A specially processed Chandra image (pink) has been combined with optical data from the Sloan Digital Sky Survey (white and blue) to highlight these spectacular arms. Researchers think that these arms -- which span at least a half million light years -- were most likely formed when smaller galaxy clusters had their gas stripped away by the head wind created by the motion of the clusters through the hot gas.

Scale: Image is 23 arcmin on a side (about 2 million light years).

Image credit: X-ray: NASA/CXC/MPE/J.Sanders et al, Optical: SDSS

Note: For more information, see Coma Cluster: Clues to the Growth of the Colossus in Coma

IC 4628, the Prawn Nebula

The glowing jumble of gas clouds visible in new image make up a huge stellar nursery nicknamed the Prawn Nebula. Taken using the VLT Survey Telescope at ESO’s Paranal Observatory in Chile, this may well be the sharpest picture ever taken of this object. It shows clumps of hot new-born stars nestled in among the clouds that make up the nebula.

Photo credit: ESO. Acknowledgement: Martin Pugh

Note: For more information, see Young Stars Cooking in the Prawn Nebula.

Coronal Mass Ejection Intensity Map

A coronal mass ejection observed by the ESA/NASA SOHO space mission on 4 January 2002 has been colored to indicate the intensity of the matter being ejected by the Sun. White represents the greatest intensity, red/orange somewhat less, and blue the least.

An extreme-ultraviolet image of the Sun captured by SOHO’s EIT (Extreme ultraviolet Imaging Telescope) instrument is superimposed on the image. The shaded blue disc surrounding the Sun at the center is a mask in SOHO’s LASCO instrument that blots out direct sunlight to allow study of the details in the Sun’s corona.

Image credit: SOHO (ESA/NASA)/S. Hill

Voyager Goes Interstellar

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the sun, is about 30 AU.

Informally, the term "solar system" is often used to mean the space out to the last planet. Scientific consensus, however, says the solar system goes out to the Oort Cloud, the source of the comets that swing by our sun on long time scales. Beyond the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the sun.

The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU.

NASA's Voyager 1, humankind's most distant spacecraft, is around 125 AU. Scientists believe it entered interstellar space, or the space between stars, on August 25, 2012. Much of interstellar space is actually inside our solar system. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it.

Alpha Centauri is currently the closest star to our solar system. But, in 40,000 years, Voyager 1 will be closer to the star AC +79 3888 than to our own sun. AC +79 3888 is actually traveling faster toward Voyager 1 than the spacecraft is traveling toward it.

Illustration credit: NASA/JPL-Caltech

Note: Of course, this story is major news, and many articles and graphics have been published. The following is some of those available:
* PIA17045: Voyager Captures Sounds of Interstellar Space
* PIA17047: Voyager Signal Spotted By Earth Radio Telescopes
* PIA17048: One Voyager Out, One Voyager In (Artist Concept)
* PIA17049: Voyager in Space (Artist Concept)
* PIA17441: Observed Change in Density Shows Voyager is in Interstellar Space
* PIA17442: Mystery of the Interstellar Magnetic Field (Artist's Concept)
* PIA17460: Moving into Interstellar Space (Artist Concept)
* PIA17461: Heading toward Gliese 445
* PIA17462: Voyager 1 Entering Interstellar Space (Artist Concept)
* PIA17463: Heliosphere Traveling Through Interstellar Space
* PIA17464: Voyager 1 Launch (1977)
* How Do We Know When Voyager Reaches Interstellar Space?
* NASA Spacecraft Embarks on Historic Journey Into Interstellar Space
* Voyager 1 Reaches Interstellar Space

From earlier in the year, see:
* NASA's Voyager 1 Explores Final Frontier of Our 'Solar Bubble'
* Voyager 1 Approaches Interstellar Space

Abell 1689

This new Hubble image shows galaxy cluster Abell 1689. It combines both visible and infrared data from Hubble's Advanced Camera for Surveys (ACS) with a combined exposure time of over 34 hours (image on left over 13 hours, image on right over 20 hours) to reveal this patch of sky in greater and striking detail than in previous observations.

This image is peppered with glowing golden clumps, bright stars, and distant, ethereal spiral galaxies. Material from some of these galaxies is being stripped away, giving the impression that the galaxy is dripping, or bleeding, into the surrounding space. Also visible are a number of electric blue streaks, circling and arcing around the fuzzy galaxies in the center.

These streaks are the telltale signs of a cosmic phenomenon known as gravitational lensing. Abell 1689 is so massive that it bends and warps the space around it, affecting how light from objects behind the cluster travels through space. These streaks are the distorted forms of galaxies that lie behind the cluster.

Photo credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Blakeslee (NRC Herzberg Astrophysics Program, Dominion Astrophysical Observatory), and H. Ford (JHU)

Note: For more information, see New Hubble Image of Galaxy \Cluster Abell 1689.

Different Perspectives of the Milky Way

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This artist’s impression shows how the Milky Way galaxy would look from very different perspectives than we get from the Earth. From some angles the central bulge shows up as a peanut shaped glowing ball of stars and from above the central narrow bar appears clearly. The many spiral arms and their associated dust clouds are also clearly seen.

Video credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt

Note: For more information, see The Peanut at the Heart of our Galaxy.

Comet Don Quixote

With the help of NASA's Spitzer Space Telescope, astronomers have discovered that what was thought to be a large asteroid called Don Quixote is in fact a comet.

In Figure 1, the left image shows Don Quixote's coma and tail -- features of comets -- as revealed in infrared light by Spitzer. The coma appears as a faint glow around the center of the body, caused by dust and gas. The tail, which appears more clearly in the right image, points towards the right-hand side of Don Quixote, into the direction opposite of the sun. The right image represents a more elaborate image processing step, in which the glow of the coma has been removed based on a model comet coma. Bright speckles around Don Quixote are background stars; the horizontal bar covers image artifacts caused by the image processing.

Image credit: NASA/JPL-Caltech/DLR/Northern Arizona University

Note: For more information, see Comet Found Hiding in Plain Sight.

Earth's Plasmasphere and the Van Allen Belts

These three panels show how the relative locations of the outer boundary of the Earth's plasmasphere, the plasmapause, (shown in blue) and the van Allen belts (shown in red) change according to geomagnetic conditions.

The plasmasphere – the innermost part of the Earth's magnetosphere – is a doughnut-shaped region of low energy charged particles (cold plasma) centered around the planet's equator and rotating along with it. Its toroidal shape is determined by the magnetic field of Earth. The plasmasphere begins above the upper ionosphere and extends outwards, with the outer boundary varying (depending on geomagnetic conditions) from 4.5 Earth radii (RE) to 8 RE.

The two Van Allen radiation belts are concentric, tire-shaped belts (shown in blue) of highly energetic (0.1–10 MeV) electrons and protons, which are trapped by the magnetic field and travel around the Earth. These radiation belts partly overlap with the plasmasphere. The inner Van Allen belt is located typically between 6000 and 12,000 km (1 - 2 Earth radii [RE]) above Earth's surface, although it dips much closer over the South Atlantic Ocean. The outer radiation belt covers altitudes of approximately 25,000 to 45,000 km (4 to 7 RE).

Both belts are separated from each other by an empty "slot" region. A temporary third belt (not shown in this image), between this slot and the outer main belt, was detected in 2013 by NASA's Van Allen Probes.

Data from the Cluster mission have shown that the position of the plasmapause – the outer boundary of the plasmasphere – is quite variable and, in addition, that the size of the slot region between the radiation belts varies with changes in geomagnetic conditions.

During periods of low geomagnetic activity (top panel) the plasmapause typically extends to around 6 RE, occasionally expanding beyond the boundary of the outer radiation belt, as far as 8 RE or even further. This result contrasts with previous studies based on observations from different spacecraft which indicated a correlation between the position of the plasmapause and the location of the inner edge of the outer belt.

During periods of higher geomagnetic activity (with moderate activity illustrated in the central panel and high activity in the lower panel) the plasmapause moves closer to the inner boundary of the outer belt, to around 4.5 RE. This behavior is similar to that observed by previous studies. (Note that the outer radiation belt also moves, but at a slower rate, towards the Earth.)

The size of the slot region also varies with geomagnetic conditions, being wider during low geomagnetic activity.

Illustration credit: ESA - C. Carreau

Note: For more information, see Cluster Shows Plasmasphere Interacting with Van Allen Belts.

ESO 540-31

A new image by the NASA/ESA Hubble Space Telescope of dwarf galaxy ESO 540-31 set against a background of distant galaxies. ESO 540-31 lies just over 11 million light-years from Earth, in the constellation of Cetus (The Whale).

Image credit: ESA/Hubble & NASA; acknowledgement: L. Limatola

Massive Black Holes Near IC 751

An optical color image of galaxies is seen here overlaid with X-ray data (magenta) from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR).

NuSTAR's serendipitous discovery in this field, indicated by the arrow (Figure 1), lies to the left of a galaxy, called IC751, at which the telescope originally intended to look. Both magenta blobs show X-rays from massive black holes buried at the hearts of galaxies.

The optical image is from the Sloan Digital Sky Survey and a color composite of images over three different optical wavebands (the G, R, and I bands). The NuSTAR data shows X-rays in the 3 to 24 keV energy range.

Image credit: NASA/JPL-Caltech

Note: For more information, see Catching Black Holes on the Fly.

Saturn

The huge storm churning through the atmosphere in Saturn's northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA's Cassini spacecraft. This storm is the largest, most intense storm observed on Saturn and is still active today. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Brown Dwarf Stars Toward Orion

The locations of brown dwarfs discovered by NASA's Wide-field Infrared Survey Explorer, or WISE, and mapped by NASA's Spitzer Space Telescope, are shown in this diagram as red circles. The red lines all link back to the location of the sun.

The view is from a vantage point about 100 light-years away from the sun, looking back toward the constellation Orion. At this distance, our sun is barely visible as a speck of light. The vastly fainter brown dwarfs would not even be visible in this view.

Image credit: NASA/JPL-Caltech

Note: For more information, see PIA17258: Free-floating Failed Star (Artist Concept) and Coldest Brown Dwarfs Blur Star, Planet Lines.

NGC 6537

This image shows an example of a bipolar planetary nebula known as NGC 6537 taken with the New Technology Telescope at ESO’s La Silla Observatory. The shape, reminiscent of a butterfly or an hourglass, was formed as a Sun-like star approached the end of its life and puffed its outer layers into the surrounding space. For bipolar nebulae, this material is funneled towards the poles of the aging star, creating the distinctive double-lobed structure.

Observations using the NTT and Hubble have found that bipolar planetary nebulae located towards the central bulge of our Milky Way appear to be strangely aligned in the sky — a surprising result given their varied and chaotic formation.

NGC 6537, which lies much closer to the Earth, was not part of the new study.

Image credit: ESO

Note: For more information, see Bizarre Alignment of Planetary Nebulae. See also Bizarre Alignment of Planetary Nebulae.

LADEE and Lunar Twilight Rays

Back in the 60s and 70s, Apollo astronauts circling the Moon saw something that still puzzles researchers today. About 10 seconds before lunar sunrise or lunar sunset, pale luminous streamers would pop up over the gray horizon. These “twilight rays” were witnessed by crewmembers of Apollo 8, 10, 15 and 17.

Back on Earth, we see twilight rays all the time as shafts of sunlight penetrate evening clouds and haze. The “airless Moon” shouldn’t have such rays, yet the men of Apollo clearly saw them.

Later this week a NASA spacecraft is going back to the Moon to investigate. Slated for launch on September 6, 2013, the Lunar Atmosphere and Dust Environment Explorer (“LADEE” for short) will seek out twilight rays and other mysteries of the lunar atmosphere.

“Yes, the Moon does have an atmosphere,” says Richard Elphic, the project scientist for LADEE at NASA Ames. “It’s just much more tenuous than ours.”

The Moon’s atmosphere is so flimsy — about ten thousand billion times less dense than Earth’s — that a good sneeze would rip through it like a hurricane. “Lunar air” is a gossamer mix of argon-40, which seeps out of the ground due to radioactive decay in the lunar interior, plus elements such as helium, sodium, and potassium, sputtered off the lunar surface by solar wind and micrometeoroids.

None of these gases appear in sufficient quantities, however, to explain the twilight rays.

“We’re missing something,” says Elphic.

The missing piece might be dust. When sunlight falls on the Moon, solar UV radiation electrifies the unprotected topsoil, possibly causing lightweight grains of moondust to rise off the ground, joining the gases already there.

“This electrically charged dust may be what the astronauts saw,” says Elphic. LADEE’s Lunar Dust Experiment will collect and analyze dust in the Moon’s atmosphere to test this hypothesis.

Researchers have a special name for atmospheres as fantastically thin as the Moon’s: an exosphere. On Earth, molecules in the thick air are constantly bumping into each other, spreading pressure and heat in all directions. In an exosphere, however, molecules are so far apart they rarely collide.

“Instead of bumping into each other,” says Elphic, “they bump into the lunar surface.”

Lunar twilight rays sketched by Apollo 17 astronauts.
Air molecules coming into contact with moondust are expected to stick, briefly, before moving on again. Hop and stick, hop and stick. At any given moment millions of molecules could be hopping like bunnies across every square inch of lunar terrain. Ultraviolet, visible light, and mass spectrometers on board LADEE will inventory the molecules present and determine how they behave.

“The dusty, flimsy mix of atoms and molecules in the lunar atmosphere is sure to have alien properties that our experience on Earth has not prepared us to anticipate,” says Elphic.

To find out, LADEE will be working on a deadline. On April 15th of next year, the sunset-colored shadow of Earth will envelop the Moon for a lunar eclipse. It will be a grand sight from Earth, but bad news for LADEE. The spacecraft is solar powered and requires sunlight to charge its batteries. An eclipse could end the mission.

"The current plan," says Elphic, "is, before the eclipse, to guide the spacecraft into the surface of the moon for a final impact that we can study. We’ll be taking data until the very end."

Video credit: NASA

PGC 10922

The NASA/ESA Hubble Space Telescope has captured this image of PGC 10922, an example of a lenticular galaxy – a galaxy type that lies on the border between ellipticals and spirals.

Seen face-on, the image shows the disc and tightly-wound spiral structures of dark dust encircling the bright center of the galaxy. There is also a remarkable outer halo of faint wide arcs or shells extending outwards, covering much of the picture. These are likely to have been formed by a gravitational encounter or even a merger with another galaxy. Some dust also appears to have escaped from the central structure and has spread out across the inner shells.

An extraordinarily rich background of more remote galaxies can also be seen in the image.

Photo credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt