NASA Astronomy Picture of the Day 1998-1

The Largest Rock Known

There, that faint dot in the center - that's the largest rock known. It is larger than every known asteroid, moon, and comet nucleus. It is larger than any other rocky planet. (Nobody knows for sure what size rocks lie at the cores of Jovian planets, or orbit other stars.) The Voyager 1 spacecraft took this picture in 1990 from the outer Solar System. This rock is so large its gravity makes it nearly spherical, and holds heavy gases near its surface. Today, this rock starts another orbit around its parent star, for roughly the 5 billionth time, spinning over 350 times during each trip. Happy Gregorian Calendar New Year to all the human inhabitants of this rock we call Earth.

Europa's Disconnected Surface

Jupiter's moon Europa is so exciting that the Galileo spacecraft orbiting Jupiter has now embarked on an extended mission to study it. Oceans that might exist beneath Europa's surface are thought to be one of the best places to look for life in our Solar System. The Galileo Europa Mission has planned eight close fly-bys frozen moon. The first close encounter of this extended mission occurred last December and the next will occur in February. The above enhanced color picture shows a small region of Europa's Conamara region. The white and blue colors highlight regions covered by ice dust from the collision that created the Pwyll Crater. Disconnected islands of ice are visible that have apparently rafted to new positions.

The Barren Moon

The above photo, taken as the Apollo 17 astronauts orbited the Moon in 1972, depicts the stark lunar surface around the Eratosthenes and Copernicus craters. Many similar images of a Moon devoid of life are familiar to denizens of the space age. Contrary to this modern perception, life on the Moon was reported in August of 1835 in a series of sensational stories first published by the New York Sun - apparently intended to improve the paper's circulation. These descriptions of lunar life received broad credence and became one of the most spectacular hoaxes in history. Supposedly based on telescopic observations, the stories featured full, lavish accounts of a Moon with oceans and beaches, teeming with plant and animal life and climaxing with the report of sightings of groups of winged, furry, human-like creatures resembling bats!

Fractal Interstellar Dust Up-Close

Our universe is a very dusty place. Dust usually shows its presence by blocking out light emitted from stars or nebulae behind it, sometimes creating the illusion of a horse's head or a sombrero hat. But nobody really knows what a typical interstellar dust grain looks like. By studying how dust absorbs, emits, and reflects light, astronomers do know that interstellar dust is much different than the cell and lint based dust found around a typical house. Interstellar dust grains are composed mostly of carbon, silicon, and oxygen and are usually less than about 1/1000 of a millimeter across. Recent work indicates that most dust grains are not spherical. The above picture shows the result of a fractal adhesion model for dust grains involving random conglomerates of spherical compounds of different properties, here artificially highlighted by different colors.

Earth's Richat Structure

What on Earth is that? The Richat Structure in the Sahara Desert of Mauritania is easily visible from space because it is nearly 50 kilometers across. Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock. Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion. Why the Richat Structure is nearly circular remains a mystery.

The Red Spider Planetary Nebula

Oh what a tangled web a planetary nebula can weave. The Red Spider Planetary Nebula shows the complex structure that can result when a normal star ejects its outer gases and becomes a white dwarf star. Officially tagged NGC 6537, this two-lobed symmetric nebula houses one of the hottest white dwarfs ever observed, probably as part of binary star system. Internal winds emanating from the central stars, shown in the central inset, have been measured in excess of 300 kilometers per second. These hot winds expand the nebula, flow along the nebula's walls, and cause gas and dust to collide. Atoms caught in these colliding shocks radiate light shown in the above representative-light picture.

The Colorful Moon

Do you recognize the Earth's Moon when you see it? The crazy, patchwork appearance of this false color image makes this nearly full view of our Moon's familiar nearside look very strange. The image was taken in 1992 by the Galileo spacecraft enroute to Jupiter. The Sea of Tranquillity (Mare Tranquillitatis) is the blue area at right, the Ocean of Storms (Oceanus Procellarum) is the extensive blue and orange area on the left, and white lines radiate from the crater Tycho at bottom center. Three filters were used to make three separate exposures, combined in an exaggerated color scheme to emphasize composition differences - blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron. NASA soon plans to conduct an extensive remote exploration of the lunar surface, including a search for water ice near the lunar South Pole, with the Lunar Prospector spacecraft.

Destination: Moon

Tuesday, January 6, at 9:28 p.m. EST, NASA's Lunar Prospector spacecraft climbed into the sky above Cape Canaveral Air Station riding an Athena II rocket. Representing NASA's first Moon mission since the 1972 flight of Apollo 17, this launch also occurred on the 30th anniversary of the launch of the Surveyor 7 lunar lander. The three stage launch vehicle's fiery trail is in the foreground of this time exposure while the Moon, near first quarter phase, is shown in the background some 250,000 miles from the Cape. Prospector will cover that distance in about 5 days, entering lunar orbit on Sunday. Prospector carries no cameras to image the well-photographed lunar surface. Instead, its array of instruments will map the lunar gravity, magnetic field, internal structure, and surface composition. The result, a detailed global view of current lunar properties, is expected to dramatically impact humanity's understanding of the origins of the Moon and the Solar System. From its vantage point in polar orbit, only 63 miles above the lunar surface, Prospector will also conduct a sensitive search for water ice which may be preserved in permanent shadow at the Moon's South Pole.

Saturnian Aurora

Girdling the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This recently released image, from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.

Disorder in Stephan's Quintet

What are five closely grouped galaxies doing in this image? The grouping is commonly known as Stephan's Quintet. Four of the galaxies show essentially the same redshift suggesting that they are at the same distance from us. The large bluish spiral below and left of center actually has a smaller redshift than the others, indicating it is much closer. It is probably a foreground object which happens to lie along the line of sight to the more distant galaxies. Of the four distant galaxies, three seem to be colliding, showing serious distortions due to gravitational tidal forces. The fourth is a normal appearing elliptical galaxy (at the lower right edge of the field). Recent results suggest that collisions play an important role in the life cycles of galaxies.

Abell 2218: A Galaxy Cluster Lens

Gravity can bend light. Almost all of the bright objects in this Hubble Space Telescope image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. As a result, multiple images of these background galaxies are distorted into faint stretched out arcs - a simple lensing effect analogous to viewing distant street lamps through a glass of wine. The Abell 2218 cluster itself is about 3 billion light-years away in the northern constellation Draco.

The Keyhole Nebula

The dark dusty Keyhole Nebula gets its name from its unusual shape. Officially designated NGC 3324, the Keyhole Nebula is a smaller region superposed on the larger Eta Carina Nebula. These nebulae were created by the dying star Eta Carina, which is prone to violent outbursts during its final centuries. Noted and discussed as early as 1840 when a spectacular explosion became visible, the Eta Carina system now appears to be undergoing an unusual period of change. An emission nebula that contains much dust, the Keyhole Nebula is roughly 9,000 light years distant. This photogenic nebula can be seen in the south with even a small telescope. The Keyhole Nebula was recently discovered to contain highly structured clouds of molecular gas.

El Niño Water Rhythm

This year's El Niño is the strongest ever recorded. The large amount of warm water in the Pacific Ocean near the equator is causing unusual weather all over planet Earth. The above false-color images of Earth show the relative levels of warm water between normal (upper left) and the comparatively mild El Niño of 1995 - 1996. Red indicates a high water level. Detailed measurements of this years El Niño indicate the warm water level is rising and falling in rhythmic fashion, possibly in response to changing wind patterns.

A Distant Destiny

Watching galaxies recede, observational astronomers of the 20th century discovered an astounding fact - the Universe is expanding. Will it continue to expand forever? The speed of light is finite so looking into the distant Universe is equivalent to looking at the distant past. These three Hubble Space Telescope images show some of the most distant detectable supernovae - stellar explosions, five to seven billion light-years away, which occurred before the Sun was formed. As recently reported, the apparent brightness and expansion velocities associated with these and other distant supernovae are beginning to suggest that the expansion rate of the Universe has not slowed but increased over time and that the expansion of the Universe is destined to continue.

Eugene Shoemaker: 1928-1997

ugene Shoemaker's passion was Astrogeology. He dreamed of going to the Moon. Credited with inventing the branch of Astrogeology within the U.S. Geological Survey, his contributions to the field and the study of impact craters, lunar science, asteroids, and comets are legendary. Though his own career as an astronaut/geologist was sidelined by a health problem, he helped train the Apollo astronauts in geology and the investigation of the lunar surface. Seen here at Meteor Crater, Arizona in the mid 1960s, Shoemaker was killed in a tragic car accident in July 1997. He is survived by his wife and professional colleague, Carolyn, and children. In a fitting tribute conceived by a former student, Eugene Shoemaker's ashes were placed on-board the Lunar Prospector spacecraft which has now successfully reached a polar mapping orbit around the Moon. After completing its scientific mission, the spacecraft will ultimately impact the lunar surface.

Dusting Spiral Galaxies

How much dust is in spiral galaxies? Does it block out much of the starlight? Because astronomers rely on an accurate knowledge of galaxy properties to investigate a wide range of problems, like galaxy and quasar evolution and the nature of dark matter, answers to simple questions like this are key. This striking, detailed Hubble Space Telescope image of dust in the outer reaches of a foreground spiral galaxy (left) back lit by an elliptical galaxy offers an elegant approach to providing the answers. As expected, dust lanes in the foreground galaxy seem to be associated with spiral arms. But surprisingly, many dust regions are not completely opaque and the dust is more smoothly distributed than anticipated. This "overlapping" pair of galaxies is cataloged as AM1316-241 and is about 400 million light-years away in the constellation Hydra.

At The Core Of M15

Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image. The star colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. The region visible here is only about 1.6 light-years across, compared to the 4.3 light-year distance to our own Sun's nearest neighbor. Imagine the night sky viewed from a planet orbiting a star near this cluster's center! M15 has long been recognized as one of the densest clusters of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

Saturn, Rings, and Two Moons

NASA's robot spacecraft Voyager 2 made this image of Saturn as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots on the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting its elaborate ring system. A robot spacecraft named Cassini was launched last October and is expected to rendezvous with the giant gas planet in 2004.

The Hubble 5 Planetary Nebula

The Hubble Double Bubble Planetary Nebula is bubbling over with excitement. More mundanely known as Hubble 5, this bipolar planetary nebula is being created by a hot wind of particles streaming away from the central star system. The hot gas expands into the surrounding interstellar medium in a fashion similar to the inflation of hot air balloons. A supersonic shock-wave can form at the boundary, causing newly excited gas there to shine as electrons recombine with resident elements. In the above picture, colors are assigned according to the energy of the recombinant radiation. This star system lies about 2200 light-years from Earth, and likely includes a Sun-like star slowly transforming itself into a white dwarf.

Arachnoids on Venus

Arachnoids are large structures of unknown origin that have been found only on the surface of Venus. Arachnoids get their name from their resemblance to spider-webs. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200 kilometers. Radar echoes from the Magellan spacecraft that orbited Venus from 1990 to 1994 built up this image. Over 30 arachnoids have been identified on Venus, so far. The Arachnoid might be a strange relative to the volcano, but possibly different arachnoids are formed by different processes.

Our Dusty Universe

What's black & white and red all over? Add our universe to this list. Adrift in a vast sea of darkness are not only familiar bright stars but dust that glows predominantly in far-infrared light. This cosmological dust was recently discovered in data taken previously by the COBE satellite, and visible as a diffuse glow visible in the above image. The amount of dust in the universe is important because it is a measure of the number of stars that created it, of the number of stars that are cloaked by it, and of the amount of distortion created in measurements of the distant universe.

Closer To Beta Pic

What did our Solar System look like as the planets were forming? Since the 1980s, astronomers have been pointing toward Beta Pictoris, a young, sun-like star a mere 50 light-years distant, as a likely example. Beta Pic is surrounded by a disk of dust which we view nearly edge-on. The dust disk shines by reflected starlight and has been examined with ever increasing detail to search for signs of planetary formation. The trick is to follow the disk as close in to the star as possible, without being overwhelmed by the direct starlight. To make this Hubble Space Telescope image, a coronagraph was used to block the direct starlight and achieve the closest view yet. The false color picture shows the inner section of the dusty disk to within nearly 1.5 billion miles of the star itself, about the scale of the orbit of Uranus. The obvious warp is indirect evidence that a planet now orbits this young sun, slightly inclined to the disk. The planet's gravitational pull would produce the visible distortion.

Jovian Aurora

These two recently released Hubble Space Telescope close-ups show the Northern and Southern lights ... on Jupiter. Like aurora on Earth, these Jovian aurora are caused by charged particles funneled into the atmosphere above the planet's North (right) and South poles by magnetic fields. But Jupiter's magnetic field is extremely large and ionized material expelled from the volcanic moon Io is trapped in it creating light shows 1,000 times more intense than Earth's auroral storms. Charged particles released by Io are also funneled along magnetic flux tubes which form a direct "bridge" to the Jovian atmosphere. The result is auroral hot spots - magnetic footprints 600 or more miles across which race over Jupiter's cloud tops. A hot spot is visible in both images as a comet-like feature just outside the polar auroral rings. In these false color ultraviolet images, Jupiter's limb (edge) appears dull brown while the auroral displays are shades of white and blue.

The Large Cloud Of Magellan (LMC)

Ferdinand Magellan and his crew had plenty of time to study the southern sky during their famous voyage around the world. As a result, two fuzzy cloud-like objects, not visible to northern hemisphere dwellers, are now known as the Clouds of Magellan. These star clouds are small irregular galaxies, satellites of our larger Milky Way spiral galaxy. The Large Magellanic Cloud (LMC) pictured above is only about 180,000 light-years distant - the only known galaxy closer is the Sagittarius Dwarf. Both the LMC and the Small Magellanic Cloud (SMC) are joined to the Milky Way by a stream of cold hydrogen gas. An unusual effect called gravitational lensing has recently been detected in a few LMC stars, and there is hope this could tell us important information about the true composition of our universe.

The Small Cloud of Magellan (SMC)

Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is not gravitationally bound to the LMC.

Interplanetary Spacecraft Passes Earth

Last Thursday an interplanetary spacecraft flew right past the Earth. The above images show sunlight momentarily reflected from this spacecraft's solar panels. No aliens were involved - the Near Earth Asteroid Rendezvous (NEAR) mission actually originated from Earth. Launched in 1996, NEAR zipped past the asteroid 253 Mathilde last June. This Earth flyby gravitationally deflects NEAR onto a trajectory passing the asteroid 433 Eros next year. Above, NEAR appears to move through the constellation of Perseus, as clouds created a changing diffuse white glow. NEAR was only visible for about 2 minutes from San Jose, California, where these image-intensified video camera observations were taken.

The Great Nebula in Orion

The Great Nebula in Orion can be found just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. This fuzzy patch contains one of the closest stellar nurseries, lying at a distance of about 1500 light years. In the above picture, the red region on the left consists of nebulae designated M42 and M43 and contains the bright Trapezium open cluster. The blue region on the right is a nebula primarily reflecting the light from internal bright stars. Recent observations of the Orion Nebula by the Hubble Space Telescope have located solar-system sized star-forming regions.

The Infrared Sky

Three major sources contribute to the far-infrared sky: our Solar System, our Galaxy, and our Universe. The above image, in representative colors, is a projection of the entire infrared sky created from years of observations by the robot spacecraft COBE. Our Solar System is evidenced most prominently by the S-shaped blue sash called zodiacal light, created by small pieces of rock and dust orbiting between the Sun and Jupiter. Our Galaxy is evidenced by the bright band of light that crossed the middle of the image, created mostly by dust that laces the disk of our Milky Way. Close inspection of the image reveals that the background is not completely dark, indicating that our Universe itself provides a diffuse glow, created by the dust left over from the star formation throughout the Universe.

The Earth-Moon System

This evocative mosaic image of the Earth-Moon system was recorded by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft earlier this month. The relative sizes shown are appropriate for viewing both the Earth and Moon from a distance of about 250,000 miles, although the apparent brightness of the Moon has been increased by about a factor of five for the sake of appearances. This space-based perspective is a unique one, the bland and somber Lunar Southern Hemisphere contrasting strongly with blue oceans, swirling clouds, and the bright icy white continent of Antarctica on planet Earth. Though its lack of atmosphere and oceans make it relatively dull looking, the Earth's moon is one of the largest moons in the solar system - even larger than the planet Pluto. During this recent flyby of the Earth-Moon system, the NEAR spacecraft used Earth's gravity to deflect it towards its ultimate destination, the Asteroid 433 Eros. It is scheduled to arrive at Eros in January 1999.

Tempel-Tuttle: The Leonid Comet

Star trails streak this composite time exposure of Comet Tempel-Tuttle recorded by T. Puckett on January 26. Presently passing through the inner solar system on its 33 year orbit around the Sun, Tempel-Tuttle has brightened unexpectedly, but binoculars or small telescopes are still required to visually observe it. Tempel-Tuttle is also called "the Leonid Comet" as the yearly Leonid meteor shower results when the Earth crosses this comet's orbital plane and encounters cometary dust. So, while not currently rivaling the spectacle of a Hyakutake or Hale-Bopp, Tempel-Tuttle may still put on a show. When the Earth plunges through Tempel-Tuttle's debris tail in November of this year, many sky-watchers are anticipating an extremely active meteor shower to result, perhaps even a meteor storm!

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