NASA Astronomy Picture of the Day 2003-3

Stereo Eros

Get out your red/blue glasses and float next to asteroid 433 Eros, 170 million kilometers away! Orbiting the Sun once every 1.8 earth-years, asteroid Eros is a diminutive 40 x 14 x 14 kilometer world of undulating horizons, craters, boulders and valleys. Its unsettling scale and bizarre shape are emphasized in this picture - a mosaic of images from the NEAR Shoemaker spacecraft processed to yield a stereo anaglyphic view. Along with dramatic chiaroscuro, NEAR's 3-D imaging provided important measurements of the asteroid's landforms and structures, and clues to the origin of this city-sized chunk of solar system. The smallest features visible here are about 30 meters across. After spending a year in orbit around Eros, the historic Near Shoemaker spacecraft made the first ever landing on an asteroid's surface February 12, 2001.

In the Center of the Trapezium

Start with the constellation of Orion. Near Orion's belt is a fuzzy area known as the Great Nebula of Orion or M42. In this nebula is a bright star cluster known as the Trapezium, shown above. New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds. Looking closely at the above picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars. The above false color image was made by combining several exposures from the orbiting Hubble Space Telescope.

Will the Universe End in a Big Rip?

How will our universe end? Recent speculation now includes a pervasive growing field of mysterious repulsive energy that rips virtually everything apart. Although the universe started with a Big Bang, analysis of recent cosmological measurements allows a possibility that it will end with a Big Rip. As soon as few billion years from now, the controversial scenario holds, dark energy will grow to such a magnitude that our own Galaxy will no longer be able to hold itself together. After that, stars, planets, and then even atoms might not be able to withstand the expansive internal force. Previously, speculation on the ultimate fate of the universe centered on either a re-collapsing Big Crunch or a Big Chill. Although the universe's fate is still a puzzle, piecing it together will likely follow from an increased understanding of the nature of dark matter and dark energy.

In the Center of the Lagoon Nebula

The center of the Lagoon Nebula is busy with the awesome spectacle of star formation. Visible in the lower left, at least two long funnel-shaped clouds, each roughly half a light-year long, have been formed by extreme stellar winds and intense energetic starlight. The tremendously bright nearby star, Hershel 36, lights the area. Vast walls of dust hide and redden other hot young stars. As energy from these stars pours into the cool dust and gas, large temperature differences in adjoining regions can be created generating shearing winds which may cause the funnels. This picture, spanning about 5 light years, was taken in 1995 by the orbiting Hubble Space Telescope. The Lagoon Nebula, also known as M8, lies about 5000 light years distant toward the constellation of Sagittarius.

Where People Live on Planet Earth

Where do people live on Planet Earth? Although people inhabit every continent, the highest population densities occur in Asia. Sparsely inhabited regions occur on virtually every continent, however, including the Sahara Desert in Africa, the Great White North of North America, the outback of Australia, the Amazon of South America, and the Himalayan Mountains of Asia. The above color-coded map was produced from populations estimates made for 1994, when the world population was about 5.5 billion. Current estimates place the world population at about 6.3 billion.

Comet NEAT in Southern Skies

After last month's dramatic swoop past the Sun, Comet NEAT (C/2002 V1) appeared as a naked-eye comet, emerging from the evening twilight in planet Earth's southern skies. On March 1st, New Zealand photographer Noel Munford captured this telephoto view of the outbound comet close to the southwestern horizon against the faint stars of the constellation Sculptor. He reports that the picture is a good representation of the comet's visual appearance on that date and estimates the impressive tail to be five or six degrees long. Discovered last November as part of the Near-Earth Asteroid Tracking program, there was some speculation that this comet would not survive its close encounter with the Sun. However, Comet NEAT is now returning to the outer solar system, diving southward and fading fast.

The Star Trails of Kilimanjaro

The night had no moon, but the stars were out. And camped at 16,000 feet on Mt. Kilimanjaro, photographer Dan Heller recorded this marvelous 3 1/2 hour long exposure. Here the landscape is lit mostly by the stars. Flashlights give the tents an erie internal radiance while the greenish glow from the distant city lights of Moshi, Tanzania filter through the clouds below. The view from this famous equatorial African mountain is toward the south, putting the South Celestial Pole close to the horizon on the far left, near the center of the graceful concentric star trail arcs. In the thin air and clear dark skies, even the ghostly Milky Way left a faint triangular glow as it swept across the middle of the dreamlike scene.

Solar Sail

Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, the illustration above represents one concept explored by NASA centers to develop an interstellar probe pushed along by sunlight reflected from an ultrathin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets -- without requiring any fuel! If launched in 2010 such a probe could overtake Voyager 1, the most distant spacecraft bound for interstellar space, in 2018 going as far in eight years as the Voyager will have journeyed in 41 years.

Farewell Jupiter

Tomorrow's picture: Big Pinwheel < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | > Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & NASA SEU Edu. Forum & Michigan Tech. U.

M101: The Pinwheel Galaxy

Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 22 million light years allow it to be studied in some detail. Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.

Iridescent Clouds

Why would clouds appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. Pictured above, iridescent clouds were photographed over Norway two months ago.

Lunar Farside from Apollo 11

The far side of the Moon is rough and filled with craters. By comparison, the near side of the Moon, the side we always see, is relatively smooth. Since the Moon is rotation locked to always point the same side toward Earth, humanity has only glimpsed the lunar farside recently -- last century. The light highlands of the far side are older than the dark Maria of the near side. A thinner crust on the near side that allowed for more dark lava flows is thought to be the cause of differences between the two sides. The cause for the crust thickness differences is still being researched, however. The large impact basin pictured above is Crater 308. It spans about 30 kilometers and was photographed by crew of Apollo 11 as they circled the Moon in 1969.

WIRO at Jupiter

Gazing out over the mountaintops from the Wyoming InfraRed Observatory (WIRO), astronomers recently recorded this bizarre looking image of the solar system's ruling planet, gas giant Jupiter. The false-color picture is a composite of images taken to test a sophisticated digital camera operating at liquid helium temperatures and sensitive to wavelengths about three times longer than visible red light. At those infrared wavelengths (near 2.1 microns) the molecular hydrogen and methane gas in Jupiter's dense lower atmosphere strongly absorb sunlight, so the normally bright, banded planet looks very dark. But particles and haze over the equator and poles rise above the absorbing layers into Jupiter's stratosphere and reflect the infrared sunlight. Also clearly extending into the Jovian stratosphere is the famous Great Red Spot seen here in yellow just under the equatorial band at the right. North is up in this view and Jupiter's rapid 10 hour rotation will soon carry the Great Red Spot behind the planet's right limb.

DEM L71: When Small Stars Explode

Large, massive stars end their furious lives in spectacular supernova explosions -- but small, low mass stars may encounter a similar fate. In fact, instead of simply cooling off and quietly fading away, some white dwarf stars in binary star systems are thought to draw enough mass from their companions to become unstable, triggering a nuclear detonation. The resulting standard candle stellar explosion is classified as a Type Ia supernova and perhaps the best example yet of the aftermath is this expanding cloud of shocked stellar debris, DEM L71, in the nearby Large Magellanic Cloud. The sharp false-color x-ray image from the orbiting Chandra Observatory shows the predicted bright edges of the outer blast wave shock region and the x-ray glow of an inner region of reverse shock heated gas. Based on the Chandra data, estimates for the composition and total mass of expanding gas strongly suggest that this is all that remains of a white dwarf star. Light from this small star's self-destructive explosion would have first reached Earth several thousand years ago.

Apollo 12: Self-Portrait

Is it art? In November of 1969, Apollo 12 astronaut-photographer Charles "Pete" Conrad recorded this masterpiece while documenting colleague Alan Bean's lunar soil collection activities on the Oceanus Procellarum. The image is dramatic and stark. Bean is faceless. The harsh environment of the Moon's Ocean of Storms is echoed in his helmet's perfectly composed reflection of Conrad and the lunar horizon. Works of photojournalists originally intent on recording the human condition on planet Earth, such as Lewis W. Hine's images from New York City in the early 20th century, or Margaret Bourke-White's magazine photography are widely regarded as art. Similarly many documentary astronomy and space images can be appreciated for their artistic and esthetic appeal.

NGC 253: The Sculptor Galaxy

NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest. Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant. NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy. Visible in the above photograph from the Hubble Space Telescope is the active central nucleus, also known to be a bright source of X-rays and gamma rays.

SN 1006: History's Brightest Supernova

Suddenly, in the year 1006 AD, a new star appeared in the sky. Over the course of just a few days, the rogue star became brighter than the planet Venus. The star, likely the talk of everyone who could see it, was recorded by people who lived in areas now known as China, Egypt, Iraq, Italy, Japan, and Switzerland. The celestial newcomer, now known to be a supernova, took months to fade. Modern observations have been used to measure the speed of the still-expanding shock wave, allowing a better estimate of its distance and hence a better estimate of the true brightness of the supernova. It turns out SN 1006 likely achieved an apparent visual magnitude of -7.5, making it the brightest supernova on record. The shock wave was imaged in 1998 from CTIO (left panel), and then subtracted from a similar image taken in 1986 (right panel), highlighting the relative expansion.

Coronal Holes on the Sun

The ominous, dark shapes haunting the left side of the Sun are coronal holes -- low density regions extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. These coronal holes, however, have just moved into view near the Sun's equator, and particles escaping them have already caused notable aurora here on Earth. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on March 9 was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory.

Jupiter's Great Dark Spot

Seventeenth century astronomer Giovanni Domenico Cassini was an astute observer of Jupiter's Great Red Spot. So it seems only fitting that his namesake, the Cassini spacecraft, has enabled detailed observations of another planet-sized blemish -- Jupiter's Great Dark Spot. Unlike the Red Spot, the Great Dark Spot lies near Jupiter's north pole and seems to appear and disappear over periods of months rather than persisting for hundreds of years. Seen at ultraviolet wavelengths, the dark feature resides in the Jovian stratosphere confined by pole-encircling winds, analogous to planet Earth's antarctic ozone hole. This image of the Dark Spot is a single frame from a movie created with data recorded during the spacecraft's year 2000 flyby of Jupiter. Projected to show Jupiter's north polar region, no data are available for the blank central area, while the Great Dark Spot lies above and just left of center. The white circle marks 60 degrees latitude and the blue contour outlines a persistent Jovian auroral zone which may be related to the formation of the Great Dark Spot.

Sunrise Analemma

Astronomically speaking, at the Equinox on March 21, 0100 UT (March 20, 8:00 PM ET) the season changes. For this Equinox the Sun rises due east as it crosses the celestial equator heading north. In celebration, consider this spectacular sunrise analemma! An analemma is the figure-8 loop you get when you mark the position of the Sun at the same time each day throughout the year. In this remarkable case, 38 separate exposures (and 1 foreground exposure) were recorded on a single piece of film between January 12 and December 21, 2002 at 0600 UT. The tilt of planet Earth's axis and the variation in speed as it moves around its elliptical orbit combine to produce the predictable analemma curve. The top and bottom of the figure-8 correspond to the Solstices -- the Northern and Southern limits of the Sun's sky motion. The two Equinoxes find the Sun at points along the anelemma curve exactly half way between the Solstices. Here, the analemma's Southern portion is partly hidden by mountains. In the foreground lie the stone ruins of the Tholos at the ancient site of Delphi, Greece.

Stars and Planets in the Halo of the Moon

Photographed on March 13th from Caledon, Ontario, Canada, a bright Moon was surrounded by this lovely halo. Planet Jupiter and stars Procyon, Castor, and Pollux also appear within the circle of lunar light. Castor and Pollux, twins in Greek Mythology, are appropriately bright stars of the constellation Gemini while Procyon is the brightest star in Canis Minor. The circular halo is produced by six-sided ice crystals in thin high-altitude clouds, which refract the moonlight and give the halo a characteristic radius of 22 degrees. For persistent skygazers such apparitions are relatively easy to see when the Moon and Sun illuminate planet Earth's skies.

M57: The Ring Nebula

cept for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Hubble Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

Alpha Centauri: The Closest Star System

The closest star system to the Sun is the Alpha Centauri system. Of the three stars in the system, the dimmest -- called Proxima Centauri -- is actually the nearest star. The bright stars Alpha Centauri A and B form a close binary as they are separated by only 23 times the Earth- Sun distance - slightly greater than the distance between Uranus and the Sun. In the above picture, the brightness of the stars overwhelm the photograph causing an illusion of great size, even though the stars are really just small points of light. The Alpha Centauri system is not visible in much of the northern hemisphere. Alpha Centauri A, also known as Rigil Kentaurus, is the brightest star in the constellation of Centaurus and is the fourth brightest star in the night sky. Sirius is the brightest even thought it is more than twice as far away. By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun, causing many to speculate that it might contain planets that harbor life.

A Digital Sunset Over Europe and Africa

No single spacecraft or astronaut took this picture. It is a digital composite of archived images taken by several Earth-orbiting satellites and ocean-faring ships. Similar images can be digitally stitched together for any Earth location by John Walker's Earth and Moon Viewer website. Specifically, the daytime land images were taken by the MODIS instrument on NASA's Terra satellite, while the nighttime images were taken by the DMSP satellites. This image is different from what an astronaut would see for reasons including a complete lack of clouds and an unrealistic exaggeration of lights and contrasts. The image has become both an internet wave in that it continues to circulate as an attachment to digital correspondence, and a modern urban legend. Another image like that is Earth at Night. The reason for the image's continued popularity might be simple: it is really cool looking.

A Slow Explosion

Why would a gamma ray burst fade so slowly? This behavior, recorded last October, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars.

A Lenticular Cloud Over Wyoming

Is that a cloud or a flying saucer? Both, although it is surely not an alien spacecraft. Lenticular clouds can be shaped like a saucer, and can fly in the sense that, like most clouds, they are composed of small water droplets that float on air. Lenticular clouds are typically formed by high winds over rugged terrain and are particularly apparent when few other clouds are in the sky. Lenticular clouds can take on particularly strange, layered shapes. Above, a couple stopped their car near Yellowstone National Park in Wyoming, USA to photograph this lenticular cloud behind picturesque windmills.

Light Echoes from V838 Mon

Tomorrow's picture: How bright was that supernova? < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | > Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Web Site Statements, Warnings, and Disclaimers NASA Official: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & Michigan Tech. U.

1006 AD: Supernova in the Sky

Tomorrow's picture: The Shadow < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | > Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Web Site Statements, Warnings, and Disclaimers NASA Official: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & Michigan Tech. U.

The Shadow of Phobos

Tomorrow's picture: Before Telescopes < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | > Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Web Site Statements, Warnings, and Disclaimers NASA Official: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & Michigan Tech. U.

Beijing Ancient Observatory

Did observatories exist before telescopes? One example that still stands today is the Beijing Ancient Observatory in China. Starting in the 1400s astronomers erected large instruments here to enable them to measure star and planet positions with increasing accuracy. Pre-telescopic observatories throughout the world date back to before recorded history, providing measurements that helped to determine when to plant crops, how to navigate ships, and when religious ceremonies should occur. It is interesting to compare the above picture to one taken in 1895.

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