The Milky Way in Infrared Credit:The COBE Project, DIRBE, NASA

At night, from a dark location, part of the clear sky looks milky. This unusual swath of dim light is generally visible during any month and from any location. Until the invention of the telescope, nobody really knew what the "Milky Way" was. About 300 years ago telescopes caused a startling revelation: the Milky Way was made of stars. Only 70 years ago, more powerful telescopes brought the further revelation that the Milky Way is only one galaxy among many. Now telescopes in space allow yet deeper understanding. The above picture was taken by the COBE satellite and shows the plane of our Galaxy in infrared light. The thin disk of our home spiral galaxy is clearly apparent, with stars appearing white and interstellar dust appearing red.

Mercury as Revealed by MESSENGER Credit: MESSENGER,NASA,JHU APL, CIW

The planet Mercury has been known since history has been recorded, but parts of the Solar System's innermost planet have never been seen like this before. Two days ago the robotic MESSENGER spacecraft buzzed past Mercury for the second time and imaged terrain mapped previously only by comparatively crude radar. The above image was recorded as MESSENGER looked back 90 minutes after passing, from an altitude of about 27,000 kilometers. Visible in the above image, among many other newly imaged features, are unusually long rays that appear to run like meridians of longitude out from a young crater near the northern limb. MESSENGER is scheduled to fly past Mercury once more before firing its thrusters to enter orbit in 2011.

Venus Unveiled Credit: Magellan Project, JPL, NASA

Explanation: The surface of Venus is perpetually covered by a veil of thick clouds and remains hidden from even the powerful telescopic eyes of earth-bound astronomers. But in the early 1990s, using imaging radar, the Venus

orbiting Magellan spacecraft was able to lift the veil from the face of Venus and produced spectacular high resolution images of the planet's surface. Colors used in this computer generated picture of Magellan radar data are

based on color images from the surface of Venus transmitted by the Soviet Venera 13 and 14 landers. The bright area running roughly across the middle represents the largest highland region of Venus known as Aphrodite Terra.

Earthrise Credit: Apollo8, NASA

In December of 1968, the Apollo 8 crew flew from the Earth to the Moon and back again. Frank Borman, James Lovell, and William Anders were launched atop a Saturn V rocket on December 21, circled the Moon ten times in their command module, and returned to Earth on December 27. The Apollo 8 mission's impressive list of firsts includes: the first humans to journey to the Earth's Moon, the first manned flight using the Saturn V, and the first to photograph the Earth from deep space. As the Apollo 8 command module rounded the farside of the Moon, the crew could look toward the lunar horizon and see the Earth appear to rise, due to their spacecraft's orbital motion. The famous picture that resulted, of a distant blue Earth above the Moon's limb, was a marvelous gift to the world.

Welcome to Planet Earth Credit: Apollo 17 Crew, NASA

Explanation: Welcome to Planet Earth, the third planet from a star named the Sun. The Earth is shaped like a sphere and composed mostly of rock. Over 70 percent of the Earth's surface is water. The

planet has a relatively thin atmosphere composed mostly of nitrogen and oxygen. The above picture of Earth, dubbed Blue Marble, was taken from Apollo 17 in 1972 and features Africa and Antarctica. It is thought to be one of the most widely distributed photographs of any kind. Earth has a single large Moon that is about 1/4 of its diameter

and, from the planet's surface, is seen to have almost exactly the same angular size as the Sun. With its abundance of liquid water, Earth supports a large variety of life forms, including potentially intelligent species such as dolphins

and humans. Please enjoy your stay on Planet Earth.

The Far Side of the Moon Credit: Apollo 16 Crew, NASA

Explanation: Does this moon look familiar? Possibly not, even though it is Earth's Moon. Locked in synchronous rotation, the Moon always presents its well-known near side to Earth. But from lunar orbit, Apollo astronauts also grew to

know the Moon's far side. This sharp picture from Apollo 16's mapping camera shows the eastern edge of the familiar near side (top) and the strange and heavily cratered far side of the Moon. Surprisingly, the rough and

battered surface of the far side looks very different from the near side which is covered with smooth dark lunar maria. The likely explanation is that the far side crust is thicker, making it harder for molten material from the interior to flow to

the surface and form the smooth maria.

The Full Moon Credit: Lick Observatory

Explanation: Earth has one moon. A symbol in famous love songs, movies, poems, and folklore, many myths about the Moon date back to ancient history. In fact, the name Monday originates from Moon-day. The Moon glows

by light it reflects from the Sun and is frequently the brightest object in the night sky. The Moon orbits the Earth about once a month (moon-th) from about 1 light second away. The above-pictured Full Moon occurs when the Moon is nearly opposite to the Sun in its orbit. The Moon's diameter is about 1/4 that of the Earth, and from the

Earth's surface appears to have almost exactly the same angular size as the Sun. Recent evidence indicates that the Moon formed from a colossal impact on the Earth about 4.5 billions of years ago, and therefore has a similar

composition to the Earth. Humans walked on the Moon for the first time in 1969.

Phobos: Doomed Moon of Mars Credit: HiRISE, MRO, LPL (U. Arizona), NASA

Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this

stunning color image from the Mars Reconnaissance Orbiter, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for

our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.

Big Mars from Hubble Credit: J. Bell (Cornell U.), M. Wolff (SSI) et al., STScI, NASA

Explanation: At about 10 am Universal Time today, Mars and Earth will pass closer than in nearly 60,000 years. Mars, noticeably red, will be the brightest object in the eastern sky just after sunset. Tonight and through much of this

week, many communities around the world are running a public Mars Watch 2003 campaign, where local telescopes will zoom in on the red planet. Pictured above is an image of Mars taken just last night from the

Hubble Space Telescope in orbit around the Earth. This image is the most detailed view of Mars ever taken from Earth. Visible features include the south polar cap in white at the image bottom, circular Huygens crater just to the right of the image center, Hellas Impact Basin - the large light circular feature at the lower right, planet-wide light

highlands dominated by many smaller craters and large sweeping dark areas dominated by relatively smooth lowlands.

Asteroids Credit: NEAR Project, Galileo Project, NASA

Explanation: No asteroid or comet is known to be on a collision course with Earth. The asteroid designated 1997 XF11 had been predicted to come uncomfortably close, but new estimates place its passing beyond the orbit of

the Moon. This earth-approaching asteroid was discovered by SpaceWatch astronomer Jim Scotti in December of last year. Orbital computations using new observations suggested that it would pass within 30,000 miles of the

Earth's center on October 26, 2028 - a very near miss considering that the radius of the Earth itself is about 4,000 miles. However, more recent and further refined calculations based on both new and archival data indicate that

the closest approach will be 600,000 miles in 2028. Imaged by NASA spacecraft, the three potato-shaped objects above are large main-belt asteroids orbiting between Mars and Jupiter. Shown to the same scale from left to right

are Mathilde, Gaspra, and Ida. Mathilde has dimensions of about 37 by 29 miles. The asteroid 1997 XF11 is much smaller, probably a mile wide, yet the impact of an asteroid of this size could have catastrophic effects. Over the last two decades, teams of astronomers have just begun to catalog and track near-earth objects.

Jupiter Eclipsing Ganymede Credit: NASA, ESA, and E. Karkoschka (U. Arizona)

Explanation: How hazy is Jupiter's upper atmosphere? To help find out, astronomers deployed the Hubble Space Telescope to watch Jupiter eclipse its moon Ganymede. Although Ganymede circles Jupiter once a week, a particularly useful occultation occurs more rarely. Such an occultation was captured in great

visual detail in April 2007. When near Jupiter's limb, Ganymede reflects sunlight though Jupiter's upper atmosphere, allowing astronomers to search for haze by noting a slight dimming at different colors. One result

of this investigation was the above spectacular image, where bands of clouds that circle Jupiter are clearly visible, as well as magnificent swirling storm systems such as the Great Red Spot. Ganymede, at the image

bottom, also shows noticeable detail on its dark icy surface. Since Jupiter and Ganymede are so bright, many eclipses can be seen right here on Earth with a small telescope.

Io's Surface: Under Construction Credit: Galileo Project, JPL, NASA

Explanation: Like the downtown area of your favorite city, the roads you drive to work on, and any self-respecting web site ... Io's surface is constantly under construction. This moon of Jupiter holds the distinction of being the Solar System's most volcanically active body -- its bizarre looking surface continuously formed and reformed by lava flows. Generated using 1996 data from NASA's Galileo spacecraft, this high resolution composite image is centered on the side of Io that always faces away from Jupiter. It has been enhanced to emphasize Io's surface brightness and color variations, revealing features as small as 1.5 miles across. The notable absence of impact craters suggests that the

entire surface is covered with new volcanic deposits much more rapidly than craters are created. What drives this volcanic powerhouse? A likely energy source is the changing gravitational tides caused by Jupiter and

the other Galilean moons as Io orbits the massive gas giant planet. Heating Io's interior, the pumping tides would generate the sulfurous volcanic activity.

In the Shadow of Saturn Credit: CICLOPS, JPL, ESA, NASA

Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a

view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when

viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the above image. Visible in spectacular detail,

however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus, and the outermost ring visible above. Far in the distance, visible on the image left just above the bright main rings, is the almost

ignorable pale blue dot of Earth.

Uranus: The Tilted Planet Credit: Voyager 2 Team, NASA

Explanation: Uranus is the third largest planet in our Solar System after Jupiter and Saturn. Uranus is composed mostly of rock and ices, but with a thick hydrogen and helium atmosphere. The blue hue of Uranus' atmosphere arises from the small amount of methane which preferentially absorbs red light. This picture was snapped by the

Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus has many moons and a ring system. Uranus, like Venus, has a rotation axis that is greatly tilted and sometimes points near the Sun. It remains an

astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are quite similar: Uranus is slightly larger but less massive.

Neptune: Big Blue Giant Credit: Voyager 2, NASA

Explanation: This picture was taken by the Voyager 2 spacecraft in 1989 - the only spacecraft ever to visit Neptune. Neptune will be the farthest planet from the Sun until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune, like Uranus, is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune's moon Triton is unlike any other and has active volcanoes. The nature of Triton's unusual orbit around Neptune is the focus of much

discussion and speculation.

Pluto: The Frozen Planet Credit: R. Albrecht (ESA/ESO), Hubble Space Telescope, NASA

Explanation: The Hubble Space Telescope imaged Pluto and its moon Charon in 1994. Pluto is usually the most distant planet from the Sun but because of its elliptic orbit Pluto crossed inside of Neptune's orbit in 1979 and will

cross back out again in 1999. Compared to the other planets, very little is known about Pluto. Pluto is smaller than any other planet and even smaller than several other planet's moons. From Pluto, the Sun is just a tiny point

of light. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Pluto has not yet been visited by a spacecraft, but a mission is being planned for the next decade.

What are Comet Tails Made Of? Credit: Jim Martin, Huntsville AL, Courtesy "Night of the Comet" (NASA / Ames)

Explanation: The tail of comet Hyakutake, visible in this recent color image, is composed of dust and gas driven off the icy comet nucleus by the Sun's heat and blown away by the solar wind. Bathed in solar ultraviolet light, the

gas molecules break down and are excited, producing a characteristic glow. This glow is responsible for visible light from the tail and astronomers using spectroscopes can identify the compounds involved. The close passage of

Hyakutake presents an excellent chance to use this technique to explore the composition of its tail. Typical comet gas tail constituents are simple combinations of hydrogen, carbon, nitrogen, and oxygen - for example, H20 (water), CO (carbon monoxide), and CN (cyanogen) are common. In fact, the poisonous CO and CN compounds were seen

in the spectrum of Halley's comet during its 1910 apparition. This caused some public concern at the time as the Earth was expected to pass through Halley's tail! However, stretching for millions of miles, comet tails are extremely

thin and tenuous and don't pose a danger to the Earth's atmosphere.