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Planets, Moons & Comets

Most pics courtesy of NASA

Mariner's Mercury

Credit:  Mariner 10, Astrogeology Team, U.S. Geological Survey

           Explanation:  Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets.  Hiding in the Sun's glare, it is a difficult target for Earth bound observers.  The only spacecraft to explore Mercury close-up was Mariner 10, which executed three flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface.  Mariner 10 deftly maneuvered to photograph part of the sunlit hemisphere during each approach, passed behind the planet, and continued to image the sun-facing side as the spacecraft receded.  Its highest resolution photographs recorded features approximately a mile across.  A reprocessing of the Mariner 10 data has resulted in this dramatic mosaic.  Like the Earth's Moon, Mercury's surface shows the scars of impact cratering.  The smooth vertical band and patches visible above represent regions where no image information is available.

Saturn:  Lord of the Rings

Credit:  Hubble Heritage Team (AURA / STScI) R.G. French (Wellesley College), J. Cuzzi (NASA/Ames), L. Dones (SwRI), J. Lissauer (NASA/Ames)

           Explanation:  Born on today's date (Feb. 15) in 1564, Galileo used a telescope to explore the Solar System.  In 1610, he became the first to be amazed by Saturn's rings.  After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights.  Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250 kilometers wide but in places only a few tens of meters thick.  Modern astronomers believe the rings are perhaps only a hundred million years young.  But accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years.  Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets.  Still mesmerized, some will take advantage of next week's (February 20, 2002) favorable lunar occultation of Saturn to search for evidence of ring material outside the well known boundaries of the ring system.  The presence of such a "lost" ring of Saturn was first hinted at in reports dating back to the early 20th century.

Saturn at the Lunar Limb

Credit & Copyright:  Tom Martinez (Astronomical Society of Kansas City)

           Explanation:  Gliding through the sky on Wednesday evening, February 20th, 2002, a first quarter Moon seemed to run over bright planet Saturn as viewed from much of North America.  In this sharp sequence of telescopic digital images from the Powell Observatory near Louisburg, Kansas, USA, Saturn is seen reappearing from behind the bright lunar limb over a period of about 2 minutes.  The ringed planet emerges above the dark, smooth lunar Mare Crisium (Sea of Crises).  This lunar occultation was widely anticipated in part because the ringed planet and the brilliant Moon are both spectacular celestial sights.  Now, European sky gazers will have their turn as the Moon occults the Solar System's largest planet, Jupiter in early morning hours on Saturday, February 23rd.

Farewell Jupiter

Credit:  Cassini Imaging Team, Cassini Project, NASA

           Explanation:  Next stop:  Saturn.  The Cassini spacecraft, launched from Earth four years ago, has now swung past Jupiter and should arrive at Saturn in the year 2004.  Pictured to the left is a parting shot from Cassini in January 2002 that would not have been possible from Earth:  Jupiter showing a crescent phase.  From the Earth and all points sunward of Jupiter, the gas giant will always appear more fully lit than a crescent.  After arriving at Saturn, Cassini will decelerate to orbit the ringed world and send a probe to its enigmatic moon Titan.

Crescent Europa

Credit:  Voyager 2, NASA

           Explanation:  Although the phase of this moon might appear familiar, the moon itself might not.  In fact, this crescent shows part of Jupiter's moon Europa.  The passing robot spacecraft Voyager 2 captured this image in 1979.  Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt.  Raised terrain is particularly apparent near the terminator, where it casts shadows.  Europa is nearly the same size as Earth's Moon, but much more smooth, showing few highlands or large impact craters.  Evidence and images from the Galileo spacecraft orbiting Jupiter, indicate that liquid oceans might exist below the icy surface.  To test speculation that these seas hold life, NASA has started preliminary development of the Europa Orbiter, a spacecraft that would use radar to help determine the thickness of the surface ice.  If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

Neptune's Great Dark Spot:  Gone But Not Forgotten

Credit:  Voyager Project, JPL, NASA

           Explanation:  When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked.  Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet.  Instead, the Voyager images revealed evidence of a dynamic and turbulent world.  One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up.  Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similarly rotating storm system.  Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet.  The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby.  When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!

Comet Ikeya-Zhang Brightens

Credit & Copyright:  Gerald Rhemann

           Explanation:  In the last week, Comet Ikeya-Zhang has become bright enough to be just visible to the unaided eye.  Based on its present activity, observers are optimistic that Ikeya-Zhang will become substantially brighter.  This composite color image from March 3rd, captured with a wide-field telescope, shows this active comet's bright, condensed coma and a delightful array of subtle structures in its developing tail.  The bluish tail stretches for 5 degrees or so against a background of stars in the constellation Pisces.  In the coming days look for the comet hanging low in the western evening sky (below a bright yellowish Mars), eventually becoming difficult to see in the March twilight.  But after April begins, Ikeya-Zhang will become a predawn object climbing higher into the morning sky as the month progresses.  Cataloged as comet C/2002 C1, improved orbit determinations now make it seem very likely that Comet Ikeya-Zhang has been around here before.  Refined calculations indicate this comet's last trip through the inner Solar System was 341 years ago, in 1661, when it was recorded as a bright comet.

The Dust and Ion Tales of Comet Hale-Bopp

Credit & Copyright:  John Gleason (Celestial Images)

           Explanation:  In 1997, Comet Hale-Bopp's intrinsic brightness exceeded any comet since 1811.  Since it peaked on the other side of the Earth's orbit, however, the comet appeared only brighter than any comet in two decades.  Visible above are the two tails shed by Comet Hale-Bopp.  The blue ion tail is composed of ionized gas molecules, of which carbon monoxide particularly glows blue when reacquiring electrons.  This tail is created by the particles from the fast solar wind interacting with gas from the comet's head.  The blue ion tail points directly away from the Sun.  The white dust tail is created by bits of grit that have come off the comet's nucleus and are being pushed away by the pressure of light from the Sun.  This tail points nearly away from the Sun.  The above photograph was taken in March 1997.

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