Here's where it all begins: home sweet Earth. While we humans have only set foot on the planet we sprang from and one solitary moon, we've constructed probes, orbiters, and rovers to saunter out into space and send back their reports. The following photos show some of the most exciting stops along humanity's (mostly vicarious) march through the solar system.
This July 2003 picture was taken by astronauts on the International Space Station, our first attempt at a permanent space habitat. The photo shows the tops of huge, anvil-shaped thunderheads far above the Pacific Ocean.
The moon is a hefty 240,000 miles away, but compared with our planetary neighbors, it's just a jump over the garden fence. Here, Apollo 16 moonwalker John Young is collecting samples that would prove that these lunar highlands were shaped more by meteorites than by volcanism.
Of the 731 rock samples the Apollo 16 astronauts brought home in April 1972, nearly all were breccia, composites formed of fragments fused together--probably by the heat and pressure of meteorite impacts. The logistics of sending astronauts on a similar mission to explore the geology of Mars are "complex, to say the least," NASA admits.
The Red Planet has fascinated earthlings since we first took to staring at the stars, so it's no surprise that, at the dawn of the space age, we started trying to send our mechanical minions to Mars to take a look. The Soviet Union and United States both had several failures with their earliest Mars probes, but on July 14, 1965 NASA's Mariner 4 sent home the first close-up images of another planet.
Fast-forwarding 30 years, this Hubble image clearly shows the icy white clouds that cap the Martian poles, as well as one sneaking along the planet's right side. The tan smudges at the lower right are massive dust storms.
In NASA's subsequent explorations of Mars, the agency has stuck to one directive: Follow the water. The Mars Phoenix Lander directly sampled water ice last year, and orbiters and rovers have found ample evidence that liquid water once flowed on the planet.
Researchers believe that the Jezero Crater housed a lake slightly larger than Lake Tahoe during Mars's Noachian epoch, between 3.8 and 4.6 billion years ago. Long-gone Martian rivers carried claylike materials, shown in green in this color-enhanced image, into the lake to form a large delta.
A crater nine kilometers across dominates weird little Phobos, the larger of the two Martian moons. Brilliant streaks on the crater walls suggest that even the feeble gravity of this moon is strong enough to erode loose material; bluish regions near the crater rim may be younger exposed surfaces. This enhanced-color photo was taken by the Mars Reconnaissance Orbiter in 2008.
Some proponents of a manned mission to Mars suggest that a trip to Phobos could serve as a trial run to the Martian neighborhood. Because Phobos has no atmosphere and only weak gravity, they say, it would be easier for a spacecraft to land there than on Mars.
In the infrared images taken by the New Horizons spacecraft in 2007, Jupiter's highest-altitude clouds appear blue, its lower cloud tops are red, and its famous Great Red Spot here looks blue-white. While space agencies are planning to send more orbiters to study Jupiter and its moons in the next decade, probes remain impractical because the gas planet has no solid surface to land on. The single probe released into its atmosphere, in 1995, was crushed by the pressure and heat after 57 minutes of descent.
On the right is Io, one of Jupiter's four giant moons. What appears as a blue bulge on its top is actually a colossal gout of incandescent sulfurous material spouting from the northern volcano Tvashtar.
Agitated by Jupiter's gravity, Io is a cauldron of volcanism, spewing out silicate lava, sulfur, and sulfur dioxide. Its irregular orbit around the gas giant creates strong tidal forces within Io, causing its surface to rise and fall by as much as 100 meters. The volcanic activity constantly remodels this small moon, flooding its surface with molten material and pockmarking it with new eruptions.
This landscape is assembled from images taken by the 1979 Voyager spacecraft and by the Galileo craft, which cruised the Jovian system in the late 1990s and early 2000s. A major international expedition to Jupiter's moons is in the planning stages, but the space agencies intend to focus on the icy moons Europa and Ganymede.
Last year, the Messenger probe--the first mission to Mercury since the 1970s--returned crystal-clear views of the 960-mile-wide Caloris impact basin, here seen in false color as yellow-orange. Messenger settled doubts about the planet's volcanic past, confirming that its smooth plains seem to be the result of ancient eruptions. The small orange specks visible at the basin's rim are thought to be volcanic features. The bright spiderlike formation in the center of Caloris, with troughs radiating outward from a central crater, has yet to be explained. Messenger will next fly past the planet in September 2009, and will begin orbiting Mercury in 2011.
NASA's first Saturn probe soared past the ringed planet in 1979, but the low-resolution images sent back to Earth revealed few details. In 2004, the Cassini spacecraft brought the planet into sharp focus when it became the first craft to orbit Saturn.
In this photo, an unusual perspective on Saturn renders almost all of its main rings transparent, visible only by the shadow they cast upon the planet. This natural-color view, captured by the Cassini spacecraft in 2007, shows the dark side of the rings from an angle slightly above the ring plane. Swirling winds blustering at more than 1,000 miles per hour, along with heat rising from the planetary interior, create the gold and yellow atmospheric bands.
This panorama was captured in 2005 as the Huygens probe plummeted through the thick nitrogen atmosphere of Titan, Saturn's largest moon. The Cassini spacecraft has since spotted lakes of ethane, making Titan the only world other than Earth known to have bodies of liquid on its surface. Methane and ethane rain down from the smoggy sky, forming vast hydrocarbon swamps.
Astrobiologists note that methane-eating microbes are found on Earth, and posit that similar life forms could evolve on Titan. Recent radar images from Cassini suggest that the moon may also have volcanoes that spew ice and frozen hydrocarbons, leading E.T.-hunters to speculate on how a subterranean heat source could aid the development of life.
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