Quotes of the Day
For a spacecraft that had spent nearly seven years in flight and journeyed more than 2.2 billion miles to get where it was going, the Cassini-Huygens Saturn probe did a funny thing when it closed in on the ringed planet last week: it hid.
Traveling at a breakneck 54,000 m.p.h. — four times its cruising speed — the ship was no longer flying toward the planet but falling toward it, on a high-speed trajectory that could send it skimming past Saturn and back out into space. If the ship was going to enter a stable orbit, it would have to fire its little braking rocket for 96 min., until it reached the right speed and position to dart upward through a gap in Saturn's rings and begin circling the giant world. But when it comes to the dense rivers of ice and rubble that form the planet's rings, the word gap is an imprecise term. Even a seemingly clear opening can be swarming with dust and particles. A collision with a bit of cosmic buckshot no bigger than a marble could destroy the ship. Making matters worse, the 930 million miles separating Saturn and Earth mean that even moving at the speed of light, radio instructions take 84 min. to travel out and another 84 to come back. Thus the ship would be operating entirely alone during its high-wire maneuver, all of its commands preloaded into its computer. As Cassini-Huygens approached the gap, it carried out, as preinstructed, one final step to protect itself: it turned around and pointed its large, dish-shaped antenna forward — a makeshift shield to protect the fragile hardware behind.
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"Engine turn-on," announced the propulsion engineer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., at 7:36 p.m. (P.T.), when the signal came down that the ship's fire had been lit. For the next hour and a half, the room was largely quiet. It was not until 9:12 that the same engineer spoke the words that meant the engine firing was over and the spacecraft had survived.
"We have burn complete," he said.
At that, the mission controllers, who in recent years have whipsawed between the devastation of the shuttle disaster and the celebration of the Mars landings, once again had reason to cheer, whoop and slap one another on the back.
For a nation struggling with war and terrorism, as well as alienation from some longtime allies, the scientific triumph was a moment to remind the world what the U.S. still does better than anyone else ever has, marshalling its imagination and technological prowess to send robotic emissaries into outer space. It didn't hurt the effort to mend international fences that the mission is being conducted in collaboration with the European and Italian space agencies. And it didn't hurt the domestic sense of pride that the arrival at Saturn occurred in the run-up to the Fourth of July.
"This is a story of human accomplishment," exulted Carolyn Porco, leader of the Cassini imaging team. "How can anyone not be excited about that?"
It would be hard, indeed. The Cassini-Huygens mission will now begin an extended tour of the glittering Saturnian system with its seven rings, 31 moons and untold cosmic secrets. By any measure, this is the most sophisticated planetary probe NASA has ever flown. About the size of a small bus, the Cassini orbiter is more than 22 ft. tall and weighs more than 6 tons when fueled. An engineering marvel, it is packed with a dozen scientific instruments and powered by a miniature nuclear generator. Carried on its side like a high-tech papoose is the Huygens lander, a 9-ft., 700-lb. wok-shaped probe that this winter will plunge through the atmosphere of Saturn's mysterious moon Titan, aiming for the most remote landing any human-made machine has ever achieved on another celestial body.
The payoff — for space scientists and curious civilians — could be staggering. Three other planets in the solar system — Jupiter, Uranus and Neptune — have rings, but they are faint and thready things, nothing like the magnificently complex cosmic jewelry that decorates Saturn. Seven of the other nine planets have moons, but none that perform the gravitational dances among themselves and within the rings that Saturn's do. And no planet has a moon anything like Titan, a world with much of the preorganic chemistry that Earth had 4.5 billion years ago — offering scientists a one-of-a-kind window into our vanished past.
The Saturnian system is, in a very real sense, the solar system writ small. And while other spacecraft have glimpsed it before — Pioneer 11 in 1979, Voyagers 1 and 2 in 1980 and 1981--they were mere flybys, quick hits by ships snapping a few pictures before whizzing off into deeper space. Cassini-Huygens — named after 17th century astronomers Jean Dominique Cassini and Christiaan Huygens — is there to stay.
"It really is like coming back to the promised land," says Torrence Johnson, a veteran of Voyager and a member of the Cassini imaging team.
The centerpiece of the Saturnian system is, of course, the planet itself, and plans call for it to get a going-over that it has never had before. The second largest of the solar system's four gas giants, Saturn — like its big brother Jupiter — is sometimes described as a starlike body with a chemistry of hydrogen and helium but without sufficient mass to light a nuclear furnace. That doesn't mean, however, that Saturn isn't roaring with activity.
By far the planet's most dramatic feature is its hellish weather. Winds blow around the Saturnian equator at 1,100 m.p.h.--five to 10 times the speed of the most powerful winds on Earth. Giant hurricanes tear through the planet's atmosphere, often two or more storms at a time, which then meet up before dying out. Displays of light similar to Earth's aurora borealis illuminate Saturn's skies, thanks to charged particles falling in from its moons. And where the auroras aren't flashing, lightning may be striking.
Cassini will use an elaborate suite of instruments to determine why all this meteorological hubbub is taking place on a planet that is so cold — with cloudtop temperatures of --218°F — that it shouldn't be able to cook up much weather. The best guess is that internal heat left over from the gravitational collapse that formed the planet in the first place is keeping things warm. Cassini will deploy its cameras, infrared sensors, chemical spectrographs and more to deconstruct the planet's atmosphere and find out for sure. Other instruments will map the planet's magnetosphere and gravitational field, perhaps confirming the theory that even so massive a ball of gas as Saturn has a solid core.
But it is by its rings that Saturn is known, and it was close-up pictures of those rings that stole the show last week. Within minutes of Cassini's arrival, the ship's camera had fired off 61 shots of the rings, and by 10 o'clock the next morning, wide-eyed Cassini scientists were showing them to the press. "I don't think we've ever seen structures like this before," said Porco.
What so moved her and the other scientists was the complexity and dynamism of the rings' internal structures. Scientists have known since Voyager that the seven broad bands that make up the ring system are not undifferentiated masses of material but rather are made up of hundreds of individual strands, like the grooves in a record album. The strands are made up of billions of bits of rubble and ice, some of them crystals smaller than a grain of sand, some of them boulders bigger than a house.
As the pictures from Cassini reveal, the interaction of this orbiting material can create bizarre effects. The edge of one ring shows elegant scalloping, presumably caused by the gravitational wake of a moon cruising alongside it. As the moon sails by at predictable intervals, the random collisions of ring particles become more rhythmic, forming tidy peaks and troughs.
Other images show that the moving moons cause equally graceful formations within the rings by tugging on particles and causing them to pile up and thin out, pile up and thin out, rippling outward in what ring scientists call a density wave. Another kind of wave known as a bending wave is caused by a moon that orbits at an angle inclined to the ring plane, warping or corrugating the ring's edge.
"We knew from Voyager that these phenomena probably existed," says Cassini planetary scientist Josh Colwell, "but there's never been anything this clear. The resolution in these pictures is unprecedented." Says an ecstatic Porco: "I'm surprised at how surprised I am by the beauty of these images."
They'll get better. Although Cassini will never again be as close to the rings as it was last week, it took only black-and-white pictures on the way into orbit. From now on, it will shoot between 100 and 200 images a day, most of them in color. The spacecraft will assemble mosaics of the rings, photographing them section by section and arranging the pictures in sequence from the center of the bands out.
Though no one is sure how the rings formed, some of the material is almost certainly the remains of small pulverized moons that were destroyed either by a cataclysmic meteor hit or when they wandered too close to a gravitational danger zone known as the Roche limit: the altitude above a planet at which the difference in gravity between the end of an object closest to the planet and the end farthest from the planet is great enough to pull the object apart while not pulling the remains out of orbit. Instead, the rubble disperses around the planet. Photographs of the debris could help confirm this phenomenon and could even turn up smaller, still undiscovered moons hiding within the rings.
Scientists already know of some moonlets that orbit inside the rings, sweeping areas clean of debris and accounting for at least two conspicuous gaps. Other tiny moons move along the outer rim of rings; these are the so-called shepherding moons that groom the ring edges and keep them sharp.
If Cassini-Huygens traveled all the way to Saturn and returned nothing but data on the planet and its rings, the mission would probably still be judged a success. Yet the true scientific goods will come when the spacecraft trains its instruments on the swirl of Saturnian moons. It would be nearly impossible for one ship to visit all 31 known satellites in Saturn's litter, so NASA has selected nine of them, both for their scientific promise and their comparatively convenient locations. The exotic names of the chosen moons — Phoebe, Titan, Iapetus, Enceladus, Mimas, Tethys, Hyperion, Dione and Rhea — hint at the exotic science that awaits.
Iapetus, for example, is a two-toned world, its leading edge dark, its trailing edge white. There are many theories advanced for this — including the possibility that there are hemisphere-wide volcanoes or that the moon is picking up dust as it moves through its orbit, staining its face and leaving the other side clean. "We have all kinds of questions," says Cassini physicist Larry Soderblum. "Were there volcanoes? Were there oceans of some mystical hydrocarbon that froze?"
Enceladus holds mysteries of its own. A bright white world with a relatively smooth face, it appears to have been repeatedly resurfaced by some kind of underground slurry or perhaps by ice volcanoes. In some places, once deep crevasses have been largely filled in and craters have been cut neatly in half, leaving one side deep and raw and the other covered, as if by snowdrifts. The area of the Saturnian ring that follows in the wake of Enceladus is slightly thicker than the rest, as if the moon were pumping out some kind of frozen exhaust, leaving a plume in its wake like the smoke from a steamship.
Other questions should be answered when Cassini flies by Hyperion, a tumbling moon that appears to have been knocked off its pins by a collision eons ago and has never regained its footing; and Tethys, a moon that bears such a massive impact scar that only the barest geological margin keeps it from shattering altogether.
It is Titan, however, that will be the main attraction. One of the largest moons in the solar system — larger than Mercury or Pluto — Titan would be a perfectly good planet if it were orbiting the sun under its own steam. NASA scientists were keenly disappointed when the Voyager 1 spacecraft flew by Titan in 1980. The moon's dense, orange atmosphere completely concealed its surface from view, revealing not a clue about what was happening on the ground.
Scientists speculate that there may be quite a bit happening. Rich in nitrogen as well as ethane, methane and other carbon-based gases, the Titanian air contains the raw chemical material believed to be needed to give rise to life — and just the kind that probably existed on the primordial Earth. Titan's frigid temperature — about --280F — would surely have prevented life from emerging. Nonetheless, over time the candlelike heat of the distant sun may have slow-cooked some of the organic materials, forming more complex molecules. What's more, if there is lightning in Titan's atmosphere, the random jolts could have shocked even bigger molecules into existence.
The Cassini-Huygens mission will investigate Titan from many angles. Of the 59 flybys of the nine selected moons, 45 will be devoted to Titan — most at a distance of just 590 miles. Preliminary images received last weekend revealed a bright cloud pattern about the size of Arizona near the south pole and what appeared to be a massive impact crater.
But there will be much more. Radar will pierce the Titanian cloud cover, mapping plains, mountains and perhaps even lakes of liquid ethane and methane — though early observations last weekend cast new doubt on the existence of the lakes. Spectrometers and other instruments will take the chemical measure of the moon's air, and cameras will again try to photograph Titan from outside in.
Cassini's best shot at the moon will come on Christmas Eve, when the Huygens probe is fired toward Titan, heading for a Jan. 14 rendezvous. On arrival, it will make a 2 1/2-hr. descent through the atmosphere by parachute. If it isn't destroyed by the landing, the probe could survive on the surface for an extra 30 minutes or so.
The brief three hours that Huygens lasts will be busy. The probe carries six instruments, including radar, an aerosol collector, a camera and wind instruments. The hardware will switch on by an altitude of 93 miles and will record data all the way down. When Huygens lands, sensors will continue to take readings — assuming it doesn't smack against a mountain or capsize in a methane lake. Even if it does, the landing — NASA's first splashdown since the return of the last Apollo spacecraft in 1975--would make space history.
Barring breakdowns or accidents, Cassini should send back data at least until 2008. If it exceeds its nominal life span it could survive for nearly a decade. When it finally does wink out, it could mark an end in more ways than one.
Cassini-Huygens is widely thought to be the last of NASA's great Cadillac probes — multibillion-dollar ships stuffed with instruments and complex backup systems. In the planning stage for 19 years, the craft cost $1.4 billion to design and build and nearly $2 billion to fly. When NASA adopted its "faster, better, cheaper" philosophy in the 1990s, it drove the cost of its unmanned ships down to the range of a couple of hundred million dollars — mostly by relying on off-the-shelf parts and eliminating redundant systems.
But the frugality came at a price. Cheap ships can't carry as many instruments as luxury models, so it may take more than one mission to bring back the same science. What's more, the lower price means more frequent breakdowns, as the string of bad luck NASA had with its Mars probes in the 1990s painfully demonstrated. The upside of flying economy is that if one spacecraft is lost, it's a relatively small matter to cobble together another. That thrift-shop technology has succeeded in getting three rovers onto the surface of Mars and will be at work again this summer when NASA launches the MESSENGER probe for a 2008 rendezvous with Mercury, and in 2006 when the New Horizons spacecraft takes off for Pluto.
Not all NASA scientists are impressed by these plans. Many believe there would be plenty of money to fly top-shelf ships if the space agency would drop its preoccupation with manned space travel. The International Space Station has been a scientific black hole, swallowing nearly $100 billion and delivering little of real value. President Bush's manned moon-Mars initiative will cost at least $170 billion — and that's from an agency that has never met a cost estimate it couldn't overrun. Forget the fixation with getting bodies in orbit or boots in the soil, critics say, and you could fairly blanket other planets with Cassini-quality landers and orbiters and still have billions left over. NASA administrator Sean O'Keefe, not surprisingly, disagrees: "Robotic missions are precursor missions." The most thorough exploration, he says, "requires the unique cognitive skills that only human beings can bring to the equation."
If Cassini really does represent the end of an era, it's a glorious end. Space scientists can justly take pride in the ship they have built and launched. They ought to be humbled too by the enormousness of the frontier they are mapping. "We have always tended to underestimate the splendor that the solar system has to offer," says physicist Soderblum. Knowing that this may be the last time — at least in our lives — that we get such a good look at Saturn makes the wonder of what we're seeing all the sweeter.
- Jeffrey Kluger