Hours after settling into orbit around Saturn, the international Cassini spacecraft on Thursday sent back unprecedented glimpses of the planet's rings, revealing patterned waves that looked like ripples in a pond.
The pictures, which imaging team leader Carolyn Porco called "absolutely mind-blowing," capped a dramatic night for the mission's U.S.-European team.
Scientists and engineers breathed a sigh of relief late Wednesday at NASA’s Jet Propulsion Laboratory as a signal indicated first that Cassini — launched nearly seven years ago — safely passed through the ring plane and then performed a crucial engine firing. It squeezed through a gap in Saturn’s shimmering rings, fired its brakes and settled into a near-perfect orbit around the giant planet.
“I can tell you it feels awfully good to be in orbit around the lord of the rings,” said Charles Elachi, JPL's director and a member of Cassini's radar science team.
Images from 900 million miles away
Early Thursday, mission officials once again huddled in front of their screens as the first raw images came in from more than 900 million miles (1.4 billion kilometers) away.
“Wow, look at that scallop on the inner edge. That’s a beauty,” said imaging scientist Jeff Cuzzi as a picture from the sunlit side of the rings was displayed.
Black-and-white images of the A ring, the outermost of Saturn's two brightest rings, showed patterns of ripples that scientists said were density waves, caused by the gravitational influences of the planet's moons.
Although the rings look like solid, flat doughnuts from Earth, they actually behave more like rivers of ice and rock, with particles ranging in size from dust specks to mountains.
"These density waves are like books, just waiting to be read," Porco said.
She estimated that some of the waves might measure as little as a quarter of a mile (half a kilometer) across. Earlier NASA probes had taken pictures of the rings as they flew by, but with nowhere near the resolution seen in the Cassini images. Some of the pictures had a resolution of 164 feet (50 meters) per pixel, Porco said.
“I'm surprised at how surprised I am at the beauty and the clarity of these images,” she told journalists. “They are shocking to me. ... They were so shocking I thought that my team was playing tricks on me and showing me a simulation of the rings and not the rings itself.”
Putting the first spacecraft into orbit around Saturn marked another major success this year for NASA, which has had two rovers operating on Mars since January and has a spacecraft heading home with samples from a comet encounter.
In a telephone call from Washington, NASA Administrator Sean O’Keefe called the reaching of orbit around Saturn an “amazing victory” and part of a “doubleheader,” following a successful spacewalk by the international space station crew earlier Wednesday.
A carefully choreographed maneuver allowed Cassini to be captured by Saturn’s gravity as it arced within 12,500 miles (20,000 kilometers) of the giant planet’s cloud tops.
Using its big radio dish as a shield against small particles, the spacecraft ascended through a gap between two of the rings, then spun around and fired its engine for more than 1½ hours to slow its acceleration. The craft then rotated again to place its shielding antenna in front as it descended back through the gap.
The maneuver had to be carried out automatically because Earth and Saturn are currently more than 900 million miles apart and radio signals take more than 80 minutes to travel each way.
Navigation team chief Jeremy Jones said initial analysis showed the orbit to be so good that a “cleanup” maneuver planned for Saturday would be very small.
Two decades of work
The orbital insertion came after two decades of work by scientists in the United States and 17 nations. The $3.3 billion mission was funded by NASA, the European Space Agency and the Italian Space Agency.
David Southwood, director of space science for the European Space Agency, called it a “world mission” but said the orbital insertion was “America doing it right.”
Cassini will now go on at least a four-year tour of Saturn and some of its 31 known moons. Cassini was scheduled to make 76 orbits and repeated fly-bys of the moons.
Scientists hope the mission will provide important clues about how the planets formed. Saturn, the sixth planet from the sun and the second-largest, intrigues scientists because it is like a model of the early solar system, when the sun was surrounded by a disk of gas and dust.
For any astronomer who wants to study how planetary disk systems work, "this is the best place to go," Porco told journalists.
Cassini and the Huygens probe it carries are named for 17th-century astronomers Giovanni Cassini and Christiaan Huygens. Cassini discovered several of Saturn's moons, as well as a gap between Saturn's rings that now bears his name: the Cassini Division. Huygens discovered Titan, Saturn's largest moon.
The piggyback Huygens probe will be sent through Titan's atmosphere in January. The moon, blanketed by a thick atmosphere of nitrogen and methane, is believed to have organic compounds resembling those on Earth billions of years before life appeared. Scientists speculate that Huygens could well splash down into a lake or ocean of methane and ethane.
Cassini was launched on Oct. 15, 1997, from Cape Canaveral, Fla., over the objections of anti-nuclear protesters who feared what might happen if the rocket exploded while carrying Cassini and its 72 pounds (33 kilograms) of plutonium, which powers the spacecraft. NASA insisted that the launch would be safe because of the numerous precautions taken with the poisonous substance.
Cassini has traveled 2.2 billion miles (3.5 billion kilometers), getting gravitational assists from Earth, Venus and Jupiter as it caromed around the solar system. The spacecraft took the roundabout route because it was too massive to be launched on a direct trajectory to Saturn.
Previous expeditions to Saturn have been brief: There were fly-bys by Pioneer 11 in 1979 and the Voyager 1 and 2 spacecraft in 1980 and 1981.