Cassini, by contrast, will become a resident, orbiting at least 76 times around Saturn. Some NASA scientists think the probe will fling an average of one gigabit of data back every day for four years, including up to 750,000 pictures. That’s “more than we’ve gotten back from any other planetary probe,” Mitchell says. Anticipating what the results will bring is a bit like anticipating a first date—you may have impressions from a glimpse of the opposite sex, but time and proximity make all the difference. “We don’t know what Cassini is going to be like,” Cuzzi says. “We don’t really have an analogue.”
When Cassini fired its rocket and eased into orbit around Saturn on June 30, one of the first orders of business became trying to make sense of those dazzling rings. The spacecraft’s namesake, Italian astronomer Giovanni Cassini, deduced in the 17th century that the rings are composed of a swarm of particles. Later studies added only a few details: The rings are composed largely of water ice and measure about 175,000 miles in diameter yet are just a few tens of yards thick. Voyagers 1 and 2 provided some amazing views of the rings but did not do much to explain two key questions: What controls the structure of the rings, and why are they there?
The why question is especially confounding. Computer simulations suggest that planetary rings are not durable. Within a half-billion years or so—scarcely more than one-tenth the age of the solar system—Saturn’s rings should have dispersed or should at least have degenerated into something much less spectacular, like the dark, wispy rings around Uranus. It wouldn’t be so surprising that we are witnessing Saturn’s spectacular display right now if planetary rings were popping up all the time, but they are not. According to what little we know, Saturn’s rings must have formed as the result of rare, highly improbable events.
There are two theories of how the rings originated—both plausible, but only barely. In one scenario, a comet or something similar struck one of Saturn’s moons, blowing it to bits. The rubble then went into orbit so close to Saturn that the planet’s gravity prevented the particles from pulling back together, and they became rings. In the second, a gigantic icy object from the extreme outer solar system swooped in, got too close to Saturn, and was ripped apart by gravitational forces.
Figuring out what explanation is correct requires knowing the precise composition of the 10 percent of the rings that isn’t water ice. To find out, Cassini will analyze radiation bouncing off the ring particles to look for silicates, carbon, ammonia, organics, and other substances. When the spacecraft passes through the rings, it will also use its instruments to sniff out their composition. In the end, neither origin theory looks terribly promising. It is extremely unlikely that a comet would strike a moon in just such a way as to destroy it, but the odds of a huge ice ball wandering close enough to Saturn to be torn apart by gravity are also long.
Which brings us to another possibility: The rings are not as short-lived as scientists think. Their stability depends on an elaborate dance between them, the surrounding moons, passing meteoroids, and Saturn. The rings contain objects ranging in scale from grains of dust to flying mountains. Regardless of size, each bit of ring follows its own orbit about the planet, suspended not only in Saturn’s gravity but also in its own minute gravitational field. Needless to say, a trillion objects tugging at each other are going to interact in complicated ways. Broadly, though, there is one overarching trend—toward destruction.
Saturn’s inner moons, orbiting just beyond, constantly steal momentum from the ring particles. As a result, the moons spiral outward, and the rings eventually fall into Saturn. This is the chief reason why scientists think they cannot be more than about 500 million years old. Another observation supports that timescale. Interplanetary dust constantly falls onto the rings, as it does onto all objects in the solar system. Whereas interplanetary dust is dark, Saturn’s rings are bright. If the rings were old, they would have darkened.
The Early Days: What Cassini Has Already Delivered
Photographs, clockwise from top left: Courtesy of NASA/JPL/Cassini Imaging Team/University of Arizona; Courtesy of NASA/JPL; Courtesy of NASA/JPL/University of Arizona
Although it has taken Cassini seven years to reach Saturn, the spacecraft has made the most of its agonizing, 2-billion-mile windup. Along the way it has set its sights on a wide range of passing solar-system sights—including our moon, Jupiter’s color-striped clouds, the Jovian moon Io, and a small asteroid called Masursky—so that engineers could test the craft’s instruments and work out any glitches. As a bonus, Cassini acquired a trove of exciting data and images long before its formal mission began.
After launch, Cassini swooped through the inner solar system to pick up gravitational assists from Earth and Venus. In the summer of 1999, it made a final swing past Earth and turned its cameras on the moon, yielding some of the sharpest images (A) ever taken of our natural satellite. Six months later, Cassini trained its cameras on the roughly 10-mile-wide asteroid Masursky as it passed. To scientists’ surprise, the object seems to have a different composition from other asteroids in that part of the solar system.
By late 2000 Cassini was closing in on another intermediate destination, Jupiter. The spacecraft’s radar instrument listened in on radio waves from the planet, producing a unique map of Jupiter’s harsh radiation environment (B). The emissions come from electrons racing around at nearly the speed of light in Jupiter’s enormously powerful magnetic field. Yellow denotes the most intense emissions in this image; blue signifies the least intense. Farther in toward Jupiter, Cassini’s cameras captured detailed panoramas of atmospheric storms and novel views of tiny moons orbiting in Jupiter’s tenuous ring. One of the most dramatic Cassini images was a portrait of the volcanic satellite Io seeming to float just above its parent planet (C). Despite appearances, Jupiter and its moon were nearly a quarter of a million miles apart.
Last year, Cassini began focusing on its final destination, taking a series of snapshots that documented Saturn growing ever larger through the narrow-angle camera. The spacecraft hasn’t looked back since.