Saturn is about to get another visitor from Earth, a joint U.S.-European spacecraft heading for a long tour of the ringed planet and seven of its 31 known moons. After its seven-year, 3.5-billion kilometer journey, the Cassini-Huygens probe will become the first to orbit Saturn next Thursday.
It has been more than 20 years since the twin U.S. craft Voyager I and Voyager II closed in on Saturn, but their inspections were brief as they passed by on the way to Uranus and Neptune. In contrast, Cassini-Huygens has an extensive itinerary around the planet over the next four years.
"The success of this mission if a matter of the highest priority in solar system exploration," said Orlando Figueroa, who is in charge of solar system exploration at the U.S. space agency NASA. "Saturn, its ring system, [and] its moons are a miniature model of the disk of gas and dust that surrounded the early sun as the planets formed in the solar system. Detailed knowledge of this environment and dynamic interactions between Saturn, the rings, and the numerous moons will provide valuable data for understanding how the solar system and its planets evolved."
Even before its arrival at Saturn, Cassini-Huygens has already exercised some of its instruments on the cratered outermost moon, Phoebe. The new analysis from a mere two-thousand kilometers away reveals it as blend of bright ice, rock, and dark carbon compounds similar to bodies much farther out in the solar system, such as Pluto and Neptune's moon Triton.
NASA scientist Torrence Johnson says this is strong evidence that Phoebe is an outsider to Saturn - a migrant from the distant region beyond Neptune called the Kuiper belt about four billion years ago, not from the disk that formed Saturn and its other moons.
"We believe at that time, the outer solar system was full of Phoebes," he said. "We believe Phoebe is a good example of the fundamental building blocks of the planets out there, an icy, rocky planetessimal, and as the big planets formed out there - that is, Jupiter, Saturn, Uranus, and Neptune - that material was either swept into these forming planets or, through gravitational interactions, ejected from that part of the solar system, some of it ending up, in fact, probably in the Kuiper Belt."
With Phoebe now far behind it, the Cassini-Huygens spacecraft's next task is to pass up through Saturn's fifth and sixth rings, slow its speed with retrorocket firings, settle into orbit, and immediately start photographing the ring system before it begins wandering around other moons.
Program manager Robert Mitchell says the rings are a collection of dust and small debris that could be fatal to the probe, so Cassini will be maneuvered into a protective position.
"The threat is that some small, fine particle would hit some sensitive part of the spacecraft," he said. "But we are orienting the spacecraft such that the high-gain antenna, four meters across, goes through first, so its diameter shields all the spacecraft behind it. We believe that we have got this worked out to where it's going to be not really a threat at all."
A highlight of the mission comes on Christmas Day when the European Huygens part of the mission breaks free of the U.S. Cassini mothership and plunges into the largest moon, Titan, the only one in the solar system with an atmosphere. Huygens is not expected to survive more than a few hours on the surface, but the payoff comes on the way down when it analyzes the atmosphere. In addition to this, Cassini will make most of its planned 76 orbits around Saturn close to Titan.
Scientists are fascinated by this satellite because its environment is believed similar to Earth's before life emerged. Titan is thought to harbor the organic compounds necessary for life and possibly an ocean. But mission official Charles Elachi says no one thinks life actually got started there.
"It's a pre-biotic environment, so we don't expect biology to be active now," he said. "So it's really more of an organic chemistry kind of mission."
Another major goal of the Cassini mission is to determine whether Saturn's core is liquid hydrogen and helium as believed. And are there more than 31 moons? If so, Cassini might find them.