Seven years ago, the largest and most expensive interplanetary probe ever built blasted off from Cape Canaveral. It was loaded with 12 advanced scientific instruments, 72 pounds of plutonium to power them, and a capsule destined to be jettisoned toward the only other object in our solar system protected by a nitrogen-based atmosphere. After launch, the spacecraft began its voyage through the void of space and was promptly forgotten by all but a few scientists and space enthusiasts.
Since then NASA has launched the International Space Station, a Mars orbiter and lander (both of which failed), the doomed shuttle Columbia and the successful Mars rovers Spirit and Opportunity. President Bush even announced plans to send humans back to the Moon. All the while, the forgotten spacecraft continued to rack up miles, about 2.2 billion of them, as it quietly made its way across the solar system.
This month, the Cassini spacecraft will finally arrive at its destination: the ringed planet Saturn. And when it does, it will steal our attention back from the Moon, Mars and even Iraq–at least for a few precious moments. On July 1, Cassini’s main engine will fire to slow down the spacecraft so that Saturn can capture it as the planet’s first artificial satellite. Cassini will fly through a gap between two of Saturn’s rings, then aim its cameras and remote sensing instruments at the rings as it passes above them. “These pictures are probably going to knock people’s socks off,” says Candice Hansen-Koharcheck, a scientist at NASA’s Jet Propulsion Laboratory, which manages the Cassini mission.
With a mission price tag of $3.3 billion, Cassini is more than four times as expensive as the Mars rovers Spirit and Opportunity combined. Cassini is the size of a school bus, while the Mars rovers are only the size of golf carts. And those rovers existed only in the imaginations of NASA scientists when Cassini began its long journey. Cassini is the biggest, baddest mission ever flown, not just because it has gone out so far–about twice as far from Earth as Jupiter–but also because of what it will do when it arrives.
When Pioneer 11 and Voyagers 1 and 2 visited Saturn more than 20 years ago, each only whipped by on its way out of the solar system. Cassini will orbit Saturn 74 times over a four-year period, studying this mysterious planet from every angle. The spacecraft will train a dozen instruments on the Saturn system–snapping pictures of the planet and its stormy atmosphere, mapping the surface of its icy moons, studying the composition and rotation of the rings to find out why strange “spokes” form and dissipate, and measuring the powerful magnetic field surrounding Saturn to learn more about its inner layer of metallic liquid hydrogen. Cassini will also drop the Huygens probe built by the European Space Agency onto Saturn’s largest moon, Titan, which may have lakes of gasoline-like liquid hidden beneath its smoggy atmosphere.
Titan is of special interest because it is the only moon in the solar system with its own atmosphere and clouds. Bigger than Mercury or Earth’s moon, it is the single largest unexplored piece of real estate in the solar system. Titan’s atmosphere, like Earth’s, is mostly
nitrogen, but the moon’s opaque brew also contains a significant amount of methane. Scientists believe that when sunlight hits this methane, the light catalyzes the gas to form a smog of hydrocarbons. Because Titan is so cold, some of the hydrocarbons condense and rain onto the moon’s surface.
This primitive atmosphere may be similar to what Earth’s was like before living organisms began pumping out oxygen. “The thing that makes Titan so exciting to scientists is that they think it probably looks a lot today like Earth did 3 or 4 billion years ago,” says Bob Mitchell, NASA’s program manager for Cassini. “Therefore, it’s a good laboratory for understanding how life began.” Scientists don’t expect to actually find life in this frigid world. Rather, they’re looking for insight into the seeds of life on ours.
The photos sent back from Titan could reveal an “otherworldly” landscape like nothing we’ve seen on Earth or even Mars, predicts Hansen-Koharcheck, who can’t wait to see the shots. She and other scientists, some of whom started working on the mission as far back as 1982 (Hansen-Koharcheck joined the project in 1990) and have patiently waited for Cassini to wend its way to Saturn, are now growing as excited as a 7-year-old on Christmas Eve. “From day one, we were thinking about 2004,” says Hansen-Koharcheck.
The fun begins even before Cassini enters orbit. On June 11, the spacecraft will fly within 1,240 miles of Phoebe, one of the planet’s most remote moons, for its first direct encounter with the Saturn system. When that happens, some 200 scientists around the world will be hovering over their computers, waiting for the first wave of data to hit their screens. (Get real-time mission updates at saturn.jpl.nasa.gov.)
“We really don’t know what we’re going to find,” says Mitchell, voicing a sentiment shared throughout the Cassini team. Planetary scientists already know enough about the Moon and Mars to focus on very specific questions–such as whether Mars once had water on its surface–and send spacecraft designed to answer those individual questions. With Saturn, however, we’re still at the reconnaissance stage. That’s why NASA is sending a multipurpose spacecraft capable of painting a broad-brush picture of Saturn and its moons.
If Cassini is a big success, it may further weaken NASA’s 12-year-old “faster, better, cheaper” mandate, which rarely hits on all three cylinders. Already NASA’s Jet Propulsion Laboratory is laying plans for an even more expensive “big science” mission: the Jupiter Icy Moons Orbiter, a nuclear-powered spacecraft that would search for signs of life hidden on three of Jupiter’s moons. The Jupiter mission is still in the early design stages, and won’t launch until 2012 at the earliest, if at all. But if Cassini and Huygens return shocking results from the outer solar system, multibillion-dollar projects will begin to seem like a bargain.