When its handlers finally steer it into a collision course with Saturn in 2017, the spacecraft Cassini will have been in space for two decades, 13 of those years spent in orbit around the planet that will eventually become its final resting place. That's a far cry from its original mission — a sprint to Saturn, followed by a scant four years in orbit.
Cassini's mission was extended — not just once, but twice. That's not an uncommon phenomenon in the world of NASA. Lots of unmanned space missions end up lasting longer than originally planned. The Voyager probes were officially just supposed to study Jupiter, Saturn, and their moons. The Galileo mission to Jupiter was extended three times. And XKCD made the epic extension of the Spirit rover's mission on Mars both famous and poignant.
Researchers say these extensions are a factor of both the budgetary process and worst-case-scenario engineering. They're also extremely important scientifically, allowing us to make important discoveries for less money. Thanks to Cassini's extensions, we were able to study a storm on Saturn that was more massive than any seen in the previous 21 years; watch the Earth-like climate physics on the moon of Titan play out through seasonal changes; and found the most-promising astrobiology hot spot in our solar system.
Engineering is the thing that makes mission extensions possible. But spacecraft aren't engineered with mission extensions in mind. Instead, the ability to keep Cassini circling Saturn and sending back information for 13 years turns out to be a happy side-effect of creating a spacecraft that's certain to last four years doing that same job.
Space is a rough place to work. For instance, when Cassini launched, everyone knew it would have to withstand exposure to radiation while in orbit around Saturn. But nobody knew how intense that radiation would be.
Earth, as you might know, is surrounded by two rings of radiation — the Van Allen belts. They're probably made from particles that were carried to our planet on the back of the solar wind and trapped in place by Earth's magnetic field. The belts pose a risk to both human and electronic space exploration, because the particles can penetrate metallic shielding, as well as skin, and damage what's underneath. Imagine throwing a bowling ball through the window of a house. Chances are good that you'll hit something and break it.
Other planets also have radiation belts, but they're different from Earth's. Jupiter's radiation belts are massive and massively powerful. Not only is its magnetic field 10 times stronger than Earth's, but the planet also produces more of its own radiation, so the belts contain more than just what happened to get trapped there from the solar wind. The radiation in Jupiter's belts is a million times more intense than Earth's. That fact basically rules out human exploration of most of Jupiter's moons and, at least for now, limits how close our spacecraft can get to the planet.
When engineers were designing Cassini, they knew the radiation belts on Saturn would be more intense than Earth, but they didn't know how much more. To get a system that would last four years, they had to design something that could last a lot longer, said Cassini program manager Earl Maize. Beefing up the electronics was key. That means adding aluminum shields — barriers that can help block the bowling ball effect — as well as designing redundant systems, and putting plenty of spacing between the different electronic components so that when a radioactive particle gets through it does less damage.
But the radiation belts of Saturn have turned out to be less intense than those of Jupiter. Engineers designed Cassini with the expectation of losing a certain number of solid state memory modules to radiation damage every year, Maize said. Instead, in the almost 17 years since the craft's 1997 launch, they've only lost a single one. Suddenly, your four-year spacecraft has a much longer potential lifespan.
All of this started to become clear about two years into Cassini's Saturn orbit, Maize said. They hadn't originally planned any life for the spacecraft past 2008. But, as they were able to create new estimates of how long the craft would last, they started to think about the future.
That's where the money part comes in. A fact of funding is that it's often easier to get the money to extend a four-year mission to 13 years than it would have been to plan a 13-year-mission to begin with, said Carolyn Porco, director of the Cassini Imaging Science team. While engineers like Maize say they don't design with extensions in mind, from Porco's perspective extensions are likely to happen and scientists plan to take advantage of them.
"It's hard to sell missions to Congress. It's easier to say to them, 'we've already been successful and you'd be fools to cut us off now,'" she said. "That's the ploy. It's been used over and over. And it works."
Case in point: Cassini's flybys of the moon Enceladus. Nobody expected the moon to be anything special, Porco said. The original mission included three or four flybys, and that was it. But those passes were enough to change everybody's minds about Enceladus and what it had to offer.
Porco thought it was possible Enceladus — a frozen ice world — might have some geysers of liquid water. But Cassini found whole forests of the things. It also found an atmosphere around Enceladus and evidence that those geysers were erupting from a relatively warm liquid sea beneath the ice. That information played a big role in securing the funds to keep Cassini going, Porco said.
The extension allowed Cassini to do more flybys of Enceladus and allowed it to do more than just take pictures. In 2008, it flew through one of the geyser plumes, sampled the material, and found some of the carbon-based molecules that are necessary to create life. They've also been able to figure out how the sea stays warm, pinning it on an elliptical orbit that creates regular changes in the gravitational forces that act on the moon. Those changes cause Enceladus to flex, creating heat energy deep in its core and cracking the surface ice enough to create those wonderful geysers.
Today, Porco says, Enceladus is the most promising place to study astrobiology in our whole solar system. In fact, it's likely to warrant its own mission in the future.
Ultimately, mission extensions are just good money management. It costs $60-70 million a year to operate Cassini, Porco said. But it cost even more to design, build and launch the thing — closer to $3 billion, she said. And that's in mid-90s dollars. It would be even more expensive to do the same thing today. That fact creates a big incentive to build spacecraft that have a possibility of living through mission extensions and to pay for those extensions when the opportunity arises. If you've already got a functioning spacecraft, why not use it to learn more?
That incentive is so strong, that it even applies to spacecraft that aren't functioning perfectly. The Spitzer Infrared Telescope launched in 2003. In 2009, it used up the last of the liquid helium coolant that kept the telescope chilled enough that it could tell the difference between heat put off from distant galaxies and its own internal temperature. At that point, it was rechristened the Spitzer Warm Mission. It can't see all the wavelengths of light it used to be able to see, but it can still see some, so we keep using it.
In fact, the only reason Cassini will end in 2017 is because that's when its supply of propellant is expected to run out. The scientists and engineers want to crash it into Saturn and get a chance to study the planet's atmosphere on the way down, Porco told me. But it can't just fall on its own as its orbit degrades. Saturn's rings would get in the way and destroy Cassini before it had a chance to reach the planet. Instead, the researchers have to budget the spacecraft's propellant, planning to use the last of it to steer Cassini past the rocks and put it "safely" on its final mission.