A Complex Mission

In October 1997, a six-ton spacecraft the size of a school bus set off on a billion-mile journey to Saturn. It was named after the seventeenth-century Italian astronomer Giovanni Domenico Cas­sini, who discovered four moons of Saturn—Tethys, Dione, Iape – tus, and Rhea—along with co-discovering Jupiter’s Great Red Spot and first spotting the gap in Saturn’s rings that bears his name. Cas­sini, along with its deployable probe Huygens, is one of the most complex and ambitious missions in NASA’s history.21 Scientists can spend their entire careers working on a planetary probe like Cas­sini. The first concept was floated in 1982 and it got a boost in the late 1980s when it was conceived of as a joint mission with the European Space Agency. That helped it survive budget-cutting by Congress in the early 1990s. More than thirty years after it was first conceived it’s still going strong. Cassini is an exemplar of international collaboration in space. More than five thousand scientists and engineers in seventeen countries have worked on the mission. It’s still the heaviest spacecraft NASA has ever launched to a destination beyond the Moon.

To date, Cassini-Huygens has cost about $3.5 billion, and the very high cost of complex planetary probes makes many people flinch. It was Cassini’s ballooning budget in part that made incom­ing NASA Administrator Dan Goldin embrace a faster, better, and cheaper approach in the early 1990s.22 During the Goldin years, NASA launched nearly 150 payloads at an average cost of $100 million, with a failure rate of less than 10 percent. But the move to crank out more frequent, stripped-down missions at lower cost was not universally popular; a NASA report in 2001 argued that the strategy had cut too many corners and produced an unaccept­ably high failure rate.23 The public took notice when the high pro­file Mars Climate Orbiter and the Polar Lander missions failed. The former was notoriously lost due to a failure to convert English to metric units, but the latter was successful in a later incarnation as the 2007 Phoenix mission to Mars.

The debate may be a false dichotomy. Special-purpose missions such as the recent LCROSS probe to the Moon and Mars Global Surveyor are only designed to do one thing. Billion-dollar missions typically have a dozen instruments and are very versatile; they’re like the Swiss army knives of the space program. Among them, the two Voyagers and Galileo lasted twelve years beyond their design lifetimes and have fulfilled all their scientific goals. Cassini has fin­ished its primary mission and in 2008 it was approved for a two – year extended phase called the Cassini Equinox Mission. In 2010, it was extended until at least 2017 and renamed the Cassini Sol­stice Mission. These names reflect the fact that the spacecraft will have witnessed an entire cycle of Saturn’s seasons by late 2017. So far, more than 1,500 research papers have been published based on Cassini and Huygens data, making this the most productive plan­etary probe ever. The mission will end with a dramatic flourish. Current plans call for Cassini to dive inside Saturn’s rings on Sep­tember 15, 2017, orbit Saturn twenty-two times, and then plunge to its death in the atmosphere. One hopes a musician will be in­spired to write a suitably operatic theme for this planetary finale.