The Cassini spacecraft is roughly halfway through its looping voyage of the Saturn system and is continuing to return a bounty of data on the ringed planet and its moons. Yet all journeys must have an end, and Cassini's eventual fate is now being discussed.
"Current planning is for a two-year mission extension that ends on July 1, 2010," said Robert Mitchell, NASA's Cassini mission program manager. "However, presuming that the spacecraft continues to function well, it's reasonable to expect that one or more further extensions will be supported."
Sometime around 2012, Cassini, like the oceangoing ships of old, will likely need to be decommissioned. However, the spacecraft cannot be towed to some nearby shore to be dismantled; she must either drop anchor, be scuttled, or cast off her gravitational moorings altogether.
"Perhaps the most likely option is to leave Cassini in a long-lived orbit that would have little to no risk of ever hitting anything," Mitchell said. "Another is to impact Saturn like Galileo did at Jupiter, although there are some complications with this one."
The complications arise from the beautiful wafer-thin rings that girdle the planet and the fact that in order to dive into Saturn, Cassini would have to pass through them — a risky maneuver that could render the spacecraft uncontrollable.
"Another option is to identify one of Saturn's icy moons as an acceptable candidate and impact the spacecraft onto it," Mitchell said.
Yet this option also holds an inherent risk arising from the three plutonium-bearing radioisotope thermoelectric generators, or RTGs, that Cassini uses as a power source.
"The issue is the heat that would be generated by the RTGs and the environment that would be created (melted ice) that could be conducive to the viability of any Earth organisms that might have survived on the spacecraft to that point," says Mitchell.
Mission planners make great effort not to contaminate alien worlds with terrestrial life.
The third option: raise anchor and escape the Saturn system altogether. Such a maneuver would require numerous flybys of Saturn's largest moon Titan to sling the spacecraft free of the ringed planet's environs.
If Cassini were to be cut adrift in this manner, her controllers have two further choices: either bring her sunward or let her escape deeper into the outer solar system.
Should Cassini be directed back toward the inner solar system, it is likely her final port of call would be Jupiter, Mitchell explained, "One possibility is to escape Saturn and then most likely put the spacecraft on an impacting trajectory with Jupiter. This appears to be feasible, but the flight times get to be rather long."
Cassini's leisurely final journey need not end at Jupiter; rather the giant planet's gravity could fling Cassini into position for an impact on Mercury. Such an impact would provide valuable data on the composition of Mercury's surface and could feasibly occur around 2021 to be observed by the BepiColombo spacecraft.
Alternatively, if Cassini were cast into the solar system's outer depths, there is a small chance she may provide further scientific reward in the form of a flyby of an outer planet or Kuiper Belt object.
"Some very preliminary analysis indicates that this might be possible. However, the flight times involved and the status of spacecraft consumables at that time make anything like this quite an unlikely option," says Mitchell.
NASA is to make the final decision on Cassini's eventual fate later in the mission.