The Cassini probe prepares for a close encounterwith the moon Titan this weekend. The target is a region called Xanadu. Scientists won’t find either Kubla Khan or Olivia Newton-John there. But they’re not quite sure just what they will find. Radar will hopefully settle a basic question: Is it a highland area or a large basin? At the moment we only have shades of infra-red radiation and the most basic of earth-based radar inklings.
Another patch of the Titan surface will be brought into clearer focus after this weekend. You might ask why Cassini doesn’t go into orbit. Why settle for these bitty strips of radar clarity? The main reason is fuel, or lack thereof.
Cassini will be passing this moon at a relative speed of 13,200 mph on Sunday afternoon. That would be enough to orbit the earth a few thousand miles up. But Titan’s gravity is about one-fourth that of earth, so that speed would need to be reduced by 75% for Cassini to have a chance of achieving orbit.
The probe already burned off an hour and a half of fuel two years ago when it entered Saturn orbit. Getting something into space always involves a trade-off of fuel versus payload. The more fuel a mission carries, the more maneuvering possibilities exist. That option means fewer scientific experiments.
Even if Cassini could make Titan orbit, it’s not likely it could ever leave the moon again to continue to explore the other sights in the Saturn system. Given the excitement at Enceladus and Iapetus, I’m not sure the mission planners would commit to a Titan orbit, even if they could.
The primary mission is planned to conclude in Summer 2008, after 76 orbits of Saturn and dozens of close fly-bys of Titan and other moons. Engineers have floated various ideas in the event (pretty likely) that the probe is still functioning well and that NASA will keep things afloat with more funding. One idea is to use Titan’s thick atmosphere to slow Cassini as it passes. This presents some challenges:
Slowing down in an atmosphere means that velocity is traded for friction. And friction raises the heat. Come in way too fast too soon and Cassini will burn up. Come in a little too fast and maybe something gets damaged. If it takes too long to set up the right approaches for aerobraking, Cassini may run out of maneuvering fuel before the target orbit is achieved.
My guess is that scientists will keep using fly-bys of Titan to tweak the orbit of the spacecraft. They can be patient and gradually complete fly-overs of unknown surface while they aim Cassini back to the interesting targets of Enceladus and Iapetus.