From the September 2015 issue

At the end of the Cassini mission, could the probe be put into orbit around Titan to observe it on a permanent basis?

Pete Cholewinski, Naperville, Illinois
By | Published: September 28, 2015
In 2017, NASA’s long-lived Cassini spacecraft will finally exhaust its fuel and crash into Saturn. This death plunge will guarantee it doesn’t smack into Enceladus or Titan, possibly contaminating unknown alien organisms with earthly microbes.
NASA/JPL/Space Science Institute
Titan is a fascinating place. It is the most massive (by a factor of 60) moon of Saturn. It is also the only moon in our solar system with a dense atmosphere and liquid on its surface, in this case liquid methane and ethane. The presence of complex hydrocarbons has even raised the possibility of extraterrestrial life. Scientists would be thrilled to have Cassini permanently remain in orbit around Titan. Unfortunately, this is impossible due to the amount of fuel available to make orbit changes.

A spacecraft orbiting above Titan’s atmosphere will travel at roughly 4,900 feet per second (1,500 m/s) in a circular orbit. Cassini currently flies by Titan on a regular basis at three times that speed and thus would need to slow down by several thousand meters per second to enter orbit.

The spacecraft would have to accomplish this change in speed, called “delta-V,” by spending fuel. After Cassini reached Saturn, it had 2,434 ft/s (742 m/s) of fuel available — not enough to accomplish this goal even then! Cassini used most of that fuel during the initial and first extended mission, leaving only 518 ft/s (158 m/s) for the entire seven-year second extended mission that began in 2010 and making a Titan orbit that much more impossible. Some spacecraft do enter orbit by using atmospheric drag to slow them down, but Cassini was not built for that.

An additional minor concern involves planetary protection. Both Titan and Saturn’s icy moon Enceladus could harbor some form of life, and it is important to protect them from external biological contamination. Although Cassini has been classified as a low-risk mission, it is nevertheless desirable to avoid impacting Titan’s surface, just in case. Any long-term mission around Titan would need to take this into consideration. The need to avoid
accidentally hitting Titan or Enceladus is the main reason that Cassini will be destroyed by flying into Saturn’s atmosphere in 2017.

Robert French
SETI Institute
Mountain View, California