[X] My Assistant

[pioneer]

According to this press release, Cassini will fly 25 Km from the surface of Enceladus in October I know the escape velocity of Enceladus is low, but just how close can Cassini come to the surface without the gravity pulling it in? What is the escape velocity of Enceladus?

[ugordan]

No matter how strong the gravity of a body you're flying by is, unless you do a braking burn at some point in the flyby you cannot get captured. From that body's point of view, you're coming in via a hyperbolic trajectory coming from "infinity" and the body's gravity accelerates you enough that by the time you pass closest point you have picked up just enough additional speed to leave the body and its gravity well with the same speed you came in. No gain, no loss, only your direction is changed, e.g. the trajectory is "bent" by a certain angle as seen from that body, in this case of Enceladus. The stronger the gravity, the more the trajectory is bent.

From the point of view of Saturn, the situation is a bit more complicated. The spacecraft actually ends up giving or stealing a bit of it's orbital energy (relative to Saturn!) to the moon, but in no practically conceivable circumstance can Cassini get captured by any moon (including much more massive Titan) just by passing by. It would need to do a propulsive maneuver to do that, and to get captured by Enceladus during these flybys it would actually have to get rid of some 18 kilometers/second velocity, which is a huge amount and well beyond Cassini's capabilities even if the propellant tanks were full again. For comparison, that's a much higher velocity than Cassini was even launched from Earth with and it took a really big rocket to do that!

[djellison]

18 kilometers/second velocity,

That would take all of Cassini's fuel tanks...and probably the Titan rocket that launched it