At last Friday’s lecture (by Dr. Jon Willis) I asked about the plans to get the probe back from Enceladus. I thought the speaker’s response that “what comes up must come down” was flippant and contrary to Newtonian physics. For instance, geosynchronous satellites do not “come back down” and have never been brought back down to low orbit for repair or refueling.
This is a non-trivial problem. Enceladus is the second most proximal moon to Saturn and is therefore deep in Saturn’s gravity well. Return to earth involves raising the probe to a heliocentric orbit (effectively Saturn escape velocity) then converting to a Hohmann transfer orbit with Earth perihelion at 1 AU. My understanding is that transfer between two circular orbits (E.G.: Earth and Saturn) using a Hohmann transfer orbit requires the same delta-v independent of the direction of the transfer. Since the probe relied on multiple gravity assist manoeuvres to get to Saturn, I doubt it has the luxury of carting a lot of fuel for the return trip.
At Earth orbit perihelion it will need to re-enter Earth’s atmosphere at close to Earth’s escape velocity. I don’t know of a successful re-entry from a heliocentric orbit, or if it is feasible.
I’m not saying return can’t be done, just that getting the probe home is as difficult as getting it up there in the first place. The project invites some interesting jiggery-pokery. Like more gravity assist or aerobraking.
My second question was about return of the plutonium battery and potential radioactive contamination upon re-entry. There have been several inadvertent re-entries of plutonium batteries (Apollo 13 Lunar Module being one). The issue has been addressed by containing the batteries within their own re-entry heat shielding so they would land in one piece (hopefully not in my backyard). Also, the Plutonium-238 isotope used in batteries has a half life of only 64 years, not the 24,000 years of weapons grade plutonium. I feel much safer now.