# How close to Jupiter does a spacecraft need to get in order to gain maximum gravity assist and Oberth effect bonus for leaving the Solar System?

Given that a spacecraft's purpose is to leave the Solar System as fast as possible, how deep into Jupiter's gravity well would be optimal for an assist in the planet's orbital direction? Jupiter's max orbital speed of 13.7 km/s sets an absolute upper limit for gravity assist, but by how much could the Oberth effect contribute?

Juno now gets to 4,200 km of the cloud tops (by some definition of cloud tops). Would it help much for a high speed spacecraft getting down to 0 km and fire the last instantaneous propulsion stage right there? Aerogravity assist would only help change the inclination, not improve the maximum Solar System escape speed, is that right?

If you are looking for pure speed your spacecraft would need to get as close to Jupiter as possible without interacting with its atmosphere too much. Remember that there is atmosphere above the cloud tops. You are right about aerogravity assists - an aerogravity assist uses aerodynamic lift to achieve the greatest change in direction, however that is at the cost of some velocity due to aerodynamic drag forces:

• Lift induced drag: this is the byproduct of the lift you generate. No wing is 100% efficient at creating lift, so you are going to lose more velocity than the change in vector you achieve
• Parasitic drag: this is the drag on the body of the spacecraft as it passes through the atmosphere

Essentially, there will be a distance where the atmospheric drag losses will exactly counteract the gains due to the oberth effect, to get the most out of the maneuver you'd want your spacecraft just above that point. To calculate this you'd need to model your spacecraft's body and do some computations on how much drag it would produce.

Some other considerations:

• If you are going to get into the atmosphere you'd need to consider how it will impact the spacecraft structure in terms of heat and stress
• Jupiter's intense radiation will require shielding, perhaps adding enough weight that the oberth gains are offset
• Do you have any suggestion about how much more Voyager or Galileo or Cassini or New Horizons or Juno could physically have gained in speed away from the Sun if they had been on a Jupiter flyby trajectory to do so, rather than using Jupiter to reach their actual science targets? Feb 13, 2017 at 14:45
• I don't really, there's too many factors I can't assess. Radiation exposure being one of them, how close could Cassino get to Jupiter without getting fried, for example.
– GdD
Feb 13, 2017 at 14:54
• Jupiter's radiation belt seems to be like a torus around it, like the van Allen belts. Juno is actually getting really close to Jupiter in part because it is safer there. However, I was going for a simpler orbital rocket science thought experiment here, even imagining there's no radiation problem. Feb 13, 2017 at 17:14
• Oberth effect calculations are beyond my capabilities I'm afraid.
– GdD
Feb 13, 2017 at 20:29