Extreme areobrakng

[Jerjare]

Whats the lowest altitude you can dip in planets with an atmosphere, and still maintain orbit?

The object is to areobrake on either Kerbin, Duna, Eve, or Jool and drop as low as possible while still able to keep orbital velocity.

[SkyRender]

Your challenge needs a lot of work. First off, incoming speed is a huge factor in aerobraking: a periapsis of 11,000m on Duna will send most ships plowing into the surface if you transfer straight from Kerbin, but if you're coming in from Eeloo at its furthest point, you'll end up in a highly elliptical orbit. Also, the mass and drag of the craft have an enormous impact as well, not to mention what control surfaces do to aerodynamics right now (which is, generally speaking, break them). See the challenge submission rules and try to re-design this into something that's up to snuff.

[Captain Sierra]

As previously mentioned, there is a lot that can go into this. In a normal Kerbin Duna transfer, you can get down to about 12.5km and still put yourself in low orbit.