When you say stable you mean it's not going to crash? Then yes (Be in the orbit for several times without crashing) or if you mean it's not perfectly circular or it's more of a line then no. - - - Updated - - - To all who were confused with the wording what i meant is it can orbit jool, sun, mum, kerbin and so on. Basicly anything that is in the same solar system so that everyones ship is in the same solar system - sorry for the confusion and even more so if i've only made your understanding worse. i probably should have worded it differently. As long as.... never mind Simply READ THIS BIT AND FORGET THE REST IF YOU DIDN'T UNDERSTAND! I can be in a orbit in any SOI you can use any natural satellite for gravity assisted sling shots (or what ever you like to call it) as long as it doesn't escape the sun's SOI. Reason for this rule is mods.