Can you achieve an elliptical orbit simply by positioning yourself right?

[SaturnVee]

This question also applies to real life, but is it possible to be close enough to a planet that it keeps you in an orbit upon entering its SOI without having to retroburn or aerobrake? Or will an intercept orbit always be parabolic or hyperbolic?

Edited August 29, 2013 by SaturnVee

[RoboRay]

You need a force to capture you into orbit. If the planet has a moon, the moon's gravity can pull you into orbit around the planet (under certain situations), but it's temporary... At some point in the future, you will re-encounter the moon and it will either collide with you, drop your orbit into an impact with the planet, or eject you right back out of the system.

[Specialist290]

Nope. Barring outside forces acting on it, a given object in an orbit has a constant amount of orbital energy. If it came into the SoI above the planet's escape velocity, it'll travel around a hyperbolic or parabolic curve and travel out of the SoI at the same velocity at the equivalent point on the other side of the orbit.

Now, in theory, if the planet has a moon that's large enough and you timed things just right, you could possibly slow down using a gravity assist from that moon to alter your trajectory in such a way that you ultimately end up around the planet, but I certainly wouldn't rely on hitting that by fluke.

EDIT: Ninja'd by RoboRay.

[Dispatcher]

To expand on the use of a massive moon (mentioned by RoboRay and Specialist290), you would need to travel against the direction of its orbit in order to slow your craft down. Otherwise you would speed up and miss your target.

Edit: Otherwise you MIGHT speed up and miss your target.

Edited August 29, 2013 by Dispatcher
Clarification

[Mr Shifty]

Sometimes, however, due to discontinuities at the SOI interface, you will find that objects do sometimes get captured in KSP when they cross the SOI border. This has happened for me particularly for communication satellites that I placed in orbit around Kerbin at Munar altitude, i.e. their Mun-relative velocity was very small. Occasionally, I'd discover that one was now orbiting Mun.

[leax256]

To expand on the use of a massive moon (mentioned by RoboRay and Specialist290), you would need to travel against the direction of its orbit in order to slow your craft down. Otherwise you would speed up and miss your target.

not quite

depends on your velosity, the moon's gravity, and the closeness of the approach

[SaturnVee]

Cool idea with the moon thing, would take a bit of math to predict that.

Thanks everyone!

[Dispatcher]

Thanks for the clarification and insight, leax256.

[maccollo]

Using gravity assists from a moon is my favorite way to get captured, because there is no uncertainty in predicting your orbit after the moon encounter.

With aerocapture errors of just a few km can be the difference between crashing into the planet and just continuing at escape velocity.

The moons you can use for this is Mun, Ike and Tylo. Now you want to get the encounter traveling in the same direction as the moon. This minimizes your relative velocity and thus maximizes the effectiveness of the gravity assist. Tylo has enough gravity to capture you and throw anywhere in the Jolian system, all the down to Laythe if necessary.

If you're actually going to the moon you're using to get captured this maneuver has another benefit. By doing an unpowered gravity capture first you can get into an orbit that is very similar to that moon. When you encounter it the second time your relative velocity will be lower, and you will safe a bit of deltaV in the capture burn. If you're going to Tylo you can save 200-300 m/s by doing this.

Edited August 29, 2013 by maccollo

[SRV Ron]

Gravity assist are an excellent way to slow down with minimal fuel usage.

Captures like this are even possible.