Is it possible to have Mun capture you in orbit around it?

[Frederf]

Gravity capture around Kerbin by transgressing the Mun's SOI is possible. Gravity capture around the Mun is not. It's tempting to think it is possible in reference frames where the Mun is in motion but consider the Mun's reference frame. The space craft heads toward the Mun hyperbolically and escapes because gravity is a conservative force. The laws of physics reach the same conclusion no matter which reference frame is chosen.

Of course if you introduce other types of forces or gravity from a complex system then all bets are off. The mechanics of capture have to include more than a 1-body gravity source for capture.

[aftershockzap]

If I am remembering correctly, the only way to get a true gravity capture in this game is from a stable mun orbit (any moon will do) flying to the moon's parent planet. The reason this works is because the moon itself is also within the parent planet's SOI, its just at close range the moon's influence overpowers the parent planet. Again this only works if you start from a stable moon orbit (I.E. your going pretty slow as is) and burn to the moon's Parent planet.

In the same respect, the sun works the same way. The second you leave a planet (Unless entering another planet's SOI) you are "captured" by the suns gravity, because all planets orbiting a star are also located within the star's SOI

[rkman]

If you enter a sphere of influence with some speed (it is impossible to enter with no speed), then you will reach that altitude again with the same speed, and exit.

Except in case of a slingshot, which causes you to exit at a speed greater than the speed at which you entered the SOI.

Is it not possible to do a 'reverse slingshot' that reduces speed rather than increase it?

[draeath]

I guess the real way to be sure is to test it...

[aftershockzap]

I'm a bit rusty on my lessons on forces and such but... We are talking about moon's with no atmosphere, so the only thing that can effect a ship is gravity. If you slingshot around a moon, you are using the gravity of the moon to make you go faster. You will keep going faster until one thing happens, you reach your lowest point in altitude. This is either by going around the planet, or crashing into it. Preforming a slingshot is attempting to ensure that you are losing altitude relative to the mun for a LONGER amount of time than you will be gaining altitude. I.E. if you get really close to the front of the moon from kerbin orbit, you will sail really close to the moon, fly past, and shoot out somewhere behind the moon's orbit. This works as a sling only because the moon is moving in an orbit as well, and because of your speed with the moon's speed you spend more time falling to the lowest altitude than "gaining altitude" in the moons SOI. (I.E. spend 10 min in mun's SOI falling to lowest altitude. After that point you spend only 5 min in the moon's SOI until exiting. This time difference is what gives you a bonus in velocity) The only force that changed the influence on your ship is mearly the speed at which the moon moved away from the vessel.

A reverse slingshot will only change your exit point and velocity, but not slow you down as you are still in the "Falling rising" trap. At worse you would gain no additional dV.

Again the only way to change your orbit is with something other than gravity. These are, ship thrust, atmosphere, or lithobreaking.

Imagine this: The moon needs as much time as possible to capture you in an orbit, SO you are on the EXACT orbit around kerbin as the moon is. Exact. You are going 1 m/s slower than the moon. The moment the moon catches you, you fall. You crash into the moon at a point, A. Gravity aided in your crash but it was only the moon itself that stopped you!

Now Jeb has dug a 10 meter by 10 meter hole on you crash site and you launch the same ship again. When you impact the moon now, you will impact 10 meters deeper than you did before! Because of the hole!

Now Jeb tunnels through the entire moon! You fall all the way to the center, and you begin your deceleration. Now its easy to think that you would be trapped by the moon because gravity has the most time to effect your ship right?

But we forgot about one thing, the moon is also moving! So RIGHT before your speed with the moon would = 0 m/s, you will find yourself stuck at the 1 m/s slower than the moon, as the speed of the moon was just enough to push you right outside of its SOI before you achieved an orbit.

I hope this was as clear and accurate as it was when it was in my head =/

[K^2]

I'm pretty sure, if a gravity sling shot is possible a gravity capture is possible also. These things work because the capturing body is moving.

No. It doesn't work like that. Only relative velocities are relevant. Once you enter SOI, your velocity relative to Mun becomes your orbital velocity in Mun's SOI. And entering SOI with basically any velocity puts you on a hyperbolic trajectory that will return you to the edge of SOI going at the same speed and you are going to escape.

Now, when you escape, you'll be traveling in different direction, so these speeds will add differently. Your orbital speed relative to Kerbin will change. For this reason, it's possible to get captured by Kerbin or any other planet with moons. But it's not possible to get captured by the Mun because there is nothing to adjust your velocity while in Mun's SOI.

[Frederf]

Except in case of a slingshot, which causes you to exit at a speed greater than the speed at which you entered the SOI.

Is it not possible to do a 'reverse slingshot' that reduces speed rather than increase it?

Ah, the classic misconception. You never leave the SOI with a greater/less speed than you entered it when the speed is measured relative to the body of interest which is the measurement that matters when considering capture.

[Spartwo]

Just happened to me I had an orbit within the mun's path which was circular.