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

[Gildon]

I've been wondering if it is possible to launch yourself on a trajectory to Mun that is just perfect enough for Mun's gravity to pull you into an orbit around it. Basically, this would eliminate the need for a large burn later to slow yourself down into an orbit.

[th3shameless]

no, this would work with an atmosphere but since the mun has none you will need rockets. lithobraking would would but that tends to lead to an unexpected rapid disassembly of your craft

[Cranium]

It's definitely possible, however unlikely. You would already have to be going the exact speed needed to orbit at your desired altitude. Some very precise calculations would be needed.

[Cashen]

It's not possible I don't think. You'd have to have a relative velocity of exactly zero when you cross the sphere of influence, so that when you loop back around in your orbit and return to the edge of the sphere of influence your velocity is again exactly zero. Any positive relative velocity to the Mun as you cross into its sphere of influence will mean you will always have a velocity above the escape velocity. The only objects that can capture you without using any propellant are bodies with atmospheres for aero-capture.

[Mr Shifty]

It's not possible because of conservation of energy. If you enter a planet or moon's SOI from outside it, that means you have enough energy to leave the SOI. You have to do a burn to get captured.

[SRV Ron]

If you hit the sphere of influence retrograde at the right speed and distance, it is possible. Or, at least, the orbit will be complete with a small retrograde burn. I almost got such a capture on Laythe last night. The Add Maneuver planner actually registered such a capture when entering the Jool system and using one of the outer moons to reduce speed on a slingshot. It didn't quite capture, the sphere of influence almost had a complete orbit. A very small burn using thrusters completed the capture to orbit.

Otherwise, aerobraking is the way to go. Very tricky to get it right.

[Frederf]

Imagine the gravity well of the Mun is a curved dish and your space craft a marble. If you introduce the marble along the edge it will roll to the bottom and come out the other side unless something slows it. It'll even come out with the identical speed as it was introduced (with respect to the Mun). It is theoretically possible with N-body gravitation because the dish isn't symmetrical and might change over time but not in KSP's 1-body gravitation.

[Evrion]

No.

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.

aerobraking, retrograde burn, lithobraking, or something else to slow you down must happen or you WILL exit again.

As SRV Ron mentioned, you can get very close, if you relative velocity when you enter is very low, then you might be able to get a stable orbit with a quick RCS thrust. But you can't get captured without something slowing you down.

[IllicitMedic]

The burn required to enter orbit around the Mun isn't that big anyway...we're talking like 200-250 m/s of Delta V.

[Coneshot]

YES you can!

Here is an experiment you can try at home:

Put yourself in a circular orbit 11,300,000 meters up (I didn't even bother matching planes, it still works):

Then timewarp ahead until you catch up to the Mun:

I'm not as bright as a lot of guys on here, but I figure you lose enough momentum being pulled up from a lower orbit to a higher one to slip into capture around the Mun.

[Cashen]

I got to thinking, and it might actually be possible in very specific circumstances. Remember, a positive gravity assist will eject you from the sphere of influence with a higher velocity than you came in with, because momentum is transferred between you and the object you are assisting with. KSP models gravity assists, or at least positive ones where you go around retrograde side (trailing edge) of the body in a counterclockwise orbit. Negative gravity assists exist as well, where you swing around in front of the leading edge of the body in a clockwise orbit, which gravity assists to reduce your velocity. In theory, then, if you entered the sphere of influence with a very very low relative velocity and on a trajectory to receive a negative gravity assist, the loss of momentum may allow a captured (but highly elliptical) orbit.

[Tw1]

It's to do with your relative velocity compared to it. Typically, we meet it too fast. But, as Coneshot has shown, by getting there will less relative velocity, you encounter the Mun with less than the velocity needed to escape it. Though I don't know if it saves you any delta V, unless you manage to get into that orbit via a gravity assist.

[PakledHostage]

But, as Coneshot has shown, by getting there will less relative velocity, you encounter the Mun with less than the velocity needed to escape it.

I wonder if Coneshot's result is an artifact of crossing SOI's at high warp? I wonder if the result would be different if the Mun's SOI were entered at a lower warp?

[Coneshot]

I wonder if Coneshot's result is an artifact of crossing SOI's at high warp? I wonder if the result would be different if the Mun's SOI were entered at a lower warp?

This is likely a large contributing factor, at high warp it's almost a sure thing.. it does occur if you stop warp just prior to capture but it is less reliable and the altitude is less forgiving (I actually played with this a lot in an unrelated project)

[K^2]

Yeah, save for glitches with warp, it's a violation of KSP physics to be gravity-captured by another object. You have only two options. Either you enter and leave SOI at the same speed, or you physically encounter the object or its atmosphere, which will certainly end in you crashing on that object sooner or later.

It might be possible to lithobrake into the tallest point on the body and bounce just right to establish an extremely low orbit, but even that should eventually lead you into crashing into the same point. Though, it's possible to match periods in a way that it'd take almost forever to happen. That is the only way to end up "captured" into an effectively stable orbit without exploiting any glitches in physics.

[Hannu]

In theory, KSP's physics model make it impossible to achieve capture without burn or other way to change velocity. Result above must be due to inaccurate calculations. There are always an error when you enter SOI with large warp factor, because time step is too large. In real life capture is possible if there are suitable three body interacion. For example if a binary asteroid fly by planet, one part can be captured and other takes that energy and get more velocity. But KSP's model does not take three body interacions into account.

Actually burning is many body interaction too. You divide your craft to parts (ship body and exhaust gases) and move momentum (and energy) from ship to exhaust gases. Gas molecules fly out of planet's SOI (at least if we ignore solar wind) and ship leaves in.

[magnemoe]

In theory, KSP's physics model make it impossible to achieve capture without burn or other way to change velocity. Result above must be due to inaccurate calculations. There are always an error when you enter SOI with large warp factor, because time step is too large. In real life capture is possible if there are suitable three body interacion. For example if a binary asteroid fly by planet, one part can be captured and other takes that energy and get more velocity. But KSP's model does not take three body interacions into account.

Actually burning is many body interaction too. You divide your craft to parts (ship body and exhaust gases) and move momentum (and energy) from ship to exhaust gases. Gas molecules fly out of planet's SOI (at least if we ignore solar wind) and ship leaves in.

Yes, but you can come very close, I had an strange encounter with Tylo, started out in orbit between Val and Tylo and did an burn for Tylo, the result looked like an capture and then entering Tylo SOI, I had an almost circular orbit with and enter and exit SOI marker next to each other.

[Juffin]

This is mathematically impossible, because your orbit can be either elliptic or hyperbolic.

If you're flying to the Mun from Kerbin then your Mun orbit is hyperbolic.

However, it might be possible with Kerbal physics.

[lammatt]

simple answer: no.

learn your high school physics...and you will understand why

well.... anyways...the fact is...

you will be approaching at a high relative speed w.r.t the Mun

and that speed is greater than the escape speed.

the Mun cant provide any mean to kill the excess speed unlike the Earth which has an atmosphere to dissipate the KE as heat.

[PakledHostage]

simple answer: no.

learn your high school physics...and you will understand why.

Fun fact: Reading comprehension and social graces are also important skills to have!

[draeath]

Couldn't you do a gravity-assist to slow yourself down? Assuming you enter SOI just over capture velocity, shouldn't this work?

[asmi]

Couldn't you do a gravity-assist to slow yourself down? Assuming you enter SOI just over capture velocity, shouldn't this work?

No, gravity assist changes your speed relative to other "outer" body, not itself.

[hellion13]

Im pretty sure IRL its not possible but this actually happened to me every once and a while, before i figured out the maneuver nodes, id be burning all nilly willy barely in control of the craft and wouldnt actually catch an encounter untill i was at apoapsis and it would give me a highly elliptical orbit around the moon(pretty sure i was just glitching the physics), ALTHOUGH it actually took more fuel to get into a position to actually do any good, than a standard Mun mission using the nodes

edit: if i bothered to take screenshots while i played id post Pics but i think in the entire time ive been playing since .16 ive taken 4 screen caps LOL

Edited July 23, 2013 by hellion13

[kahlzun]

This is a discussion that's been bandied around previously. Essentially, it's only possible because KSP isn't 100% accurate with its orbits (we've all seen the orbit line jittering around). Here's an example of an unaided Kerbin capture from IP space that I did a while ago:

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[rosenkranz]

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.

If you can burn so that your encounter is along the leading edge of the SOI such that the SOI overtakes you could enter the SOI with less than escape velocity.

In real life space is not a friction-less void and the scale of things is much bigger. Gravity captures happen all the time in our solar system.

In KSP however, the scale makes this less likely and the fact that KSP 'space' is a true friction-less void may make this situation all but impossible.