I was wondering...

[Geo793]

How low can a spacecraft possibly orbit the Mun without smashing into it?

[CCKinnison]

Wow. While I know a few references, the first thing that popped to my mind isn't the math for the mun's radius and topography, or an "it depends where you pass Periapsis", etc. No, no.... it was Tootsie Roll Tootsie pops, Kerbal-style.

"Let's find out. *zoom* 3000M... *zoom* 2500 M... *zoom* 2000M... *BANG* Hmm. 2000 M."

(note: that's not an actual answer... it really would depend on where you did your close approach, and what your speed was - this is more of a Challenge-type question "How close can YOU get?")

[Sir Nahme]

Theoretically you could orbit at sea level, you just won't last long due to hitting the first hill that comes along, it's highest elevation is listed on the wiki as 3340 so 3341 would be the theoretical minimum orbit height as long as you don't have parts projecting too much below that

[Dizzle]

However, too low and your orbit will decay at a noticeable speed, so I don't think just being above max elevation will survive a full orbit.

[Sir Nahme]

However, too low and your orbit will decay at a noticeable speed, so I don't think just being above max elevation will survive a full orbit.

Not at all, there is no atmosphere to cause drag and the mun's moon is negligable a 4km orbit will work fine and a 8km orbit would be effectively the same as a 70km kerbin orbit

[Flixxbeatz]

How low can a spacecraft possibly orbit the Mun without smashing into it?

I'll put the limit at 3500. Go lower than that and you'll find out your spacecraft had suddenly disappeared on the map view, only to find out that it has crashed already on the surface.

[FITorion]

However, too low and your orbit will decay at a noticeable speed, so I don't think just being above max elevation will survive a full orbit.

umm... no... it won't. no atmosphere to worry about means no drag and no orbit degradation.

[CCKinnison]

No, no, no. That's the HIGHEST elevation on the Mun (3340) It's like saying you can't pass through Earth's atmosphere under the altitude of Everest's peak, because you'd hit it.

I'm quite certain you can make a lower pass than 3340. The question is, HOW much lower? "...do you feel lucky, punk?"

This is why I said it'd make a good Kerbal Challenge.

[Dizzle]

What game are you playing? When I orbit at less than 5km, I lose about 10 meters per second from my periapsis.

[Sir Nahme]

The problem is the only way to get a reliable orbit is equatorial, any variance means more terrain variance over time. So you'd have to determine the highest point around the equator

[Schmoogleedorf]

Do note that Mun's topography is different depending where you are. Mun's lowest point, or sea level, is at the bottom of the lowest crater; whereas Mun's highest is in the hilly regions at the south pole. So if you are going for a flat equatorial orbit, you can be safe anywhere up to about 2500, but even that is pushing it. For max safety a 4km orbit is perfect.

[stupid_chris]

What game are you playing? When I orbit at less than 5km, I lose about 10 meters per second from my periapsis.

I think you are having a problem There's no atmosphere and KSP doesn't simulate gravitational drag in any way, so you might want to double check your things, you might have a bug because of a mod or something.

[NeoMorph]

Use ISA Mapsat to plan your low orbit. It helps because it shows the peaks to avoid.

I think you are having a problem There's no atmosphere and KSP doesn't simulate gravitational drag in any way, so you might want to double check your things, you might have a bug because of a mod or something.

Sounds like he is experiencing clipping and it's generating phantom forces. I once left a ship in circular orbit around Minmus and went to get dinner... came back to find I now had an eliptical orbit that was impossible. I first thought I had left the throttle open slightly (done that before now) but nope... throttle down, engines deactivated even... just was getting slightly larger each orbit. That was down to a ladder clipping a docking hatch.

Edited June 24, 2013 by NeoMorph

[Spatzimaus]

Sounds like he is experiencing clipping and it's generating phantom forces.

Most likely this. There's no drag, but the game doesn't always zero things out correctly. Clipping's the most common culprit, but you can also get it happening if your RCS jets were firing when you went into time acceleration.

As to the original question, it's like this. If you do a perfectly equatorial orbit, then obviously your radius will need to be higher than the highest point on the equator; this should be easy enough to visualize. But any non-equatorial orbit will eventually pass over every point on the surface between its minimum and maximum latitude, as the orbit precesses around the surface. So, just because your orbit doesn't hit that 3000m mountain on THIS pass doesn't really mean much, because eventually it'll run into it unless it's too far north/south for your orbit. In theory there'd be altitudes where you could have some sort of synchronous orbit (passing over only a few discrete lines), but in practice that won't happen because of simple rounding errors.

So, the answer given earlier is correct: whatever the highest point is, add a little to that, and that's what your periapsis would need to be.

[Kanga]

You should be able to orbit below the highest point, by making sure that the highest point is not in your orbital path. I think it was mentioned above that the highest peak is on the south pole or something, so if you have an equatorial orbit then your orbit can be lower. Unless of course the Mun librated a lot, which it doesn't in the game (but does in real life).

[Flixxbeatz]

No, no, no. That's the HIGHEST elevation on the Mun (3340) It's like saying you can't pass through Earth's atmosphere under the altitude of Everest's peak, because you'd hit it.

I'm quite certain you can make a lower pass than 3340. The question is, HOW much lower? "...do you feel lucky, punk?"

This is why I said it'd make a good Kerbal Challenge.

Done, and done right.

You can't timewarp, at that altitude, unfortunately

[bonyetty]

The problem is the only way to get a reliable orbit is equatorial, any variance means more terrain variance over time. So you'd have to determine the highest point around the equator

Sir Nahme in my humble opinion has proposed the most reasoned answer to the question so far. Not hard to test. I'm gona go do some Mun equatorial testing now. Will do one orbit at 3500m then drop one meter each orbit till smash. It will probably take a while as I will not use time warp and require me to save a few times too. I'll report back after the SMASH.

[maackey]

1m at a time is a terribly inefficient way to go about it. Around the equator the highest hill is just over 2km. 2500 is a good orbit to keep reasonably safe from the Mun. You just have to be careful not to accidentally get too below that at the wrong time while burning to circularize or lower periapsis. With MechJeb you can see your true altitude, so you can get into a low orbit and then watch the altitude until it reaches a minimum point and then subtract the value from your current orbit. Unless your time means absolutely nothing I'd suggest at least trying it at 5km so you can take advantage of timewarp.