Just a quick orbital mechanics question for the gurus (just a technical question, I don't believe there's any funny business going on):

I understand the basic concept of the descent from parking orbit to lunar surface, with a short impulse-type burn at descent orbit insertion (DOI) followed by a powered descent starting 1/2 orbit later. I also understand they would have planned the descent with contingencies for aborting and immediately redocking with the CSM in case something went wrong. I'd like it if someone could explain why, after undocking and separation, the LMs orbit allowed pictures like AS11-37-5545 and AS11-37-5447 (pictures of the CSM with the lunar surface in the background).

I guess the short version is: did the LM actually climb to a slightly higher orbit after separation, or is this an optical illusion of some kind (ie the CSM is actually ahead of or behind the LM, it just appears to be directly below it), and if the LM ascended, why was this necessary (or better) than, for example, descending. Any thoughts are appreciated.

What you are seeing is the LM and CSM staying in the same position in relation to each other but both orbiting the moon and a package...uh sort of. They are both in their own orbits but they stay at the same position relative to each other. It's easier to see it for yourself by playing around with Orbiter.

I'll have to let somebody else, that can make more sense, explain it.

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Yeah their orbits are nearly identical because the change in velocity caused by undocking was very small, that's why they are nearly stationary relative to one another, but what I'm asking is which direction is the LM relative to the CSM, is it ahead, behind, above, below? Apparently it's at least slightly above (relative to the moon) the CSM, and although maybe this choice is just arbitrary and inconsequential, I was wondering if there was a reason behind it.

I know that after the DOI burn the LM descended and therefore raced ahead of the CSM in its parking orbit by a few minutes, perhaps the LM climbed above and slightly behind the CSM after undocking to keep them closer together for communication purposes and to make for an easier redocking in case something went wrong, I don't know. I'm just speculating that there's a reason and looking for confirmation on that. Maybe it's just arbitrary, but surely someone, somewhere must know, right?

Wow, sorry, it looks like I didn't even read your post before I answered. I did but apparantly didn't. Oh well...

I assume they thrust out a ways and check the legs and inspect the ship prior to slowing down once they get to the right side of the moon. Looks like it's going to be 1/4th of an orbit and the LM will be behind the CSM then they can pitch and slow down for decent. At least that's what they did in From Earth to the Moon.

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My guess is going to have to be lighting conditions for the inspection of the LRV. In the pictures, the CSM would be easier to view, but if the inspection was done at a different part of the orbit, it may have been easier to see the craft "on top".

From the Apollo 11 Press Kit (9 Mb pdf), page 33:

Lunar Module Descent, Lunar Landing

The lunar module will be manned and checked out for undocking and subsequent landing on the lunar surface at Apollo site 2. Undocking will take place at 100:09:50 GET prior to the MSFN acquisition of signal. A radially downward service module RCS burn of 2.5 fps will place the CSM on an equiperiod orbit with a maximum separation of 2.2 nm one half revolution after the separation maneuver. At this point, on lunar farside, the descent orbit insertion burn (DOI) will be made with the lunar module descent engine firing retrograde 74.2 fps at 101:38:48 GET. The burn will start at 10 per cent throttle for 15 seconds and the remainder at 40 per cent throttle.

The DOI maneuver lowers LM pericynthion to 50,000 feet at a point about 14 degrees uprange of landing site 2.

Unfortunately, it doesn't say why this is done. There are a few illustrations in the press kit which might help.

There is a nice photo taken from Eagle of Columbia almost over the landing site, AS11-37-5447.

Caption from the ALSJ: CSM separation sequence. North is to the right. The Apollo 11 landing site is situated NNW of the CSM, SSW of the Cat's Paw landmark, at the center of the photo. The sharp-rimmed crater at the lower left edge is Moltke. Hypatia Rille extends through the top left corner of the image. The crater directly to the right of the CSM has been named Collins following the Apollo 11 mission. The crater Aldrin is in the top right corner of this image. The third crater named in honor of the Apollo 11 crew, Armstrong, would be to the lower right of this photo and cannot be seen here. During the Apollo 11 mission, the craters Armstrong, Aldrin and Collins were still referred to as Sabine E, D and B respectively, with Sabine C being the crater that forms a triangle with B and D in this photo.

Then there's also the lovely one taken during Apollo 17, AS17-147-22465, above the landing site. Note the tiny CSM above Bear Mountain and heading towards South Massif, 7 o'clock from the centre fiducial.

Wow, I just learned something. I have seen that picture before and never really knew what it was. I thought it was the LM, sitting on the surface or something. But this thread made me double check, and it turns out thats the CSM still in orbit as viewed from the LM. It's just an optical illusion that the object is on the surface. Thanks, Kiwi.

Good idea =D> I second that recommendation! After you've read about Apollo for a while, go fly it yourself.

Did you consider the scale of that view of the Taurus-Littrow valley -- the heights of the mountains and sizes of the craters? Each of the craters in the trio at right on the valley floor is about half a mile in diameter. There's no way, from that height, that the object could possibly be an Apollo craft on the surface. Besides, the very next photo shows the CSM superimposed against the top of South Massif.

Here's the Apollo 17 lunar module on the surface as viewed from orbit:
http://www.boulder.swri.edu/~durda/A...ls_17_5aa.html

Use the buttons at the bottom of the page to see a sequence that starts with a view from earth then moves in to the LM. It's worth saving all the pictures for all the landing sites -- they help us understand why it's so hard to view the sites with the Hubble or from other than a low lunar orbit. Remember that even with a viewing aid, Collins couldn't see Eagle on the surface from where he was.

Main page for all the landing sites:
http://www.boulder.swri.edu/~durda/A...ing_sites.html

Isn't a seperation of 2.2 nanometres cutting things just a wee bit close!!!!!

Maurice

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nm = nautical miles as well as nanometers. I always loved the old "Battlestar Galactica" where the base unit of distance was the "micron" -- as in, "Enemy closing: 20 microns." (Apparently closing very slowly.)

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Clavius Moon Base

Yeah well they also used Computrons in BSG.

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Eagle Eye Observatory

I always loved the old "Battlestar Galactica" where the base unit of distance was the "micron"
At the risk of exposing myself as an unrecalcitrant geek, the "micron" was a unit of time. A subdivision of "Centon" roughly equivalent to a minute.

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Kiwi, I knew something about that photo didn't look right. The object is far too large to be on the surface and rather blocky to be the LM. I just never asked the right clarifying questions.