# What's with all of the gold foil on the Lunar Lander? **URL:** \ **Category:** Factual Questions\ **Created:** [April 16, 2016, 3:45am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335 "2016-04-16T03:45:36Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![chacoguy](https://avatars.discourse-cdn.com/v4/letter/c/50afbb/32.png) [@chacoguy](https://boards.straightdope.com/u/chacoguy)\ **Post date:** [April 16, 2016, 3:45am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/1 "2016-04-16T03:45:36Z")
[Take a look at this picture.](https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTsC-TtqfZFUHFqfQkDPou8n6AS6BO_s0GxA758dtuFf6QjS9gU) There’s gold even on the pads and legs; less so further up. What is the purpose of that? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [April 16, 2016, 3:58am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/2 "2016-04-16T03:58:10Z")
Gold is a pretty good radiation and heat shield, and can be hammered into very thin foil, so it’s light. ---
**Author:** ![chacoguy](https://avatars.discourse-cdn.com/v4/letter/c/50afbb/32.png) [@chacoguy](https://boards.straightdope.com/u/chacoguy)\ **Post date:** [April 16, 2016, 4:08am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/3 "2016-04-16T04:08:53Z")
Fair enough, still, the part where the people live is _not_ gold plated. The legs and the pads are; I’m trying to figure why the part that stayed on the Moon needed to be shielded, whilst the part that had to go back into Lunar Orbit was left unprotected. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [April 16, 2016, 4:13am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/4 "2016-04-16T04:13:52Z")
Use the gold to barter with in case the Moon was inhabited. No room for a box of paddleball games in the Lunar Lander. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [April 16, 2016, 4:50am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/5 "2016-04-16T04:50:37Z")
The foil is actually not gold but metallized Mylar sheeting, chosen because it’s lightweight, not very conductive of heat, and has a low thermal mass. It serves to reject solar radiation from the sun, to reduce the rate at which the structure heats up or cools off, and also to reduce the rate at which the structure cools off when it’s in the shadow of the moon or earth and out of the sunlight. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [April 16, 2016, 6:33am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/6 "2016-04-16T06:33:21Z")
After some googling, it appears to be something called [Multi-Layer Insulation (MLI)](https://en.wikipedia.org/wiki/Multi-layer_insulation). A Lunar Module-specific quote [here](https://www.flickr.com/photos/jurvetson/6861117148): > [@](#): > > A low-cost technique was needed to insulate and protect the LEM’s structural materials, including the landing feet. The team decided to use 12 to 18 layers of Kapton or aluminized Mylar material sandwiched together in a 70°F earth clean room and trap the air with a special sealing tape. This trapped air remained permanently at 70°F and was used in many areas of the LEM, including the cupped landing feet. The ‘foil’ around much of the LEM was made with 2- and 5-mil aluminized Kapton film.” ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 16, 2016, 11:29am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/7 "2016-04-16T11:29:54Z")
The above quote is almost hilariously bad. It is correct in that the insulation is indeed Mylar and Kapton. But the bit about trapping air at a permanant temperature is so nuts it boggles the imagination. It is a case of where does one start with the mistakes and misaprehansions. The LEM was origionally designed with more solid heat sheilds, but when it was clealr overwight some significant attention was paid to reducing mass. To quote from [NASA’s Chariots for Apollo](http://www.hq.nasa.gov/office/pao/History/SP-4205/ch7-3.html) > [@](#): > > One of the more striking changes to come from this drive for a lighter spacecraft was the substitution of aluminum-mylar foil thermal blankets for rigid heatshields. The gold wrapping characteristic of the lander’s exterior saved 50 kilograms ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [April 16, 2016, 12:39pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/8 "2016-04-16T12:39:25Z")
> [@chacoguy](#): > > Fair enough, still, the part where the people live is _not_ gold plated. The legs and the pads are; I’m trying to figure why the part that stayed on the Moon needed to be shielded, whilst the part that had to go back into Lunar Orbit was left unprotected. That part was also shielded with similar foil insulation, you just don’t see it because of the panels covering it. See LM during assembly: [http://www.hq.nasa.gov/office/pao/History/alsj/misc/apmisc-LM-noID-18.jpg](http://www.hq.nasa.gov/office/pao/History/alsj/misc/apmisc-LM-noID-18.jpg) ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [April 16, 2016, 1:05pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/9 "2016-04-16T13:05:40Z")
But why insulate the legs? I can’t think of any reason why they would need to be protected from the sun. Or is it just a matter of insulating all the structural-but-not-heat-sensitive things that could conduct heat to more sensitive equipment? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 16, 2016, 1:42pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/10 "2016-04-16T13:42:47Z")
> [@lazybratsche](#): > > But why insulate the legs? I can’t think of any reason why they would need to be protected from the sun. > > Or is it just a matter of insulating all the structural-but-not-heat-sensitive things that could conduct heat to more sensitive equipment? Probably to ensure there was no more thermal stress on anything than could be avoided. The lander still spent a couple of days lazily barbecuing itself between full unfiltered sun and pure dark space as the LEM/CSM rolled through space. The aggregate stress on anything could cause cracks. They had enough trouble finding and stopping newly assembled components of the LEM from spontaneously cracking, let alone giving them a few thousand heat/cold cycles. Having a leg develop a fracture might have been somewhat unfortunate. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 16, 2016, 4:39pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/11 "2016-04-16T16:39:26Z")
Althou protecting the legs from thermal cycling may have been a consideration, the primary reason for covering virtually all of the external structure of the descent stage with MLI is to assure that there isn’t a thermal path. As you are aware, metal is a good conductor of heat, so any exposed part would receive thermal radiation and the conduct the heat to other parts of the structure. The reason it only covers the descent stage is because the most extreme heating isn’t solar radiation but the plume from the engine during descent, which is underexpanded in vacuum due to limits on the envelope length of the nozzle and will turnback and heat the nearby structure that is within view. The ascent stage is far enough away that it doesn’t require MLI, and of course it has a thermal regulation system for the habitat. The ascent engine is much smaller and fires for a shorter duration, so it doesn’t require as much protection on the ascent stage. And slight nitpick, while early studies of the lunar lander referred to is as the “Lunar Excursion Module” or “Lunar Exploration Module”, the craft built and used for the Apollo program was just the “Lunar Module” or LM, although it was colloquially referred to as “the lem”. Stranger ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 17, 2016, 2:22am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/12 "2016-04-17T02:22:16Z")
> [@Stranger\_On\_A\_Train](#): > > And slight nitpick, while early studies of the lunar lander referred to is as the “Lunar Excursion Module” or “Lunar Exploration Module”, the craft built and used for the Apollo program was just the “Lunar Module” or LM, although it was colloquially referred to as “the lem”. Yeah, I can never remember which came first, and hence which to use. I grew up with it called the LEM in everything I read, and in my head it is still always “the lem”. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [April 17, 2016, 4:47am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/13 "2016-04-17T04:47:26Z")
The LM could only be used when the sun was between 5 and 15 degrees in the sky, basically either early morning or late evening. Why was that. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 17, 2016, 6:09am UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/14 "2016-04-17T06:09:10Z")
I’m not sure where you get 5 and 15 degrees, but it is true that the LM was designed for a specific heating profile, with thr cabin of the ascent module oriented so as to prevent it from absorbing too much solar radiation. The LM would not have been able to operate in complete dark, as there was insufficient power to heat the habitat section, and ascent engine propellant and batteries would have frozen and stopped functioning. Space enthusiasts like to talk about how we got to the Moon six times as if it became an almost trivial effort, but in fact we were barely able to explore very small areas with some significant limitations on where and how the astronauts could operate. Only Apollo 17 carried an actual geologist and did “real science” (i.e. making scientific observations rather than just following the mission script) and even that was very limited. To build a continuously occupied habitat on the Moon today would require advances in many areas, including power generation/storage, recycling technology about an order of magnitude more efficient that what we currently have on the ISS, and mitigation of the noxious and potentially hazardous electrostatic dust that covers the regolith and sticks to everything like hot glue. Stranger ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [April 17, 2016, 12:19pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/15 "2016-04-17T12:19:47Z")
> [@Stranger\_On\_A\_Train](#): > > I’m not sure where you get 5 and 15 degrees… The number I’ve seen is sun angle of 15 to 45 degrees above the horizon. There were multiple factors including pilot visibility during landing, and visibility for the lunar explorers on the surface. The moon has no atmosphere to scatter light and therefore shadows are completely black; at either low or high sun angles, visual observations can be difficult. While there were preferred optimal mission parameters for the LM, it generally had considerable margin in most areas. E.g, the stated maximum inclination on landing was 12 degrees from local vertical. This was supposedly to ensure the ascent stage could clear the descent stage and correct the trajectory before striking the lunar surface. However further study has shown the LM ascent stage could probably have landed and taken off successfully at a 43 degree tilt angle. The original NASA specs indicate if it landed with one foot pad caught inside a crater rim with the vehicle hanging down the outside tilted at 70 degrees from vertical, the ascent stage could still successfully lift off. ---
**Author:** ![aldiboronti](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@aldiboronti](https://boards.straightdope.com/u/aldiboronti)\ **Post date:** [April 17, 2016, 2:25pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/16 "2016-04-17T14:25:12Z")
> [@Francis\_Vaughan](#): > > The above quote is almost hilariously bad. It is correct in that the insulation is indeed Mylar and Kapton. But the bit about trapping air at a permanant temperature is so nuts it boggles the imagination. It is a case of where does one start with the mistakes and misaprehansions. > > The LEM was origionally designed with more solid heat sheilds, but when it was clealr overwight some significant attention was paid to reducing mass. To quote from [NASA’s Chariots for Apollo](http://www.hq.nasa.gov/office/pao/History/SP-4205/ch7-3.html) The quote apparently is from an EDN Network interview with Grumman’s Ross Bracco, one of 25 engineers who began development of the LEM, as it was first called. The thing that I find mind-boggling is how the hell the engineer who helped design the thing could get the science so wrong. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 17, 2016, 2:33pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/17 "2016-04-17T14:33:25Z")
That description reminds me of a plumber once at my apartment who knew that ACs have a fluid that cycles through it, and it stays cold from the factory, and after a few years has to be replaced because it finally warms up. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [April 17, 2016, 3:23pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/18 "2016-04-17T15:23:13Z")
> [@Leo\_Bloom](#): > > That description reminds me of a plumber once at my apartment who knew that ACs have a fluid that cycles through it, and it stays cold from the factory, and after a few years has to be replaced because it finally warms up. When it finally warms up it’s called freoff. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 17, 2016, 3:31pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/19 "2016-04-17T15:31:08Z")
> [@aldiboronti](#): > > The quote apparently is from an EDN Network interview with Grumman’s Ross Bracco, one of 25 engineers who began development of the LEM, as it was first called. The thing that I find mind-boggling is how the hell the engineer who helped design the thing could get the science so wrong. I could see a lot of ways the statement could have come out wrong in an interview. The context may be missing, or the statement never got completed, or he just said the wrong thing without realizing it or getting a chance to correct it. Or he really didn’t know and he was just reciting something he or someone else got wrong before. ---
**Author:** ![Sasquatch\_1](https://avatars.discourse-cdn.com/v4/letter/s/8491ac/32.png) [@Sasquatch\_1](https://boards.straightdope.com/u/Sasquatch_1)\ **Post date:** [April 17, 2016, 4:21pm UTC](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335/20 "2016-04-17T16:21:20Z")
> [@TriPolar](#): > > I could see a lot of ways the statement could have come out wrong in an interview. The context may be missing, or the statement never got completed, or he just said the wrong thing without realizing it or getting a chance to correct it. Or he really didn’t know and he was just reciting something he or someone else got wrong before. I could make a charitable assumption he meant that with the conductivity/reflectivity of the multiple layers, the air inside the innermost layer stayed pretty close to 70 degrees on average despite the thermal cycling of the environment. [Next page](https://boards.straightdope.com/t/whats-with-all-of-the-gold-foil-on-the-lunar-lander/752335.md?page=2)