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#226 2020-11-08 18:29:52

SpaceNut
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From: New Hampshire
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Re: Colonizing / terraforming small asteroids

So where is it now
outbound-cruise.png

3-computingpat.jpg

It gets pulled and pushed by not only Earth but Venus and Mars as it goes around the sun along its orbital path.

The YORP Effect and Bennu

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#227 2020-11-16 19:14:53

Void
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Re: Colonizing / terraforming small asteroids

------
I feel compelled to make some commets on the nature of asteroids, information that I have understood to be true.
Many have water and hydroxyl bound to minerals.  Typically these are the ones that also have Carbon in quantity.  The numbers are about 10% to 15% of mass, and rarely even 20% of mass.
It is true, that most of those are in the outer asteroid belt.  But a few NEO asteroids, including Bennu should have some reasonable recoverable quantiy of Hydrated substances.  This is not ice.  It is Hydrogen related materials bound chemically to regolith.
The Trojan asteroids are not well understood, but it seems  a likely guess, that as a rule they are rather more hydrated.
Vesta which was investigated by the Dawn mission, along with Ceres, is quite an interesting subject.  It seems that it has hydrated minerals about it's equator.  It may also be that 16 Psyche, also has some such deposits.  In each case this could bode well for human activities on those tiny worlds.  Most likely things like Bennu crashed into them at slow enough speeds that the hydration did not get baked out of the materials.
It offers just a hope that on our Moon, their may have deposited rubble piles of materials that did not give up their Carbon or maybe even all of their hydrated materials.  But that needs proof and discovery.  The chances are just a bit better for an impact in the night cold, and maybe at a high latitude I would think.
I would love to see a human presence for Vesta, and 16 Psyche.  The mineral wealth may be unbelievable.  It may also be that these worlds are small enough that synthetic gravity machines could be put inside of them.  How far down could you drill into Vesta?  To the core?  If so, then unbelievable amounts of metals most likely available.
How many cities inside such a world?  Well space and energy and matter availbility would determine that.  Presuming humans can remain human or even a bit more than that.
There are possibly a few asteroids of the main belt that do have ice.
If we go to the Moons of Mars, there are various theories of how they came to be.  I think they are composits.  Some part Mars materials, some part asteroid materials, and perhaps even some part of their origination was from the formation of Mars, where it could have had an original moon(s) formed along with it.   Some say that how would they have equitorial circular orbits?  I think that they respond to tidal forces, and so are drawn to that.  They appear to be rubble piles, at least in part.  So, they would have tides.  The Martian world is binary as most of what is south of the equator is higher and most of what is north of the equator is lower.  I suspect that this could cause rubble pile objects to tidally become equatorial.
But then NEO's.   I would not like the space program to divert to these immediatly.  Not unless rational thinking justified it.
Most of these are not at this time a threat to the Earth, but some are.
433 Eros, is a rather large object, for a NEO.  It is said to be about 17 km / 11 mi in diameter.
It is stony and may have lots of useful minerals.
1036 Ganymed, is a bit larger.   35 km / 22 mi.
Stony also, lots of iron, it seems.  Magnisium silicates it is said.
So, not so much water or Carbon.
I don't think that either of the above are much of a threat, thankfully due to their current orbits.  But they would require an input of organic chemicals to be locally useful, I would think.
And then there are objects like Bennu.  I have thoughts on that.
I would consider:
Put a shell around such.  Have solar energy.  Apply a magnetic field to allow thrust from the solar wind to move them back outward.  Try to synchronoze them with the orbit of Earth, but make sure that they could not collide.   A surrounding shell might be thin as aluminum foil, but would not need to be of special metals.  The European device that extracts Oxygen from Lunar regolith simulant also secretes a mixture of metals.   A foil of that should be good enough, to encourage Bennu regilith to stay inside, provided the forces applied to it are reasonably small.
A 1 to 1.5 or 1 to 2 year orbit relitive to Earth may be quite useful.
You have photons or solar wind to push these objects to longer more desirable orbits.  I prefer the solar wind.   We speak of making artificial magnitism for Mars to protect it.  How about giving Bennu a whopping big magnetic field?   Very likely a great thrust.  One that could be throttled.
For the European device or other equivalant, there would be no need to process them in microgravity.  You could have synthetic gravity machines in place.
The European device could likely remove the Hydrogen compounds and the Carbon from the ore, and then the Oxygen.  Reduced metals may be present in the remainder.  You could then use micobes to remove Gold and Copper, perhaps other metals.  Then make local structure from the remainder.
It would be nice to make such a device a "Moon" of a larger NEO that does not have hydrated minerals, so to utilize it as well.
Along with magnetic fields, you may also possibly use plasma bubbles, where you might inject plasma and even dust into the "Bubble" to increase efficiency/effectivness.
Done.


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#228 2020-11-17 10:21:17

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 603

Re: Colonizing / terraforming small asteroids

Many Trojan asteroids have a reddish hue and appear to be centaurs captured by Jupiter's gravity.  If so, they should be a 50-50 mix by mass of ices and silicates.  Such bodies would be candidates for ice covered lakes.  A nuclear heat source could melt an area and a layer of silica aerogel could be overlaid by a thick floating layer of ice, which would keep the water under enough static pressure to prevent it from boiling.

An ecosystem could develop under the ice, powered by artificial light.  If the body has surface gravity about 1% of Earth, hydrostatic pressure would be 1bar at a depth of 1km.  That is the ideal depth for underwater cities.

Last edited by Calliban (2020-11-17 10:26:33)


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#229 2020-11-18 07:05:47

tahanson43206
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Re: Colonizing / terraforming small asteroids

For Calliban re #228 .... nice vision .... at 1 kilometer depth, the residents would be protected from radiation, and they would have the aqueous environment for exercise despite the low gravity.  (th)

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#230 2020-11-24 17:05:45

Calliban
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Re: Colonizing / terraforming small asteroids

A link to the original Shell Worlds article.
http://www.ultimax.com/whitepapers/Shel … Sfinal.pdf

This would work in one way or another for any world larger than a few tens of km in diameter.  Asteroids of this size would have ~1bar static pressure close to their centres.  Eros and Phobos may be the first two objects that humans attempt to colonise in this way.  A stony asteroid 100km in diameter, would have static pressure of 1bar at a depth of 2km.  The thing that may actually impede the creation  of shell worlds on bodies such as this, is the disposal of waste heat.  The rate of conduction through 2km of rock would be miniscule.  How this problem can be dealt with, I do not know.   Any surface radiators would be vulnerable to meteorite damage.

Last edited by Calliban (2020-11-24 17:07:37)


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#231 2020-11-24 18:37:07

tahanson43206
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Re: Colonizing / terraforming small asteroids

For Calliban re #230

The thing that may actually impede the creation  of shell worlds on bodies such as this, is the disposal of waste heat.  The rate of conduction through 2km of rock would be miniscule.  How this problem can be dealt with, I do not know.   Any surface radiators would be vulnerable to meteorite damage.

This quote is an example of why it's fun having working engineers posting in the forum!

I'd like to point out that any of these bodies are going to be at close to absolute zero.

I would have thought a spherical mass 2 km in diameter sitting at absolute zero (or close to it) would serve admirably as a heat sink.

I posed a question to Google .... Google is an AI under constant development .... I figured this question would tax it severely ...

how much energy is required to heat a 2 km diameter asteroid from absolute zero to 400 Celsius

To my amazement, the following link showed up in the third citation results:

https://arxiv.org/pdf/1604.05384.pdf

This lengthy paper reports on observations by the Spitzer space telescope ... about half way through, I found a paragraph that included the assumption that the "night" (anti-Sun) side of an asteroid would have a temperature of Absolute Zero  (T=0).

I ** think ** your prediction of difficulty with heat transfer is related to your expectation of poor heat conductivity inside an asteroid.  However, it occurs to me that in order for the entire asteroid to have reached Absolute Zero throughout, there ** had ** to have been heat conductivity. 

Edit#1: A related question that may not yet have been answered by direct observation is: What is the coefficient of heat conductivity inside an asteroid?

Wikipedia came up with what appears (at first glance) to be a reasonably well written summary of the field ...

https://en.wikipedia.org/wiki/Thermal_conductivity

While scanning this document, I found explanations of the difference in thermal conductivity in metals as compared to non-metals.

The interior of a rocky asteroid would (presumably) not conduct as a metal would .... Your observation (to which I am responding) implied ( as I read it) that you anticipate that the thermal conductivity of the loosely bound material inside an asteroid would be low.  However, (by deduction), I would assert that the thermal conductivity of such an asteroid is not zero.  The question to be answered by direct experiment would be:

Can the interior of an asteroid carry away whatever heat is generated by a nuclear power plant in the heart of a human habitat in the middle of such an asteroid?

I noted that the Wikipedia article explains (or at least shows) the difference between heat transfer in metal (by electrons) and in non-metals (by phonons).

Edit#2: Investigating further, I asked Google for the thermal conductivity of a pile of rock ...

An early citation seemed to fit the request fairly well .... https://link.springer.com/article/10.10 … 6459011919

This paper reports on thermal conductivity measurements of piles of waste rock.  As I interpret the results, the values look low, but I'm not sure how to interpret them.  One thing I ** did ** note was that the presence of water in one of the piles increased the thermal conductivity of the mass.

Water would not be present inside a rubble pile asteroid.

Oh! Of course!  the astronauts could just spray an amount of water onto the inside of the cavity, to increase the thermal conductivity of the surface.

Radiation from radiators connected to the nuclear reactor in the habitat would reach the water in the wall, and (I presume) act to insure a flow of thermal energy into the wall beyond the layer of water.

(th)

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#232 2020-11-24 19:06:22

SpaceNut
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Posts: 20,263

Re: Colonizing / terraforming small asteroids

We know that the sun lit side at full lunar day and night, the temperature on the Moon can vary wildly, from around +200 to -200 degrees Celsius (+392 to -328 degrees Fahrenheit), And we know that with distance from the sun / earth of approximate 1,200w to mars drops the energy to 430 watt so its temperature high is going to drop quite a bit for asteroids for each sq. meter that its arrives at.

https://mars.nasa.gov/insight/weather/
Mars has very little atmosphere to absorb heat with or to slow heat loss with so the temps seen by insight each day would be what to expect.
-150 'F during the night and for the day is 20'F at a high....

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#233 2020-11-24 19:14:26

tahanson43206
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Re: Colonizing / terraforming small asteroids

For SpaceNut re #232

Thank you for this interesting addition to the topic!

The Lunar regolith clearly conducts heat ... your example of the temperature swings confirms that ....

The question at the heart of Calliban's observation is whether the material inside an asteroid can absorb heat (phonons) at a rate sufficient to satisfy the waste heat disposal problem of a nuclear reactor in a human habitat in a cavity inside the asteroid.

Would you be interested in pursuing this a little bit further?  Is there any actual research/observation that considers the thermal conductivity of lunar regolith?

The gravity of the Moon may skew the results ... inside an asteroid rubble pile there would be vacuum between particles of rock.

(th)

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#234 2020-11-25 02:51:38

Calliban
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From: Northern England, UK
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Re: Colonizing / terraforming small asteroids

Fouriers law of conduction states that: Q = K dT/dX

Let us assume that the average surface temperature of our terraformed 100km asteroid is 200K; the shell is 2000m thick and the biosphere inside has a constant temperature of 300K.  Typical thermal conductivity of rock is about 1W/m.K.  So the equilibrium rate of conduction through the rock would be:

Q = 1 x (300-200)/2000 = 0.05W.

Insolation averaged over night and day, summer and winter in northern temperate climates, typically exceeds 100W/m2.  So you see the dilemma.  Initially of course, we could use the asteroid itself as a heat sink for the waste heat.  We would do this by drilling deep bore holes into it.  But over long periods of time, it would eventually heat up.

A few scoping calculations.  Let us assume that the asteroid starts out at a temperature of 150K.  It is 100km in diameter, has a density of 2.6tonne per cubic metre and a specific heat of 1KJ/kg.K.  How long would it take to heat it up to 300K?

Let us assume that the entire subsurface at a depth of 2km is riddled with terraformed caves and tunnels, with a light flux of 100W/m2.  That's a total energy flux of 2.9E12 watts.

Heat capacity of the entire asteroid is 1.36E21 J/K.  The amount of heat required to raise its temperature by 150K would be 2E23J.

Tge time taken to reach a temperature of 300K is therefore 2E23/2.9E12 = 7E10 seconds (2200 years).  So we could in principle rely on the heat capacity of the asteroid for quite some time.  But eventually rising temperatures would pose a severe problem for a shell world.  The only way around the problem would be to introduce active cooling, and to cover the surface with a layer pipes that allow heat to be dumped into space.  We could cover the pipes with a layer of regolith to protect them from micrometeorites.

Rather than attempt to build giant heat exchangers within the biosphere, it would be more sensible to rely on the ability of rain and natural water bodies to soak up heat.  Water from internal seas would be pumped up through the shell to surface pipes where they would dump heat.  Heat would be transfered into the seas internally via wind and precipitation.

We would probably choose to put the fusion reactors we would need on the surface of the shell (or even in stationary orbit).  That way, we don't need to contend with waste heat from the power source overheating the biosphere.

Last edited by Calliban (2020-11-25 03:15:17)


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#235 2020-11-25 04:02:12

Calliban
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From: Northern England, UK
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Re: Colonizing / terraforming small asteroids

tahanson43206 wrote:

For SpaceNut re #232

Thank you for this interesting addition to the topic!

The Lunar regolith clearly conducts heat ... your example of the temperature swings confirms that ....

The question at the heart of Calliban's observation is whether the material inside an asteroid can absorb heat (phonons) at a rate sufficient to satisfy the waste heat disposal problem of a nuclear reactor in a human habitat in a cavity inside the asteroid.

Would you be interested in pursuing this a little bit further?  Is there any actual research/observation that considers the thermal conductivity of lunar regolith?

The gravity of the Moon may skew the results ... inside an asteroid rubble pile there would be vacuum between particles of rock.

(th)

On the moon, one would need a shell of rock about 30m thick to contain a 1bar atmosphere.  The rate of conduction through that shell, assuming a 100K temperature difference, would be 3.3W.  So active cooling would still be required for any serious habitat.  The shell itself would take over 2 years to heat up from 200K to 300K.  So there would be no problem dumping heat into the shell during lunar day and waiting until lunar night to radiate it into space.

If atmospheric pressure were reduced to say 5PSI, then a layer of rock 10m thick would be sufficient.  The rate of heat transfer through this would be 10W/m2.  Maybe we could enhance that further through the selective use of heat pipes and tune the light frequency in the habitat to be better suited to plant growth.  Under these conditions, it might be possible to dispense with active cooling.

On Mars, with higher gravity, conduction through the shells should be sufficient to dispose of internal waste heat, as the shell can be much thinner.

Last edited by Calliban (2020-11-25 04:04:24)


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#236 2020-11-25 07:16:55

tahanson43206
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Re: Colonizing / terraforming small asteroids

For Calliban re #235

Thank you for continuing this discussion of heat flow in various materials.  I find it surprising (not having studied the topic before) that heat flows poorly through rock, since heat obviously ** does ** flow through rock.  The question you posed, and the one I'd like to pursue since this ** is ** one of the asteroid topics, is the mechanism by which heat from a reactor in the central cavity of a 2 km diameter asteroid rubble pile can be "dumped" into the mass of the asteroid.

Heat can be conducted from the reactor to radiators outside the body of the habitat, and these will produce infrared radiation that will flow to the interior walls of the cavity.  I would expect the walls to absorb that radiation, and for the thermal energy to be communicated by direct contact between the molecules comprising individual particles in the wall. 

It should be possible to model the process with computer code.  Once again the need for members with specialized knowledge and access to the appropriate hardware and software becomes clear.  This forum recently reached a potential turning point.  The constant and unceasing flood of (mostly Russian) spam ID's led to a decision to close off open registration.  The forum is now able to accept new members who are sponsored by one of the Administrators or by one of the Moderators.   It is up to the existing members of the forum to nominate new members for consideration by SpaceNut or kbd512, or one of the Moderators.

Because of your (unique in this forum) situation of being an active engineer, with a Masters in reactor design, I am hoping you will know folks who would be able to assist with various projects available for development within the context of the forum.

Terraformer provides an example of a recruiting effort which had some success.  Other members of this forum are potentially capable of persuading high value individuals to consider participating in support of RobertDyck's Large Ship, or one of your several interesting projects.

SearchTerm:CallForParticipation
SearchTerm:CFP

(th)

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#237 2020-11-25 07:59:52

Calliban
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From: Northern England, UK
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Posts: 603

Re: Colonizing / terraforming small asteroids

The rate of heat flow through a piece of material, is proportional to the temperature difference at each end, but inversely proportional to thickness.  Double the temperature difference, double the rate of flow.  Double the thickness, half the rate of flow.  Even materials with good thermal conductivity will have low rates of heat flow through thicknesses measured in kilometres.  Consider that the centre of the Earth is as hot as the surface of the sun.  Yet average heat flows at the surface is measured in milliwatts per square metre.  The Earth's mantle is 5000km thick.  The crust is 50km thick.  This is how the Earth has managed to retain the heat of its formation even after 4.5 billion years.

I made some oversimplification in my earlier calculations.  At 300K, the specific heat for granite is 775J/KgK.  The thermal conductivity is 2.79W/m.K.  So, heat capacity of the asteroid would likely be only 75% what I calculated, but it would lose heat through its shell twice as quickly.  Not enough to change the conclusion, but worth noting for future calcs.


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#238 2020-11-25 08:38:35

tahanson43206
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Re: Colonizing / terraforming small asteroids

For Calliban re #237 and topic ...

Thanks for your follow up ... The Wikipedia article (mentioned a post or two ago) about thermal conductivity made reference to the difference between heat conductivity in metals and non-metals.  The methods of heat conductivity are (apparently) quite different.  However, there is another wrinkle in the asteroid rubble pile scenario ... while the individual particles are likely to be stony, they are NOT bound tightly as they would be in granite.  The heat conductivity between particles in a rubble pile would (presumably) involve radiation in every transaction between particles.  I would imagine this situation would decrease conductivity from the interior of the rubble pile to the exterior, where radiation to space would occur.

Thinking out loud here .... radiation impinging upon loose particles in the interior wall of a cavity in an asteroid would (presumably) cause some agitation of the particles with respect to their neighbors, in addition to excitation of molecules chemically bound to each other inside the particle.  The collisions of particles in a loosely bound mass may advance the rate of heat transfer, as compared to materials tightly bound, as would be the case inside your interesting example of the Earth.

I think that an experiment is called for ...

The theory that there is a cavity in the center of Bennu is untested (to the best of my knowledge)

Bennu is (presumably) a flying rubble pile, so excavation of a pathway into the interior would (presumably) be less difficult that would be the case if the components of the mass were tightly bound to each other throughout.

The ability of the interior of a (hypothetical) cavity to accept infrared radiation at a rate agreeable to a human habitat is probably subject to computer modeling, but a test with a real vehicle would settle the matter.

Expeditions such as this one would be are often funded by individuals with motivations other than profit.  I think it is reasonable to suppose there are people on Earth today with both the means and the scientific curiosity to be willing to fund a well designed expedition to find out what conditions exist inside Bennu.

(th)

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#239 2020-11-25 10:35:29

SpaceNut
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Re: Colonizing / terraforming small asteroids

The shell of a rubble pile could be ground and then re-laid to make a harder shell as the thermite or arcing of current through it to meld or make the metal particles adhere to each other. This new outer shell would help to maintain internal pressure.

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#240 2020-11-25 18:08:17

Calliban
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From: Northern England, UK
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Posts: 603

Re: Colonizing / terraforming small asteroids

SpaceNut wrote:

The shell of a rubble pile could be ground and then re-laid to make a harder shell as the thermite or arcing of current through it to meld or make the metal particles adhere to each other. This new outer shell would help to maintain internal pressure.

A thin shell made up from sintered blocks, with shell compression maintained by basalt fibre cables, would be a far more material efficient option than relying on gravity and thick rubble layers.  It would make heat rejection easier too.  It would require continuous maintenance, as the cables would have a fatigue life and would need to be replaced.  Thin shells are also more vulnerable to meteorite damage.  So pros and cons with both approaches.

A classical shell world is an incremental approach that can be built up over millenia by gradually tunnelling out the interior of a world.  It starts small and gradually extends itself until the entire body is colonised and terraformed.  The waste heat problem is tough to overcome.  On the moon, there is enough gravity for convection to work efficiently and closed heat pipes could carry heat to the surface.  On smaller bodies, it is more difficult to see how this would work.  Maybe sunlight pipes in the shell could be filled with gas allowing them to transport heat to the surface.  Natural convection could be boosted through the use of fans.  In closed pipes, oxygen could be used as heat transfer fluid.

Last edited by Calliban (2020-11-25 18:13:35)


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#241 2020-11-25 19:59:11

tahanson43206
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Re: Colonizing / terraforming small asteroids

For Calliban re heat conductivity in particular, and properties of lunar soil in general ...

I'm hoping you might enjoy this presentation ... Dr. Carrier helped to prepare Apollo astronauts for their expeditions.  His specialty was lunar soil.

In the course of the first 50 minutes, he mentioned the extremely low heat conductivity of the lunar regolith.


https://www.thespaceshow.com/show/13-no … id-carrier

We welcomed Dr. David Carrier to the program for a two part 92 minutes discussion about his new book, "Apollo Memories," and his work in training the Apollo astronauts to work on the lunar surface.  Our discussion started with Dr. Carrier telling us about his book and why now.  We then talked about the state known lunar science in the late 60s and all during the Apollo era.  For example, one of his tasks was to figure how far into the lunar soil might the lander and the astronauts sink when they were on the lunar surface.  Don't miss how he figured this out then finding out he was very accurate.  This was just one of the many fascinating stories he told us during the show about his working with the astronauts in preparation for walking and working on the lunar surface.

(th)

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