scrape_r_rational/data/cache2/2kj147.md

## [Q] Traveling from the Moon to the Earth

* Author: u/thecloud2 *
* URL: https://www.reddit.com/r/rational/comments/2kj147/q_traveling_from_the_moon_to_the_earth/
* Score: 25

* Created: 2014-10-28T02:06:56

### Post:

You are an otherwise physically-baseline human who is immortal, invincible, does not need to eat, drink, breathe, nor sleep, nor gets physically tired, feels pain, nor suffers from boredom or tedium. You know the entirety of current-day science and engineering and can recall information at will. 

You find yourself naked, on the Moon, in an universe in which humanity has never existed (so you cannot cannibalize technology left over from Moon missions, or contact Earth.) Your goal is to travel from the Moon to the Earth, using only [materials available on the Moon](https://en.wikipedia.org/wiki/Internal_structure_of_the_Moon), and in as little time as possible. Bonus points if you can do it before the Sun expands into a red giant (~5.4 billion years.)

### Comments:

> **u/Charlie___** [+12]  (an hour later)
> 
> Polish moonrocks on my immortal butt to make a huge parabolic solar concentrator to melt stone with. Since I'm invincible, I can just melt stone and shape it in my hands to make containers.
> 
> Apparently there is a fair amount of iron on the moon as free metal (if mostly as tiny grains). Into the solar death ray with you!
> 
> There's also water on the moon, as ice. So a steel rocket powered by O-H seems like a good goal to aim for. But there are so many intermediate technical challenges.
> 
> Alternately, maybe I could use the iron to make absolutely monster-sized capacitor banks, and charge them up by, you guessed it, rubbing my immortal butt. ("Dear Randall Munroe. How long does it take to charge up a 20 MWh capacitor bank by static electricity using only your butt and a metal plate?") Then take a quick railgun ride to earth and I'm done.

>> **u/SharkMolester** [+3]  (22 hours later)
>> 
>> giant iron container + ice + parabolic mirrors to heat container full of ice + release valve = steam cannon

>> **u/mhd-hbd** [+2]  *Writes 'The World is Your Oyster, The Universe is Your Namesake'* (15 days later)
>> 
>> This will be the new standard reply for "you are invincible, immortal, etc. and need to do X"
>> 
>> "Well, first I am going to do Y by rubbing it on My Immortal Butt."

> **u/alexanderwales** [+11]  *Time flies like an arrow* (22 minutes later)
> 
> That's a tough one. I'm actually not sure that it's even possible. The escape velocity of the Moon is 2.4 km/s. We have no real fuels - no trees, no organic matter, nothing that we can really burn, and no oxygen to burn anything anyway. That also means that refining metals is going to be nearly impossible.
> 
> The best bet is probably to dig down to the molten core and use that energy somehow, but it seems like even if you were invincible you'd run the risk of a tunnel collapse and getting trapped miles below the surface until the sun burns out.

>> **u/ulyssessword** [+6]  (an hour later)
>> 
>> >The escape velocity of the Moon is 2.4 **k**m/s.
>> 
>> FTFY.

>>> **u/alexanderwales** [+8]  *Time flies like an arrow* (an hour later)
>>> 
>>> You saw nothing!

>> **u/duffmancd** [+2]  (an hour later)
>> 
>> You're missing a "k" in 2.4 m/s. I read that and thought, why not just run?  If only it were that easy.

>> **u/AmeteurOpinions** [+1]  *Finally, everyone was working together.* (9 hours later)
>> 
>> [/u/Charlie____ had a pretty good solution to the melting problem](http://www.reddit.com/r/rational/comments/2kj147/q_traveling_from_the_moon_to_the_earth/cllu1k6).

> **u/capsless** [+11]  *despiser of hpmor* (2 hours later)
> 
> y'all're scrubs. i, too, spent like twenty minutes trying to figure out how to split minerals without any chemically reactive compounds, or how to transform the energy from a giant solar array into explosives -- but actually, there's a far simpler solution. you only need to apply, at apogee, something like 2600m/s of dv (to a small projectile, too); why bother with anything fancy? a giant trebuchet ought to do the trick. sure, it'll be a hell of an engineering nightmare, especially since you're working with materials that are flimsy and dense, but it shouldn't take more than a few decades.
> 
> also possible: predicting meteor strikes, positioning self so that you will be struck mostly upwards by the resulting explosion. from cursory research, impacts of that size happen every few millenia.
> 
> i will do the requisite calculations in the morning. in the meantime, if you can clarify a few things i'd deeply appreciate it:
> 
> -can you self-modify? to what degree? what's the mechanism of the invunerability?
> 
> -can things that are a part of your body leave your body? do those parts also remain in your body? (eg, can you produce saliva indefinitely, can you use blood and so on)
> 
> -does "current science" extend to trivia? do you have, say, a topographical map of the moon?
> 
> -what time are you placed on the moon? at formation, nowish, or something else?
> 
> -what happens to material that enters/exits your body?
> 
> thanks

>> **u/None** [+2]  (4 hours later)
>> 
>> 2600m/s gets you to lunar orbit. You need a bit more than that to get to the earth. On the plus side, you have no atmosphere to worry about. And friction is based on the normal force, no speed component, so a faster-swinging fulcrum doesn't cause problems.

>>> **u/capsless** [+2]  *despiser of hpmor* (10 hours later)
>>> 
>>> 2600m/s is actually a little too much. the gravitational potential of a 50kg object on the surface of the moon is something like -141MJ; adding energy from the "maneuver" (which is close to impulse, so we don't have to worry about gravity drag) leaves roughly 28MJ at arbitrary distance from the moon. transfer that to velocity and you get ~1062m/s; since the speed of the moon at apogee is (iirc) 999.6 m/s, that's more than enough to get to earth.
>>> 
>>> on friction: normal force is a reaction to any (contact) force, not just gravity. this means that centripetal force (which is obviously pretty extreme at the peak of the arc) also contributes to friction and such. not only that, but rock simply doesn't have the tensile strength to stay intact at rotational speeds of more than a few hundred m/s, so you'll need to use invincible materials (hair) for the last leg of the contraption.
>>> 
>>> still doable.

>> **u/thecloud2** [+2]  (5 hours later)
>> 
>> 1) Hadn't thought about it, but if you can do it with just moon materials and modern science/tech, I don't see why not. The invulnerability is mostly so you can survive in vacuum and Earth reentry, so if I had to explicitly define it, you cannot get hurt if you don't wish it. As for biological functions, you are magically kept alive with "life force". 
>> 
>> 2) I feel like it's against the spirit of the thought experiment to use yourself as unlimited raw materials. You're probably not consuming any water or nutrients, so you'd be limited to however much raw material is in your person at t=0, and no more than that (with the caveat that you don't cripple yourself from draining all of your blood or whatever.) 
>> 
>> 3) Yes, you know everything about the moon that is currently known. 
>> 
>> 4) Now-ish.
>> 
>> 5) It stays in existence.

>>> **u/Bowbreaker** [+1]  *Solitary Locust* (a day later)
>>> 
>>> > As for biological functions, you are magically kept alive with "life force". 
>>> 
>>> Does that mean your blood infinitely replenishes? Also, what about functioning after having temporarily removed vital organs?
>>> 
>>> >5) It stays in existence.
>>> 
>>> Is it still invulnerable to involuntary damage though?

>>>> **u/thecloud2** [+1]  (a day later)
>>>> 
>>>> Pardon the tone, but what do you plan to do with this information? Do you actually have a scheme in mind?

>>>>> **u/Bowbreaker** [+1]  *Solitary Locust* (a day later)
>>>>> 
>>>>> I haven't yet looked into the consistencies of bodily fluids and how you can extract anything from it. But dependent on your answer you either have infinite liquids or at the very least can lower your own weight considerably, both of which make the whole endeavor a tad easier. And in the best case scenario you have infinite indestructible materials. Building a small vessel out of guts and coagulated blood would make the required fuel to launch much less. And I'm sure *something* in our body can be used as fuel as long as we find a way to produce oxygen from the water.
>>>>> 
>>>>> But mostly these questions are there to give ideas to the more scientifically literate than me. And to refine the questions /u/capsless already stated.

> **u/ulyssessword** [+7]  (a day later)
> 
> Okay, I think I have one that would work now.
> 
> First, collect two rocks made of the strongest material you can find (iron meteorites are a good candidate).
> 
> Make sure that they are the same mass (ideally ~20kg), and drill a hole through each of them.
> 
> Next, wait for about 10 years.  In this time, your (indestructible) hair will grow to about 2m in length.
> 
> Separate your hair in half, left and right.  Loop the left half through the hole in one rock, and tie the end securely to your left arm.  Loop the right half through the hole in the other rock, and tie the end to your right arm, making sure that you can later untie it.
> 
> Stand on top of a mountain, and start spinning clockwise like a top.  You've had time to practice, so I'm assuming you match the human record of 300 rpm.  Each 20kg rock is traveling 5 full revolutions around a 1m radius circle per second, for a speed of ~30m/s, and it has 600 kgm/s of momentum.
> 
> We need about 240 000 kgm/s, to escape with 100 kg of yourself + one of the rocks, which translates into a 20 kg mass at 12km/s.  This is equal to traveling around a 400m radius circle at 300 rpm.
> 
> Keep spinning for 8000 years, maintaining the same rotation speed.  You will need to constantly add small amounts of energy to the system as your hair grows and the rocks get further away (and their speed increases).  You are now spinning two rocks, each 400m away from you.
> 
> Wait until you are facing Earth, and then untie your left hand rock.  It will slip off your hair and fly away from the earth at 12km/s, carrying 1.44 * 10^9 J of kinetic energy, about the same as 1/3 of a ton of TNT.  Your right hand rock, with nothing counterbalancing it, will try to go straight at the earth, yanking you along for the ride.  This should be enough to break lunar orbit and eventually crash to earth.

>> **u/capsless** [+2]  *despiser of hpmor* (2 days later)
>> 
>> i have no idea whether the energy is sufficient, but i do know that it's impossible for the rocks to stay intact (!!!) at those speeds. the maximum speed at which the edge (the boulders) of something spinning can stay intact is equal to the square-root of its specific tensile strength; for impure iron, this is ~ 200 m/s. with tight enough hair weave it's certainly possible to keep them in place for a few seconds, but eight thousand years is out of the picture.

>>> **u/ulyssessword** [+1]  (2 days later)
>>> 
>>> >the maximum speed at which the edge (the boulders) of something spinning can stay intact is equal to the square-root of its specific tensile strength
>>> 
>>> Isn't this only if you have a solid bar extending from the center to the edge, such as for building a centrifuge?  
>>> 
>>> It's probably far too much acceleration anyways, but let's find out.  A calculator says that the system reaches accelerations of about 40 000 g, and let's call the ultimate tensile strength of meteoric iron 200 MPa (the same as cast iron), with a density of 8 g/cm^3.  200 MPa means it can hold against 20kN per cm^2.  At our acceleration, 50g exerts 20kN of force due to the acceleration.  A 1cm square piece of iron is about 6cm long when it weighs 50g, so that's the max thickness of a piece of iron before it rips itself off.
>>> 
>>> It's still too much force to work as I described it, but it's possible with some minor modifications.

> **u/ulyssessword** [+8]  (an hour later)
> 
> I'm not sure if this could actually work or not, but here goes:
> 
> First, exercise and get into at least as good of shape as the best pro athletes.  
> 
> Next, dig a hole through the center of the moon, to give yourself a good runway.
> 
> Lastly, gather a large amount of baseball-sized rocks, bind them together (somehow) and get the entire thing falling back and forth through the hole (with no air resistance, it shouldn't stop).
> 
> During one of the passes next to an entrance, hop onto the back of the conglomerate boulder, and throw a rock as hard as you can backwards, increasing your velocity a tiny bit.
> 
> Repeat as needed, making sure not to block off the borehole in the process.
> 
> With any luck, you have transferred enough kinetic energy to yourself (and a small fraction of the remaining rocks) to escape the moon's gravity.  Then it is a simple matter of forcing your orbit to degrade into the Earth's atmosphere.

>> **u/capsless** [+3]  *despiser of hpmor* (11 hours later)
>> 
>> the problem with digging through the center is that you've got pretty insane pressures. while you don't have to deal with gravity at the point you're digging, you do have to worry about the gravity effecting every /other/ point -- that is, pressures of > millions of pascals. and part of the stuff you have to dig through is /liquid/.
>> 
>> idk if its possible (probably, given enough time) but it'd certainly require materials that aren't available.

>> **u/Charlie___** [+2]  (15 hours later)
>> 
>> If you tried to dig a hole in the moon, you could only keep going until the pressure from all the rock above you is more than the strength of the stone you're digging through - and worse, as you go to the center the rock gets hotter and more liquid.

> **u/puesyomero** [+1]  *The Culture* (14 hours later)
> 
> dig a pit next to a cliff, and make rope out of hair. tie the rope to a big weight near the cliff and polish the contact surfaces of your pit and where the rope will slide. make sure to have a release knot so the rock doesn't drag you to the bottom of the cliff.  stand at the bottom of the hole release the counterweight and fling yourself to earth.

> **u/None** [-2]  (12 hours later)
> 
> [deleted]

>> **u/capsless** [+2]  *despiser of hpmor* (13 hours later)
>> 
>> the velocity imparted to the moon when you take off is exactly equal in magnitude to the velocity imparted when you land, but opposite in direction. this will do nothing.

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