
Earth-Moon Fire Pole - mooreds
https://what-if.xkcd.com/157/
======
amelius
> There's one more problem: The Moon doesn't always stay the same distance
> from Earth. Its orbit takes it closer and farther away.

Then use a pole that is strong enough to eliminate this problem.

~~~
laumars
What material would you suggest is strong enough to handle the orbit of two
massive bodies (relative to girth of a poll one could realistically hold
onto)? And what do you suppose the consequences of altering the moons orbit
would be?

Something with elasticity would be a better option than something that is
strong but even that wouldn't resolve some of the other issues presented; such
as the moons surface not being geostationary to any point on the Earth.

~~~
amelius
> such as the moons surface not being geostationary to any point on the Earth.

The pole would also solve _that_ problem ...

~~~
laumars
...not without tearing the moon apart in the process. At which point you have
much bigger issues (as well as no need for a pole in the first place).

But given you're just making absurd "just do X" style statements I'm sure
you'll reply with some remark about gluing the moon back together or wrapping
it in clingfilm before you insert the pole to strengthen it's integrity.

~~~
arghwhat
Glue and clingfilm? Don't be silly.

The solution is obviously attaching rockets to the moon to accelerate it
first.

------
Sharlin
As always, remember to check the title texts of the images as well!

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Roritharr
Reading these makes me very happy, I'm so grateful Mr Munroe is still working
on them.

~~~
ragazzina
Do you know of any similar websites? I have a couple of questions that I would
like to see answered. I emailed mr Munroe in the past but he never replied,
I'm sure he has thousands of questions from readers to choose from.

~~~
davedx
[https://waitbutwhy.com/](https://waitbutwhy.com/) is pretty good, you need to
set aside a decent chunk of time for each article typically.

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yosito
I would imagine that what the five year old who asked wanted was an intuitive
sense of how far away the moon is. My answer would have been far less
interesting. I would assume a reasonable rate at which a child slides down a
pole, say 1 meter per second, multiplied by roughly 384,000km from the moon to
Earth, and it would take a little over 12 years.

~~~
jameshart
Let me guess - your favorite map projection is Goode Homolosine. You like easy
solutions. You think we wouldn't have so many problems if we'd just elect
normal people to Congress instead of Politicians. You think airlines should
just buy food from the restaurants near the gates and serve that on board. You
change your car's oil, but secretly wonder if you really need to.

~~~
saagarjha
For reference: [https://www.xkcd.com/977/](https://www.xkcd.com/977/)

~~~
hyperpallium
For mobile: [https://m.xkcd.com/977/](https://m.xkcd.com/977/)

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aravindet
Wouldn't attaching the pole at a pole (heh) be a better strategy to avoid
uncomfortable relative velocities?

It would also have the advantage of providing a fixed place on the earth to
find a stash of warm clothes and an energy drink.

~~~
albertgoeswoof
If you fix it to a point on the earth, how will it get to the moon?

~~~
RyJones
the pole doesn't need to be straight. It could be hook shaped.

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tomxor
It must be so fun to have a Dad that entertains your foolish ideas as a child,
and play them out in immense detail for you :) I hope I can be that Dad one
day.

~~~
jameshart
The question asker’s father (Ramon Schönborn) is not the same person as the
answerer (Randall Munroe). But nonetheless it would be fun to have a dad who
sends your questions to random internet cartoonists to entertain your foolish
ideas for them, too.

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codeulike
So he doesn't actually answer the question - how long would it take? He goes
off on a tangent about disembarking from the pole

~~~
newlunarfire
He does, indirectly. He just can't give a precise estimate. A few years to
climb to L1, a matter of weeks afterwards.

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HopDavid
There has actually been a lunar elevator proposed, the most massive part lying
at Earth Moon Lagrange 1. In my opinion, not practical. I took a look using
Xylon: [http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-
teth...](http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-tether.html)

For the numbers I used a safety factor of 1. The slightest nick or scratch
would cause a break. Safety factor of 3 is more sensible.

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ngvrnd
Hey Randall! If you put a microwave in a sub-zero walk-in fridge, and after
the device had reached equilibrium with ambient conditions, you connected the
microwave to mains power, then put a chunk of ice in the microwave and turned
it on at full power, would the ice ever melt?

~~~
probably_wrong
I guess the answer would be similar to the toaster-in-fridge question[1] from
a previous what-if, namely: yes, it would melt.

[1] [https://what-if.xkcd.com/155/](https://what-if.xkcd.com/155/)

~~~
ngvrnd
but what if microwaves don't heat solid ice at all, they heat the water that
forms on the ice because the microwave is at room temperature? google "how to
melt glass in a microwave".

~~~
sp332
That's very cool. But if you had a standard 1000W microwave, heat from the
electronics would probably get the job done eventually.

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hiccuphippo
Reminded me of the Space Elevator:
[https://en.m.wikipedia.org/wiki/Space_elevator](https://en.m.wikipedia.org/wiki/Space_elevator)

~~~
HopDavid
[http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-
teth...](http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-tether.html)

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jobigoud
Fun fact: the orbit of the Moon around the Sun is concave in every point.

~~~
ColinWright
Convex.

~~~
jerf
Pish, details.

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swyx
wait but he didn't answer how long it would take! i was reading for a big
payoff at the end when he takes an integral to all those stages!

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archagon
I wonder how long these things take to math out. Is there a lot of preliminary
research involved?

~~~
rimunroe
The "mathing it out" takes about as long as he has before he needs to actually
start writing and putting the post together. The problems can involve a lot of
research since there's basically no end to how far you can take a given
question. The whole thing usually takes less than a week of on and off work.

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htgb
Wow, great to see a new what-if! After no updates for a couple of years (?), I
thought we had seen the final one…

~~~
owenversteeg
Looks like 156 and 155 were both in March 2017, so about one year.

~~~
htgb
I see. I guess it just feels longer :)

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Angostura
I initially assumed this was Randall's suggestion for a new password.

~~~
icc97
It would need 'staple' on the end:

Earth moon firepole staple

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jgrahamc
Randall Munroe is everything Neil deGrasse Tyson isn't. And by that I mean...
not a bore.

~~~
HopDavid
XKCD is usually accurate as well. Tyson on the other hand...

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lmilcin
Unfortunately it is inaccurate.

Actually, the person sliding the pole would be like an object changing orbit.
Remember, Moon orbits Earth. Even at L1 you can't just push yourself and coast
along the pole. The movement along the pole is changing orbits and as we know
this requires energy. In this case if you push yourself along the pole you
would suddenly find yourself drifting away from it.

That's of course if you could create a rigid pole. As every piece of the pole
would be on a different orbit there would be huge forces acting on the pole.

~~~
ColinWright
> _Actually, the person sliding the pole would be like an object changing
> orbit. Remember, Moon orbits Earth. Even at L1 you can 't just push yourself
> and coast along the pole. The movement along the pole is changing orbits and
> as we know this requires energy. In this case if you push yourself along the
> pole you would suddenly find yourself drifting away from it._

Suppose you tether yourself to the pole with a ring, and are closer to the
Earth than L1. You now are not moving fast enough to stay in the orbit implied
by your current altitude, so you move in an orbit that brings you closer to
the Earth.

You also try to move away from the pole, but you are tethered to it, so there
is a force holding you by the pole. That force provides energy to you.

So it seems to me that it does work, and I don't understand your reasoning.

~~~
lmilcin
The mentioned loop around the pole attached to your waist would do but since
none of the extremely detailed drawings contained the loop we must assume that
the person travaled the pole without the loop.

Moon orbits Earth at about 1km/s and you have to loose orbital speed as you
move along the pole.

Since the pole distance is about 400000km and the orbital speed to loose is
1km/s it means you need to loose about 2.5mm/s for every km of the pole that
you travel. It may not sound much, but if you propel yourself along the pole
in the direction of Earth at L1 you would be surprised to find yourself
travelling away from the pole at 2.5mm/s after 1km. If you push yourself hard
at say 36km/h (or 10m/s) and decide to get some sleep then you would travel
almost 290km but you would also find yourself moving away from the pole at
almost 1m/s and at about 10km from the pole.

~~~
ColinWright
Quoting:

> _Note: While you 're flinging yourself along, be careful not to drift out of
> reach of the pole. Hopefully you brought some kind of safety line so you can
> recover if that happens._

Of _course_ you have lateral forces on the pole, that's not in question. But
if you're sliding down the pole then you're, you know, _sliding down the
pole._ That means you are hanging on to it somehow, or tethered to it, or
something.

The point is that if you are actually sliding down the pole then it really
does actually work.

> _... you would be surprised to find yourself travelling away from the pole
> at 2.5mm /s after 1km._

No, I wouldn't be surprised at all, because I do know a little about orbital
mechanics. Possibly not as much as you do, but I do give talks on it, so if
you can explain clearly how sliding down a pole (once closer than L1) won't
work, I'd be really interested to hear it, because so far what you're saying
hasn't convinced me.

If all you're doing is saying "If you're not tethered to the pole then you'll
drift away from it and it won't work." then we have no problem. But I'm
considering the case where we are, in fact, sliding down the pole.

~~~
lmilcin
Colin, you are right. Of course if you are sliding along it you have some
option to grip it and thus provide additional force needed to keep you in the
vicinity of it.

Of course, if you decide to actually fall to Earth from L1 that starts
becoming problem. I'm too lazy to calculate exact terminal velocity you would
reach when falling L1 to Earth (it won't be the 11km/s you would reach if you
were falling from Moon _orbit_ because you also have Moon tugging you). I
estimate it not lower than 9km/s (Moon has 2.4km/s escape velocity).

Now, if you moved along the pole at 9km/s you would feel much more tug
(noticable 2.5cm/s2) or about 1/40th of Earth surface gravity. If you were
80kg person that would mean about 2kg apparent force.

Of course, by that time you wouldn't be thinking much about it as you would
have other problems in mind...

~~~
HopDavid
Altitude of EML1 is about 320,000 kilometers. An ordinary 320,000 x 300 km
earth orbit would be moving 10.81 km/s at perigee.

As an object falls towards earth it will surge ahead of the moon and the moon
will be pulling it backwards. This makes effective apogee more in the range of
300,000 km.

A 300,000 x 300 km orbit has velocity of about 10.8 km/s at perigee.

In other words, dropping from the neighborhood of L1 will still give you speed
very close to escape velocity by the time it reaches earth.

In the case of orbital tether, an elevator car would have gripping wheels in
contact with the tether. If its's descending the wheels can power a generator
to charge the elevator car's battery.

