
Spaceflight from Super-Earths is difficult - walkingolof
https://arxiv.org/abs/1804.04727
======
pavel_lishin
Scott Manley has a pretty accessible Kerbal Space Program video about this:
[https://www.youtube.com/watch?v=amjuJJwI3iM](https://www.youtube.com/watch?v=amjuJJwI3iM)

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alistproducer2
Can someone elaborate on why escaping a planet's gravity is about velocity and
not just attaining a certain height from the surface? More concretely, why
isn't getting to space as easy as taking a balloon to a certain height?

~~~
nilkn
Escape velocity is actually only necessary for launching ballistic objects
such that they will never return to the surface of the planet over an infinite
amount of time (assuming no other interactions occur). It's not necessary when
you have a method of providing continual propulsion and acceleration. Speed is
mostly irrelevant if you can provide propulsion indefinitely. Escape velocity
is an important concept because, realistically, you can't actually do that or
it's extremely cost inefficient to attempt to do so.

~~~
garmaine
This is very confused. To leave the Earth’s gravitational grip you need to
impart a certain minimum change in velocity. You can do this all at once or or
we time but it doesn’t matter. The delta-V is the same.

~~~
nilkn
This is simply false. The needed escape velocity decreases with distance from
the planet, so applying a change over a long period of time actually can very
materially change the requirements.

To give an example, if you provided propulsion to move an object at 1 mile per
hour, you could turn off all propulsion and the object would have escaped
Earth's gravity once it was 4 * 10^12 km away. This is, of course, an
extraordinary waste and probably practically impossible.

~~~
larkeith
Your "example" seems to predicate on a world without gravity. The reason
escape velocity is reduced based on the distance from the surface is that you
have already provided enough energy to overcome the vertical difference - you
can see this reflected in the difference in gravitational potential energy.

~~~
nilkn
Escape velocity is reduced based on distance because gravity itself is reduced
based on distance. This is obvious from the the expression sqrt(2GM/r), which
is the escape velocity at a distance r from the center of a spherically
symmetric body of mass M. From this expression one can easily calculate the
distance I provided. This computation is completely based on Newtonian gravity
and would make no sense in a world without gravity.

~~~
garmaine
That’s not how physics works. I suggest reading up on conservation of energy
and gravitational potential energy.

~~~
nilkn
With all due respect, this reply isn’t going to fly. This is a pretty
pretentious copout that attempts to end the exchange while saving you from
needing to provide any details or engage with the underlying material in any
way.

I provided a very specific computational counter-example to your statement.
You’ll need to start there if you wish to compose a serious reply.

I’ll restate the counter-example for your convenience. One can provide just
enough continuous thrust to move an object at a fixed unchanging speed of 1mph
for a long time (about 283 million years), at which point it will have reached
a distance where the escape velocity is only 1mph. Once it has surpassed that
distance, one can disable the thrust and the object will have completely
escaped Earth’s gravity without ever having moved at more than 1mph. It will
then asymptotically slow down to 0 mph over an infinite amount of time.

~~~
garmaine
Multiple people have pointed out the mistakes you are making, to no avail. I
really don’t know what else can be said. It’s like you’re claiming 2+2 is 5.
What else can I say?

There is ample introductory material on Newtonian physics that you could learn
from, only a google search away. It’s disrespectful of my time to assume I
must be on the hook to handhold you through it.

~~~
zpconn
I found a number of sources online that disagree with you.

[https://www.quora.com/Escape-velocity-is-supposed-to-
be-24-0...](https://www.quora.com/Escape-velocity-is-supposed-to-
be-24-000-mph-but-our-rockets-never-achieve-this-speed-How-does-that-
work?share=1)

Multiple answers explain that there isn't a fixed minimum change in velocity
required for an object to escape Earth's gravity.

[https://www.quora.com/Does-rocket-always-need-the-escape-
spe...](https://www.quora.com/Does-rocket-always-need-the-escape-speed-11-2km-
sec-to-escape-the-earth?share=1)

The first answer here explains the same thing, using basically the same
example (1 m/s instead of 1 mph).

[https://space.stackexchange.com/questions/4688/couldnt-i-
esc...](https://space.stackexchange.com/questions/4688/couldnt-i-escape-
earths-gravity-traveling-only-1-mph-0-45-m-s)

The answers here overwhelmingly reiterate and support the same example in this
thread.

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hinkley
On a planet with high gravity and a thick atmosphere, doesn’t the Space Ship
One launch protocol become much more cost effective than launching from sea
level?

And similarly don’t launches from the equator become more critical to success,
due to delta-v increasing faster than v due to rotation?

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scj
I've always been interested in the other case: What advances in spaceflight
would be made by a technologically similar civilization if they had much lower
gravity?

~~~
pronoiac
I read some musings by Warren Ellis [1] which brushed up against easier space
travel, very briefly:

"The Olympus Mons mountain on Mars is so tall and yet so gently sloped that,
were you suited and supplied correctly, ascending it would allow you to walk
most of the way to space. Mars has a big, puffy atmosphere, taller than ours,
but there’s barely anything to it at that level. 30 Pascals of pressure, which
is what we get in an industrial vacuum furnace here on Earth. You may as well
be in space. Imagine that. Imagine a world where you could quite literally
walk to space."

It's mostly not about your question, but I liked this part.

[1] [http://www.warrenellis.com/?p=14314](http://www.warrenellis.com/?p=14314)

~~~
LeifCarrotson
Walking to the altitude of space and standing on something is very, very
different than accelerating to the velocity necessary to stay in space.

But that is an interesting point. On a super-Earth, with super-sized mountains
or super-thick atmosphere, it might be more useful to start from an extremely
high place so that you don't have as much atmosphere to deal with.

~~~
stcredzero
_Walking to the altitude of space and standing on something is very, very
different than accelerating to the velocity necessary to stay in space._

But an Olympus Mons sized mountain with a summit in 30 pascals atmosphere
would allow someone to more easily build a very dandy and useful mass driver.

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antisthenes
A bit disappointing. I was expecting them to elaborate on the nuclear
propulsion that might be used to achieve escape velocity for a typical Super-
Earth.

Wouldn't such a spaceship still need to have materials that are able to
withstand the heat/energy produced by nuclear propulsion?

~~~
abecedarius
Start with
[https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...](https://en.wikipedia.org/wiki/Project_Orion_\(nuclear_propulsion\))

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njarboe
Spaceflight from super-Earths might be easier (for a civilization to
accomplish) than from smaller planets as the difficulty of not using nuclear
power will cause nuclear tech to be developed. Development of this tech could
make spaceflight easier than the chemical rockets we have on Earth.

~~~
azernik
Why would non-nuclear power on such a planet be more difficult?

~~~
cfadvan
I think the idea is that non-nuclear power would be insufficient given the
rocket equation, so the only way they could have rocketry is if it was nuclear
powered.

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hinkley
This came up a couple weeks ago and blew my mind.

Doesn’t this mean the Drake equation needs to include the rocket equation?

~~~
inteleng
Would love to see your formulation of it!

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code51
Imagine a higher culture formed in a super-earth, keeping peaceful with the
earth and not leaving their home without a sufficiently eco-friendly
technology being discovered.

They are used to their gravity.

It is hard to leave so they find value in sustainability earlier on.

They only leave with super-advanced form of green technologies.

~~~
nickik
Going to space and living their forces the devlopment of advanced technology
that can make the world eco-friendly. Water recycling, factory farming,
resistant plants, nuclear power innovation, advanced solar power, syntetic
fuel production, fuel cells and so on.

The attitude that we should never do anything because its not green enough is
just naive. Would you have been sitting around in 1800 and tried to stop
people from explointing coal?

> They only leave with super-advanced form of green technologies.

We have that technology.

We could use nuclear energy as Nuclear Thermal rockets, we can use it as a
space battery, we can use it as a nuclear reactor to drive ion enignes, we can
use it on mars to make rocket fuel and so.

Ironoically envoirmentalist are partly responable that we almost use no
nuclear technology anymore. They advocated for 40 years against all use of
nuclear energy, and the most harmful effect is widespread misunderstanding of
risks. The political deadlock about all issues nuclear make it a nightmare to
deal with, and that's threw your supply chain.

A Nuclear Thermal rocket was planned to be a major part of Mars exploration
when von Braun thought about it in the 60s. Robert Zubrin who planned 'Mars
Direct' in the early 90s, want to use a nuclear reactor to make fuel on mars
and he was strongly avocating for NTRs in the second generation. Elon Musk has
said that he would want nuclear reactors on mars and NTRs in later
generations.

That's exactly the advanced green technology you are looking for. After our
first steps we should have moved onto that next level. Sadly it has not
happened and it will take a while longer. It is currently practically
impossible to devlop such a technology, unless you have direct support from
NASA and other governments agencies. They themselfs have little interest in
doing much in this space.

So, I think space people are happy to do it as greenly as you like, but then a
whole set of regulations have to change.

~~~
code51
I think most people understand the benefits and greenness of nuclear compared
to other forms of energy generation.

Yet it is also superficial to plainly say "nuclear is safe". That's obviously
more complicated than that. Chernobyl, Fukushima (widely known, 2011),
Marcoule (2011), Ibaraki (1999), WIPP (2014) etc.

These are not events like "Uber car killed a person but self-driving cars are
the future" thing, these events are actual evidence that human error plays a
huge part in dealing with nuclear systems.

Plus, when you think about the waste, it doesn't seem so green anymore. Maybe
greener when you're throwing radioactive stuff into space.

What I'm saying is, it's not a simple issue that "average people don't
understand" Actually we see that average people pick most pragmatic options
when economically pressed.

Regulations are there for a reason. Personally, I don't want spacecraft
throwing radioactive material onto our own atmosphere.

~~~
nickik
> I think most people understand the benefits and greenness of nuclear
> compared to other forms of energy generation.

No they do not. Even in France the majoirty of people believe that nuclear
causes more CO2. Another example is the impact of Uranium mining, there is a
waste overestimation of the imact on uranium mining compared to the mining you
would have to do for ANY other energy source.

There is a waste spread of misinformation about nuclear. Organisation like
Greenpeace, Sierra Club have spent the last 50 years spreading misinformation
with really very little pro-nuclear opposition.

> Yet it is also superficial to plainly say "nuclear is safe". That's
> obviously more complicated than that. Chernobyl, Fukushima (widely known,
> 2011), Marcoule (2011), Ibaraki (1999), WIPP (2014) etc.

Nothing is 100% safe. People say solar is safe all the time, yet it kills more
people then nuclear by a factor of 10x. And that includes all the deaths from
Chernobyl.

> Plus, when you think about the waste, it doesn't seem so green anymore.
> Maybe greener when you're throwing radioactive stuff into space.

Nuclear waste does not hurt nature or anybody at all. A lot of that waste will
be fuel for nuclear reactors of the future. We need a small amount of long
term storage, and the money for that has already been payed.

Its just a political dead lock that prevents a soluiton with practically zero
impact on the envoirment threwout the whole supply chain.

> What I'm saying is, it's not a simple issue that "average people don't
> understand" Actually we see that average people pick most pragmatic options
> when economically pressed.

That is actually totally false. Avg peoples opinions are not right outside a
'wisdom of crowds' kind of knowlage. If you have bias then the avg people are
wrong. The dangers of radiation and/or nuclear waste are a perfect example.

The relative fear of nuclear vs coal. Coal plants, if they were nuclear, culd
not operate because they are to radioactive.

Regulation exist for a reason, but that does not mean that they are not
harmful, self-contradicting or efficent.

Your blue-eyed view about government processes is contradicted by political
sience.

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lainga
dang, could you please unshout the title

~~~
dang
yes

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qrazhan
Are there any harmful effects the increased surface gravity would have on the
human body and if we were to live on these Super-Earths how would we mitigate
them?

~~~
headcanon
Humans raised on earth would find living there extremely unpleasant. The body
would adapt by becoming stronger but it would be painful simply existing, and
would wear out joints significantly. People with already poor health may be
unable to survive, depending on how strong the gravity is.

We obviously don't have any data on how humans raised in different gravities
would react, but its likely that humans raised on a higher surface gravity
would grow shorter, stocker, and more muscular. Humans on lower surface
gravity would likely grow taller, wispier, and more elongated, and would by
the same token find life on earth equally unpleasant.

 _The Expanse_ series does a good job of highlighting these differences in a
human way, since there are many people who grow up in asteroid colonies that
cannot live on Earth easily.

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soperj
First I ever heard of this idea was on this site. Crazy to see a paper about
it now.

~~~
dogma1138
My own estimate was 1.5-2 earth masses it's so awesome that there is now
research that proves something similar.

[https://news.ycombinator.com/item?id=12280432](https://news.ycombinator.com/item?id=12280432)

