Spaceflight from Super-Earths is difficult | Hacker News Reader

Spaceflight from Super-Earths is difficult

arxiv.org

58 points_lbaq88 comments

Scott Manley has a pretty accessible Kerbal Space Program video about this: https://www.youtube.com/watch?v=amjuJJwI3iM

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?

It's because you need to not just get to space, you need to be in orbit.

If you just take a magic balloon up to a certain height, once you're up there and let go fo the balloon - you just start falling back down.

You need to have a lot of sideways velocity in order to actually get into orbit, to be revolving around whatever body you're trying to get off of.

(It didn't used to make sense to me, either.)

Imagine one of those games where you put a nickel in and watch it circle around a slope until it falls into a hole (for charity!). What's happening is the nickel's forward momentum being lost, resulting in succumbing to the gravity well.

Launching a rocket to orbit is like starting from the bottom of the well, and spinning around the slope really fast until the nickel is returned to your hand.

Xkcd explains orbital speed in a much detailed explanation.

https://what-if.xkcd.com/58/

The gist is that gravity pulls you. To go to space you have to escape gravity. Even if you go thousands of miles above earth, gravity will still pull you. To counteract gravity, you need to go really fast sideways.

When you spin a ball attached to a string, the faster you spin, the more the ball pulls the string away from your hand. If you spin it really really fast, the string breaks eventually and the ball flies away.

The string is gravity.

That's the best ELI5 analogy for orbit I've ever heard!

See here for early 20th Century experiment validating the outcome when you let go of the balloon: https://lizaknapp.files.wordpress.com/2015/10/pooh-with-bees...

Couldn't you theoretically escape a planet's gravity without going into orbit? I mean, gravity weakens by the square of distance, so it wouldn't be long before Earth didn't have much influence on you

Sure, just reach escape velocity (well, in the right direction). It's higher than orbital velocity, obviously. To escape the solar system you'll need a bit more velocity on top of that.

We do this all the time with probes to other parts of the solar system. Manned missions (Apollo) used a small number of earth orbits before doing the burn to get to the moon primarily to ensure that systems checked out before making the bigger step.

Escape velocity will work in any direction (assuming no wind resistance)

Unless you hit the ground, or a mountain, or a plane, or the moon, etc.

Hitting the ground was exactly what I was trying to get across. Otherwise, discounting pesky atmospheric drag, your vector doesn't matter.

The amount of fuel (delta-v specifically) to reach orbit from a given altitude is always less than the amount of fuel needed to escape the planet's gravity.

No, you would just be in a different kind of orbit. You would be in Earth orbit until reaching escape velocity, at which point you would be in Solar orbit.

so are you saying that even though I could escape Earth's gravity going at sub-orbital velocities that I would still need to be going fast enough to avoid being pulled into the sun?

Disclaimer: the entirety of my orbital mechanics knowledge derives from Kerbal Space Program. I cant recommend it enough if you're really interested in building an intuitive understanding for this stuff.

But no, it's not that you would be pulled into the sun. You would simply be in a solar orbit just like the earth, unless you continued accelerating and reached solar escape velocity (~600km/s). You can think of gravity like successively larger "wells" which you must escape from if you want to go anywhere. You can't just point a spaceship at a planet and go there.

> solar escape velocity (~600km/s)

That's for escaping from the surface of the Sun. At Earth's distance from the Sun, the velocity needed to escape from the Sun's gravity is only 42 km/s.

> You can't just point a spaceship at a planet and go there.

Disclaimer: You can, you just need so much power it’s like flying with a helicopter from your bedroom to the kitchen, absolutely wasteful.

The main question at that point becomes: how can you carry that much fuel?

Depends on how efficient the engine is. If you're riding a nuclear rocket, you don't need that much fuel.

Theoretically? Fusion reactor + ion drive. Throw enough power at those and you can accelerate for a long time.

One thing you learn from KSP is it requires less energy to fly into to alpha centauri than fly into the sun.

Getting the Parker Solar Probe down to the sun will take 7 flybys of Venus to use its gravity to slow down, over 6 1/2 years.

http://parkersolarprobe.jhuapl.edu/The-Mission/index.php 'Journey to the Sun.'

You don't escape Earth's gravity at sub-orbital velocities. Escape velocity is always larger than orbit velocity.

What the GP said is that if when you reach Earth's escape velocity, there is still a long way to go until you have Sun's escape velocity (so you are on a solar orbit), and after you reach that, there is a huge distance until Milk-Way's escape velocity, and an even huger way until you escape the Local Group.

> even though I could escape Earth's gravity going at sub-orbital velocities

That's not what he said. Escape velocity is higher than orbital velocity (as ythn pointed out upthread). And escape velocity from Earth is still slower than Earth's orbital velocity around the Sun, which in turn is lower than escape velocity from the Sun's gravity at Earth's distance from the Sun.

You can’t escape Earth’s gravity going at sub-orbital velocities. Earth escape velocity is higher than any orbital velocity.

This is what I was getting at. The article mentions escaping a planet's gravity, not orbiting it.

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.

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.

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.

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.

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.

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

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.

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.

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

https://www.quora.com/Escape-velocity-is-supposed-to-be-24-0...

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...

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...

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

Ok, let me ask a very straight and simple question.

Imagine that I have a spacecraft with imaginable engine which is able to indefinitely provide thrust enough for this spacecraft to move straight up from the planet’s surface at the constant speed of 1 mph.

Will this spacecraft leave the planet in indefinite amount of time or not? If not - what is going to stop it?

You will have no trouble then pointing out the precise mistake in the calculation.

You can reduce delta-v it a little by floating to the edge of the atmosphere. Beyond that, there is no method of getting higher that does not involve gaining some speed.

But, ignoring things like air resistance, the lower the thrust is, the more delta-V/energy it takes to escape?

For instance, if you had a rocket with a fixed delta-V and it used all of its energy almost instantaneously, say it escapes from Earth. But if you lower its thrust so that it is not accelerating upward (lower than the force of gravity), it could use the same delta-V without leaving the ground.

No, it’s the things you’re “ignoring” which keep it on the ground. A solar electric thruster provides less thrust than the surface weight of the spacecraft it is pushing, but it is nevertheless able to push a probe to escape velocity and beyond over months or years of continuous thrust.

If it's in orbit, yes. I thought the discussion was based on something sitting on the surface of the Earth.

That’s what I was talking about.

The further you get from the planet, the lower escape velocity becomes. At some point you will be going faster than escape velocity.

Indeed -- this is illustrated in my example below where one moves an object at 1mph (fixed speed) for about 283 million years until it's 4 * 10^12 km away from the Earth, where the escape velocity is right around 1mph.

Escape velocity is the total amount of acceleration necessary to escape a gravity well, aka the minimum amount of energy.

No, it's explicitly defined as a speed or velocity, not as acceleration and not as energy.

A velocity is the total of acceleration over time (as was referred to). A mass (the spacecraft) moving at a velocity has a given amount of energy, and in this context may be used interchangeably.

This is just pointlessly pedantic. You are going to get into all sorts of problems if you arbitrarily substitute any of velocity, acceleration or energy for any of the others. Having unstated dependencies is only going to lead to avoidable confusion.

The key insight revealed in the term 'escape velocity' is that it is independent of the mass of the body. It is also independent of how quickly it acquires that velocity.

>Speed is mostly irrelevant if you can provide propulsion indefinitely

Except you can't, so speed is very definitely not irrelevant.

It would probably be enlightening to read the sentence that immediately followed.

You are in space right now, you just happen to have 100km of atmosphere above you. When we say "go to space" we really mean "go to orbit". To go to orbit, you need tangential velocity to get away from the body you're currently attracted to. We use rockets to gain that velocity because no other technology we know of can provide it with the required thrust/weight ratio.

The absolutely best thing you can do to really internalize what achieving orbit make is to play a bit of Kerbal space program. No sarcasm, it's an eye opening experience the first time you make it.

Gravity + tyranny of the rocket equation.

http://www.physlink.com/Education/AskExperts/ae158.cfm

Escape velocity is the inverse of terminal velocity of an object falling from infinite height (discounting air resistance).

https://youtube.com/watch?v=uWjdnvYok4I

Besides what others have said, the surface of the Earth is already quite high (distant from the center). Getting above the atmosphere only adds ~1% to your height (if you're 40 miles up). OTOH the Earth spins at only a small fraction of the speed you'd need: in low Earth orbit you're going around in a little over an hour, versus 24 for the spin.

It basically amounts to falling down, except you're moving sideways at a fast enough clip to miss the earth entirely. Just being up the right distance isn't enough, You need the sideways velocity to make sure you don't hit the planet.

This is the simplest way to explain it, I think. https://www.youtube.com/watch?v=xfZHUwyh59k

Here read this. https://what-if.xkcd.com/58/

That was a good read. Thanks

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?

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?

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

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.

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.

> 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.

The paper tackles this. Mountains should be shorter on a super-Earth.

The next logical step after this is a space elevator. Because the structure is mechanically linked to the rotation of the planet below, you can achieve orbital velocity simply by climbing up to geostationary altitude and pushing off. I'm almost certain that there are physical limits that prevent such a thing from forming naturally, but I don't know how close to it something like a mountain can get without collapsing under its own weight.

Definitely, though being able to walk to the edge of the atmosphere could let you do things like use rail guns to get payloads up close to orbital speeds

No, when you walk up to space using a sufficiently tall mountain you automatically get the necessary speed. You just need a mountain that takes you up to geosynchronous orbit. That's the whole point of a space elevator. Even if the mountain doesn't take you up to that elevation, you gain speed in the same way as starting from the equator helps.

> On a super-Earth, with super-sized mountains or super-thick atmosphere

Not a physicist nor a geologist, but I feel a super earth would have a harder time cresting higher peaks due to gravity’s increased effect. It’s not going to look like “earth, only uniformly bigger.”

I would imagine spaceflight would be much cheaper, more reusable, and widespread in general - space elevators would be much easier to construct, as would fully reusable SSTOs; even if the gravity remains too strong or material sciences too undeveloped for either to be feasible, since the cost of reaching orbit changes exponentially based on surface gravity.

Also of note is that interplanetary transfers would be cheaper, thanks to a lower escape velocity.

A rather utopian view envisions a society of many space elevators scattered across the world, with orbital stations attached via monorail-esque shuttles. Spaceborne hubs with electromagnetic launchers could provide cheap transit beyond the immediate gravity well, while autonomous solar sail harvesters could collect asteroids for resources. Orbital solar farms might provide power on a massive scale for the surface, and lenses or mirrors could be used to assist in terraforming bodies further from the system's sun (for example, a Europa-equivalent).

For planets without a significant atmosphere, you get to do fun stuff like Arthur C Clarke's Lunar mass driver [1].

[1] https://en.wikipedia.org/wiki/Mass_driver

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?

Start with https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

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.

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

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.

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

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

Would love to see your formulation of it!

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.

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.

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.

> 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.

We can send a person to space, but we can't colonize another planet yet, so our capabilities are virtually the same as in the premise you state.

dang, could you please unshout the title

yes

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?

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.

Imagine hanging a weight on every cell and fluid in your body. I don't think people would live very long. We already know there's damage (eyes) after months in space due to a reduction in gravitational effects.

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

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

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