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Comment by transreal | Hacker News Reader

With a planet this large, visiting it would be a one way trip due to the "The Tyranny of the Rocket Equation" [0]. I'm looking forward to the day we start finding exo-planets that are closer to Earth in size and which could potentially have space-faring races (and which we could leave if we were ever to visit them).

[0] - https://www.nasa.gov/mission_pages/station/expeditions/exped...

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It's 111 light years away. For all intents and purposes, that's a one way trip right there; irregardless of the rocket equation. Even at relativistic speeds, you can't come home anymore.

Theoretically, you can. At 0.99c 111 light years take about 15 years of ship time to travel.

You can make the trip arbitrarily small from your perspective (in theory at least :) ). The issue is that by the time you've made it back home, it's 222 years later. Hence "you can't go home anymore". :)

If you put some money in an interest-bearing account you could at least buy a new home when you got back.

You know where I can find one of these interest-bearing accounts that can outpace housing cost inflation?

While not an interest bearing account, investing in an index fund of global stocks should outpace housing. Investing in a REIT should roughly keep pace with housing costs.

Better do it in different countries and in different currencies and hope none of them get confiscated or have hyperinflation.

Or maybe a meal at a nice restaurant?

> If you put some money in an interest-bearing account you could at least buy a new home when you got back.

If you do it in a country that doesn't allow you to be declared legally dead when you are out of contact for a couple centuries, and if the institutions involved don't collapse, and if the interest on the account outpaces inflation, sure.

What percentage of the banks that were around 222 years ago have not failed?

From [0] we can see that there are 10 banks founded before 1700 and 24 from 1700 to 1800.

So it might not be that far fetched, assuming that the banks keep your accounts open after more than 100 years without use.

[0] https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

10 banks out of how many founded in 300 years?

I don't argue that it's probably a tiny fraction.

I was just pointing out that it might be somewhat possible for banks to exist for a span of time big enough for this.

Probably tiny?

Lloyds bank founded 1765

Barclays founded 1736

I doubt any banks in the new world are as old.

Berenberg Bank founded 1590 (429 years ago)

Monte dei Paschi di Siena 1472 (547 years ago)

So.... All western European then?

Bitcoin! :)

What if they release Bitcoin 2.0 the year you leave?

That's not the right question, though, because the expected rate of failure isn't constant throughout the lifetime of a bank. See, e.g., https://en.wikipedia.org/wiki/Lindy_effect

Seems like your best bet would be to park your money with several very old banks: https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

Also might want to think about purchasing bonds: https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

2nd link was supposed to be https://en.wikipedia.org/wiki/Perpetual_bond with information about a bond from 1648 with the issuer still paying annual interest.

As pointed out by other commenters: a non zero number have survived. A small investment made with many different institutions would substantially increase your chances of a massive return upon... return.

Not if you have negative interest rates. I heard on the radio that some European banks want to introduce this.

So you can survive the trip but even if you come back home, it’s not home anymore? Imagine leaving 222 years ago and coming back now ...

Many european cities would still exist and the mountains I grew up in front of too.

But granted, all people I know would be dead and some I could find in the history books maybe..

> and some I could find in the history books maybe..

Where you would also be, at that stage.

When the French invade Fishguard, Wales in 1797 our hero Gruffyd Rhys Llewellyn goes to get help in the only place where help may be forthcoming. The Heavens!

I should make a kickstarter, but I am already in my jammies.

I would recommend reading Noumenon by Marina J. Lostetter if you'd like to scratch that theoretical itch playing out one specific way.

I mean that sounds pretty awesome if you invested your funds well. I'd definitely be willing to be the first person who did that.

Where can you invest that would still recognize those funds as yours after 222 years?

You'd have to form some kind of trust, the "Swizec is in space for 200 years trust" to handle your investments for you with a constitution that lays out how long it is expected for you to be gone.

You might come back to discover that after several generations without oversight your trust has invested in bombing children and enriching the fund managers. I dunno.

You still get in trouble I think. Have any political regimes been stable enough in the past 222 yaers for the trust to survive?

All of Europe has gone through multiple revolutions, USA was only 30 years old back then so you wouldn’t have considered it ... that leaves what, the UK? Any parts of Asia that survived since 1797?

With travel time that long , you have to consider technological advancement at home. Just imagine that you look out of your window after travelling for 70 years and there is this other ship overtaking yours with ease.

Going down in history as "First to leave, last to arrive". Brutal

The colony of New South Wales was founded in 1788 and there is a fairly direct lineage of laws from there to modern-day Australia, and going backwards also a direct lineage of laws in Britain.

Nothing is risk-free, diversify! The likeliest outcome aside from coming back to nothing is you come back to an institution that is nothing like what it started.

It's been done actually. Ben Franklin established a trust that lasted over two hundred years by design.

https://www.nytimes.com/paidpost/franklin-templeton/investin...

I admit I have some bias here being British, and just assuming the banking system won’t go under. I think if I was actually going to execute on this I’d end up finding a bunch of countries with well-established trust law and spread it between them.

I'd like a pint of mead and a stout woman! /s

Wait, is this correct? Doesn't 111 light-years mean even light takes 111 years to travel through? And light travels at 1c.

It takes 111 light years from our reference point to travel that distance. If photons could perceive things, it would not take them 111 years by their perception.

i'm pretty sure the assertions means that the 111 years needed would only age the traveler by 15 years

Light does not experience time at light speed - it is stuck in a static moment. So from our stationary perspective, light takes 111 years to come from a 111 light year distance, but from light's perspective it takes an instant.

Every time I think I’m closer to understanding matters of time and space...

Go watch a Carlo Rovelli lecture on time and loop quantum gravity.

Time might not even exist in and of itself.

Just view light as like being in line at the DMV.

So, actually, like, it's actually really intuitive I feel like:

From the perspective of an object accelerating, newtonian physics works totally intuitively. If you had a rocket that could accelerate at 1g indefinitely, you just go faster and faster and faster and you get to any destination you want (even far away!) pretty quickly. And it would be a rather comfortable trip! You'd have Earth-like gravity the whole way.

It's really only the observer's perspective that things get confusing. When an observer watches something accelerate, they see it never going faster than the speed of light, no matter how fast it "actually" goes.

The trick is time. Time for slow things passes faster than time for fast things. A clock on a very fast rocket ticks much more slowly than clocks on (relatively) stationary things. That's how the paradox is solved.

Let's say you wanted to visit the Andromeda galaxy, which is around 2,500,000 light years away. If you had a rocket that could travel at 1g indefinitely, you'd get there in a comfortable 29 years! However, observers on Earth would see the trip taking around 2,500,000 years.

If you'd like to play with these numbers yourself, feel free to check out this neat calculator (not made by me)

http://nathangeffen.webfactional.com/spacetravel/spacetravel...

There is nothing intuitive about relativity unless you understand the physics and math behind it. Intuition can only be as good as your knowledge and experience.

Hey, look at this guy over here who's never been accelerated to a relativistic velocity here! But in all seriousness, you don't need to know all the math behind how relativity works to have a general idea of its effects.

Here's a quick analogy to help (loosely adapted from Brian Greene):

You're on a grass field, sitting on one of those riding lawnmower thingies, with a broken throttle. It's moving at a fixed speed of 1mph. You can't ever change its velocity. But you can steer it. If you are going precisely east-west, then it means you're not going north-south. The more you go north-south, the less you'll be going east-west. If you're going precisely north-south, it means you're not going east-west at all. One direction is traded against the other.

Pretty straightforward, right?

So here's the analogy: that grass field is a "dimension" in the same way that "spacetime" is a "dimension". The two "directions" of spacetime aren't "east-west" and "north-south", but "space" and "time". These are inherently traded against each other. The more you're moving through one, the less you're moving through the other.

So what about that constant-velocity rideable lawnmower? That's "c" -- the speed of light. You're always traveling at this velocity. If you are sitting still in space, then you are nonetheless moving through time. Your rate of movement through time is "c". But as soon as you start moving through space, it means you are moving less through time. This is exactly the same tradeoff as moving north-south vs. east-west. If you devote 100% of your "c" to moving in the direction of the "space" axis, then it means you're not moving on the "time" axis at all.

(This is basically all it means for something to be a "dimension": different axes that are traded against one another.)

This analogy can be used to understand quite precisely how movement relates to time dilation. (It also helped me understand e=mc^2. Why is "c" there? What does the speed of light have to do with the embodied energy of matter at rest? Answer: nothing is ever at rest; all static matter is moving through through time at the velocity of "c", and obviously that movement must have kinetic energy.) But it's not a completely perfect analogy. Weirder relativistic effects like length contraction and frame dragging need much weirder analogies.

I like this analogy, but I think an even bigger problem than distance contraction is that in fact the lawnmower can go east-west on one axis, but only towards the north on the other axis, towards the future, at least as far as we have observed in macroscopic systems.

This may not be a problem for some definitions of time, but for the notion of time which goes from past to future, I don't think the analogy holds very well.

Actually the analogy is a bit poorer than that, but also more consistent. You can only go one direction on either axis. Any movement in 3D space advances you along the "+space" axis, to the the detriment of your default movement along the "+time" axis. But just as there's no "-time" axis, there's also no "-space" axis.

Let me put it another way: from any point in space, we know you can move to any other point in space, in a finite amount of time (disregarding the accelerating expansion of the universe).

As far as we have observed, the same is not true with (the common-language definition of) time - I can't go back to the moment I was born, for example.

I think your comment about disregarding the accelerating expansion of the universe is a key point. We can't actually move anywhere in space; reacheable space is constrained by our reference frame relative to the reference frame of some other part of the universe that we're accelerating away from. At a certain point, the parts of the universe become unreacheable, because we would have to accelerate faster than the speed of light in order to reach them.

If my understanding is correct, this same condition exists inside the photon limit of a black hole. Technically you're still in navigable space -- not inside the singularity yet -- and can move in any direction. But to actually escape the black hole would require accelerating faster than the speed of light.

Again, if my understanding is correct -- and I'm definitely not a phycisist by any stretch of the imagination -- our movement through time is exactly the same phenomena. We can slow our velocity through time (by moving through space instead), but we can't escape our local reference frame without moving faster than the speed of light. If we could exceed the speed of light, then we would be moving into spatial regions which are otherwise causally inaccessible to us; in other words, we'd be going backwards in time.

So: if we could go FTL, we could escape from black holes, visit parts of the universe beyond the locally-observable limit, and go backwards in time. I think (IANAP) that these are all describing precisely the same thing.

Dunno if this helps. The lawnmower analogy has definitely broken down by this point.

Also, it makes me think: if our experience of time is navigationally equivalent to the experience of space for someone getting sucked into a black hole, does that imply the existence of a higher-dimensional universe where ordinary, non-accelerating time is as fully navigable as our ordinary "non-accelerating" space? And in that higher-dimension universe, are we living near the surface of some kind of singularity? Do the inhabitants of that universe wonder about how sad it must be for poor creatures like us, forced to live on a time gradient which inexorably slopes in just one direction, the way we might commiserate the fate of those sucked into a black hole?

Yeah, the experience of time to travel a distance approaches zero as your speed approaches the speed of light. It's really crazy to think about.

It is worth adding for the sake of clarity that light has no perspective or frame of reference because photons are non-inertial.

>but from light's perspective it takes an instant.

And for that reason they don't experience distance either. So the term 'sun-kissed' isn't actually that far off...from the photon's perspective the sun IS giving you a kiss.

Super fun. Great one. Actual replying to save this thread so I can go back and check some of this out. Looks neat.

And how much time will it take a person to reach that speed at human-safe acceleration? And how much energy would it take?

I calculate it to take about a year, with 1 g acceleration, but don't take my word for it, am curious to compare with a more authoritative answer.

Just under a year at 1g (according to google. I'm on mobile and would have to derive from F = ma on paper to solve). Energy is variable. I'd assume energy isn't relevant, as it's currently impossible, so we're taking that for granted.

Not really that long.

Not a physicist, but I think it might not be that simple. From our point of view, a ship with constant 1g acceleration increases its speed by 9.8 m/s every second, but people on a ship moving at a significant fraction of c take more than one of our seconds to experience one of their seconds. During the time they experience one of their seconds, the speed increases more than 9.8 m/s, so they must experience greater than 1g acceleration. I haven't done the math but it would be interesting to work out how the acceleration in our reference frame needs to reduce to ensure constant acceleration in the ship's reference frame, and how long it would really take to reach .99c without squashing the passengers.

Isn't a second now officially defined as a single period of atomic decay of a certain atom (I think Cessium?)? So this theoretical near-lightspeed craft could adjust it's accelerators based on an onboard atomic clock that would not be based on seconds as we tend to perceive them, but on SI seconds. I assume these would be getting longer as velocity approached 1c. Unless radioactive decay is also affected by relativity (obviously I am not a scientist), in which case of course that wouldn't work.

One of the consequences of the theory of relativity is that there is no "absolute time". The atomic clock will perceive time just as you will–that is, slower than an inertial reference frame.

Err, that isn't a correct understanding of physics, I believe. The ship wouldn't need to reduce its output as it gets faster relative to some other object in the universe. It can happily keep accelerating at 9.8 m/s for as long as it likes (or has fuel). No passengers would get squashed.

If the ship's means of propulsion is set to impart a constant force, then I understand that the passengers could experience a constant acceleration indefinitely, but that constant acceleration would be with respect to the ship's reference frame, not ours. A constant acceleration of 1g in our reference frame would mean by definition that the speed reaches 2c in two years, which can't be right. Subject again to the disclaimer that I'm not a physicist and haven't done the math, the only outcome I can picture is that in our reference frame the speed asymptotically approaches c in some interesting way.

Oh I see! 1G from our reference frame! Fascinating! That’s a perspective I haven’t seen before, I didn’t realize that that’s what you meant. Indeed, it isn’t possible. We’d see the ship slowly approach but never reach the speed of light, no matter how hard or long they step on the accelerator.

Length contraction is something outside observers would see; the people on the ship would see themselves as normal (and hence not be "squashed".)

It will take longer that people here have calculated. As you approach c the energy required to accelerate quickly rises. To reach c you need infinite energy.

Maintaining 1g of acceleration for a useful amount of time would require an extraordinary amount of propellant.

All of those estimates are tongue-in-cheek and are accurate if your energy expenditure is actually unlimited.

This paper has some interesting ideas. They want to manufacture really tiny black holes (radii of "a few attometers") and use Hawking radiation (which is inversely related to mass) to drive the vehicle. The black hole serves as both engine and fuel tank.

https://arxiv.org/abs/0908.1803

Which would put you in your 50s to 60s if all you did was go there and back.

And? Seems unlikely that we would send a small number of people on a 30 year trip. Given 30 years in a tin can with little to do, I'd expect the age range on departure to be ~18+ (with some people knowingly making a one way trip), and the age range on arrival back at Earth to be 0+, due to new additions to the crew mid-flight.

Heck, I managed that without interstellar travel.

accelerating at 1G to the mid point, then decelerating at 1g, will mean you’ll get there in about 15.8 ship years, or 115 earth years.

Spend twice as long on boats though (32 years) and you can do about 10,000 light years.

A constant acceleration of 1g would cause the ship to hit c in about one ship year.

The ship will never hit c.

If the ship travels tens of billions of light years will it eventually be traveling away from it's starting point at greater than c due to the dark-energy expansion of space itself? Or will the ship just red-shift more and more but never disappear from an observer with a very good telescope who stayed home?

And what if you drove your ship straight into a super-massive black hole?

One theory of the far future (Big Rip) is eventually nearby galaxies will be receeding faster then light, then stars, planets, then even atoms.

There's a short story about the Big Rip

https://web.archive.org/web/20080725045740/http://www.solari...

The fastest spacecraft humans have produced is the Parker Solar Probe which when it zips around the sun will reach 430,000 mph. At that speed it would take 173,000 years to reach this planet.

Great! , do that gravity assist a few more hundred times around the sun and we are there in 500 years.

You can't do a gravity assist around the sun once you've reached solar escape velocity, because you won't be visiting the sun again.

If we could pull that trick so easily, we already would have.

Well... you could wrap around another star, or two, and come back to this sun for another assist a la Rendezvous with Rama.

It is also the slowest spacecraft. Speed is relative. To get it going that 'fast' when close to the sun the other half of its orbit is correspondingly slower than earth. When launched it accelerated away from earth at speed, but from the perspective of the sun is was actually decelerating, and was moving much slower that it was when attached to the earth.

We're talking about just escaping the surface gravity, not returning to Earth.

If you can get there from 111 light-years away, you can probably also solve the higher surface gravity. While the former problem is theoretically solvable in a straightforward way, both are impractical with our current understanding of physics and material science.

regardless

But that's assuming you're traveling at the speed of light, right? In reality it would take much much longer than 111 years.

The fastest man-made object: https://en.wikipedia.org/wiki/Parker_Solar_Probe

Travelling at that speed (0,00064c - no relativistic effects) would take ~170,000 years.

At 111 light-years, we need to create a vehicle that can sustain several generations.

> I'm looking forward to the day we start finding exo-planets that are closer to Earth in size and which could potentially have space-faring races (and which we could leave if we were ever to visit them).

I just realized I have no real concept of how many stars there even are within, say, a 100 light year radius of our sun (I guess that's a more realistic thing to find out than the number of planets).

A quick search provided some estimates and they're kinda... disappointingly low, at around 20000 stars. That's a number where some "1% of 1% of 1%" kinda filter quickly ends up in a scenario where a planet fitting all our criteria might simply never be in reach. For something more "realistic" (I know, heh!) like 20 light years, there are only 150 solar systems. I've seen different numbers and have no idea how they're calculated but for the usual astronomic scales which quickly go into "billions" territory, it seems we're kinda stuck with a comparably small list of candidates.

It might cheer you up to to think we have yet to prove there is a star with no planets.

On the plus side there are more moons than planets and many of them may be habitable. We've barely started looking at extra solar Jupiter like planets because of the much longer orbital periods.

> That's a number where some "1% of 1% of 1%" kinda filter quickly ends up in a scenario where a planet fitting all our criteria might simply never be in reach.

It might cheer you up to think that's the only reason the human race happens to be the one in our neighborhood that made it into space, without being stepped on by an Old One.

I think people are beginning to realize, that just as there are more asteroids than planets, and more space junk than large asteroids, there are a lot more free floating planets than stars.

It could well be the universe is filled with life, but the dominant mode is underground chemo/radiotrophic microbes on planets without stars.

I made it well into this decade before I was made aware of this fact. It's kind of a shock, still. At some point your planet is massive enough that you can't get into orbit with chemical rockets (even, I think, by flying them up like Burt Rutan?).

The implications for the Drake equation are pretty big.

Rockets without any promise of ever being able to break orbit are good for what, war? Would you keep developing them? Would you give up dreams of the stars? Would you look for intelligent life you couldn't ever possibly meet?

"At some point your planet is massive enough that you can't get into orbit with chemical rockets (even, I think, by flying them up like Burt Rutan?)."

Nuclear rockets don't seem to be very hard. They're somewhat dangerous if they explode, but they aren't very hard. Fairly solid prototypes were built decades ago and there's little to suggest they couldn't have been made production-grade [1]. We'd have them now if we didn't find the risk/reward to be too highly slanted to the "risk". Other species and other ecosystems may come to different conclusions, e.g., an ecosystem already more exposed to radiation and evolved to deal with much higher levels of it may judge it much less "risk" for some radionuclides to be scattered across the landscape in case of failure.

What can be more of a problem is being in a place where you have no obvious access to technology at all. However smart our cetacean buddies may be, it is not clear even at this point in the 21st century what path to technology they could possibly have from their starting point. "The literature", a.k.a. "science fiction" has hypothesized breeding programs to develop various tools, but it's still not entirely clear how they'd get from "breeding useful jellyfish" to, well, anything like technology as we know it. It's possible we're just not solving this problem because we don't have to, maybe there's some easy path with the right development path, but it's still not clear what that would be.

[1]: One of my markers for "the space age is truly here" is when we lift a nuclear rocket into space, sans fuel, and fuel it with space-sourced radionuclides. Earth-bound citizens will still complain, because "NUCLEAR BAD!", but their complaints will be ignorable at that point.

Do solids settle out of air on a high gravity environment faster? The air would be thicker. Does gravity or buoyancy win than tug of war?

I spent a day once trying to figure out what the Bronze Age would be like for marine creatures. Oxidation is less of a problem but galvanic action is huge. Fire pretty much doesn't work, which blocks a whole bunch of precursors like ceramics.

Atmospheric density it a little more complicated: Venus has the same gravity as Earth but a thicker atmosphere, while Titan is a lot smaller and still has a surface pressure 50% more than we do at sea level.

Couldn't marine creatures learn to collect air bubbles underwater in order to use fire?

How do they know that will lead to fire?

The big problem with marine technology isn't that it's totally impossible, it's that there's vast gulfs between various achievements and little sign that continued progress on some matter will lead somewhere. You could raise jellyfish to be transparent and lens shaped and build some telescopes, but how do you figure out that's a thing that might be a good idea? You might be able to turn an ocean vent into a forge, but how do you figure out that's a good idea? We had a path where we noticed certain rocks in a fire ooze useful metal, for instance. We didn't deduce from first principles the Periodic Table, guess the properties of metals from logic and maybe our interactions with (very soft!) silver and gold, determine it was likely that some of those colorful rocks are metallic salts, and then determine they might be useful to mine. We found they were useful to mine, then after thousands of years of civilization built on top of the resulting tools, only then figured out the why of a lot of those things.

This is one of those places where it's really a good idea to understand that despite the pretty Just So stories where science pre-dates engineering, in reality, engineering extremely frequently has predated science, at times by centuries. How are water-bound creatures going to figure out enough engineering to even get science going?

Certainly, as I said, they can breed things, but how do they even know where to try to go? How do they maintain the discipline to breed things over hundreds or thousands of generations? How do they get to genetic engineering?

There may be answers to this question but they sure aren't obvious.

So long as you're carrying all the fuel on you, right? If I can launch 1000 auxiliaries that resupply you and a 1,000,000 auxiliaries to resupply the 1000 auxiliaries I can go up farther.

So that's purely for a chemical rocket, right?

Is there any combination of tricks that can realistically push the envelope there? For example can we use a space elevator to start higher/faster (or, I don't know, balloons? a catapult or railgun or something?), laser power delivery from the ground, so we don't have to carry all the fuel, and an orbiting way-station for refueling, etc.?

Solar sails might be able to beat it.

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

Aren't solar sails just for propelling yourself once you're in space? I can't imagine how you could launch off the surface of a planet with a solar sail.

Yes, and even then they're incredibly low thrust devices. A light sail (whether solar or laser powered) may well be viable for sending a tiny unmanned probe to another star system (see [0]). It's really unlikely that it will ever scale up enough to take an average communication satellite the same distance, much less a manned craft.

[0] https://en.wikipedia.org/wiki/Breakthrough_Starshot

M2P2 was going to test out magnetic sails, but Wikipedia is telling me they generate less thrust per kilowatt hour than ion thrusters. Explains why I haven't heard anything further about it.

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

Solar sails are good for inter-planetary travel, but they aren't going to move the needle for launching off of a planet with 2g gravity! You need something that is compact and would give you a big impulse.

As a KSP engineer would say, it "needs more boosters" https://i.redd.it/zuymxc5bb7s21.jpg

Exactly

From the reference article:

> Travelling from the surface of Earth to Earth orbit is one of the most energy intensive steps of going anywhere else. This first step, about 400 kilometers away from Earth, requires half of the total energy needed to go to the surface of Mars.

Which means that if we use something like a balloon/blimp in the first stage, it would be a lot more energy efficient.

Anyone knows why it's not done that way already?

Also, whatever happened with the plane+rocket Virgin Galactic project?

Orbit isn't a height, it's a speed. 7 km/sec for low earth orbit. A balloon floating you higher while not making mach 20 isn't much of a shortcut.

How poor of a shortcut is it? I'm not really clear on what the math would look like. Would it just be better by approximately the ratio of gravity's strength at the surface vs the height of the balloon?

Basically just saves what you would've lost to wind resistance and gravity while getting to that height, while adding the complication of accelerating from 0 to orbital before falling back into the atmo.

Space elevator ideas usually have the hop-off point all the way out at geosynchronous orbit to solve the velocity problem. Which is.. a really tall elevator.

Better than the miniscule gain in potential energy (height x mass x gravity) because you also save a lot of aerodynamic drag not punching through the lower atmosphere at high speed and some "hovering losses" (rise time x mass x gravity, I'm sure there is a better term for that). But still not worthwhile due to the difficulty of floating a full size rocket. If you had a very very high mountain or tower, moving that full size rocket to the peak could be worthwhile. Theoretically, a civilisation trapped in a gravity well too deep for a solution to the rocket equation could dig themselves out by reshaping their planet from a roughly spherical geoid into a disc or rod (or into a torus for the really adventurous)

I doubt such a planet would be hydrostatically stable, though…

> "hovering losses"

Gravity drag

most of the energy spent getting to that 400km low earth orbit is spent accelerating to very very fast lateral speeds. If you use a balloon to get your rocket up as high as you can before launching it you limit the weight of the rocket you can lift up there, and most of the weight of rockets is used to gain that lateral speed. The plane + rocket idea has been shelved for now, as reusable rockets have decreased the price much more than launching a small rocket from 40,000 feet.

Isn’t that orthogonal to interplanetary travel though?

If you go straight up far enough you’ll be out of Earth’s gravity well.

You’ll still need acceleration to escape the solar gravity well, but you’ll never need horizontal acceleration necessary for Earth orbit.

>If you go straight up far enough you’ll be out of Earth’s gravity well.

While you could theoretically do this by accelerating directly up, You still have to accelerate somehow.

Understood, but somehow the discussion ended up around achieving orbital velocity instead of escape velocity (the one relevant to the original sub-thread topic), two very different numbers (though escape velocity is ultimately a higher number.)

https://physicsabout.com/orbital-escape-velocity/

Sure, but they're not orthogonal at all. One is just a lot more delta-v than the other. It's the same basic thing. Point the rocket nozzle in the opposite direction that you want to go and turn it on (modulo the fact that the "direction you want to go" might not be intuitive because orbital mechanics).

You want to accelerate lower down in the gravity well, however not too low due to atmosphere losses. It’s more efficient that way.

Launching from stationary altitude doesn’t save much at all

Because the problem with space is not at all the altitude necessary to reach it, but it is the speed necessary to remain in it.

So a balloon/plane/blimp/very high building will only help very marginally.

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

The Pegasus rocket used to launch satellites from a modified 747 for between 1990 and 2016 [1]. It just doesn't work out well in practice because the cost of the modified plane far exceeds the cost savings from a first stage unless you're doing half a dozen or more launches per year.

[1] https://en.wikipedia.org/wiki/Pegasus_(rocket)

Virgin Orbital are doing something similar - satellite launching from a 747.

Launching from a mountain is a possibility ( I think China does it)

Realistically, but not plausibly unless its an emergency, Thermonuclear bombs:

https://www.youtube.com/watch?v=EzZGPCyrpSU

Nuclear bomb rockets would probably do https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

> For example can we use a space elevator to start higher/faster (or, I don't know, balloons? a catapult or railgun or something?), laser power delivery from the ground, so we don't have to carry all the fuel, and an orbiting way-station for refueling, etc.?

These methods will all help with the first 1% of your problem, getting off of the earth.

But you need so many orders of magnitude more energy to reach the kinds of velocity needed to get to another star in less than a million years. It's just an unfathomable amount of energy per kg. Put simply: if you can get to another star, getting off the planet is nothing.

There's also the Wait Calculation to consider:

https://en.wikipedia.org/wiki/Interstellar_travel#Wait_calcu...

Hey, I like that idea.

Maybe it implies that every interstellar mission is just an in-flight rescue mission.

That's interesting in theory, but as far as I know our interstellar propulsion technology hasn't advanced significantly at all in the last 50+ years. You can't do theory in a vacuum forever. I'd argue that unless we launch an interstellar something, we're never going to see any technological advances in the field.

The wait calculation implies that the returns from economic growth will eventually be converted into rocket fuel. A fun equation, but economics doesn’t translate well on time scales where depreciation of capital is 100% and becomes another expense.

Surface gravity will be around 2g, so even if it has a rocky surface it's not going to be a very comfortable home from home.

So a 150lb person will weigh 300lb. Sure that's gonna suck on day one but there's plenty of people who weigh that much who get by. Without all the health complications from high body fat it wouldn't be that bad. You'd probably die young but I don't think that would bother people.

Not sure the two are comparable. 300lb obese person's bone structure weighs the same, their heart and lungs weigh the same. I suspect 2x the weight on every organ is going to be seriously detrimental for sustained periods.

Not to mention you'd go from years and years of near 0G gravity to twice the gravity of earth. You wouldn't even be able to walk.

Compare to the difficulty of getting there in the first place, getting spin gravity to work right isn’t a big problem.

I would assume if we can figure out accelerating to near the speed of light, we'd also be able to make a viable rotating spacecraft to use centripetal force to solve that problem.

If you're making this trip in any reasonable amount of time, you're probably accelerating during the journey at least half a G anyway.

You're not the only thing that weighs twice as much; everything else does too.

I would imagine the atmospheric pressure would be the most noticeable consequence of this.

The article notes that the planet is substantially larger:

> K2-18b is very unlike our home world: It’s more than eight times the mass of Earth, which means it’s either an icy giant like Neptune or a rocky world with a thick, hydrogen-rich atmosphere.

And the Wikipedia article for "Super-Earth" mentions something relevant:

> a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar

For comparison, the atmosphere on Earth (sea level) is approximately 1 bar.

It looks like the apparent atmospheric pressure on such a planet might be similar to being approximately 200 kilometers below the surface of the ocean on Earth. For additional perspective, the Mariana trench is (I believe) the lowest point on the planet, and is only like 11 kilometers deep.

So I guess what I'm saying, is that the apparent doubling of one's weight would be an insignificant concern in the grand scheme of things.

Given the published figures for size and mass, it will be closer to 1.5 - 1.6 g. Still tricksy to walk for unmodified people, but not impossible, especially given how long it will take to reach it. Plenty of time to work out how to change people.

I wonder if we could work around the stronger gravity by using a spaceship as the anchor for a space elevator or skyhook? Both of those technologies are pretty far off, but so is getting humans to another star system.

As gravity increases the tensile strength necessary for the elevator increases. We haven't even figured out how to mass produce materials that would suffice for an elevator on earth yet.

I mean, we're already talking about getting humans to another star 111 lightyears away; I'd assume by the time we're ready to do that we'll have figured out how to mass-produced nanotubes or something.

currently we have no known materials that would actually be strong enough to support their own weight at the length of a space elevator on earth. double the gravity and the length will also at least double (possibly quadruple? not sure on the math here) so it may also be impossible to build a space elevator on one of these planets, at least without the use of active suspension (possible? theoretically.)

Currently we have no known materials that would actually survive a 111 light year trip to another solar system, so that fact that we can't actually build the space elevator when we get there seems irrelevant. They're both roughly the same order of magnitude of impossible with our current technology.

Sure we do. Chuck a rock in the right direction, it'll still be a rock 111 light years later.

Okay, I should know I have to be 100% pedantic when discussing these kinds of topics, so that's my bad. I amend my claim to the following:

Assuming we could generate a sufficiently focused laser or other communication mechanism to communicate over 111 light years (which we can't), currently we have no known material that we could build the comm device out of that would survive a 111 light year trip to another solar system intact enough for the device to actually function.

Still a bit wishy-washy since it involves magical technology that we have no idea how to build, and if we had the communications technology we would probably have the materials science too, but hopefully that gets the point I was originally trying to make across.

Thanks for the clarification :)

If the rock is moving at a relativistic velocity then it will be gradually ablated by collisions with interstellar particles. That's why any realistic design for a starship requires some kind of bow shield.

I mean, as far as I am aware, that problem is due to the propulsion method. Would a nuclear based rocket not be able to solve this issue? My understanding is that it would.

A nuclear rocket still needs propellant, doesn't it?

Yes, but significantly less of it by weight. You get significantly more energy per kg of nuclear fuel than you do traditional chemical propellant.

Perhaps, but my intuition is it wouldn't make a whole lot of difference, because the reason you can't get off a heavy planet is because of an exponential runaway effect.

https://www.nasa.gov/mission_pages/station/expeditions/exped...

An experimental nuclear rocket from the 70s nearly doubled our "payment energy". It should greatly reduce the initial and total mass portions of the rocket equation as well. I don't have all of the numbers to punch into the rocket equation to figure out things exactly, but the exhaust velocity and initial/total mass make up significant portions of the equation, and increasing the former while decreasing the latter will make significant impact on the ability to leave a more massive planet.

Or we could shift to an other side of the universe by "inverting" the mass of the ship where the speed of light is much higher[1].

Cosmology is so cool... too bad we do not have time for that: we cannot even cure the common cold!

[1]: https://januscosmologicalmodel.com/pdf/2014-ModPhysLettA.pdf Cosmological bimetric model with interacting positive andnegative masses and two different speeds of light,in agreement with the observed acceleration of the Universe

> Special Relativity can be summed up in the sentence: “We live in a spacetime which is an M4 manifold with a hyperbolic Lorentz metric of signature (+−−−)”. General Relativity can be stated accordingly: “The Universe is an M4 manifold with a Riemannian metric of signature (+−−−)” which is a solution of the Einstein equation:Rμν−12Rgμν+Λgμν=χTμν.

It sounds like you get anti-gravity for free along the way to getting superluminal travel.

No because no "run away" effect. "inverted" mass means that the mass goes on the other side... gravity works the same there.

I couldn't find anything with its approximate radius (it mentions about-earth sized and 8x the mass but that could mean anything) to make an evaluation of the likely gravitational force at the surface in order to make the evaluation of whether you could get off the planet cheaply.

> With a planet this large, visiting it would be a one way trip

Assuming chemical propulsion and no refuelling at the destination.

If it's chemical propulsion, you're dead when you get there. If there's no refuelling (and you need refuelling) with some other travel mechanism, you can't get back.

So we have an "or" assumption, not "and", with an additional and assumption about refuelling. That's how I read it.

I think what the GP refers to is that there isn’t enough energy in chemical rockets to overcome its gravity, even flight might not be even possible albeit that’s also dependent on the density of the atmosphere to some extent.

Flight would actually be easier on such a planet. The advantage of increased atmospheric density is greater than the downside of the increased gravity.

So... if one made a very aerodynamic craft, and launched at a more extreme lateral angle, do you think it would supplement the chemical rockets enough that escape would be possible?

not really going to orbit is about velocity not altitude the gravity at low earth orbit is pretty much the same as at sea level.

The savings you get when launching from say an aircraft at 40,000 feet mainly come from not having to go through max-q at sea level the relative amount of propellant you’ll need to get to orbit is the same you can just use a smaller rocket but it doesn’t help to overcome the rocket equation trap.

But at altitude you do get higher ISP, which is a greater limiting factor than drag/max-Q. The Saturn-V would not even get off the pad in an even slightly denser atmosphere. But lift it up to where the air is thinner, where its engines can generate more thrust, and it might have enough DeltaV to get something into an orbit.

That is of the atmospheric density is higher, we don’t know the composition and temperature don’t forget that at higher pressures the boiling point of water is higher so it might not actually have water vapor in the atmosphere.

The atmosphere can also be much more shallow than earth.

Also I was more referring to winged flight than balloons balloons might be a problem of their own if the pressure at ground level is too high for them to inflate normally.

Between earth and Venus there are a lot of options so if the atmosphere is similar to earths sans the water vapor I’m not entirely sure flight would be actually easier I can probably do some napkin maths over the weekend for this.

The amount of fuel at the destination doesn't change the rocket equation for being able to get into orbit from the planet. From the provided link:

"If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. <snip> That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

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