3 hours ago, swansont said:

Further, I think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true. You can’t engineer your way around e.g. a violation of conservation of energy, or momentum

Or the actual money available. I once had a struggle with someone who told me it was feasible to move Earth's present population to other planets (making Earth a sort of wilderness park). Other aspects aside, I pointed out the math on amounts of propellant, technology and resources needed just to keep pace with net population growth of 190,000 per day, i.e. just hold population at 8.3 billion.

The lowest price currently to get a mass to LEO is $1000/lb. Say through various engineering miracles it was cut tenfold to $100/lb. So, just to get to LEO (let alone suitable locations for space habitats or other planets) would in that reckoning cost an average adult person (140 lb) $14,000. A journey to a viable habitat and the cost of building it would easily be a hundred times that.* So, $1.4 million per person, an extremely conservative estimate. The median net worth of a US citizen is $190,000. One may safely assume it's lower for the global population.

This kind of disconnect with actual costs of anything space travel related is probably due in part to the human tendency to fill in gaps in knowledge with "a miracle occurs here," - in my example there are both engineering and economic miracles.

*(for a sense of the scale on this, consider the cost of moving somewhere on Earth to the cost of the home you're moving to - now think on how that ratio might compare to moving to Mars or an O'Neill colony or whatever)

3 hours ago, Moontanman said:

I agree, but how do these things prohibit star travel? There are ways to star travel without violating the laws of physics. Slow boats are an example, we might not be able to do it now but it's mostly engineering problems not the laws of nature.

And there’s no science here to back this up.

But there could be. You still have to have fuel and reaction mass to travel, to slow down at the destination, to do a controlled entry of a gravity well and to leave. None of that requires knowing the specific energy source, but the analysis could tell you how efficient of a method you’d have to use, which rules out certain (perhaps all) approaches

But a blanket statement? Not scientific.

5 hours ago, dimreepr said:

Really?

Intelligent life has to evole somewhere, that retains an atmosphere to live in, long enough to develope an intelligent approach to escaping a gravity well that retains an atmosphere, but prohibits an escape,,,

Seems like a fine balance...

I agree the balance may indeed be fine, maybe there are planets that require nuclear power to escape... my idea of colonizing space happens after the the beings escape their planet. In fact you have hit upon why a space faring civilization might want to avoid planets. coming and going from a Earth sized planet require enormous amounts of energy, it would be much easier to travel between space colonies than between planets.

Once a civilization begins to colonize via space habitats the game changes from going in and out of deep gravity wells to using things like ion drive or hall thrusters or maybe things we do not see happening yet.

As the integrity of space colonies gets better and better at total recycling, probably won't ever be 100%, the idea of spanning space in the home you already live in becomes much more attractive to certain types of people at least. It also make choosing a goal much easier since you are not looking for habitable planets.

8 hours ago, swansont said:

think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true

And sometimes we think we have good reason to believe principles are true because we might be missing information ...

2 hours ago, swansont said:

And there’s no science here to back this up.

But there could be. You still have to have fuel and reaction mass to travel, to slow down at the destination, to do a controlled entry of a gravity well and to leave. None of that requires knowing the specific energy source, but the analysis could tell you how efficient of a method you’d have to use, which rules out certain (perhaps all) approaches

But a blanket statement? Not scientific.

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

48 minutes ago, Moontanman said:

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

None of that analysis is included in saying “slow boat” or “we’ll refuel along the way”

The science is in the details.

2 hours ago, Moontanman said:

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

Every change in velocity (speed and/or direction) requires energy. Each single change takes a piece of your finite energy budget and efficiency is never 100%.

19 hours ago, MigL said:

22 hours ago, dimreepr said:

Isn't there a maximum size of planet from which it's impossible to escape the planet with known propellant's?

A 'reaction' motor provides thrust by ejecting energized mass rearward, at velocity, to provide forward motion according to Newtonian mechanics ( essentially conservation of momentum ).
Escape velocity is proportional to the square root of the mass you are attempting 'escape' from ( assuming a fixed radius of departure ).

I don't see how your statement is valid.

But dim is right in this case: I think the key in his statement is "with known propellants". It is the ratio of chemical energy and mass of the propellant, and the gravity of the planet that makes escape from a 'too heavy planet' impossible. Of course you can add more propellant, but if you need more propellant to lift the portion you added just before, you'll never get very far.

Escape velocity has not much to do with that: this is rocket science (really, this time!), not ballistic mechanics. Only when the rocket engines are turned off escape velocity (from the point where the rocket is at that moment) is relevant.

Edited yesterday at 09:58 AM1 day by Eise

15 hours ago, Moontanman said:

I agree the balance may indeed be fine, maybe there are planets that require nuclear power to escape... my idea of colonizing space happens after the the beings escape their planet. In fact you have hit upon why a space faring civilization might want to avoid planets. coming and going from a Earth sized planet require enormous amounts of energy, it would be much easier to travel between space colonies than between planets.

Doesn't that explain the Fermi paradox?

You may as well be talking about the teapot orbiting Jupiter...

16 hours ago, swansont said:

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

None of that analysis is included in saying “slow boat” or “we’ll refuel along the way”

The science is in the details.

Wouldn't that depend in part on how fast your slow boat was going? To people who live inside space habitats the trip would just be life as usual, so the speed could be quite low. The time of travel would be very long of course but if you used the same amount of energy to slow down as you used to speed up...

Then again this could seem trivial to beings with much longer life spans than ours, or technology that allows them to spend most of the time in hibernation, combine that with long life spans... Technology is a wonderful thing.

Also i can see a system similar to staged rockets, the engine could be dropped after the habitat hits a certain speed, things like magnetic sails could be used to decelerate, or carry a second stage capable of slowing down in conjunction with mag sails.

I am sure that thousands of years in the future technology could be somewhat more advanced than today and be capable of things like antimatter drives or fusion drives or more likely something we haven't thought of yet.

1 hour ago, dimreepr said:

Doesn't that explain the Fermi paradox?

You may as well be talking about the teapot orbiting Jupiter...

I'm not sure how that could explain the Fermi paradox, are you suggesting that most planets are too big to escape from?

Teapot orbiting jupiter? I have not suggested such a thing, only logical extensions of current technology not physics breaking Clark Tech!

16 hours ago, swansont said:

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

I'm not sure I understand this, things like Ion drives use much less fuel to do the same thing as a chemical rocket, other methods like fusion or antimatter could hypothetically be even more efficient, wouldn't not requiring high speeds be another way to requre less fuel?

8 minutes ago, Moontanman said:

Then again this could seem trivial to beings with much longer life spans than ours, or technology that allows them to spend most of the time in hibernation, combine that with long life spans... Technology is a wonderful thing.

Also i can see a system similar to staged rockets, the engine could be dropped after the habitat hits a certain speed, things like magnetic sails could be used to decelerate, or carry a second stage capable of slowing down in conjunction with mag sails.

How do magnetic sails work, or provide the ∆v required for something which was accelerated with a rocket?

Also, this slow boat speculation shifts the topic from aliens visiting Earth to aliens who dwell in space and are willing to move their city to the Sol system over several millennia (and be separated from a handy radiant energy source for a very long time). Hardly a "visit."

2 hours ago, Moontanman said:

I'm not sure I understand this, things like Ion drives use much less fuel to do the same thing as a chemical rocket, other methods like fusion or antimatter could hypothetically be even more efficient, wouldn't not requiring high speeds be another way to requre less fuel?

The energy to move from one place to another in a gravitational field is not affected by the method,

A 1 kg object moving up 1 meter on earth requires 9.8 Joules because that’s the change in potential energy. You can devise methods that use different forces acting for different distances or times, but the minimum energy you need to supply is the same.

The method affects how much energy is wasted on whatever it is you’re throwing out of the back of the rocket. That dictates the efficiency.

9 hours ago, Eise said:

But dim is right in this case: I think the key in his statement is "with known propellants".

Maybe not.
There are various ways to get to a higher radius, where less energy is required to break free.
The solid rocket booster with Hydrogen/Oxygen combustion has been more convenient for Earth, but other methods have been studied ( mainly to increase payload capacity ).
The air breathing first stage, to get to a height of about 70000 ft, has been considered since the 70s, with the most advanced concept being the British HOTOL single stage to orbit system

British Aerospace HOTOL - Wikipedia

Recently Germany is examining a similar system for payload launching.

Germany's POLARIS Spaceplanes Develops Hypersonic Test Vehicle for Defense and Space Research | Mahir Zeynalov posted on the topic | LinkedIn

The point is, that if you can 'gather' propellant as you go, you only need one of the propellants ( Hydrogen in HOTOL's case ).
Even in the case of gas giants like Jupiter, the amount of combustible methane available in its atmosphere is a resource that shouldn't be ignored ( necessitating only oxidizer, no teapots required ).

Contextual Efficiency

Contextual efficiency seems to be a potential key.

Which energy source is more efficient (and safe & clean) for a motion of things (e.g. like a spacecraft) with specified speeds (e.g. slow, average, and fast; Pros and cons for each speed)?

14 minutes ago, tylers100 said:

Contextual Efficiency

Contextual efficiency seems to be a potential key.

Which energy source is more efficient (and safe & clean) for a motion of things (e.g. like a spacecraft) with specified speeds (e.g. slow, average, and fast; Pros and cons for each speed)?

It also a matter where you are; you aren’t going to accelerate upward unless your thrust or lift exceeds the gravitational force. It’s why ion drives aren’t used until you get away from the planet. NASA’s best gives you about 100 milliNewtons of thrust

https://science.nasa.gov/mission/dawn/technology/ion-propulsion/

(similar argument for using photons)

22 hours ago, TheVat said:

How do magnetic sails work, or provide the ∆v required for something which was accelerated with a rocket?

Also, this slow boat speculation shifts the topic from aliens visiting Earth to aliens who dwell in space and are willing to move their city to the Sol system over several millennia (and be separated from a handy radiant energy source for a very long time). Hardly a "visit."

This describes the basics of a magnetic sail.

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

On 6/8/2026 at 7:43 AM, dimreepr said:

Doesn't that explain the Fermi paradox?

You may as well be talking about the teapot orbiting Jupiter...

I wasn't aware we were talking about the Fermi Paradox but you do have a point, there is a sweet spot of planetary mass where a planet is really too small to hold onto an atmosphere long enough to allow complex life (Mars?) and and so big that getting off the surface is next to if not impossible.

I'm not how large or small that sweet spot should be. I would think that nuclear power might expand that sweet spot.

16 hours ago, MigL said:

Maybe not.
There are various ways to get to a higher radius, where less energy is required to break free.
The solid rocket booster with Hydrogen/Oxygen combustion has been more convenient for Earth, but other methods have been studied ( mainly to increase payload capacity ).
The air breathing first stage, to get to a height of about 70000 ft, has been considered since the 70s, with the most advanced concept being the British HOTOL single stage to orbit system

British Aerospace HOTOL - Wikipedia

Recently Germany is examining a similar system for payload launching.

Germany's POLARIS Spaceplanes Develops Hypersonic Test Vehicle for Defense and Space Research | Mahir Zeynalov posted on the topic | LinkedIn

The point is, that if you can 'gather' propellant as you go, you only need one of the propellants ( Hydrogen in HOTOL's case ).
Even in the case of gas giants like Jupiter, the amount of combustible methane available in its atmosphere is a resource that shouldn't be ignored ( necessitating only oxidizer, no teapots required ).

Would nuclear power change this equation?

20 hours ago, swansont said:

The energy to move from one place to another in a gravitational field is not affected by the method,

A 1 kg object moving up 1 meter on earth requires 9.8 Joules because that’s the change in potential energy. You can devise methods that use different forces acting for different distances or times, but the minimum energy you need to supply is the same.

The method affects how much energy is wasted on whatever it is you’re throwing out of the back of the rocket. That dictates the efficiency.

Thanks, see what you mean, but wouldn't the efficiency of the engine at least cut down on reaction mass required to reach a certain speed?

Edited 8 hours ago8 hr by Moontanman

23 hours ago, Moontanman said:

I'm not sure how that could explain the Fermi paradox, are you suggesting that most planets are too big to escape from?

Teapot orbiting jupiter? I have not suggested such a thing, only logical extensions of current technology not physics breaking Clark Tech!

You said yourself, I'm paraphrasing, 'Why would a space faring civilisation waste resources visiting a gravity well, when all their needs are met'.

Besides, if travelling slowly wouldn't they be more susceptible to being dragged into the well?

A teapot orbiting Jupiter, is perfectly possible but since I can't interact with it, it's not worth the effort to justify it;s existence.

34 minutes ago, Moontanman said:

Thanks, see what you mean, but wouldn't the efficiency of the engine at least cut down on reaction mass required to reach a certain speed?

Yes, but only as compared to a less-efficient method. There’s a minimum, i.e. you can’t be more than 100% efficient, but it can’t even be 100% efficient. No matter what you throw out of the back of a rocket, it has energy. You can’t transfer all the energy to the payload.

46 minutes ago, Moontanman said:

but wouldn't the efficiency of the engine at least cut down on reaction mass required to reach a certain speed?

Current rocket technology involves two 'propellants', to not only supply the ejected mass, but also to provide the energy to the mass through a redox reaction ( LOX/LH usually ).
If you provide the energy through other means, such as a nuclear reactor, you could eject rocks, waste, or parts of the ship, as you are only using it for its mass. You would think that would be more efficient, but the means to provide the energy, along with associated shielding and equipment, probably add more weight than the second propellant would.
If you eliminate the requirement for ejectable mass, by 'scooping' it up as you move ( Bussard ramjet ), and energize it with the most efficient reaction possible ( matter-antimatter ), you would have the most efficient propulsion system.

However, the technology is way beyond us.
Maybe someday ...

One of the issues with matter-antimatter is that it’s an energy storage medium, like a battery that needs charging, rather than tapping into already-stored energy. You need to create all the antimatter ahead of time, so that whole energy investment is up front.

—

Let’s look at a snapshot of a rocket that carries its own reaction mass. The ejected mass m is sent away at speed v, and the rest of the rocket has mass M and recoils at V. This is in the rest frame of the system as the mass is ejected.

Conservation of momentum dictates mv = MV, so (mv)^2 = (MV)^2

Meaning the ratio of kinetic energy of each is KE_rocket/K_Eexhaust = m/M

i.e. if, in some small time increment, you eject a kg of exhaust, your million kg of rocket (payload+fuel+superstructure, etc) gets one millionth of the energy of the exhaust.

That’s a reason why rickets are multi-stage - you are shedding mass to improve the efficiency. But you can only do that if you don’t need that superstructure anymore, which is not the case if you want to, say, enter and leave some gravity well (or wells) after your trip. It only works as a one-off, like our trips to LEO, or to the moon, or probes/orbiters

All energy is stored by reconfiguring the system creating potential ( matter-antimatter has potential equal to 2mc² ).
Chemical energy that powers rockets is 'stored' in the bound electrons of the constituent molecules.

But we do know that, at one time, the universe made more matter than antimatter; we just don't know the mechanism ... yet.
If we could use some excess energy from the reactor, and this mechanism ( but in reverse ) to make more antimatter than matter, and then react this excess antimatter with Hydrogen scooped up by the Bussard ramjet, we would have source of antimatter that is replenisheable in flight.

The mentioned mechanism that created the universe's matter-antimatter imbalance is already an area of Physics under much scrutiny.

5 hours ago, dimreepr said:

You said yourself, I'm paraphrasing, 'Why would a space faring civilisation waste resources visiting a gravity well, when all their needs are met'.

Besides, if travelling slowly wouldn't they be more susceptible to being dragged into the well?

A teapot orbiting Jupiter, is perfectly possible but since I can't interact with it, it's not worth the effort to justify it;s existence.

@dimreepr I'm not sure what you mean, I agree that simply visiting another star is hardly worth it unless you intend to colonize that stars "debrie" .

The gravity well I am alluding to is the gravity field of planets, going to and fro the surface of planets is silly if you live in space in artificial habitats.

In my idea planets would be pretty much ignored in favor of ice and rocks that orbit the star.

Some small subset of these "aliens" might be interested in what is going on any specific planet but colonizing them seems unlikely for reasons that really off topic.

4 hours ago, swansont said:

Yes, but only as compared to a less-efficient method. There’s a minimum, i.e. you can’t be more than 100% efficient, but it can’t even be 100% efficient. No matter what you throw out of the back of a rocket, it has energy. You can’t transfer all the energy to the payload.

Wouldn't a more efficient methods require significantly less propellent? Or are we talking about waste heat?

4 hours ago, MigL said:

Current rocket technology involves two 'propellants', to not only supply the ejected mass, but also to provide the energy to the mass through a redox reaction ( LOX/LH usually ).
If you provide the energy through other means, such as a nuclear reactor, you could eject rocks, waste, or parts of the ship, as you are only using it for its mass. You would think that would be more efficient, but the means to provide the energy, along with associated shielding and equipment, probably add more weight than the second propellant would.
If you eliminate the requirement for ejectable mass, by 'scooping' it up as you move ( Bussard ramjet ), and energize it with the most efficient reaction possible ( matter-antimatter ), you would have the most efficient propulsion system.

However, the technology is way beyond us.
Maybe someday ...

Hmmm... I'm not sure all power sources need as much shielding as others but I'm pretty sure a nuclear light bulb rocket gets around this. Studies I've seen would seem to show an engine several times as efficient as chemical rockets not to mention safe to launch from a planetary surface.

I know this would not be appropriate for interstellar missions but it shows that there are alternatives to chemical and solid core nuclear rockets.

For my money aneutronic fusion seems the best bet but of course that is only 20 years away.

https://beyondnerva.wordpress.com/2020/03/21/the-nuclear-lightbulb-a-brief-introduction/

Edited 2 hours ago2 hr by Moontanman

3 minutes ago, Moontanman said:

Wouldn't a more efficient methods require significantly less propellent? Or are we talking about waste heat?

My equations are for conservation of momentum and energy; they already assume the actual device is as efficient as possible.

The way to maximize efficiency is to have the rocket mass be small and the ejected mass be large. Such as a bomb set in between a payload and a really, really big rock. You drastically change the ratio - if you have a 1 kg payload and 9 kg rock, your efficiency is now 90%.

But that also means a really large acceleration, and you have no more reaction mass (if you did, that has to be included with the payload mass so it’s not 90% efficient anymore), so no more thrust or maneuvering. Doesn’t work for the visitation scenario.

Just now, swansont said:

My equations are for conservation of momentum and energy; they already assume the actual device is as efficient as possible.

The way to maximize efficiency is to have the rocket mass be small and the ejected mass be large. Such as a bomb set in between a payload and a really, really big rock. You drastically change the ratio - if you have a 1 kg payload and 9 kg rock, your efficiency is now 90%.

But that also means a really large acceleration, and you have no more reaction mass, so no more thrust or maneuvering. Doesn’t work for the visitation scenario.

IMHO, ignoring the possibility of "Clark Tech" the only scenario that I can think of is that aliens colonize the debri of stars like our kuiper belt or oort cloud, taking advantage of the resources available to create their civilization. They could have specialists who study planets with life, much like we have specialists who study remote tribes here on earth but for the most part I would think they would ignore us.

5 hours ago, Moontanman said:

describes the basics of a magnetic sail

Thanks. The big strobing message I'm getting is this needs testing out in the interstellar medium, to see how much drag compared to thrust.

That, and the electric sail, sound like systems that could be used in tandem with a photon lightsail, where one hands off to another for braking.

Visitation really seems like something where propellantless systems are the most feasible. (And you might want smaller "shuttles" for going deep into a planetary system - for one thing, risks like micrometeorite damage would increase markedly, so you wouldn't send a delicate interstellar butterfly of a craft in there)