We have the occasional thread on alleged aliens but nobody ever seems willing to tackle the physics and engineering discussion of how they got here

This discusses various methods of propulsion, assuming a cruising speed of 0.1c

https://theconversation.com/could-aliens-ever-visit-earth-an-aerospace-scientist-unpacks-the-challenges-of-interstellar-spaceflight-280657

“Consequently, using chemical propulsion on a spacecraft with a cruise velocity of 19,000 miles per second (30,000 km/s) would require more fuel than all the mass in the observable universe”

Of course you could go slower but that amplifies some other issues raised, like how equipment breaks down over time. It’d be interesting to see an analysis of what the maximum cruising speed is with chemical rockets, for a reasonable fuel load.

It doesn’t investigate other problems you’d encounter that relate to biology, nor does it investigate the problems of visiting planets if you somehow got to a destination.

Well does the 'alien' have to be sentient ?

I am observing that non-dead life from at least 5,000 years ago has survived and been revived here on Earth (eg seeds from the Pyramids)

So could spores, seeds, whatever travel on rocks or other natural interstellar objects in say twice that or 10⁴ years ?

We have the occasional thread on alleged aliens but nobody ever seems willing to tackle the physics and engineering discussion of how they got here

5 hours ago, swansont said:

This discusses various methods of propulsion, assuming a cruising speed of 0.1c

https://theconversation.com/could-aliens-ever-visit-earth-an-aerospace-scientist-unpacks-the-challenges-of-interstellar-spaceflight-280657

“Consequently, using chemical propulsion on a spacecraft with a cruise velocity of 19,000 miles per second (30,000 km/s) would require more fuel than all the mass in the observable universe”

Of course you could go slower but that amplifies some other issues raised, like how equipment breaks down over time. It’d be interesting to see an analysis of what the maximum cruising speed is with chemical rockets, for a reasonable fuel load.

It doesn’t investigate other problems you’d encounter that relate to biology, nor does it investigate the problems of visiting planets if you somehow got to a destination.

Interstellar travel would indeed be a challenge but I see no reason to assume it's not possible. From robotic craft carrying data that could be used to reconstruct living beings upon arrival to long term space habitat travel I would hesitate to claim it can't be done.

I would think a slow colonization of places like Keiper belts and oort clouds being used as a source of raw materials to build artificial habitats capable of slowly traveling from one star to another to using space objects similar to kuiper belt objects which are thought to occur in interstellar space but if not you could take a chunk of a kuiper belt object along with you to use as a store of raw materials for the trip. If small comet like objects exist in interstellar space they could be used as spepping stones from one star to another. I would think that slowly spreading across space using these methods would be preferable to only looking for habitable planets and trying to travel directly to them from one star system to another.

I would expect searching for habitable planets and then taking off to colonise them would be very difficult but slowly spreading from one star system to another using small bodies as resources makes much more sense. In fact I have doubts that planets would be desirable at all, deep gravity wells, possible disease from alien microbes and native life forms would make the controlled conditions of a space habitats more desirable and would also make nearly every star colonisable whether it has planets or not.

Why are you assuming chemical rockets? What about other more efficient means of propulsion

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

Propulsion technology

Spacecraft propulsion technology can be of several types, such as chemical, electric or nuclear. They are distinguished based on the physics of the propulsion system and how thrust is generated. Other experimental and more theoretical types are also included, depending on their technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.^[37]

Almost all types are reaction engines, which produce thrust by expelling reaction mass, in accordance with Newton's third law of motion.^[38][39][40] Examples include jet engines, rocket engines, pump-jet, and more uncommon variations such as Hall–effect thrusters, ion drives, mass drivers, and nuclear pulse propulsion.^[41]

Recently I've seen a fusion engine that (supposedly) can be strapped to another space craft and reused (I'm not really sure how serious this one is) https://pulsarfusion.com/sunbird-fusion-propulsion/

The fact remains that technology is advancing and I doubt that chemical rockets will be the limiting factor.

One more https://www.msn.com/en-us/news/other/nasa-tests-record-power-lithium-plasma-engine-for-mars-travel/gm-GMA58E3587

Edited Thursday at 08:24 PM1 day by Moontanman
data

2 hours ago, Moontanman said:

Interstellar travel would indeed be a challenge but I see no reason to assume it's not possible.

Perhaps a universe capable of engineering itself in such a way as to allow for the development of life on a very few isolated celestial bodies, is equally capable of ensuring that a population on any one of those bodies is incapable of adversely impacting life on any other. The Second Law is very fond of diversity.

Perhaps not impossible in an absolute sense but so difficult as to be impossible in practice.

Alien visitations. like interstellar colonialism almost always assumes 'magic' grade technology. And whilst the Clarke observation that sufficiently advanced technologies look like magic I do think we are circling in on a complete understand of the physics of the universe - and we are not finding room for FTL. The gaps in understanding where such possibilities may still exist seem more likely to close than to open.

Even the mass advantages of fission and fusion over chemical rockets comes up against the need for a lot of reaction mass - whilst likely to require greater 'dry' mass as hardware. The gains, significant as they can be are likely to still be far short of fixing the problem. Boosting a whole asteroid doesn't fix the fuel and resources problem, it makes it worse by increasing the total mass that has to be accelerated.

I think leapfrogging between deep space objects, building an economy capable of refitting and resupplying, over and over again for thousands of generations is the most feasible, yet, again, I think that would be unachievable in practice. If such economies were possible there would be no need to go onwards towards other stars - it would take indoctrination to sustain the goal. Without knowing the composition of such interstellar bodies we don't know if stone and nickel-iron will be easy to find; not non-existent but exceptionally rare or deep within icy objects.

If it is possible to build an advanced industrial economy amidst the resource scarcity between stars there is likely to be dependence on robotics and virtual expertise and the sum of human knowledge; societies and economies large enough to support a sufficiency of humans with high level expertise to have deep and comprehensive understanding of the technologies and systems they depend on is going to be difficult. It does seem like there is high potential for ongoing loss of capabilities.

18 hours ago, studiot said:

Well does the 'alien' have to be sentient ?

I am observing that non-dead life from at least 5,000 years ago has survived and been revived here on Earth (eg seeds from the Pyramids)

So could spores, seeds, whatever travel on rocks or other natural interstellar objects in say twice that or 10⁴ years ?

How do these spores or seeds reach cruising speed, and slow down when they reach their destination?

Surviving the vacuum and cold and radiation in space, and then not burning up in the atmosphere when you hit it at high speed.

15 hours ago, Moontanman said:

Interstellar travel would indeed be a challenge but I see no reason to assume it's not possible. From robotic craft carrying data that could be used to reconstruct living beings upon arrival to long term space habitat travel I would hesitate to claim it can't be done.

Robotic craft does not solve the propulsion issues raised in the article.

15 hours ago, Moontanman said:

I would think a slow colonization of places like Keiper belts and oort clouds being used as a source of raw materials to build artificial habitats capable of slowly traveling from one star to another to using space objects similar to kuiper belt objects which are thought to occur in interstellar space but if not you could take a chunk of a kuiper belt object along with you to use as a store of raw materials for the trip. If small comet like objects exist in interstellar space they could be used as spepping stones from one star to another. I would think that slowly spreading across space using these methods would be preferable to only looking for habitable planets and trying to travel directly to them from one star system to another.

This is the issue of the physics of visiting planets I mentioned. You have to slow down and speed up again to land on a planet or asteroid and then leave, so all you’ve done is amplify the problem that hasn’t been solved.

5 hours ago, Ken Fabian said:

Alien visitations. like interstellar colonialism almost always assumes 'magic' grade technology. And whilst the Clarke observation that sufficiently advanced technologies look like magic I do think we are circling in on a complete understand of the physics of the universe - and we are not finding room for FTL. The gaps in understanding where such possibilities may still exist seem more likely to close than to open.

Right. One needs to distinguish between engineering solutions and hard physics limits. The speed of light is a hard limit, as is the requirement for expelling mass for propulsion (unless you use photons, but that carries its own severe limitations) Having to accelerate all of the fuel you take with you is a hard physics limit; you can do analysis without worrying about the specific way you engineer the product.

2 hours ago, swansont said:

How do these spores or seeds reach cruising speed, and slow down when they reach their destination?

Surviving the vacuum and cold and radiation in space, and then not burning up in the atmosphere when you hit it at high speed.

Robotic craft does not solve the propulsion issues raised in the article.

This is the issue of the physics of visiting planets I mentioned. You have to slow down and speed up again to land on a planet or asteroid and then leave, so all you’ve done is amplify the problem that hasn’t been solved.

Right. One needs to distinguish between engineering solutions and hard physics limits. The speed of light is a hard limit, as is the requirement for expelling mass for propulsion (unless you use photons, but that carries its own severe limitations) Having to accelerate all of the fuel you take with you is a hard physics limit; you can do analysis without worrying about the specific way you engineer the product.

Right, delta-v can't be ignored and is a major factor in considering the feasibility of space missions iirc.

2 hours ago, swansont said:

Robotic craft does not solve the propulsion issues raised in the article.

I think it helps quite a bit, a robotic craft doesn't have to have life support running for thousands of years, all it needs is the ability to find resources in another planetary system and use them to build... it could be quite small compared to a traveling colony of living humans, a self replicating Von Neumann type probe.

I think a lot of this depends on exactly what we are talking about. Are we talking about sending a live human to another star system with that person's lifetime? A slow colony ship to another star? An unmanned probe?

I think the most obvious way would be a slow colonization of a stellar system via artificial habitats and then venturing outward as the technology matures maybe snagging interstellar comets as they pass the solar system then possibly onto other objects in interstellar space hopping off when they pass other star systems. Once humans get used to inhabiting habitats in space and improve the technology whether or not a star has an habitable planet becomes meaningless, debris becomes the goal not planets.

Edited yesterday at 02:46 PM1 day by Moontanman

Took pictures of a couple of pages from a document...

Sorry, somehow one is 90 degrees

This discussion is very much couched in terms of human experience. What if said aliens had a lifespan measured in millennia instead of years?

Also, turning mass into energy is likely not the most efficient method of interstellar travel. One method I have not seen discussed is solar sails, using a star's output similar to the way a ship uses the wind. With much of the known universe being way older than our little corner of it, I think it is not only likely that if there are any aliens with a high degree of technology, they know about our solar system and quite possibly have visited us. Whether that has happened in the time of humans and with anywhere near the frequency of sightings, I am highly skeptical.

The reaction mass issue with any impulse engine seems hopeless for anything practical and fast. Ground-push from lasers makes a bit more sense, if you can work out tacking for the deceleration. Otherwise you have the bootstrapping problem with the deceleration phase of a journey, e.g. how to put laser decelerators on your destination planet or system. And also any trip beyond a close neighbor like Proxima C requires extraordinary feats of alignment, targeting and collimation to deal with beam divergence. So even approaches that seem grounded in real physics can run up against mindboggling engineering hurdles. And using natural solar output for a lightsail as @npts2020 mentions will have the inverse square law to reckon with when traveling interstellar distances. (Might be okay for small UM probes, though, if you're not in a hurry.)

24 minutes ago, Externet said:

Took pictures of a couple of pages from a document...

I have some pictures of a hair-sprouting mole. From someone. How would you rank its evidentiary value compared to your documents?

13 minutes ago, TheVat said:

...I have some pictures of a hair-sprouting mole. From someone. How would you rank its evidentiary value compared to your documents?

Ask the military, not me.

(This is not the "popular" Roswell)

4 hours ago, Moontanman said:

think it helps quite a bit, a robotic craft doesn't have to have life support running for thousands of years, all it needs is the ability to find resources in another planetary system and use them to build... it could be quite small compared to a traveling colony of living humans, a self replicating Von Neumann type probe.

There’s nothing in the discussion of the analysis that covers this. It’s about how much fuel you need to get to the cruising speed and slow down again, and how the problem is that as you add fuel, you require even more fuel to accelerate it.

Any maneuvering of the craft suffers from this problem. Ignoring it doesn’t make it go away.

Even for short-range rockets, going relatively slowly, the vast majority of the rocket’s mass is fuel, the container, and the engines

4 hours ago, Moontanman said:

I think a lot of this depends on exactly what we are talking about. Are we talking about sending a live human to another star system with that person's lifetime? A slow colony ship to another star? An unmanned probe?

The analysis stands regardless of which scenario you apply it to, though a live human going to another system would likely require an even higher speed

1 hour ago, npts2020 said:

This discussion is very much couched in terms of human experience. What if said aliens had a lifespan measured in millennia instead of years?

Doesn’t change the analysis that was given.

It means you could re-evaluate based on a lower speed, but the underlying problems still have to be addressed.

1 hour ago, npts2020 said:

Also, turning mass into energy is likely not the most efficient method of interstellar travel. One method I have not seen discussed is solar sails, using a star's output similar to the way a ship uses the wind. With much of the known universe being way older than our little corner of it, I think it is not only likely that if there are any aliens with a high degree of technology, they know about our solar system and quite possibly have visited us. Whether that has happened in the time of humans and with anywhere near the frequency of sightings, I am highly skeptical.

Photons have a spectacularly small momentum relative to their energy, and 1/r^2 is a big problem. Near earth it’s about 4.5 micronewtons per square meter

8 hours ago, swansont said:

How do these spores or seeds reach cruising speed, and slow down when they reach their destination?

Surviving the vacuum and cold and radiation in space, and then not burning up in the atmosphere when you hit it at high speed.

What would be an appropriate 'cruising speed' for spores or seeds, given that we have not idea of their longevity ?

Who said anything about the conditions in space or an atmousphere ?

Spores/seeds embedded deeply enough in a large enough rock would not experience the former or suffer from the latter as the outer layers of rock would protect them.

Such a journey would also take care of the deceleration issue.

42 minutes ago, studiot said:

What would be an appropriate 'cruising speed' for spores or seeds, given that we have not idea of their longevity ?

Who said anything about the conditions in space or an atmousphere ?

Spores/seeds embedded deeply enough in a large enough rock would not experience the former or suffer from the latter as the outer layers of rock would protect them.

Such a journey would also take care of the deceleration issue.

OK. You still have to get the rock up to whatever speed is necessary, but we know lower speeds are possible. If the rock is too big, the impact at the destination is going to be catastrophic, but that’s also going to depend on the gravity and atmosphere. Plus you’re describing an unguided payload, so this is not responsible for any of the sightings that prompted the analysis.

42 minutes ago, swansont said:

OK. You still have to get the rock up to whatever speed is necessary, but we know lower speeds are possible. If the rock is too big, the impact at the destination is going to be catastrophic, but that’s also going to depend on the gravity and atmosphere. Plus you’re describing an unguided payload, so this is not responsible for any of the sightings that prompted the analysis.

Which is why I asked at the beginning

On 5/28/2026 at 6:10 PM, studiot said:

Well does the 'alien' have to be sentient ?

I am observing that non-dead life from at least 5,000 years ago has survived and been revived here on Earth (eg seeds from the Pyramids)

So could spores, seeds, whatever travel on rocks or other natural interstellar objects in say twice that or 10⁴ years ?

The analysis was a response to the release of UFO/UAP reports by the US government, so the visits have to be something that would be reported as such. I think arrive, with the ability to maneuver and leave is implied. Something that would look like a regular meteor strike would not qualify, regardless of whether the occupant was sentient.

@studiot I would read the topic as specific to intelligent aliens doing it on purpose. Enlarging the scope to non-sentient life I would not say known types of spores surviving such a trip is impossible - just may be impossible. I would say we don't know. It seems like something we might be able to determine. I didn't look very hard but none jumped out that comprehensively studied it.

Spores are known to be very hardy but that is a relative term; we are talking about thousands of years and possibly as much as millions at extreme cold temperatures. Moss spores successfully propagated from ISS hull residues managed a couple of years in LEO conditions but the temperatures, cold as they got, were nothing near what interstellar space would present.

How will cosmic ray exposure affect them? Small packages will be less shielded and get more exposure. But wouldn't high energy particles hitting the surrounding material have a cascading effect?

Given enough time would radioactive decay within the spores cause significant damage? Would the radioactive decay within the packaging material affect the spores?

And there is the atmospheric entry - faster speeds reduce time in interstellar space but make it hit harder and burn hotter in atmosphere and on impact. Bigger objects offering more protection are harder to accelerate to interstellar speeds.

Possibly the nature of the packaging will make a difference - some material might shed away in a controlled way to dissipate the extreme heat.

Can life spread that way? I think we don't know enough to say.

======

As a variant possibility - might a rogue planet with sufficient internal heat to sustain liquid water for millions/billions of year carry life across interstellar distances and much shorter distance/duration exchanges from meteorite impacts within a solar system during a fly-by passage cross contaminate them?

Edited 16 hours ago16 hr by Ken Fabian

10 hours ago, Ken Fabian said:

Can life spread that way? I think we don't know enough to say.

But what we do know is that if any ecosystem were able to 'terraform' say an entire galaxy to make it uniformly hospitable to the very narrow range of physical conditions necessary to support that ecosystem, harvesting and converting all asteroids and Kuyper belt type objects into purpose built space-ships, living spaces, factories etc., this would have very profound thermodynamic consequences due to the galactic scale reordering and reduction of diversity such a process would represent.

The production and disposal of collosal amounts of kinetic energy, comparable with the energy equivalent of the masses involved as they are manoeuvred between different gravity wells has already been discussed. But that is really only the start. Such a grand scale reordering of orbitals, structures, and chemistry into an ever lower entropy state is going to produce a truly epic intensity of waste heat. Where is all that going to go? We're beginning to get our heads around the causes and effects of global warming. Now imagine galactic warming.

Nice to be able to log in again ...

On 5/28/2026 at 9:49 AM, swansont said:

Consequently, using chemical propulsion on a spacecraft with a cruise velocity of 19,000 miles per second (30,000 km/s) would require more fuel than all the mass in the observable universe”

Simply solved by electromagnetically scooping up interstellar Hydrogen, feeding it to a ramjet that 'energizes' the expelled mass using a matter-antimatter reactor, and expelling it rearwards to accelerate, and forward to decelerate.
Of course the hydrogen density in this part of the galaxy is not sufficient to provide adequate thrust, nor is the technology available for a matter-antimatter reactor, but that doesn't mean it wouldn't be possible for a more advanced civilization in a Hydrogen dense spiral arm of the galaxy.

On 5/28/2026 at 9:49 AM, swansont said:

We have the occasional thread on alleged aliens but nobody ever seems willing to tackle the physics and engineering discussion of how they got here

Sure, and we also have threads on perpetual motion machines or 'free' energy, intelligent 'creation', gravity by electromagnetism, global warming 'hoaxes', and ToE explained by pictures.
Nobody that proposes those is willing to tackle the Physics and Engineering either.
Even when you present the Physics/Engineering issues to them , they refuse to see/acknowledge said issues.

On 5/28/2026 at 9:08 PM, Moontanman said:

We have the occasional thread on alleged aliens but nobody ever seems willing to tackle the physics and engineering discussion of how they got here

Interstellar travel would indeed be a challenge but I see no reason to assume it's not possible. From robotic craft carrying data that could be used to reconstruct living beings upon arrival to long term space habitat travel I would hesitate to claim it can't be done.

I would think a slow colonization of places like Keiper belts and oort clouds being used as a source of raw materials to build artificial habitats capable of slowly traveling from one star to another to using space objects similar to kuiper belt objects which are thought to occur in interstellar space but if not you could take a chunk of a kuiper belt object along with you to use as a store of raw materials for the trip. If small comet like objects exist in interstellar space they could be used as spepping stones from one star to another. I would think that slowly spreading across space using these methods would be preferable to only looking for habitable planets and trying to travel directly to them from one star system to another.

I would expect searching for habitable planets and then taking off to colonise them would be very difficult but slowly spreading from one star system to another using small bodies as resources makes much more sense. In fact I have doubts that planets would be desirable at all, deep gravity wells, possible disease from alien microbes and native life forms would make the controlled conditions of a space habitats more desirable and would also make nearly every star colonisable whether it has planets or not.

Why are you assuming chemical rockets? What about other more efficient means of propulsion

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

Propulsion technology

Spacecraft propulsion technology can be of several types, such as chemical, electric or nuclear. They are distinguished based on the physics of the propulsion system and how thrust is generated. Other experimental and more theoretical types are also included, depending on their technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.^[37]

Almost all types are reaction engines, which produce thrust by expelling reaction mass, in accordance with Newton's third law of motion.^[38][39][40] Examples include jet engines, rocket engines, pump-jet, and more uncommon variations such as Hall–effect thrusters, ion drives, mass drivers, and nuclear pulse propulsion.^[41]

Recently I've seen a fusion engine that (supposedly) can be strapped to another space craft and reused (I'm not really sure how serious this one is) https://pulsarfusion.com/sunbird-fusion-propulsion/

The fact remains that technology is advancing and I doubt that chemical rockets will be the limiting factor.

One more https://www.msn.com/en-us/news/other/nasa-tests-record-power-lithium-plasma-engine-for-mars-travel/gm-GMA58E3587

To me the key point is that any reaction engine will be subject to the rocket equation and thus you get a requirement for an absurd amount of mass. Unless the thing travels for millennia.

17 hours ago, swansont said:

The analysis was a response to the release of UFO/UAP reports by the US government, so the visits have to be something that would be reported as such. I think arrive, with the ability to maneuver and leave is implied. Something that would look like a regular meteor strike would not qualify, regardless of whether the occupant was sentient.

Well it's your thread so I will bow out with the observation that you are limiting the scope.

For instance why is it necessary to leave ?

The Founding Fathers of New England had nothing to go back to and did not expect to leave so what would be wrong with a one way journey ?

Further why should the journey not outlast the lifespan of those who started, if proper provision for continuity was made ?

Everyone seems to be discounting slow, but slow could be goog, given these details.