13 hours ago, TheVat said:

From the Rocheworld article in wiki: To catch the energy, Forward uses a 1,000-km-diameter circular aluminum sail. The sail resembles a flattened disk with a 300-km-diameter removable center portion. When traveling to Rocheworld, the entire sail is used. When the ship needs to decelerate, the smaller sail is separated from the larger outer sail. The large sail is used as a reflecting lens, focusing light onto the smaller sail, slowing the craft. (end quote)

Aside from the absorptive heating issues, I am unclear how this double-reflection gimmick would be sufficient delta-v. I recall Forward as the sort of SF writer to really thrash out all the RW science issues, so maybe reading the novel would tell me more. Maybe there's other shipboard mass that gets dumped.

I don’t see how this would work. The detached sail will now have a higher acceleration, so it will separate from the craft. It will reflect less light, so any deceleration from it is smaller, and the remaining sail is still reflecting light, giving forward propulsion. The detached sail will be much more prone to tumble if there are any intensity variations over its profile.

20 hours ago, TheVat said:

In any case, the thread topic is on the possibility of aliens visiting here. Whatever the nature of UAPs, they don't seem to colonizing Earth. (and we would be a poor choice for a colony - imagine if we humans went looking for a terrestrial planet to colonize; overpopulated, polluted, bristling with nukes, and constantly having wars would be our last choice)

With low speed travelling, they couldn't know at the departure that the Earth is inhabited by an intelligent species. I don't know about them, but I wouldn't colonize a planet already occupied. So, I'd either abort the mission completely or choose some moon to dig and build habitable places and so on. Therefore it is possible to have aliens on some moon in our solar system.

16 hours ago, swansont said:

The limitation is that reflective surfaces are not 100% reflective, and the high power needed for any appreciable acceleration makes absorptive heating a huge obstacle

We may solve this problem by improving and extending the surface of the sails, by increasing de time for acceleration (using not only the lasers near the Sun, but also lasers situated much further, and powered from the belt of power stations near the Sun) and by improving the resilience of the shipment (robots, devices, etc.), in order to withstand longer travel time, with lower speed.

I also thought about a way to flip the sails when overheated (if sails are divided in smaller sections).

15 hours ago, exchemist said:

How would you decelerate the spacecraft at its intended destination?

Due to the fact that the acceleration is provided by external sources, we can send a big multistage rocket for the deceleration. After that, we redeploy the solar sails. If we still need deceleration, we may consider skimming through some planet(s) atmosphere.

I want to add another use for the solar power stations belt: we can use it to direct heat to some frozen moon, in order to make it habitable. So, the investment would not be solely for colonizing other solar systems.

4 hours ago, swansont said:

I don’t see how this would work. The detached sail will now have a higher acceleration, so it will separate from the craft. It will reflect less light, so any deceleration from it is smaller, and the remaining sail is still reflecting light, giving forward propulsion. The detached sail will be much more prone to tumble if there are any intensity variations over its profile.

Thanks, I had wondered how that could work with still retaining some forward thrust. And I still am not clear about research on embrittlement of sail materials from ionizing radiation. Both polymers and metals are subject to embrittlement. This could be fatal for a structure like a sail.

I don't have access to the full paper here but it may have some cited results that address materials such as sails could be composed of.

https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B9780323524728000290

23 hours ago, DanMP said:

we can send a big multistage rocket for the deceleration

In fact, such rockets would be obsolete. I'm pretty sure that some lighter and more efficient thrusters would be developed/available.

I learned that there are more methods for deceleration: Combining Magnetic and Electric Sails for Interstellar Deceleration

Edited Wednesday at 01:26 PM2 days by DanMP

2 hours ago, DanMP said:

In fact, such rockets would be obsolete. I'm pretty sure that some lighter and more efficient thrusters would be developed/available.

I made the point earlier that there’s a lot of analysis that’s independent of the technology; conservation of momentum doesn’t care how you impart the momentum. You have to use up a lot of your energy budget throwing the mass out of the back of the rocket, even if there’s no energy lost to anything but thrust and payload KE.

9 minutes ago, swansont said:

I made the point earlier that there’s a lot of analysis that’s independent of the technology; conservation of momentum doesn’t care how you impart the momentum. You have to use up a lot of your energy budget throwing the mass out of the back of the rocket, even if there’s no energy lost to anything but thrust and payload KE.

Ok, but you may use nuclear energy ... The fuel mass should decrease substantially if you switch from chemical to nuclear.

Did you read the arxiv article? The acceleration phase is pretty much what I suggested and they proposed magnetic and electric sails for deceleration. If you add that to what I already proposed, including atmosphere skimming, you may have a viable solution.

2 hours ago, DanMP said:

Ok, but you may use nuclear energy ... The fuel mass should decrease substantially if you switch from chemical to nuclear.

I’m talking about the reaction mass

2 hours ago, DanMP said:

If you add that to what I already proposed, including atmosphere skimming, you may have a viable solution.

How do you maneuver in the atmosphere and get back into space?

4 hours ago, DanMP said:

Ok, but you may use nuclear energy ... The fuel mass should decrease substantially if you switch from chemical to nuclear.

I think the whole point of this thread was to establish the technical how this might be achieved, not whether or not there is some source of energy or not, such as nuclear.

Just having energy doesn't automatically make it available.

As Australians are wont to say - tell 'em they're dreaming.

As thought experiment it can be fun considering possibilities and exposing the impossibilities. As a serious goal to commit vast resources too... in an age where our overuse of Earth resources and carelessness are turning around to bite us? No. And perhaps the most essential thing for keeping such possibilities possible is a healthy, wealthy Earth society. I would be concerned for its mental health if interstellar expansion became an obsession.

As threads grow longer the older posts get overlooked and a lot of what DanMP is suggesting has been discussed and I'm probably reiterating some of my own by saying -

Saving humanity from extinction is to my mind one of the most unrealistic reasons for interstellar travel. We'll go to space and succeed there from the economic opportunities that require on-site human labour and without them succeeding in delivering more back than the investments they will whither. I am still struggling to see what those opportunities could be.

If surviving extreme disasters is the overarching intent then artificial habitats closer to hand, beginning with bunkers before space colonies and space habitats before planets and within the solar system before other stars makes more sense - if you can really call it sense. (Switzerland has (had?) nuclear war bunkers for the entire population - using their abundance of tunnels - but I would find it objectionable for my nation's government to commit to them for only a select few.)

Try building a bunker that can last a thousand years without outside resources and see how that works out.

I'm more inclined to being 'philosophical' about species mortality; I don't like my impending mortality and putting up a good fight against it is in our nature but so is acceptance of the inevitability. Making the best of what we get and not spending the best of it chasing dreams of immortality is sane and reasonable too.

And I think any worlds with existing life are far more likely to be uninhabitable than present as colonial opportunities - nothing makes more varieties of poisons, toxins, allegens than biochemistry and it is big stretch to assume compatibility, even aside from potential for predators and plagues; for most of Earth's living history humans would find Earth's atmosphere un-breathable.

I would think if it were somehow possible to go there such living alien life and biochemistry, uncontaminated, is worth more to humanity for what we can learn from it - from an in-space habitat, deploying robotic sampling - than as something to displace and extinct on purpose or by accident with Earth life in pursuit of dreams of land ownership. I hope humanity can rise above such self indulgence.

Edited Wednesday at 11:20 PM1 day by Ken Fabian

On 6/24/2026 at 8:02 PM, swansont said:

I’m talking about the reaction mass

Yes, if we use nuclear power, apart from the fuel itself, we need a material to eject + a nuclear reactor and the ejector. That would add to the rocket/thruster mass ... But we don't necessarily have to carry everything from Earth. Since we are already deploying a massive magnetic field (the magsail), we could theoretically use it in reverse during the cruise/early deceleration phase as a magnetic scoop, similar to a Bussard Ramjet concept. We collect interstellar hydrogen and than eject it.

Also, a nuclear/ion system has a much higher exhaust velocity, so you need fewer particles to get the same kick.

Anyway, we send the most efficient rocket/thrusters we get at the time, nuclear or/and chemical.

On 6/24/2026 at 8:02 PM, swansont said:

How do you maneuver in the atmosphere and get back into space?

You don't need to maneuver in the atmosphere. You adjust your trajectory far before the entry, with minimum use of thrusters. And then you go "straight" (free fall).

The planet may be a gas giant. And you can do it multiple times, until you can land on an appropriate moon or planet. Yes, you lose time on elongated orbits, but you have no living creatures in the ship, so it doesn't matter too much.

2 hours ago, DanMP said:

Also, a nuclear/ion system has a much higher exhaust velocity, so you need fewer particles to get the same kick.

KE scales as v^2, momentum as v. If you double v, you quadruple the KE you impart. But that’s true of the payload as well.

As I derived earlier:

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

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

The exhaust velocity doesn’t enter into this limit

An advantage of the ion thrusters we use is the fuel need not be carried onboard - you use solar panels. Also, we often use Xenon, mass of ~130 u, as opposed to might lighter molecules typical of chemical rockets (e.g. water). Rockets are limited also by thermodynamic efficiency, while ion thrusters are not based on heat transfer