If cost were no object: Interstellar travel

I’m thinking using a space elevator, making the parts on Earth, assembling it in orbit, and using nuclear power for propulsion.

What are the technical complications?

Could you use fission to stream matter/energy fast enough to accelerate up to one G?

Or would the amount of matter needed make that physically impossible?

I’m talking about a one-way trip: accelerating to the midway point, turning, and decelerating until you got there. Maybe ten-ish light-years, perhaps 15.

Thank you.

Don’t need answer fast.

I think we’re close to being technically able to make a space elevator, if cost is no object.

You could probably use a solar sail to decelerate after you arrive, so that part might be feasible.

The “using nuclear power for propulsion” part, I’m not sure how feasible that is anytime soon. We can make large nuclear power plants, but they need water for cooling. We can use nuclear materials to generate heat for thermo-electric power sources, but at much less power.

You also need some way of generating propulsion, once you have a power source. Plasma drives, last I heard, only generated about as much force as the weight of a piece of paper.

Of course, you didn’t specify a time frame for when we need to get there, so if you don’t mind waiting a few thousand years, maybe we can do it.

I believe the most plausible method of interstellar travel, using technology mostly available today, is with nuclear pulse propulsion. Basically, the spacecraft will carry a bunch of small nuclear bombs and detonate them periodically behind the craft. One of the early studies of the concept is “Project Orion”. An Orion based craft (or one of the later developments of the concept) could conceivably make it to Alpha Centauri in 50-100 years.

Really more a debate than a GQ type question/answer, but since I love the subject:

I’d guess the biggest one would be radiation. I think that we could, if price wasn’t an object, produce a craft with at least a small chance of success that could get to the nearest star, but radiation protection for the crew, who would be on the craft for at least one generation would be problematic.

Well, there are nuclear rockets, but I’m not sure you could get continuous thrust of 1G for any long duration with the current technology. What you COULD do is something like an Orion, where you toss fission bombs out the back to provide you with thrust. In an Orion system the bigger the ship the better, as well, so that’s a plus.

I seem to recall that an Orion drive could get you up to a fraction of the speed of light (like .3 C IIRC, at maximum thrust), so you are talking over a thousand years to get 15 light years out. That’s a pretty long trip.

The big problem with rockets is not power, but reaction mass. It doesn’t matter that you have a fusion reactor or total conversion torch that can produce essentially unlimited power. You still move by heating reaction mass and squirting it out that way so your rocket goes this way. Simple matter of Newtonian physics. And carrying more reaction mass around with you doesn’t help, because the more reaction mass you carry the more reaction mass you have to use to move the reaction mass that you’re carrying around.

So you can’t just fire up your fusion rocket and blast off to Tau Ceti and 1g until you get halfway there. You’ve only got so much reaction mass to spew out the ass of you rocket and once it’s gone it’s gone.

Your quickest way may very well be wait here a bit longer, wait for faster ways of travel to be developed, then leave on the faster ones, you will most likely pass the ship you would have boarded if you left now on the way and get there much sooner. Even though that is a bit of a crap shoot, it may be much better then the only known option of the orion ship, which is also a crap shoot if it can safely transport people across the stars. I would say your odds are better to wait till the technology improves.

I do recall a sci-fi story, that I didn’t read, just this principal was relayed to me. That we discovered a earth twin around another star, produced a stasis ship and sent it on it’s way, meanwhile drive technology improved that we were able to send other ships to this system, colonize the planet. Many decades later that first ship arrived at this new planet, expecting to be the first interstellar travelers from earth, and very disappointed to see human cities on their new world.

Less reaction mass is not a problem if that reaction mass can be pushed out the back a lot faster. If you smack a single ping pong ball hard enough, you’ll be pushed the other way at 0.99c.

The problem is that high specific impulse engines (e.g. ion thrusters) tend to be very low thrust engines, so you’ll be accelerating at 0.0001g for a very, very long time, but you’ll be very reaction-mass efficient doing it.

Is this true? 100 years to Alpha Centauri? If so, that is much faster than I thought it would be possible with anything resembling our current level of tech.

Well, according to the Wiki link above:

So, either 1330 years or 133 years. I’m assuming this is due to having to accelerate and then decelerate.

In Starman Jones, by Heinlein, early space travel involved using some big rocket technology to accelerate to a critical speed, then make some kind of warp jump. The ship ended up lost and was found a while later by the next generation of starship; the replacement parts to upgrade their ship to the new technology fit into a briefcase.

For my vote, one big problem would be that we can’t make a closed-enough system to last 10 or 15 years, and I’d be worried about running out of air, water or food partway along.

This is sort of what I’m asking about. I’m no physicist (duh) but I’m curious if there’s a physical law that would constrain you from carrying enough fuel for continuous 1G acceleration - you’re saying if you hit a ping pong ball hard enough you could do it. I know we don’t have the tech for it, but what I’m imagining is something that could accelerate small bits of matter at a very high speed to push the ship. When you say “high specific impulse engines tend to be very low thrust” is that a constraint of physics, or something that theoretically could be overcome?

There are several problems that will prevent us from interstellar transit in the foreseeable future. The most fundamental is just the mass of propellant required to even achieve speeds to travel the distance between stars in a human lifespan. As every freshman physics student discovers, rocket propulsion (regardless of power source) requires that you eject mass rearward in order to make the vessel go forward, maintaining conservation of momentum per the rocket equation. This means that some of the initial mass of your vessel includes this material, which is generically called propellant. In the chemical rockets that we use to launch satellites into space this is composed of fuel (such as kerosene, liquid hydrogen, or some form of hydrazine) and oxidizer (typically liquid oxygen, but can be stabilized nitric acid or even more exotic substances) which also provide the power source in the form of exothermic chemical combustion which heats the products to high temperatures and pressures, forcing them out the nozzle. Nuclear thermal rockets are more of the same, except the propellant is generally hydrogen (favored in vacuum for the low molecular weight). Other systems, like fission fragment, fissionable salt water, nuclear pulse propulsion, et cetera use other propellants such as the fission products themselves combined with water or other propellants, but in the end, there has to be mass ejected in order to effect a change in momentum.

The efficacy of propellants, called specific impulse (Isp) is measured in terms of thrust divided by the rate of consumption by weight or mass of propllant per interval of time, giving a mass-independent result in units of seconds. The best chemical rockets have a hard time achieving an Isp of 400 seconds even with the best possible mass fraction (mass of propellant over the total gross liftoff weight of the vehicle). Nuclear thermal may give on close order of 1000 seconds. Fission fragment and nuclear pulse propulsion could potentially offer up to around 5000 seconds. Using a hypothetical nuclear fusion power source could give specific impulse (using diatomic hydrogen as the propellant) of somewhere around 10,000 to 20,000 seconds. By comparison, to have a vessel with a reasonable mass fraction–say, 1 kilogram of spacecraft/payload to 2000 kilograms of propellant–capable of achieving even 0.01c and then decelerating would require a specific impulse on the order of 80,000 seconds, and that is still a four century transit to our nearest interstellar neighbor. Anything less will strand the spacecraft and its hapless inhabitants in the interstellar void for thousands of years. This capability is radically beyond anything we could hope to develop in ten years, or indeed, likely within the next century, and would require some kind of fundamental breakthrough in either energy or propulsion, it just isn’t feasible to send any craft, manned or unmanned, to another star in less than millennia.

The other major issue is the devil that pops up to cause no end of trouble in every area of natural science, thermodynamics. Specifically, the inevitable buildup of heat–not just from this energetic propulsion system, but also from all the other life-sustaining and vehicle maintaining activities–which has to be transferred away lest amount of the heat and thus temperature of the system keeps increasing until it physically breaks down. On Earth, this is not problem; the air and water carry heat away by convection, and the Earth as a whole spends about twelve hours a day radiating all of the heat energy it absorbs from the Sun (except for that used in driving the hydrological cycle) which dwarfs the production by any human activity (provided we don’t significantly alter the composition and resulting emissivity of the atmosphere). In space, however, the only way to reject (get rid of) heat that is produced without having to exhaust more mass to carry it way is to radiate it away. Also the space background is very cold (at 2.7 Kelvin) for a vessel of any significant volume the amount of radiative area required to exhaust excess heat is going to be huge, not withstanding all of the systems needed to convey heat to the radiators. Basically, the vessel would consist of a habitat of some suitable size surrounded by a giant spheroid bubble of radiating surface.

Mind you, this is not a theoretical problem; spacecraft that have onboard energy generation systems or high power avionics have to be actively cooled, and the Shuttle Orbiter of the now retired Space Transportation System always had the cargo bay doors open in orbit not because it looked cool but so that the radiators on the inside could keep the Shuttle at a liveable temperature. These systems are good for missions of limited duration or low energy output, but for a habitat of indefinite duration that has to produce food, distill water, and propel itself it becomes a massive hurdle which is not readily overcome by any kind of brute force methods, e.g. just make it bigger or throw more power at it. Adding a high power propulsion system or nuclear power plant just magnifies the problem by many orders of magnitude.

There are, of course, other more prosaic issues, such as how you would provision for such a long journey, ensure the health and survival of the crew against hazards, boredom, and personal strife, ensure the functionality of electrical and mechanical devices over such a long duration without impractical levels of redundancy, et cetera, but these are the two fundamental issues which no extant or developing technology will address in the foreseeable future.

Although Dyson promoted Project ORION based upon an advertised capability to achieve interstellar speeds, a very simple simulation using realistic impulse values demonstrates that it just isn’t feasible over a human lifespan, and the amount of nuclear “fuel pods” required for a single transit radically dwarfs the yield of all nuclear weapons every produced. Nuclear pulse propulsion is adequate for exploring the planetary system, but traveling to the stars will squire a fundamentally more efficient form of propulsion, or a much longer tolerance for duration.


This is a little tangential, but it’s something I’ve always wanted to ask. Just never got around to it. What would happen if you were cruising along at some ridiculously unobtainable speed such as 0.99 c, and you hit something the size and mass of a grain of sand?

Frankly, I don’t know of any physical law that would prevent a high I_sp, high thrust engine, but as SoaT notes, our current and projected technologies are orders of magnitude away from fitting the bill.

An alternative would be to use a solar sail/laser propulsion system where the power is provided externally, but that presents other engineering challenges.

Well, here’s some food for thought, on .10 c. At .99 c, I think you’d similarly be having a very bad day.

Probably Far Centaurus:


Do you have a cite for “realistic impulse values”? Dyson apparently thought that exit velocities on the order of 10[sup]3[/sup] to 10[sup]4[/sup] km/s were achievable, which corresponds to an Isp of 100,000 to 1,000,000 seconds–rather more than the 5,000 you mentioned. Have there been more recent analyses that show the system to be far less efficient than expected?

Assuming a grain of sand is 1 mg with a relative velocity 0.99 c, the amount of kinetic energy is 44 GJ. This approximately the same as the yield as 10 MT of TNT equivalent, but concentrated in one the size of a pinhead. No reasonable material could withstand that kind of impact without enormous damage. Fortunately, the incidence of even grain-of-sand size material in interstellar space is almost negligible (as far as we can estimate) but obviously even a one in thousands chance of impact for a mission like this would be unacceptable. The obvious solution is to precede the vessel with a sacrificial barrier which can absorb the impact and redistribute the resulting energy via ablation. A large slug of water ice (say, about 100 metric tons) would serve to both intercept any small mass and absorb the resulting energy by ablative evaporation. Of course, impact with a larger object would be catastrophically destructive at such speeds regardless of the size of a sacrificial barrier.


That isn’t what your cite says; Dyson calculates the maximum “debris velocity” of a pure deuterium fusion explosion as U’<3x10[SUP]4[/SUP] km/s and of an actual nuclear device as U’>3x10[SUP]4[/SUP] km/s. These are not effective exhaust velocities; these are the initial rate of expansion of the products from the nuclear weapon. He calculates the “maximum available exhaust velocity” (relative to the spacecraft) as U’/4<U<U’/2, with a range of 750 km/s to 15,000 km/s. However, it should be understood that these are related only to the bomblet itself and not the vehicle, a point that Dyson glosses over. The effective exhaust velocity pertaining to the spacecraft is the average rearward axial velocity of the exhaust products that impact the back of the spacecraft minus the instantaneous forward velocity of the spacecraft. All of the radial velocity of the exhaust is wasted, as are all of the products that do not impact the pusher (i.e. the half of the explosion that is expanding hemispherically rearward, and all of the forward hemisphere that “leaks past” the pusher). Realistically, just transferring even 10% of the axial impulse to the pusher is highly optimistic. Realistically a maximum specific impulse of between 10,000 and 20,000 seconds is theoretically feasible, but without having an enormous area on the pusher plate, 5,000 is a more reasonable estimate.


Just adding more fuel does, in fact, help, and in principle, you can reach any speed you like, using any drive technology you like, if you just bring enough fuel. The problem is that the scaling is really, really unfavorable, such that, no matter what your standard of practicality is, you very quickly exceed it.