You can do a back-of-the-envelope without accounting for relativity, but you can’t do it without accounting for the mass of the fuel. Fuel required for any given delta-V is exponential, not quadratic. It’s still theoretically possible to reach any speed using any propulsion technology: You can plug the numbers into the rocket equation and calculate an amount of fuel needed. But exponential growth means that the fuel ratio (ratio of mass of fuel needed to payload mass) very quickly becomes ludicrous. Heck, you can argue that even just the numbers needed to get into orbit right now are already ludicrous, and you’re just debating about how ludicrous you want to get. But it can get a lot more ludicrous.
@HoneyBadgerDC , you absolutely can refuel a craft in orbit. We’re already looking at doing that, even for tasks as relatively trivial as going to Mars. By the time you’re even thinking about an interstellar mission, it’s safe to take that as a given. But at some point, it doesn’t even help. At some point, you stop even thinking about your ship being in orbit, because the mass needed is so high that your ship is the thing that other things are orbiting.
I agree. I recognized that once the fuel becomes a significant portion of vessel weight, then the calculations explode dramatically. (Someone joked that New Glenn, for example, uses half its fuel clearing the launch tower - except when it uses all of it). Pournelle mentioned with the prototype to the X-33 that the vehicle would be around 95% fuel by weight just to get to orbit.
Interplanetary travel would become feasible when someone manages to produce fuel in orbit or beyond (perhaps, if sufficient ice were found on the moon). But the amount of fuel needed to propel a crafft to relativistic speed is orders of magnitude beyond that, even using fusion technology. I just used the basic kinetic energy calculation to show how immense the number can be. You can fiddle with the numbers, smaller craft (but for a 10-year mission, how small can you go?), fusion instead of fission, etc.
And as others pointed out, radiation, micro particles of interstellar matter or gases, there’s still a real problem with relativistic speed.
It has been suggested that you could largely forgo fuel by using a system of rotating tethers as essentially kinetic energy batteries. Outgoing ships would be accelerated by the tether, slowing its rotation down; incoming ships would latch onto the tether and be slowed down by it, increasing its own rotation again.
Like a lot of things in space, interplanetary travel should become far easier once there’s an infrastructure for it.
I suspect you could achieve interstellar travel with relatively near future technology if you were willing to accept a “nomadic” model of travel and not care about how long it takes. Interstellar space apparently has a lot of matter in it in absolute terms, even if widely spread out. A large ship or fleet of ships moving at (in interstellar terms) slow speed from one drifting rogue planet or random iceball could travel indefinitely far within the galaxy. Basically traveling from one oasis to another in the interstellar desert. It would be able to repair and replenish itself from what it found as it went, not try to pack in all the needed resources at the start.
It might take thousands of years to get from one star to another assuming they bother, but since the fleet is their home it doesn’t really matter to them.
An interstellar ship would likely be more than large enough to get around that by imitating gravity via spin.
Good news: That’s already resolved. It’s easy to produce centrifugal artificial gravity. The only reason why the ISS is zero-g is because they want to study the effects of zero-g. And they want to study zero-g because that’s the environment that space missions like the ISS are in.
Once you set an actual goal that doesn’t have zero-g as an inherent part of it, you just give up on zero-g and just spin the thing.
There are several “little” issues that we really do have to resolve, even if spin gravity is not necessarily one of them.
Sticking with life support, water reclamation and retention is going to be one of them. We use water for a lot of things, including manufacturing and industrial processes and there’s not really a way to 100% efficiently recover it. Any generation ship is going to have to deal with that, especially the expected efficiency. Some SF stories double that with radiation and impact shielding by using ice shields.
Whatever the case, not necessarily an intractable problem but also not trivial. And it’s just one of many.
Things like slingshot tethers or worrying about refuelling forget the insane speeds involved. For interplanetary travel maybe these might help. For interstellar it doesn’t even begin to make sense. Take everything you know about space travel and multiply velocities by at least 10,000, and thus energies by 100,000,000.
At 0.1c your spacecraft covers the distance from the Earth to the Moon in 12.5 seconds. Any sort of intermediate interaction with objects is going to result in highly undesirable results with energies that will result in large expanding clouds of plasma and gamma rays where the spacecraft one was. Any idea that puny inter-atomic bonds will hold stuff together is fanciful.
For a well known example only a little faster see
Getting your spacecraft into space and exiting the solar system is by far and away the trivial part. That lives at the banging the rocks together end of the technology spectrum.
Yeah, at this point, there are parallel discussions going on in this thread about interplanetary travel and interstellar. I think that the people who mentioned things like tethers know that they’re just for in-system work, but it’s good to remind everyone of the vast differences. The kinds of things that work great for in-system travel can only, at best, get you to something like twice the escape speed from the Sun (around 1200 km/s), which would still take you around thousand years to get to the next nearest star.
At some point after interplanetary travel becomes easy, someone will develop a self-sufficient space habitat. Once we have that, and once they’ve proven themselves for decades or a century (and once we’ve mastered controlled fusion, if that hasn’t also happened meanwhile), then will be the time to start considering interstellar travel.
Something else to consider in terms of how close nearby stars are is that the number is always changing. Wait 26000 years and Proxima Centauri will be a whole light year closer. Wait 1.2 million years and the star Gliese 710 will be within 60 light days distance.
A few minor changes to the timing of human evolution and we could have been this conversation with our nearest stellar neighbor basically on top of us.
We’re thinking of what’s practical from the point of view of creatures with a lifespan of 100 years and experiencing the world on a scale of fractions of seconds. A species of organisms that live for 1,000,000 years and take a week to have a thought or experience a sensation might have a very different perspective on interstellar travel. No relativistic speeds required.
Be that as it may, the reality is that the average distance between stars in our galaxy is still incredibly vast; indeed, by any human standards, even the distance between planets in our own solar system is unimaginably vast.
Typical interstellar distances are greater by many orders of magnitude. The average distance between stars in our galaxy’s spiral arms is on the order of about 5 light-years. In the galactic core stars are packed much closer together, as close as a few light-months. While that may seem like high-density housing, a single light-month is still 777,062,051,136 km, or about 486 trillion miles. The interstellar distances between habitable planets are going to be much, much greater than any of those numbers.
And yet, the size of our Galaxy is tiny, compared to its age. Which leads to the “Fermi paradox”: If any intelligence anywhere in the Galaxy ever reaches spacefaring technology, the time for that species to fill the Galaxy would be much less than the time for other intelligent species to evolve. So where are the aliens?
Of course there are many possible resolutions to this question. Most of them are some sort of “Great Filter”, something that prevents spacefaring tech from arising in the vast majority of cases, resulting in an average of only one per galaxy, or less. In that case, the question is whether we’re past the Filter yet (in which case we’re probably heirs to the entire Galaxy) or not (in which case we’re probably doomed soon). Or maybe the aliens are out there, but have some sort of Prime Directive-like policy of not interfering with other species. Or maybe there are lots of different species working their way towards intelligence, but we just happen to be the first, and we will become the Old Ones that all the others are awash in. Or maybe some combination of those: Like, maybe the Great Filter isn’t quite so efficient, and lets an average of ten species per galaxy through, and we just happen to be the first of those ten.
There’s a pretty good video on the Fermi paradox that I recently posted at the link below, which among other things discusses variations on the Great Filter hypothesis as a possible explanation for the apparent lack of evidence of intelligent alien life …
Personally, my hunch is that there’s not one Great Filter, but two, and that they’re both the Great Oxygenation Event. Most life-bearing planets never go through anything like the Great Oxygenation, and so life continues blissfully on them, but slowly, without the vast reservoir of energy our oxygen atmosphere provides. And of the small fraction that ever does have a Great Oxygenation, in most of them, it results in extinction of life on that planet. We’re unusual in that we’ve not only had an oxygenation, but we’ve also survived it. We’re the species that literally breathes rocket fuel.
A fascinating question. There are many references out there. One says:
“But some galaxies pack stars even tighter. M32, one of the Andromeda Galaxy’s satellites, has the highest measured stellar density of any nearby galaxy — around 20 million stars per cubic parsec in its core! Not even HST can resolve M32’s core into individual stars. A typical stellar separation at this density works out to 0.008 light-year, or 500 AU — about 12 times the Sun-Pluto distance — between stars.”
Well, Proxima Centauri is “only” 4.25 light years away, but it would still take tens of thousands of years to get there using the best form of propulsion that we currently have.
This now begs the question: even in the relatively crowded center of our galaxy, how close can stars in separate solar systems get before creating instability. I found several sources that basically said this: “If the Centauri system and the Solar system would get to about 1 light-year distance there already would be disturbances in the Oort-cloud of our system.”
So, for the sake of discussion, let’s go with 1 light year. With our current technology, it will still take over 15,000 years to travel that one light year.
So, in reality, a civilization comparable to ours wouldn’t have any kind of advantage regarding interstellar space travel even if it was located in a relatively crowded part of our galaxy.
Yep, just watched a physics show that said the sun’s gravitational influence extends about 1ly in each direction. Although I suspect that any advanced civilization that evolved in those conditions would likely be able to survive all the volatility. Assuming their planets aren’t being blown apart constantly from collisions.