I had in mind a sci-fi setting where humans have begun terraforming nearby planets without the benefit of faster-than-light travel. For example, Wikipedia lists a handful of terrestrial planets within 15 light years. What I have in mind is that humans have the capability to send unmanned probes to those planets after about a 50-year voyage, and manned probes after slightly longer.

I had hoped to set these stories in the last 22nd century, roughly 150 years from today. I could go as far as 250 years in the future, but I don't want the space-farers to be too far removed from modern Earth culture.

  • How could I make it plausible that by that time, human beings have created spacecraft of those speeds?
  • What are the major barriers mankind would have to overcome to reach those speeds?
  • 1
    $\begingroup$ What kind of "probes" are you looking at? If the goal of the probe is to build an earth styled Manhattan district on each planet, this would require more resources (and thus more time). If the goal is to seed each planet with a few hundred people and let them adapt to a foreign planet, the timeline gets closer, but you wont see as much Earth culture because the humans spend MUCH more effort adapting themselves to the planet, while the former spends more time adapting the planet to the people $\endgroup$
    – Cort Ammon
    Commented Nov 9, 2014 at 20:42
  • 1
    $\begingroup$ You're asking for sub-light-speed travel? $\endgroup$
    – HDE 226868
    Commented Nov 9, 2014 at 21:10
  • $\begingroup$ The gist of my last comment is that there's a difference between two possible questions here: 1) How fast can people go (i.e. approach the speed of light) and 2) How long until people can achieve FTL travel (which I would disregard as not being science-based). $\endgroup$
    – HDE 226868
    Commented Nov 9, 2014 at 21:49
  • $\begingroup$ I seem to remember reading a Popular Science article a while ago about a proposed ship that might do *X*% (20? 30?) of light speed. I'll see if I can find it. $\endgroup$
    – Shokhet
    Commented Nov 9, 2014 at 23:19
  • 1
    $\begingroup$ Are you asking about FTL (faster than light) travel, or about interstellar travel? In context with the text of your question and HDE 226868's answer, the question title seems to be a bit confusing.. $\endgroup$
    – Irigi
    Commented Nov 11, 2014 at 12:10

8 Answers 8


Let's do a calculation or two, shall we? (Note: All those wishing to skip the boring math can just read the lower portion of this answer.)

Finding the speed you're going for: $$v=\frac{15 \text { light-years}}{50 \text { years}} \times \frac{10,000,000,000,000,000 \text { meters}}{1 \text { light-year}} \times \frac{1 \text { year}}{31,500,000 \text { seconds}}= 95238095.24 \text { m/s}$$

Finding the energy needed to get to that speed: $$E_k=\frac{mc^2}{\sqrt{1-(v/ c)^2}}-mc^2$$ Assuming a mass of about $150,000$ tonnes (taking the Project Daedalus figure and tripling it), $$E_k=\frac{(150,000,000)(300,000,000)^2}{\sqrt{1-((95238095.24)/(300,000,000))^2}}-(150,000,000)(300,000,000)^2$$ $$=7.36 \times 10^{24} \text { Joules}$$

Can we do it? . . .

According to Wikipedia

In 2011, total world energy consumption was 549 exajoules

Multiply that by 50 years, and we still don't have enough.

I suppose we could use the Kardashev scale to figure out when humanity will have energy at that rate. Using this formula: $$K=\frac{\log_{10}P-6}{10}$$ and substituting in humanity's energy usage per year, we could calculate just when we'll reach that point. Just use $$P= \text { Joules/year}=2.34 \times 10^{16} \text { Watts}$$ $$K=\frac{\log_{10}7.36 \times 10^{24}-6}{10}$$

and we find we should be a Type 1.03692 civilization. Given that we're currently a Type 0.7 civilization, we should be at that point in about 100 to 200 years. Note, though, that this figure is for a ship using all of humanity's total energy, which is unrealistic. However, it shows that we won't be able to harness that energy at all for a while.

I would also be obliged if someone could check my calculations. I used this logarithm calculator, in case anyone wants to use it. Conversion ratios can be found easily online.

What are the major barriers mankind would have to overcome to reach those speeds?

If we're talking purely about travel at this speed, i.e. assuming we've figured out everything else, the main problem is propulsion. As the above result showed (for those who skipped it, you just have to know that we'd need a lot of power to reach these speeds.), it's not easy to travel fast in space. Here are some options:

  • Bussard Ramjet - Take in hydrogen gas floating in interstellar space, compress it, begin fusion, and convert that into thrust. Obstacles: It has to be really big, and you have to be sure you have hydrogen in space. Oh, and you have to master fusion.

  • Nuclear Pulse - Chuck nuclear bombs out the back of the spacecraft, and ride the shockwaves. This was used in the hypothetical Project Daedalus and the equally hypothetical Project Orion. Neither idea got off the ground (pun definitely intended). Obstacles: You have to make loads of nuclear bombs. Also, you have to find a nation that's willing to sacrifice a bunch of land for the launch pad, because that launch pad and the surrounding area will be completely destroyed.

    Alternatively, you could just use this idea in space, as Lohoris suggested.

  • Solar Sail - Ride the radiation pressure from a star using a sail. Obstacles: You've got to make and deploy an enormous sail kilometers wide. Also, the acceleration is incredibly slow.

  • Antimatter Rocket - I've saved the best for last. Put together matter and antimatter and harness the energy from the annihilation. Obstacles: Very expensive, because antimatter is expensive. Also, you'll have to make quite a lot of it at first, although you can start to coast at some point. You have to be very delicate, though. Veeeeery delicate.

  • 5
    $\begingroup$ +1 for a clever, clear, no-nonsense answer. It may be worth noting that we are assuming a smooth, continuous growth in our technology. A serendipitous discovery (paradigm shift type) could accelerate this process. $\endgroup$
    – abcdefg
    Commented Nov 9, 2014 at 23:29
  • 1
    $\begingroup$ @NickR True, a discovery like that would change everything. Perhaps if the Alcubierre drive becomes something more than a fantasy, we can be at Alpha Centauri by teatime tomorrow. . . $\endgroup$
    – HDE 226868
    Commented Nov 9, 2014 at 23:33
  • $\begingroup$ It certainly would, but that would mean FLT, and the drive is specified within our current scientific paradigm. As Kuhn noted, about every 100-200 years, our scientific orthodoxy is overturned and replaced with a new orthodoxy. $\endgroup$
    – abcdefg
    Commented Nov 9, 2014 at 23:38
  • $\begingroup$ I did a quick calc using the linked log calculator and got $ K = 1.808... $ as $ log_{10}(7.36) + 24log_{10}(10) - 6 $ all divided by $ 10 $. ??? SORRY EDIT I mis-read the calculation. I'll try again. No, I think it 1.808... $\endgroup$
    – abcdefg
    Commented Nov 9, 2014 at 23:47
  • 1
    $\begingroup$ About the nuclear bombs, can't it just "take off" normally, and start using the bomb-propulsion once it's already in space? $\endgroup$
    – o0'.
    Commented Apr 16, 2015 at 21:38

Most likely space colonization will not go as you envisage.

Actually there could be hundreds and thousands of planets within 1 light-year from the Sun. If there are no stars that close does not necessary mean there are no planets, both wandering and orbiting the Sun. Statistics predict that there are much more wandering planets in the galaxy than stars.

As such, future people may not think it is necessary to travel to other stars before they colonized at least the solar system. As they explore it they find more and more celestial bodies suitable for colonization, gradually approaching other stars.

  • 1
    $\begingroup$ This makes a lot of sense for a calm and collected society, who is willing to take the time to make sure the colonization goes right. A massive spam of probes to every planet within range at one time would more likely indicate a panicking culture trying desperately to survive. $\endgroup$
    – Cort Ammon
    Commented Nov 10, 2014 at 0:04
  • 1
    $\begingroup$ @Cort Ammon sending unmanned probes is actually not than costly and people actually can send probes to any substantial planetoid they discover. But still it is most likely well far from truth that the Wikipedia;'s list of currently known planets is exhautive even at 1%. $\endgroup$
    – Anixx
    Commented Nov 10, 2014 at 0:10
  • 1
    $\begingroup$ I'm not thinking of cost. I'm thinking of social impacts that would drive a society to colonize slower than their most breakneck speed possible. Each colony WILL be different and WILL have different politics. Turning those into a unified federation is a challenge. It is easier to start small, with a few colonies, and grow as time progresses. Because it's easier to start small, there would have to be a substantial reason which gives people the impetus to light a large number of rockets and colonize the galaxy in one big step. $\endgroup$
    – Cort Ammon
    Commented Nov 10, 2014 at 0:22
  • 10
    $\begingroup$ rogue planets would not make good places for humans to live. Without a star to provide energy it's going to be very cold, no atmosphere (what gasses there ever were will be frozen solid and/or stripped away), etc. etc. They might make way stations, but not having predictable orbits that's not at all a trivial matter either. $\endgroup$
    – jwenting
    Commented Nov 11, 2014 at 14:50
  • 2
    $\begingroup$ @jwenting A large part of a planet's thermal energy comes from the original formation. Also, I believe it's actually the sun that strips away atmosphere. But there may be other factors in play. $\endgroup$ Commented Jan 29, 2015 at 6:53

Tough but possible.

$15ly$ in $50$ years would require $0.3$ times the speed of light. To reach this speed, we could accelerate with $4g\approx 40m/s^2$ for 26 days (or 51 days with $20m/s^2$ or 13 with $80m/s^2$ .. the last one would be very unpleasent for the passengers).

How much energy would it take? Lets stick at $150000$ tons. So its $E_1=\frac{1}{2}mv^2=\frac{1}{2}m(0.3c)^2\approx6e23\,\text{Joule}$. If we use relativistic formulas, we would need $6.5e23 \text{Joule}$. Close enough for our estimates. But wait! Now we reach the target system but still travel with a notable fraction of the speed of light. We need to get rid of this velocity and need the same amount of energy again. This sums up to $E=E_1\times 2=1.2e24 \text{Joule}$. That's nearly 8 tons of antimatter by the way.

How do we get the energy? Lets try solar power. The solar constant is $E_0=1367\frac{W}{m^2}$ near earth. This constant is proportional to the inverse of the distance to sun squared. At $0.1AE$ (about one third of the distance from sun to mercury), its $E_{0}^\text{Mercury}=137\frac{kW}{m^2}$. To get enough energy in one year, we need to cover an area of $\frac{E}{E_{0}^\text{Mercury}}\times \text{seconds per year}\approx 7e12m^2$ (that's arround $2700km \times 27000km$ - tough but possible). You have to scale it up for general loss due conversion and inefficencies though. There are other ways like fusion reactors, magic cold fusion, etc. Your choice.

Now the hardest part: how to convert the energy in velocity?

  • Solar panels? The acceleration is painfully slow in our solar system and goes down with the distance squared (double the distance, acceleration goes down by a factor of 4).
  • You don't want to use an FTL so you can't invent a classical warp drive. Maybe you can invent something like a slower-than-light-warp but this would render all calculations useless as there is no knowledge about it. If you want to stick at hard SciFi without warp, you need something to exhaust to push you forward.
  • Classical propolusion drives are out of question too. Take a look at the rocket equation. The Ariane 5 rocket have a specific impulse of $440s$. Putting in our numbers, we come up with a mass around $e^{20850}kg$ - more than double precision can handle. You need something to push away with a notable fraction of $c$.
  • The idea with nuclear bombs (antimatter?) driving you forward is tempting. But explosions are short bursts and we need many of them over a big time (remember the $4g$ over 26 days?). This is a big challenge for the structure of tyour space ship.
  • I think you have to invent something new like a large scale particel accelerator shooting matter out with the speed of light. You still need quite an amount of this particels ... for the needed $\Delta v=0.6c$ $0.8$ times your ship's mass - $123'000t$. Remeber to scale up your solar panels.

Every SciFi I know reaching out for other solar systems - even the hard ones - have some magical device/ thruster without real science behind them (warp drive, warp tunnels, instant teleportation over light years, ...). I don't think you can get around it.

Now the social component. Try cryo sleep chambers, thats the easiest (I'm more a technical guy).

  • 1
    $\begingroup$ I think it was determined that a ship that used nuclear bombs dropped behind a big thick shield could get you going fairly well with current tech. You just need life support for those years. $\endgroup$
    – Oldcat
    Commented Nov 15, 2014 at 0:19
  • $\begingroup$ Re hard SF: you’ve not read Robert L. Forward it appears. $\endgroup$
    – JDługosz
    Commented Sep 22, 2016 at 5:22

Ok, so the best and most enlightened answers here have considered the amount of energy required to travel these distances.

If you want to shortcut over the hundreds of years it will take us to get to this point you could try the following:

  • External Influence

Somebody like the Vulcans visit us from another planet and share advanced space flight technology. Or perhaps we steal the technology after they visit.

  • Gift from Space

Something like a meteor strikes the Earth carrying large quantities of some new type of fuel compound which is more effective than anything we have on Earth.

  • Time Travel

People from the far future come to our time to share with us the advanced technology required for space flight.

  • Messages from Space

An Alien race sends us the designs for building interstellar craft.

  • Unknown Technology

At some point in the not too distant future a breakthrough is made that changes our understanding of Space and Time, allowing us to get to this level of technology faster


Yes, based on our calculations for energy requirements, which is right on, and the Kardashev scale 300 to 350 years would be a good guess.

This would be for a sublight starship using 7 x10 ^24 joules... likely fusion powered. Still very "pre-warp", as warp drive would likely require Kardashev mid level two civilization...

Though as Nick R stated, new "multiple" discoveries resulting in paradigm shifts could accelerate that Daedalus type ship by perhaps shaving 100 years off that 300 - 350 year estimate...


With an electricmagnetic thruster like VASIMIR and a nice small fission reactor like SNAP, you're more like 50 years from starting an unmanned probe. OK, you still need a lightweight, space-rated >200W generator.

A manned probe? Think again. And forget about transporting terraforming equipment. A probe with ten times improved exhaust velocity from VASIMIR (500km/s from 50km/s, which is not too ambitious for 50yrs imo) needs to have less than 1% payload ratio to get to 0.01 c.

With 1000km/s,things would look like they become practically useful, like having enough fuel to slow down after the cruise (Initial payload ratio for acceleration is square root of absolute payload ratio). But still more like 250 years travel time. You would need a fusion reactor here, admittedly. Apart from it's far better efficiency (energy/kg fuel) it has the added benefit that you might use the fusion product as additional propulsion medium.

Useful: http://www.wolframalpha.com/input/?i=rocket+speed

  • $\begingroup$ As I recall The Starflight Handbook summarizes that fusion is barely practical for launching an interstellar mission. Fission would be too low power. $\endgroup$
    – JDługosz
    Commented Sep 22, 2016 at 0:58
  • $\begingroup$ @JDługosz You're right of course, basically. ;-) A nuclear reactor at full power spends his fuel rather fast, so you'll have to bring along a rather large supply of fuel rods, which are heavy. Fusion does have the power, if you're OK with less than 5% c travel. Trouble is, if we cannot miniaturise the fusion reactor, then any spaceship will have to be huge, because it definitely needs to start with >95 wt.% fuel $\endgroup$
    – Karl
    Commented Sep 22, 2016 at 1:49

One unified field theory suggests 11 dimensions. If true, maybe one could traverse a 3-dimentional distance by passing through one or more higher dimensions. Folding space, or folding yourself to slip between dimensions. You might need a gate to enable a return trip unless the device was small enough to take with you.

  • $\begingroup$ So? How close are we to that? $\endgroup$
    – JDługosz
    Commented Feb 15, 2017 at 8:13
  • $\begingroup$ It would require a new invention, paradigm shift, alien intervention sort of thing. If we could scale quantum mechanical effects that allow a photon or electron to be in two places at once we might get this to work. Maybe it will be something entirely unrelated to that phenomena. So this is in the 20 to 2,000+ years in the future, or possibly never. $\endgroup$ Commented Feb 16, 2017 at 19:28
  • $\begingroup$ Put the information in the answer, not a comment. My remark is to point out that you’re not answering the question. Your new statement isn’t a useful answer. $\endgroup$
    – JDługosz
    Commented Feb 16, 2017 at 21:10
  • $\begingroup$ How is my answer less useful/relevant than HDE's suggestion of a Bussard Ramjet where we have to develop fusion and figure out how much hydrogen is in interstellar space? $\endgroup$ Commented Feb 17, 2017 at 2:08
  • $\begingroup$ HDE offers concrete math to describe his approach and gives a definite answer. Your answer only describes a possible approach that doesn't actually provide a number as an answer. Currently, warp gates are considered pure science fiction and this is a science-based question. $\endgroup$
    – Green
    Commented Feb 17, 2017 at 5:18

NASA plans to have their Warp vessel built by 2100 so it is unlikely we would try any interstellar travel until proves to be unfeasible. Given that as a starter point we should expect failure between now and then in your universe.

A Mars mission is planed in the mid 2030s and might get pushed back by as much as a decade. You can then expect at least a decade or two dealing with Mars.

This puts you at 2050 before we get to large scale warp trials. I'd expect at least 10-30 years of trials and this gives us a 2080 date of when probes and such are going to start to be planned.

within a decade or 2 of this we'll launch probes which will be equipped with ion thrusters or solar sails or something and there are a lot of stars within 50-100 light years and each take more than 8 years to get to and 4 to communicate back. 12 years is the minimum for that which means you're now up 2112 before we get info back on any star. Assuming a perfect hit we then have a decade or 2 to plane the mission and more than 8 years to get to the planet, because the ship is going to probably have to go slower, but forgetting that, the minimum time year that we'll land on another planet to colonize it is 2140 if we don't have warp of any kind.

Ironically Warp would probably push this back because we then wouldn't rush to colonize because time wouldn't really be as big an issue and you run into that thing about speed of ships would constantly outpace each other so it's best to build a significantly advanced warp ship where the outpaced development wouldn't matter.

A more realistic timeline would be 2200s or 2300s simply to get all the data back and do a lot of testing of close range vessels for longer periods to make sure they get to where they're meant to be going and also transporting building materials.

  • 1
    $\begingroup$ «plans to have their Warp vessel built by 2100 » citation needed! $\endgroup$
    – JDługosz
    Commented Sep 21, 2016 at 7:02
  • $\begingroup$ @JDługosz Every time I've heard of this it was NASA specifically, but it's DARPA and NASA working together with a pretty low budget. NASA seems to have an official stance of it's not possible even though preliminary tests have been promising. space.com/11200-nasa-100-year-starship-interstellar-travel.html $\endgroup$
    – Durakken
    Commented Sep 21, 2016 at 8:07
  • 1
    $\begingroup$ So their official policy is “it's not possible” yet you state that they plan to have it developed by 2100? I still don't beleive that any of their planning includes breakthrough physics, but only estimatable engineering efforts. $\endgroup$
    – JDługosz
    Commented Sep 21, 2016 at 17:06
  • $\begingroup$ Your link to a brief on the 100 Year Starship Project does not state that anyone is planning on warp drive. It’s all about budget and comittment to effort. The closest thing is the note that a writer named Kloor says they should see unforseen breakthroughs in some fields (medicine, etc.) over that time scale. $\endgroup$
    – JDługosz
    Commented Sep 21, 2016 at 17:14
  • $\begingroup$ @JDługosz Yeah. I can't recall where I heard it, but my bet is that it is based on this partnership. The "not possible" is nonsensical given this partnership and I know there is research somewhere which shows Negative energy on a small scale which I've heard, but again, I can't find it. Part of the problem is google's algorythms mess with me looking for things part of it is just so much bullshit to get through. $\endgroup$
    – Durakken
    Commented Sep 21, 2016 at 17:44

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