So humanity has decided to reach for the stars, they can't get anywhere near light speed but that hasn't stopped them.

The plan is to create a self sustaining ship and attach it to a large sungrazer comet which is used to exit the solar system by altering its angle of orbit.

What speed could it get to using conventional means (defined as proven technologies at our current level that could be foresee-ably be improved upon in terms of efficiency) and how long would it take to get to the determined location — approximately 25 light years away. The crew won't mind how long it takes as they'll be reconstructed at the end of the journey from stores of carbon and other materials.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Aug 11, 2016 at 12:19

3 Answers 3


Comets are rocket fuel! (But rockets are no good)

The problem with rockets is the Tyranny of the Rocket Equation. It means there are diminishing returns on rockets, and that means you can only go so fast. One of the things stopping us from getting to Mars is that the Tyranny prevents us from taking enough fuel with us for the return trip. So, long story short, you won't be able to take enough fuel with you when you launch.

Attaching yourself to a comet is only a good idea if you intend to use the comet for fuel. Now, this is mostly a pretty good idea, because comets generally contain ice, which can be split into hydrogen and oxygen which is - you guessed it - rocket fuel! Comets also contain a lot of waste material, however, and that will just slow you down, so a reasonable mission approach might be something along the lines of this:

  1. Send a swarm of unmanned drones to process the comet ahead of time, breaking it down into its constituent parts, discarding the unnecessary elements and processing the water content into fuel. This could take years, but comet orbits are often in the region of decades or centuries, so that's okay.
  2. On the comet's (well, at this point it's really just a big fuel tank) next orbit, send up your crew and have them rendezvous and bunker down for the long sleep.
  3. Wait until perihelion (your closest approach to the Sun) and fire the engines. Due to the Oberth effect this is the most efficient place to fire your engines. This will eject you from the solar system and send you on your merry way.

Just how fast can we go with chemical rockets?

Let's get some numbers together. On average, comets weigh in at around 1013 kg, and are thought to generally be about 80% water. This gives us about the comet's mass (80% is pretty much all of it, the calculations we'll be doing aren't going to be completely exact. You can use a bigger or smaller comet as you please) as reaction mass.

The most efficient rocket motor in a vacuum today using hydrogen as fuel and oxygen as oxidiser has a specific impulse (a measure of efficiency for rocket motors) of 438 seconds. Note that this is near the theoretical limit of efficiency for chemical rocket motors. Now we just need a dry (without fuel) mass for our spacecraft. This is a number that you can change wildly as well, so I'm just going to go with a nice round 100,000, or 105 kg. Now we can plug our numbers into the rocket equation! Using the approximate numbers I've come up with here, I get a delta-V of 79 km/s. Since you need to slow down at the other end, this is more like 40km/s of acceleration. Keep in mind for the next paragraph that this hypothetical comet-fuelled spacecraft outperforms every spacecraft we've ever built in terms of delta-V.

Sungrazing comets from the Oort cloud are travelling very close to escape velocity at perihelion, which is about 615 km/s. Yep, our feeble chemical engines are barely going to change this speed at all. We can escape the Sun's gravity, however, and will end up travelling at 655 km/s at our fastest. This turns out to be about 0.2% of the speed of light. A 25 lightyear journey at this speed will take 12,500 years.

However, this isn't even the end of the story! Because of this very marginal increase in velocity at perihelion, the CometShip will be moving very slowly relative to the Sun. It will orbit the centre of the Milky Way along with everything else at pretty much the speed the Sun does, ~200km/s. This results in a journey time of about 40,000 years.

As you can see, chemical rockets are simply not viable for interstellar travel. You may want to look at nuclear pulse propulsion systems, or things like solar sails, which falls into a similar category as laser propulsion. While none of these exist right now, there is nothing in our current understanding of physics (besides developing some interesting materials) to say they wouldn't work.

  • $\begingroup$ really great and well thought out answer :) My story is focussing on not getting there in any rush and the problems associated with it $\endgroup$
    – Chris J
    Aug 10, 2016 at 13:16
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    $\begingroup$ @ChrisJ thanks for accepting my answer! You might be interested in the wait calculation for interstellar missions. It basically says that when dealing with mission timespans of absurd length it's probably better to wait for sufficient advances in technology, as a mission launched later could still get there first if it goes faster, making the first mission useless. I'd be pretty miffed if was on a ship for 12,500 years to find that the destination has been inhabited for 10,000 because of a faster ship. $\endgroup$
    – ktyldev
    Aug 10, 2016 at 13:33
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    $\begingroup$ The Starflight Handbook, which you can find as a PDF now, goes over this in detail. Bottom line: you need antimatter. Fusion is just barely practical for some configurations. $\endgroup$
    – JDługosz
    Aug 10, 2016 at 15:10
  • $\begingroup$ This makes very little sense to me. A comet's speed at the far end of the orbit (aphelion) should be basically zero relative to the sun. You don't get to 0.0018c in interstellar space simply by riding a comet. Focusing on the perihelion is like saying you're going very fast at the bottom of a crater when your speed drops to zero near the rim, i.e. irrelevant. Moreover you'd need to speed up to the comet's intercept speed. $\endgroup$ Aug 10, 2016 at 19:26
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    $\begingroup$ @SerbanTanasa what you say is true, what you actually get is very nearly the Sun's velocity in very nearly the same direction, which puts the actual final speed at about ~200km/s, or 0.007c. At this speed the 25 light year trip would be 37,500 years, or about how long it's been since flutes were invented in Germany! $\endgroup$
    – ktyldev
    Aug 10, 2016 at 19:31

Actually if you could go fast enough to catch up to a sungrazer, you wouldn't need it in the first place :)

However, if your spaceship speed is not that high, then you can wait at the place at which a sungrazer will pass by, (after lots of observation and calculations) and then you attach yourself to it. That way, you get a free ride at 0.0018c. Note that you will have to detach yourself from the comet, as most sungrazers vaporise because they get too close to the sun. You can adjust the trajectory of the comet easily enough, so you won't end up returning to the same location again. (By using your own spaceships thrusters)

So to travel 25 lightyears, it will take approx 13888.89 years if you travel at only .0018c (.18 PERCENT of the speed of light) :(

Explanation: Instead of walking to a place by yourself, you can instead wait for a bus. This is a somewhat limited analogy, but it has it's advantages. You don't have to be that advanced. Even if your rocket's speed is low, you can just attach yourself to a comet which is much faster than you. That way you can accelerate yourself while barely spending any fuel. That way, you can greatly reduce costs (adding extra fuel tanks, and the cost of the fuel itself).

Note: You will have to calculate where to be in order to attach yourself to the comet, but this is near-future, so it should be easy enough.

The other advantage would be, that if you could somehow burrow inside (possible, considering that the OP stated that the ship has replicators, and that this is near future (and in order to go to another star system, you would have to be far more advanced)) you could protect yourself from other space debris. Instead of wasting energy by creating a force field to divert objects from puncturing your ship, just use the comet itself as a shield.

  • $\begingroup$ @Chinu : Damn I can't believe I made that mistake lol $\endgroup$ Aug 10, 2016 at 11:12
  • $\begingroup$ thanks both, @Chinu that's around the time frame I was expecting (10's of thousands of years) - is there anything I could do to increase the speed of the comet/spacecraft? I'm not sure throwing more mass out at that point $\endgroup$
    – Chris J
    Aug 10, 2016 at 11:18
  • $\begingroup$ @ChrisJ : Well you can use your own thrusters. It sounds like you don't need the comet in the first place, but actually it saves fuel - you first attach yourself to a comet (just by sitting there, so no fuel) then you speed up the comet. A lot of fuel will get saved really. $\endgroup$ Aug 10, 2016 at 11:20
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    $\begingroup$ @KingofSnakes you still have to expend energy to catch up with the comet. Buses stop for you, comets don't. In the same way that you can't jump onto a moving bus, you can't attach your spaceship to a comet moving with any significant relative velocity - imagine what would happen if you tried to jump onto a bus moving at dozens of kilometres a second. $\endgroup$
    – ktyldev
    Aug 10, 2016 at 12:14
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    $\begingroup$ The comet will drop to zero velocity at the far end of its orbit, so the calculation is flawed. At just escape velocity your speed becomes asymptotically slow as you get away from the solar system. $\endgroup$
    – JDługosz
    Aug 10, 2016 at 15:12

If we wanted to reach another star system we can currently reach .8c but it would cost a ton of money and isn't really practical to do, with matter-antimatter annihilation rockets. A bit less expensive and more practical would be the ~.1c of nuclear fusion bombs... using the Orion Project design. Which means we can go 25 light years in roughly 31 years with the former and 250 years in the latter.

There are technologies I think I heard about, but I can't recall, that are faster than the .1c range and more practical, being developed so really you don't need to do the whole reform at the other end thing considering it could be done in a life time.

  • $\begingroup$ I've looked into antimatter and it's hella expensive and nuclear fusion bombs will end up with the ship either being irradiated or sent massively off-course. $\endgroup$
    – Chris J
    Aug 10, 2016 at 11:40
  • $\begingroup$ We haven't actually developed antimatter rocket motors yet, there's still a fair bit of development to do in that field... $\endgroup$
    – ktyldev
    Aug 10, 2016 at 11:51
  • $\begingroup$ It's possible to do using what we know today though. Sure there are development issues, but that's every such project like the one described. And yes it would be massively expensive or have other problems but, we aren't talking about those things. I'd never recommend either as both are would generate radiation problems that it's probably better to avoid than work out a solution. $\endgroup$
    – Durakken
    Aug 10, 2016 at 12:53

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