# How long for a spacecraft to reach Tau Ceti at 1 g? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [April 6, 2026, 1:25pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064 "2026-04-06T13:25:16Z")\ **Posts on this page:** 16\ **Page:** 3
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [April 8, 2026, 12:36pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/41 "2026-04-08T12:36:09Z")
> [@Chronos](#): > > Of course, the longer you keep it up, the more magical a constant-acceleration drive needs to be. A constant 1 g from here to tau Ceti is just maybe plausible, with an antimatter-based fuel. To the Andromeda galaxy, though, even with pure antimatter fuel, the fuel ratio (amount of fuel needed to total payload) gets ludicrous, and for the edge of the observable Universe (as observed from Earth right now)? Fuhgettaboutit. Right, which is why many of the theoretical propulsion systems proposed for interstellar travel generally don’t involve the ship carrying its own fuel. Besides solar sails, another theoretical interstellar propulsion system (introduced to me by science fiction writer Larry Niven) is the [Bussard ramjet](https://en.wikipedia.org/wiki/Bussard_ramjet). ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [April 8, 2026, 1:57pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/42 "2026-04-08T13:57:18Z")
> [@dtilque](#): > > > [@Tim\_T-Bonham.net](#): > > > > How does this transmission work when the earth is revolving so that half the day the transmitter is pointed the wrong way? > > I think the transmitter is supposed to be in orbit. Probably solar orbit. Note that the transmitter experiences the same thrust as as the receiver, which means it has to be counter-balanced to maintain orbit. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [April 8, 2026, 7:47pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/43 "2026-04-08T19:47:28Z")
> [@Chronos](#): > > Of course, the longer you keep it up, the more magical a constant-acceleration drive needs to be. A constant 1 g from here to tau Ceti is just maybe plausible, with an antimatter-based fuel. To the Andromeda galaxy, though, even with pure antimatter fuel, the fuel ratio (amount of fuel needed to total payload) gets ludicrous, and for the edge of the observable Universe (as observed from Earth right now)? Fuhgettaboutit. I’m probably missing something obvious, but why does the ship have to keep accelerating the whole way there? Couldn’t it stop accelerating once its reached 99 plus percent of the speed of light relative to its destination? ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [April 8, 2026, 7:57pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/44 "2026-04-08T19:57:43Z")
> [@FlikTheBlue](#): > > > [@Chronos](#): > > > > Of course, the longer you keep it up, the more magical a constant-acceleration drive needs to be. A constant 1 g from here to tau Ceti is just maybe plausible, with an antimatter-based fuel. To the Andromeda galaxy, though, even with pure antimatter fuel, the fuel ratio (amount of fuel needed to total payload) gets ludicrous, and for the edge of the observable Universe (as observed from Earth right now)? Fuhgettaboutit. > > I’m probably missing something obvious, but why does the ship have to keep accelerating the whole way there? Couldn’t it stop accelerating once its reached 99 plus percent of the speed of light relative to its destination? Sure you could. But as @Chronos pointed out in another thread, continuous acceleration increases the time dilation for the ship. The Andromeda Galaxy is 2.5 million light years away. So there’s no way to get there in a human lifetime without continuous acceleration. (The amount of time dilation and duration of the trip _as experienced by the crew of the ship_ is left as an exercise for the reader. 😉 Of course, to an outside observer here on Earth, the journey will take at least 2.5 million years.) ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [April 8, 2026, 8:12pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/45 "2026-04-08T20:12:49Z")
> [@robby](#): > > The Andromeda Galaxy is 2.5 million light years away. So there’s no way to get there in a human lifetime without continuous acceleration. Depending on how you define ‘human lifetime’, it wouldn’t necessarily have to be continuous acceleration the entire way. But it would still be several years of 1g acceleration at a minimum to get close enough to c for sufficient time dilation. But for the edge of the observable Universe? Yeah, that’s more like decades of 1g acceleration to get there within a normal human lifetime. The other caveats about fuel ratio and engineering challenges still apply, of course, which should include the human physiological need for gravity, which a constant 1g acceleration would artificially provide. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [April 8, 2026, 8:41pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/46 "2026-04-08T20:41:41Z")
> [@robby](#): > > > [@FlikTheBlue](#): > > > > I’m probably missing something obvious, but why does the ship have to keep accelerating the whole way there? Couldn’t it stop accelerating once its reached 99 plus percent of the speed of light relative to its destination? > > Sure you could. But as @Chronos pointed out in another thread, continuous acceleration increases the time dilation for the ship. The Andromeda Galaxy is 2.5 million light years away. So there’s no way to get there in a human lifetime without continuous acceleration. Yep. With truly uniform acceleration, the time dilation can reach absurd values. The farther the destination, the more it matters. > [@Great\_Antibob](#): > > Depending on how you define ‘human lifetime’, it wouldn’t necessarily have to be continuous acceleration the entire way. But it would still be several years of 1g acceleration at a minimum to get close enough to c for sufficient time dilation. > > But for the edge of the observable Universe? Yeah, that’s more like decades of 1g acceleration to get there within a normal human lifetime. It won’t feel that long to the accelerating travellers. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 8, 2026, 8:45pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/47 "2026-04-08T20:45:23Z")
> [@Great\_Antibob](#): > > The other caveats about fuel ratio and engineering challenges still apply, of course, which should include the human physiological need for gravity, which a constant 1g acceleration would artificially provide. I mean, yeah, that’s a pleasant side effect, but if that’s your only reason for constant acceleration, there are much cheaper ways to get the same effect. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [April 8, 2026, 8:46pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/48 "2026-04-08T20:46:38Z")
Hey, I’ll take it, as long as we’re ignoring only some but not all physically realistic constraints on space travel ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 8, 2026, 8:58pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/49 "2026-04-08T20:58:04Z")
It should be mentioned, by the way, that we know for sure that zero g is bad for humans: We’ve had lots of experience with lots of humans at 0 g, some of them for over a year at a time. But we don’t know how much gravity is necessary for health, since the longest any human has ever spent at any gravity between 0 and 1 is the few days the Apollo astronauts spent on the Moon. So maybe your starship only needs to maintain a fraction of a g. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [April 9, 2026, 12:43am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/50 "2026-04-09T00:43:20Z")
> [@Great\_Antibob](#): > > But for the edge of the observable Universe? Yeah, that’s more like decades of 1g acceleration to get there within a normal human lifetime. Time again to mention Poul Anderson’s [_Tau Zero_](https://en.wikipedia.org/wiki/Tau_Zero), which explores time dilation in spades. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [April 10, 2026, 12:42am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/51 "2026-04-10T00:42:15Z")
Indeed! Larry Niven also had several stories that featured a Bussard ramjet that experienced extreme time dilation, but not to the extent of _Tau Zero_. One of them (_The Ethics of Madness_ from 1967) was a story about a murderer who tries to escape the consequences of his crimes by taking off in a Bussard ramjet, chased by a grieving relative of the deceased in another ship. The murderer can’t outrun the ship chasing him, so the two ships accelerate to extreme relativistic speeds toward the edge of the universe. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 10, 2026, 1:01am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/52 "2026-04-10T01:01:41Z")
Of course, a real Bussard ramjet (assuming it’s fusion-powered and maximally efficient) couldn’t get to more than 0.12 c, due to the necessary drag force from collecting the hydrogen. Which is still an impressive speed, of course, but relativistic effects are still small at that speed. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [April 10, 2026, 1:28am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/53 "2026-04-10T01:28:45Z")
> [@Chronos](#): > > Of course, a real Bussard ramjet (assuming it’s fusion-powered and maximally efficient) couldn’t get to more than 0.12 c, due to the necessary drag force from collecting the hydrogen. The perils of creating a setting in the 1960s that keeps being read into the 2020s. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 10, 2026, 1:23pm UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/54 "2026-04-10T13:23:00Z")
Right from the start, it could have been anticipated that a ramscoop would produce drag, and everything needed to do the calculation of maximum speed derived therefrom was known in the 1960s. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [April 11, 2026, 5:38am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/55 "2026-04-11T05:38:57Z")
> [@Mangetout](#): > > Not so much if they encounter a little pebble at 99% of the speed of light, I imagine I’d be more worried about the 5.7 GeV proton (hydrogen nuclei) flux being constantly encountered at .99c… ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 11, 2026, 7:29am UTC](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064/56 "2026-04-11T07:29:57Z")
> [@Marvin\_the\_Martian](#): > > 5.7 GeV At some point in the acceleration, it might be possible to use the incoming particles to take X rays to help diagnose the cancers they are also causing. [Previous page](https://boards.straightdope.com/t/how-long-for-a-spacecraft-to-reach-tau-ceti-at-1-g/1029064.md?page=2)