In the SyFy mini-series Ascension, as one of the plot points reference is made to the Ascension space ship reaching a "Rubicon Point" at which it will take more resources to return to Earth as opposed to continuing the remaining distance to the Proxima system. I.e. it is perceived as a point of no return. However, out of an approximately 100 year journey, wouldn't that have occurred much earlier? I am looking for an in-universe explanation of why that would not be the case.

I am assuming the Ascension is accelerating at some percentage of ~1g to maintain the appearance of an Earth gravitation. The ship is not spinning. Too, the appearance inside the ship seems to have something approaching Earth gravity. As such, after 51 years, the ship is traveling at or near its maximum expected speed. Note, this is supported by the scene during the radiation storm when the question is asked "why should this storm be any different?" The response is that the ship is now traveling faster than it ever has, thus the radiation will have more impact.

However, if that is so, then wouldn't the ship be expected to require another ~50 years of DECELERATION at the same level of ~g to slow the ship down as it approaches the Proxima system? Too, if that assumption is correct would that not also imply that it would take at least 50 years to slow down enough to then be able to THEN reverse course to return to Earth making?

If that is correct, then wouldn't the true Rubicon point on an estimated 100 year journey actually be at ~25 years give or take? 25 years accelerate, 25 years to slow down, 25 years to reverse course to Earth, 25 years to decelerate going back to Earth?

The only thing I can think of which might mitigate this is if the plan is to somehow use the Proxima system stars in series of gravitational slowdown maneuvers at higher than 1g effects rather than simply decelerate for 50+ years. But, I would think to shed that much speed in a short time after reaching Proxima would require a significant increase in the "perceived" gravity within the ship. More than what would be expected people could take for an extended period of time. Remember there are babies and small children on board as well as presumably elderly.

BTW, though I am not good enough at the math to show it, I am setting aside the fact that after 50 years of ~1g acceleration, wouldn't the ship be traveling at an appreciable fraction of the speed of light, hence relativistic effects would start to become noticeable as well?

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    @Richard slowing from near the speed of light to 0 using aero braking (or any kind pf braking that took less than 10 years - for a very uncomfortable time) would be quite lethal. The issue isn't the energy requirements, its the crew surviving
    – user20310
    Dec 21, 2014 at 17:17
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    Your instincts about relativistic effects are justified. It only takes about a year of 1g acceleration to approach the speed of light. Continuing to accelerate at 1g for a perceived 50 years would mean an absolutely absurd amount of time dilation from the perspective of the rest of the universe.
    – Micah
    Dec 21, 2014 at 17:20
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    @user20310: The problem is, it's been 50 years in the ship's frame. So either they haven't been accelerating the whole time, or the star they're aiming at is long dead and they're off in intergalactic space somewhere...
    – Micah
    Dec 21, 2014 at 17:25
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    I haven't seen the series, but how do you know the ship isn't spinning? Even if they show an external shot, it could be internal parts are rotating inside a non-rotating shell. Also, I heard some spoilers about the ending of the first episode which might be relevant to this question, but I don't want to reveal anything if you haven't seen it yet.
    – Hypnosifl
    Dec 21, 2014 at 17:53
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    @Hypnosifl. From the shots of the ship looking down from the top, it shows that the angle of acceleration (hence apparent "gravitational" effect) is along the length of the ship. I.e. "down" is towards the engines and "up" is towards the top of the ship. If the ship was spinning, I would expect "down" to be to the outside of the ship's hull and that towards the central core "up" one would encounter weightlessness. That doesn't seem to be the case here. Too, when looking outside the ship, the stars do not appear to be moving which would be expected if the ship were spinning.
    – beichst
    Dec 21, 2014 at 18:02

1 Answer 1


At 1g or 9.8m/s^2, it takes about 36 days to reach a speed of about 10% the speed of light. At this speed, it would take about 44 years to reach the closest stars, which are the binary star system Alpha Centauri. For a practical purpose, the acceleration phase of 36 days doesn't really change anything important when compared to the total length of the journey.

For a one hundred years journey, the maximum speed should be about 4.4% of the speed of light, which requires about 16 days when accelerating at 1g; which is well inside the parameters of an Orion thermonuclear starship; see the article http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) ; which states a maximum of 8% to 10% of the speed of light for a thermonuclear powered Orion and 3% to 5% for an atomic (fission) version.

Of course, we could use a lower rate of acceleration. For making a 100 years journey to Alpha Centauri, the slowest rate you can have would be to continuously accelerate at 0.00175g until reaching 8.8% the speed of light at the half point after 50 years; then decelerate at the same rate until reaching your final destination.

If this rate of 0.00175g was what they would have used then yes, you are absolutely right about the fact that the point of no return should have been set at around the end of the 25th year. However, they never said in the story that this was the case and as far as we know, they could have used a much more higher rate of acceleration.

Of course, we should take into account any relativist effect but with a maximum speed of 4.4% or 8.8% the speed of light, they are not big enough to have any drastic effect on the overall result.

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    So, basically what you are saying is that they screwed up the math and physics?
    – Moo
    Jan 3, 2015 at 11:32
  • @Moo: I've said nothing of that. There are other limitations than just the rate of acceleration and maximum speed; like the amount of energy for lighting and growing up food or to repair and maintain the ship. These ressources are probably not limitless and may barely cover their needs for a one hundred years journey.
    – SylvainL
    Jan 3, 2015 at 12:12
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    Yeah, but we are looking at a crew which definitely are still under 1G acceleration half way into the trip - hence my point about math and physics.
    – Moo
    Jan 3, 2015 at 12:40
  • Unless they are accelerating/decelerating continuously or are following some kind of spiral as their trajectory, with both options requiring an excessive lot of fuel, there is no way to substain an acceleration of 1g (or something slightly less like 0.3g) for more than a couple of weeks.
    – SylvainL
    Jan 3, 2015 at 12:53
  • "Of course, we could use a lower rate of acceleration." - it seems that the Earth-like gravity aboard the ship is supposed to come from the ship's acceleration. I have created a separate question to find out for sure, though. Mar 31, 2019 at 21:39

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