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I'm an English teacher with a modest science background teaching Science Through Science Fiction this fall, and I need some help with the physics in Andy Weir's Project Hail Mary. Here's one question I have. (Light spoilers for the book.)

The ship is going to Tau Ceti (11.9 light years from Earth). Astrophage (their propulsion system) can achieve avelocity of 0.92c. Weir tells us the time that will pass from Earth's frame of reference during the ship's journey is about 13 years.

I don't know the equations, but my intuition says that would make sense for cruising at 0.92c the whole way, but does it account for acceleration and deceleration? The ship would have to accelerate and decelerate with low enough $g$'s not to kill the human crew, which I believe is at least less than 5 $g$'s (borrowing from Kim Stanley Robinson's Aurora). It seems to me this would significantly extend the travel time. But I haven't seen any science critique call this out, so maybe I'm just that hazy on how the math actually works out.

Does Weir's stated time frame make sense or is it not fully accounting for acceleration?

Edit: Catiffiny reminds me that the book states acceleration is 1.5 g. I'm not sure it states this is constant for the whole trip (including when the crew is asleep), but it does at least imply constancy by not mentioning changes anywhere else.

Arwen Spicer
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  • Probably a more complete answer would be gotten from Sci Fi & Fantasy Stack Exchange, but I suspect the weird quasi-suspended-animation sleep the crew is intended to be in while in transit handwaves away some of the normal human acceleration limits. – notovny Sep 01 '22 at 23:24
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    You may find this page helpful: https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html The relativistic equations for constant acceleration are fairly simple, but they do use hyperbolic trig functions. – PM 2Ring Sep 02 '22 at 00:29
  • The travel time estimate is probably one of the few "SciFi Physics" things in the book that doesn't require willing suspension of disbelief. On a related note, Weir's tendancy to write a cliff-hanger into every chapter gets boring rapidly. – Carl Witthoft Sep 02 '22 at 14:34

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The book clearly states 1.5g acceleration outbound from Earth, & 1.5g deceleration approaching Tau Ceti.

Catiffany
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