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During the journey from Earth to Homestead II, Jim tries to send a message back to Earth. The computer informs him that it will take 19 years for the message to reach Earth.

Since we know that the ship is travelling at 50% of lightspeed and since we know that it's been 30 years since the ship set off, shouldn't the message should reach Earth in 15 years rather than 19?

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    Most likely something about them traveling and sending the message at the same time, they were moving farther as the message was going towards earth.
    – user57650
    Jan 11, 2017 at 20:50
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    @Cascabel - That was my instinct. We see them slingshotting a star to pick up speed and in the original script at one point they plough through a small nebula. I'm guessing the ship was traveling faster for the first few years of the journey and is gradually slowing down as they collide with interstellar dust.
    – Valorum
    Jan 11, 2017 at 21:09
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    Well, the ship has engines. Perhaps they were at max burn for the first few years, got up to .8 of lightspeed and have been gradually slowing down due to interstellar friction.
    – Valorum
    Jan 11, 2017 at 21:34
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    The fact that you can only justify marking this question as a duplicate if you reference a particular answer, is a huge screaming clue that the questions are not, in fact, duplicates. (Also "the second answer" is totally devoid of meaning in a Stack Exchange context.)
    – Martha
    Jan 11, 2017 at 21:52
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    @Martha - I've amended my answer (on the dupe) in order to more specifically address the point raised above.
    – Valorum
    Jan 11, 2017 at 22:09

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