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It probably is, but what you can do instead is rework it as two books, and then when shopping for an agent, present it as book-plus-sequel. Science fiction (and fantasy) in particular are forgiving...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/21901 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/21901 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/