Nor will they be useful if they remain too expensive to implement.
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Mostly. This is because, theoretically, we could still save the ISS and move it into higher orbit. NASA has calculated that propelling the station more than 640 kilometers above Earth would keep it alive for 100 years—and also require at least 18.9 metric tons of propellant. That’s roughly 2,000 airline carry-ons. A thousand years would require at least 36 metric tons. If that doesn’t seem like very much, consider the fact that, right now, no vehicle can transport that amount of gas to the station. The still-in-development SpaceX megarocket Starship might be able to haul a significant percentage, but it would struggle to dock with the station, according to the space agency’s estimations.