> But to reach say 0.25c, a tiny ion engine over a long enough time would suffice. Tsiolkovsky says otherwise, by a factor of over 10^663 (not even counting re

> But to reach say 0.25c, a tiny ion engine over a long enough time would suffice. Tsiolkovsky says otherwise, by a factor of over 10^663 (not even counting relativity): https://www.wolframalpha.com/input?i=1%2Fe%5E%2874900km%2Fs%2F%285000s%29%2F%289.81m%2Fs%5E2%29%29 https://www.wolframalpha.com/input?i=1%2Fe%5E%2874900km%2Fs%... …Seriously. I tried to figure out just how much xenon you'd need to make that work. But you'd need to be able to store it in something like 15-dimensional space to even fit it within the diameter of the observable universe. And even if your ion engine and Hubble-scale fuel tank weighed less than the mass of the lightest quarks, the amount of propellant you'd need for it to reach 0.25c is still well over 10^600 times the combined mass of the entire observable universe, and would also collapse somewhere around 10^600 times the diameter of the observable universe into a single black hole: https://www.wolframalpha.com/input?i=2G%282.2MeV%2Fc%C2%B2%2F%287e-664%29%29%2Fc%5E2&assumption=%7B%22C%22%2C+%22G%22%7D+-%3E+%7B%22Unit%22%7D&assumption=%22UnitClash%22+-%3E+%7B%22c%22%2C+%7B%22SpeedOfLight%22%2C+%22dflt%22%7D%7D https://www.wolframalpha.com/input?i=2G%282.2MeV%2Fc%C2%B2%2... Of course, this also shows the intent of my original comment: Energy density matters, and somebody packing enough to casually cross interstellar densities isn't going to struggle with a planetary gravity well unless Idk they're doing like a low-tech off-grid trend or something. Also, skyhooks!