David L Babcock <olb...@gmail.com> wrote: The bad news negates itself: > > Considering C of E, a miss-aimed craft could not apply more energy to a > planet than was originally applied to the craft to bring it up to speed. A > continent-melting crash requires that more than a continent-melting supply > of fuel has been applied to/used by the craft. >
I don't think this adds up. First, the fuel would be used slowly, over many hours or days as the ship speeds up. Or if it were incoming from another star, the fuel might have been used up over years. Suppose, for example, something goes wrong and it never reverses thrust to slow down. It comes in out of control after accelerating for 5 years in a 10-year trip. Wham! Second, you might actually use up a continent-melting supply of fuel, if it was external to the ship. This statement does not follow: > Another consideration: The craft has to carry with it a similar amount of > energy, stored, to use for deceleration. And twice that again, to come home. > The fuel might be expended at a fixed orbiting base that accelerates the ship with something like a laser beam. Or, as mentioned, the fuel might be scooped out of interstellar space. The hydrogen in the flight path might be deployed in space before the flight, by robots that keep the space lanes filled with gigantic quantities of hydrogen from surrounding star systems. You might find a way to tap a significant fraction of the sun's output to power a ship without vaporizing the ship. I cannot imagine how, but there might be a way. This is enough solar energy to easily accelerate a million-ton ship to 0.9 c. It might power the ship for 0.5 light years out, with another gigantic machine at the destination star to slow it down. The point is, the ship would not have to carry the fuel or the reactors. You might need a gigantic mass of fuel, or building material, for some of these schemes. More than you can conveniently mine from planets and asteroids. In the far distant future suppose people figure out a way to convert the energy from the sun back into mass. The sun loses 4.7 million tons of matter per second in mass-energy conversion. If you could intercept a small part of that and convert it to mass, you would soon have plenty of free hydrogen to deploy in the space lanes, or enough to build a gigantic shell to intercept and concentrate solar energy, and other construction projects as large as the solar system. - Jed
