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It is some small number of years into the future. We are regularly using beamed energy, as defined in this answer: large arrays of microwave transmitters. We have developed this so that these arrays are now positioned in orbit around Earth (powered by fuelled engines, regularly refuelled by probes).

Our space programs, however, have not developed that much. We're still stuck using liquid and solid fuelled rocket motors to get things into space. The one thing that has changed is that we now have access to an ion drive using electrostatic ion thrusters.

Our civilization has realised that in the not-too-distant future, the planet is going to die. They want to get out of there, as far away as possible - in contrast to the linked question, the Earth is going to explode so getting a long distance away is a good idea.

The problem with our ion drive is its energy use, which is high. We could just take huge batteries but we'd rather not.

So the problem I pose is this: assuming a sufficiently-sized competent team left on Earth to operate the beaming satellites, how much will the satellites help us? At what range are they no longer effective - and most importantly, can we get far enough away to avoid an Earth-destroying explosion?

This is . I'd like to see figures and equations where possible, but note if something cannot be calculated.

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    $\begingroup$ This sounds like a modified Solar Sail to me, and my intuition says the help is really small since you're competing with the amount of energy beamed out all day long by the sun. $\endgroup$
    – Erik
    May 26, 2015 at 13:03
  • $\begingroup$ This reminded me of gundam x (Japanese anime) a manned robot capable of absorbing microwave energy beamed from a lunar base(unmanned) but the co-pilot is a psychic. Unlike sunlight this beam resembles laser so the only difficulty encounter would be accuracy and precision across vast space. Ideally use chemical propellant for pickup speed follows by microwave drive for acceleration in space. $\endgroup$
    – user6760
    May 26, 2015 at 13:11
  • $\begingroup$ True, a plasma or laser sail would probably have better range (much bigger target) than "receive and power an ion drive" would. So a laser sail and some of the reflected energy used to power system including ion based "attitude thrusters". There should be some data on laser sails on the internet. $\endgroup$
    – Ville Niemi
    May 26, 2015 at 13:12
  • $\begingroup$ How violent is the explosion? Is it specifically the Earth that's going to blow, or will it just be consumed in the explosion? $\endgroup$
    – Frostfyre
    May 26, 2015 at 13:16
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    $\begingroup$ Don't we already have access to ion thrusters? They've been used since before 2000 for orbit-raising and stationkeeping on geostationary sats, and Deep Space 1 and (more recently) Dawn both used ion thrusters to reach their interplanetary targets. $\endgroup$
    – 2012rcampion
    May 26, 2015 at 14:07

1 Answer 1

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The two key elements are how much divergence the beam has, and how efficient the microwave receivers are.

Part two is actually rather easy, microwave "rectenna" have been researched since the late 1970s in conjunction with the idea of beamed power from space or the ground, and are quite efficient, converting microwave energy to electricity with 80% + efficiency. The downside is converting electrical power to microwaves is not efficient at all (especially at the scales we are talking about), so this is the main reason we have not seen things like nuclear aircraft carriers beaming microwaves to their aircraft to power them.

Part one is more difficult. As noted, conversion efficiencies are not especially high, and solar power satellites in orbit would need huge radiators for their microwave systems. Given the reason for building and operating these systems is to evacuate the earth, most of the efficiency and engineering difficulties would be attacked by large teams of engineers, and a lot of losses wold be considered acceptable (just build more beaming stations). Once the beams are being sent to cooperative targets on the spacecraft, the divergence of the beam becomes important. Large "lenses" would have to be scattered around the Solar System to focus the microwave beams (although they would be in the form of Fresnel lenses, with alternating rings of foil and empty space. The beam would diffract through the spaces and be refocused. The actual range of the beam before needing refocusing depends on the initial wavelength and (I think) the diameter of the emitter, so there are a lot of variables to consider to determine the range of the beam and the distance between lenses.

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