If fusion, assisted by magnetic sails, gets us to Alpha Centauri in ~50 years, then how do we get there faster? Absent annihilation drives, powered by gamma-ray lasing matter-antimatter reactions or Hawking decaying force-fed mini-black holes, then we need to get the power-supply off the space vehicle and send fuel, momentum and energy to the vehicle as it accelerates. “Centauri Dreams” has covered a number of notable options just recently – the laser-powered ramjet, the laser-powered rocket and, of course, the Bussard ramjet itself.
Then there’s the various light, laser, microwave and momentum sails that have been proposed over time. Jim Benford, twin brother of SF-writer Greg Benford, and high-power microwave expert, has studied in some detail the economics of microwave propelled interstellar sails. The costs are extra-ordinary for all but the most primitive interstellar probes, but such figures are somewhat misleading. A basic assumption is that the energy generating and emitting systems will be installed in much the same way we do things at present – Jim factors in economies of scale, but not revolutions in technique.
Let’s have a look at the raw requirements. We’ll assume a 1,000 tonne payload, 1,000 tonne mag-sail and 400 tonnes of laser-sail. A 5,000 terawatt laser accelerates the sail to 0.5c in about 0.8 years – a total energy expenditure of 1.26E+23 joules. How much power is 5,000 terawatts? Earth receives 174,400 terawatts from the Sun, absorbing 122,200 terawatts of that. Balancing out the heat-flows in Earth’s atmosphere and oceans, equator-wards of the Tropics is a region that gains energy, while pole-wards of the Tropics are regions which lose net energy back into space. Energy flows northwards and southwards via the winds and oceans – the winds carrying about 5,000 terawatts in both directions. Thus our laser-sail needs about 50% of the Earth’s wind-power available.
We can’t power a starship with Earth-based energies, unless we mine heroic amounts of deuterium or boron from the oceans and land. We must turn to what’s available in space – the most abundant source being the Sun. In radiant energy alone, the Sun puts out ~384.7 tera-terawatts (384.7 yottawatts), but also sends forth immense amounts of energy in the Solar Wind. Tapping either is a non-trivial task. In the late 1970s NASA and the US DoE studied Solar Power Satellites (SPS) – one estimate was that a 5 gigawatt SPS would mass ~50,000 tonnes. Thus 5,000 terawatts would require 1 million SPS with a total mass of ~50 billion tonnes. Of course techniques have improved considerably since the 1970s – some ultra-light SPS designs approach ~1,000 tonnes per gigawatt. To go much lighter we need to move them closer to the Sun – if we can operate them at 1,000 K then we can park them just 0.1 AU from the Sun. There our “1 gigawatt” SPS can generate 100 gigawatts. Thus ~5 million tonnes of near-Solar SPS will power the lasers for our starships.
How fast can we get there with 5,000 terawatts of laser-power pushing us? I’ll have some answers in Part II.