First we have to develop photonic laser thrusters (amplifying laser propulsion by bouncing between mirrors to recycle photons). Photonic laser thrusters are described in a previous nextbigfuture article. There has been experimental work that indicates that it is feasible.
The systems described here are an improvements by several orders of magnitude over regular power beaming. Recycling photons between mirrors reduces the energy requirements as does lightening the spacecraft and shortening the wavelength of the laser. There are other technological solutions to interstellar travel but this looks like one potential feasible technological solution.
Then we can develop a Photonic Railway, a permanent transport structure based on photon propulsion, has a potential to enable routine interstellar commutes via Spacetrains.
The Photonic Railway, as the transcontinental railway systems did, is projected to inspire sustainable economic interest and return investment, and to potentially achieve the goal: rountrip manned interstellar flight potentially within a century.
In this section, a four-phased developmental pathway of the Photonic Railway toward interstellar manned roundtrip is proposed:
1) Development of PLTs for satellite and NEO maneuvering,
2) Interlunar Photonic Railway,
3) Interplanetary Photonic Railway, and
4) Interstellar Photonic Railway.
It is projected that these developmental phases will result in systematic evolutionary applications, such as satellite formation flying, NEO mitigation, lunar mining, and Space Solar Power, which is projected to generate sufficient sustainable economic interest and return investment to the development pathway.
Development of PLTs for satellite and NEO maneuvering
Over the 5 – 30 year time frame, PLT would be capable of providing thrusts in the range of 1 mN – 1 kN, which requires the operation power of 100 W – 100 MW. The solar panel based space power currently can provide electrical powers up to 100 kW, therefore, the PLT capable of providing thrusts up to 1 N can be readily implemented in the near future. Further scaling up of PLT with 1 kN thrust will require a large solar power system capable of providing power up to 100 MW, of which development and implementation may depend on the space solar power or space nuclear power development in the future.
In Phase I, the operation distance of PLT is projected to be up to 100,000 km, which can cover a wide range of spacecraft maneuvering at LEO, MEO, and GEO. For example, once the spacecraft is in orbit, a 1 ton vehicle will take about 2.3 hours to cover 100,000 km via a 100 MW PLT with a thrust amplification factor of 1,000 for flying by. The diffraction limited size of the beaming mirror/lens should be on the order of 100 m and the spacecraft mirror diameter 2.44 m. The maximum thrust of such a system would be on the order of 1 kN.
Interlunar photonic railway
This is predicted to evolve over the 30 – 50 year time frame. PLT would be capable of providing thrusts in the range of 1 – 100 kN, which requires the operation power of 100 MW –10 GW. The operation distance of PLT is projected to be up to 1,000,000 km, which can cover a wide range of spacecraft maneuvering over lunar-scale distances. The diffraction limited size of the beaming lens should be on the order of 200 m and the spacecraft mirror diameter 50 m. The sizes the lens and mirror will decrease proportionally as the laser wavelength decreases. For example, a 100 time reduction in the laser wavelength will result in the lens diameter 20 m and the mirror diameter 5 m.
The Interlunar Photonic Railway with PLTs is predicted to meet the needs of the future generation of space industry market by enabling a wide range of innovative space applications involving the moon as a second step towards interstellar manned roundtrip commutes. For example, a 10 GW PLT with a thrust multiplication factor of 1,000, will generate a thrust of 66.7 kN, which can accelerate 6.8 ton Spacetrain at 1.0 g, a comfortable cruising acceleration.
Interplanetary Photonic Railway
Probably in the 50 – 70 year time frame, PLT will be capable of providing thrusts in the range of 100 kN – 10 MN, which will require the operation power of 10 GW – 1 TW. By this time frame, it is projected that high-power short wavelength laser will be fully developed for the required PLT power level. The operation distance of PLT is projected to be up to 10 billion km, the EarthPluto distance. One of the important milestone of this phase is the construction of Earth-Mars Photonic Railway.
With the Earth-Mars distance of 225 million km, the diffraction limit sets the beaming lens diameter 2.5 km, and the spacecraft mirror diameter 220 m with 1 µm lasers. For the Earth-Pluto Photonic Railway with a distance of 7.3 billion km, the diffraction limit sets the beaming lens diameter 35 km, and the spacecraft mirror diameter 500 m with 1 µm lasers. A 1,000 times reduction in wavelength will reduce both the lens and mirror diameters by a factor of 32 respectively, and the lens and mirror diameters required for Earth-Pluto Railway will be 1 km and 16 m, respectively.
Interstellar Photonic Railway
Predicted for the 70 – 100 year time frame, PLT-BLP is projected to be capable of providing thrusts greater than 10 MN, which requires the operation power of 1 – 100 TW. The operation distance of PLT-BLP is projected to be up to 100 trillion km, which can cover a wide range of spacecraft maneuvering over earth-nearby-star distance. For the Earth-ε-Eridani BLP with an operation distance of 10.8 ly (~100 trillion km), the diffraction limit sets the beaming lens diameter 1,000 km, and the spacecraft mirror diameter 252 km with 1 µm lasers as mentioned for the earlier systems.