A laser erected on a mountain peak aims its beam toward a spacecraft and heats up propellant carried by the spacecraft. On a bad day clouds absorb more laser light than the propellant. Since the spacecraft is subject to extreme temperature, it is not durable. The minimum mass is 106 tons.
Hydrogen propellant produces the highest specific impulse, but it does not absorb laser light well. Either it has to be seeded with light-absorbing soot, or it has to be heated to 15,000 K to enable inverse bremsstrahlung absorption. The most practical propellant is a solid block of black plastic.
Ronald J. Glumb and Herman Krier, "Concepts and Status of Laser-Supported Rocket Propulsion," Journal of Spacecraft and Rockets, Vol. 21, No. 1, January-February 1984, pp. 70-79.
Mitat A. Birkan, "Laser Propulsion: Research Status and Needs," Journal of Propulsion and Power, Vol. 8, No. 2, March-April 1992, pp. 354-360.
Oleg A. Volkovitsky, Yuri S. Sedunov, and Leonid P. Semenov, Propagation of Intensive Laser Radiation in Clouds, AIAA, 1992, ISBN 1-56347-020-9.
Laser propulsion bibliography by Jordin Kare.
Laser propulsion page at Lawrence Livermore National Laboratory.
A microwave beam produced by a very large (>1km) phased array transmitter is aimed toward a spacecraft and heats up hydrogen propellant carried by the spacecraft. Unlike the laser beam, the microwaves are not absorbed by the clouds. The cost of generating microwaves is much lower than the cost of generating laser light of the same power. On the other hand, the microwaves cannot be focused as easily as the laser light, so the transmitter antenna must be very large. The receiver antenna on the spacecraft must be cooled by the liquid hydrogen propellant.
It may be possible to reduce the size of the transmitter antenna if the receiver antenna is a cloud of hydrogen plasma held in place by several superconductive electromagnets. Plasma is unstable, so designing such a device is difficult.
J. Benford and J. Swegle, "High Power Microwaves," Artech House, Boston, MA, 1992.
Leik N. Myrabo and J. Benford, "Propulsion of Small Launch Vehicles using High Power Millimeter Waves," SPIE Paper 2154-23, Proceedings of OE/LASE '94 Conference, Los Angeles, CA, January 22-29, 1994.