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"If you turn away from the natural gifts that God has given you, or the universe has given you, you're going to grow old too soon."
- Ray Bradbury

Interstellar Laser Propulsion System  
  A means of traveling to the stars, using a Sol-based laser system and light sail-equipped ship.  

The payload sent to the Barnard system consisted of the crew of twenty persons and their consumables, totalling about 300 metric tons; four landing rockets for the various planets and moons at 500 tons each; four nuclear powered VTOL exploration airplanes at 80 tons each; and the interstellar habitat for the crew that made up the remainder of the 3500 tons that needed to be transported to the star system.

This payload was carried by a large light sail 300 kilometers in diameter. The sail was of very light construction, a thin film of finely perforated metal, stretched over a lightweight frame. Although the sail averaged only one-tenth of a gram per square meter of area, the total mass of the sail was over 7000 tons. The payload sail was not only used to decelerate the payload at the Barnard system, but also for propulsion within the Barnard system.

The 300 kilometer payload sail was surrounded by a larger ring sail, 1000 kilometers in diameter, with a hole in the center where the payload sail was attached during launch from the solar system. The ring sail had a total mass of 71,500 tons, giving a total launch weight of the sails and the payload of over 82,000 tons.

The laser power needed to accelerate the 82,000 ton interstellar vehicle at one percent of earth gravity was just over 1300 terawatts. As is shown in Figure 5, this was obtained from an array of 1000 laser generators orbiting around Mercury. Each laser generator used a thirty kilometer diameter lightweight reflector that collected 6.5 terawatts of sunlight and reflected into its solar-pumped laser the 1.5 terawatts of sunlight that was at the right wavelength for the laser to use...

The accelerating lasers were left on for eighteen years while the spacecraft continued to gain speed. The lasers were turned off, back in the solar system, in 2044. The last of the light from the lasers traveled for two more years before it finally reached the interstellar spacecraft. Thrust at the spacecraft stopped in 2046, just short of twenty years after launch. The spacecraft was now at two lightyears distance from the Sun and four lightyears from Barnard, and was traveling at twenty percent of the speed of light. The mission now entered the coast phase.

For the next 20 years the spacecraft and its drugged crew coasted through interstellar space, covering a lightyear every five years. Back in the solar system, the laser array was used to launch another manned interstellar expedition. During this period, the Barnard lens was increased in diameter to 300 kilometers. Then, in 2060, the laser array was turned on again at a power level of 1500 terawatts and a tripled frequency. The combined beams from the lasers filled the 300 kilometer diameter Fresnel lens and beamed out toward the distant star. After two years, the lasers were turned off, and used elsewhere. The two-light-year long pulse of high energy laser light traveled across the six lightyears to the Barnard system, where it caught up with the spacecraft as it was 0.2 lightyears away from its destination.

Before the pulse of laser light had reached the interstellar vehicle, the vehicle had separated into two pieces. The inner 300 kilometer payload sail detached itself and turned around to face the ring-shaped sail. The ring sail had computer-controlled actuators to give it the proper optical curvature. When the laser beam from the distant solar system arrived at the spacecraft, the beam struck the large 1000 kilometer ring sail, bounced off the mirrored surface, and was focused onto the smaller 300 kilometer payload sail as shown in the lower portion of Figure 6. The laser light accelerated the massive 71,500 ton ring sail at 1.2 percent of Earth gravity and during the two year period the ring sail increased its velocity slightly. The same laser power reflecting back on the much lighter payload sail, however, decelerated the smaller sail and the exploration crew at nearly ten percent of Earth gravity. In the two years that the laser beam was on, the payload sail slowed from its interstellar velocity of twenty percent of the speed of light to come to rest in the Barnard system. Meanwhile, the ring sail sped on into deep space, its job done.

From Rocheworld, by Robert Forward.
Published by Not known in 1985
Additional resources -

The basic idea was described by Dr. Forward in a 1961 paper Ground-Based Lasers For Propulsion In Space. It was used by Larry Niven in his 1966 story Neutron Star; he called it laser cannon.

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Additional resources:
  More Ideas and Technology from Rocheworld
  More Ideas and Technology by Robert Forward
  Tech news articles related to Rocheworld
  Tech news articles related to works by Robert Forward

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