Going Interstellar (collection)

There is another approach that takes advantage of the Earth’s well-established manufacturing infrastructure and the unique environment of space to solve the manufacturing and launch problems: build the sail on Earth, but make it more robust—thicker—than the mission requires and make the extra thickness out of materials that won’t easily tear when in the Earth’s gravity and that will not damage easily during launch. But, design the more robust sail so that the heaviest part will evaporate when exposed to a selected portion of the Sun’s ultraviolet light—which only happens when we are above the Earth’s atmosphere. Voila! The thick and heavy sail that was easier to make and launch quickly becomes the wispy, lightweight sail needed for rapid propulsion through interstellar space.

This might just work.

The single largest constraint on an interstellar spacecraft propelled by a solar sail is the “solar” part. If the ship must get all of its thrust from the Sun, then it is constrained to do so before it passes the orbit of Jupiter (in just a couple of weeks) because the Sun gets very dim at this point and the additional thrust the ship would obtain from the ever-more-distant Sun is minimal. It is very difficult to get enough energy from the Sun for a voyage to another star—especially in a few days or weeks. How then can we build a sail and continue to use light pressure to accelerate even after the sail is beyond the reach of sunlight?

Lasers may solve this problem. A laser provides a tightly focused beam of light across large distances and might be capable of providing enough light to continue pushing our sail during its journey through interstellar space. An interesting approach to using laser energy for interstellar solar sailing was described by the late physicist, engineer and author extraordinaire, Dr. Robert Forward. As early as 1962 Forward was publishing technical papers describing how a future sail might be pushed through deep interstellar space by a powerful laser orbiting the Sun.

On the scales that we typically use lasers, say in the few tens of feet or less, the beam appears to be tightly focused without significant divergence, or beam spread. But over millions of miles, even the best laser beam will diverge and become more diffuse. In order to keep a relatively small beam focused on our interstellar sail, we will need to build a six hundred mile diameter focusing lens at about the orbit of Jupiter through which we will shine our laser.

Using a spacecraft of similar weight to the one described above for the Sun-only solar sail, and using a sail of about the same size, Forward calculated that a sixty-five Gigawatt laser could accelerate our sail to a velocity of one-tenth the speed of light. This would enable our spacecraft to reach Alpha Centauri in only a little more than forty years after launch. A substantial improvement over three thousand years!

Unfortunately, we don’t know how to build continuously operating sixty-five GW lasers, nor do we know how to build six hundred mile diameter lenses orbiting the Sun near Jupiter. Our physics is once again ahead of our engineering—but we won’t let that stop us!

Forward went on to show that a sail craft of much more interesting (from the point of view of future human interstellar exploration) sizes, say six hundred miles in diameter and weighing almost two million pounds, could have the same forty-year trip time if a seven-Terawatt laser were used (Figure 7). I should point out that the annual total power output for the human race is approximately 1 TW. Again, there is no physical reason this cannot be done. The challenge, as physicists are often fond of saying, is in the engineering. BUT IT IS POSSIBLE.

Figure 7. Robert Forward’s interstellar light sail concept shown as it appeared in his Advanced Space Propulsion Study for the Air Force Astronautics Laboratory in 1986.

Figure 8. A possible roadmap for developing solar sail propulsion from that we can build today to that which will be required to take us to the stars. (Image courtesy of NASA.)

Forward further proved that we could slow down and rendezvous with a target star’s planets by having a detachable inner sail that uses laser light reflected from the outer ring (of the sail) to slow it down. This same approach could be used to send spacecraft to virtually any nearby star system with commensurately longer trip times—though they will be measured in decades rather than millennia.