Will We Travel To The Stars?

By Freeman Dyson

If you ask whether we will travel to the stars, the answer is probably yes, but it will take a long time. Maybe 500 years. If you ask whether any human being will travel to the stars within the 21st century, the answer is certainly no. The difference between traveling to the nearest star and traveling around our own solar system is about the same as the difference between swimming across the Atlantic and swimming across the Potomac. To get across the Atlantic you need to have a boat or an aircraft. To get to the nearest star you need to have a spacecraft that we have no hope of building within 100 years.

To scoot around the solar system and return within a few years, you need a spacecraft that will cruise at 100 miles a second. At that speed you will get to Mars in 10 days, to Pluto in 16 months. We can imagine a spacecraft carrying a big area of thin film to collect solar energy, with an ion-jet engine to produce thrust powerful enough to boost a spacecraft to a speed of 100 miles a second. It is also possible to build a nuclear-powered jet to do the same job, if the political objections to nuclear spacecraft can be overcome. The quantity of energy available from sunlight or from a nuclear reactor is large enough to take us on trips around our solar system, if we decide to spend the money to do it. We may or may not decide to build a 100-mile-a-second spacecraft within 100 years, but we know that it is technically possible. The cost of developing a 100-mile-a-second spacecraft would be large but not absurd.

On the other hand, the nearest star is about 10,000 times as far away as Pluto. A trip to the stars within a human lifetime requires a spacecraft that cruises at more than 10,000 miles a second and accelerates to this speed within 10 years. The engine would have to deliver about a megawatt of power for every pound of weight of the ship. There is no way an engine that small and that powerful could keep itself cool. Even if the fuel is something exotic like antimatter, carrying far more energy than sunlight or uranium, the problem of cooling the engine remains insuperable. Travel to the stars within this century, using any kind of engine we know how to build, is not going to happen.

How about unmanned space probes going to the stars? Unmanned probes can be much smaller and lighter than manned spaceships. That means the total power required for a probe to reach the stars is much less. But the unmanned probe still needs an engine delivering one megawatt per pound. The problem of cooling the engine remains the same, whether the ship is manned or unmanned, and the conclusion is the same. Unmanned probes are not going to reach the stars within this century.

Robert Forward, an engineer who used to work for Hughes Aircraft and now works independently, has designed a space probe that might reach the stars, not within this century but a little later. It avoids the problem of cooling the engine by not having an engine. It is a sailing ship, not a steamship. He calls it Starwisp. It is a fishnet made of very fine wires and weighing less than an ounce. The net acts as a sail and is driven by the pressure of radio waves generated by a huge radio transmitter. The transmitter stays put, with its radio beam pointing in the direction we wish to explore, and the sail travels along the beam, picking up momentum from the radio waves. This scheme works beautifully in theory, but there are some practical difficulties to be overcome. The transmitter has to be gigantic and must focus the energy of the beam on the fishnet as it accelerates. The fishnet must absorb only a tiny fraction of the radio waves to avoid being vaporized. The probe must carry instruments to collect information and transmit signals back to earth, and those instruments must weigh less than an ounce. There are enough problems here to keep engineers busy for several centuries, but one day a ship like this will fly.

Ultimately, one can imagine scaling up the Starwisp by a factor of 1 million, so that the fishnet is big enough to carry human passengers to the stars. The radio transmitter to drive it would use far more power than all the power stations on earth now generate. Some day we may have this much power to spare for voyages of exploration, but not soon. Perhaps around the middle of the third millennium.

Freeman Dyson, retired professor of physics at the Institute for Advanced Study, is author of The Sun, the Genome and the Internet