COULD YOU EVER FLY TO THE STARS? 2. Interstellar Overdrive
Figure 1. Artist impression of the Daedalus probe arriving at Barnard's Star. Artwork courtesy of Adrian Mann.
Figure 2. The main parts of the Daedalus star probe.
Figure 3.In this fusion reaction, two types of hydrogen, deuterium and tritium, combine to form helium nuclei. In the process, tiny particles called neutrons are released.
Figure 4. The "Sunflower" sailing craft has an aluminized sail made of 480 individual petals. The spacecraft itself consists of a two-by-four-foot cylinder, tethered to the sail by wires and a beam.
Figure 5. Artist's impression of an interstellar ramjet.
Artwork courtesy of Adrian Mann.
The year is 2090. From its fueling base near the giant planet Jupiter, a remarkable spacecraft is setting out on the first mission ever attempted to another star. It is the robot probe Daedalus, and its target is Barnard's Star (see Figure 1). This is a small, dim star, 5.9 light-years away, that may have planets circling around it.
Daedalus's journey will take only 50 years at a top speed of 22,000 miles per second, or one-eighth the speed of light. To go that fast, the spacecraft will rely on a totally new method of propulsion. Each second, 250 small nuclear explosions, like miniature hydrogen bombs, will be set off in the starship's engine chamber. The hot gas produced by these explosions will be focused by a powerful force of magnetism into an exhaust jet that streams out behind the spacecraft. The speed of the escaping gases will be around 6,250 miles per second – 2,500 times faster than the exhaust of an ordinary chemical rocket. The fast-moving jet of gas will drive the 54,000-ton Daedalus forward with a thrust of 1.7 million pounds.
For more than two years, the big, first stage engines of Daedalus will fire, accelerating the probe to a speed of 13,200 miles per second. Then the empty fuel tanks and motors of the first stage will be released into space, and the second stage engines will be started. Exactly three years and 290 days into the mission, these smaller engines, too, will stop firing. The spacecraft will have reached its final cruising speed, ready for a 47-year coast to Barnard's Star.
All this may seem to be just another fantastic idea from science fiction, like the starships in the movies and on television. But, in fact, it is more than that. Daedalus may not have been built yet, and perhaps it never will be. But a detailed design for it does exist, and there is no reason why a spacecraft like it could not be launched sometime in this century (see Figure 2).
Blueprint for a Starship
The idea for Daedalus comes from a team of scientists and engineers whose efforts were coordinated by the British Interplanetary Society. Their goal was to see if a spacecraft, capable of reaching a nearby star in 50 years or less, could be designed and built in the near future.
The main problem was to decide what kind of propulsion system – the system used to push the spacecraft forward – to use. In the end, the design team settled for a system that worked by NUCLEAR FUSION. This is the same process by which the Sun makes its heat and light.
In nuclear fusion, particles of a lighter substance, such as hydrogen, are slammed together at very high temperatures to form particles of a heavier substance, such as helium (see Figure 3). In the process, a tiny bit of mass is lost. That mass is turned into a huge amount of energy, including the energy of motion of the particles created in the reaction. Since these fast-moving particles also carry an electric charge, they can be guided and focused by a magnetic field. This would be an ideal means, the Daedalus scientists reasoned, to drive the spacecraft to the stars.
Little pellets of fusion fuel would be shot into the engine chamber at the back of the spacecraft at the rate of 250 a second. Each would be met by a short, but incredibly powerful burst of particles called electrons. This burst would trigger the fusion reaction. In an instant the pellet would explode with the force of several tons of TNT, a high explosive. Most of the hot, charged gas from the explosion would be channeled into an exhaust jet to push the spacecraft forward. A small fraction of the gas would be used to supply the energy for the next electron burst.
When it reached Barnard's Star, Daedalus would start its busy observation program. Small scoutships would head out from the main spacecraft to explore the surface of any newfound worlds. By means of a powerful onboard computer, Daedalus would examine all of the incoming data and radio back the most important findings to Earth.
But the one thing Daedalus could not do, once it had arrived at its destination, is stop. Within a few days of its encounter, the spacecraft would be heading away from Barnard's Star, still moving at one eighth the speed of light.
|Sailing on a Wind of Light
Hold your hand in front of a flashlight, and the light from it causes a very slight pressure against your hand. Although you cannot possibly feel it, because the pressure is so small, light does push. This has led to an interesting idea for a robot spacecraft (see Figure 4).
Imagine that, orbiting around the Sun, is a fantastically strong source light. It is a laser, a device for producing light in intense, pure, narrow beams. This particular laser is billions of times more powerful than any in use today.
Parked some distance in front of the laser is a very unusual spaceship. The main part of it consists of a round sail of aluminum, 62 miles across but less than a millionth of an inch thick! Attached to the center of this fine, metallic sail is the spacecraft's scientific and communications equipment. There are no fuel tanks or rocket engines, because they are not needed. This is a spaceship made to catch a wind of light.
Suddenly, the laser bursts into life. Its powerful rays, trained accurately on the giant sail, begin to push the strange spacecraft away. Steadily, the spacecraft gathers speed. After 18 months of being pushed by the laser beam, it is moving at half the speed of light. Now the laser is turned off, allowing the ship to coast toward its destination – one of the stars nearest the Sun.
This remarkable idea is not without its problems. For instance, it would be extremely difficult to keep a laser beam tightly focused on the probe's light-sail over great distances. Also, tiny particles of dust in space would tend to scatter and weaken the laser light. Finally, if the star probe could reach very high speeds, its slender sail would be in danger of being wrecked by dust particles as it collided with them.
Another idea for sailing on a wind of light, called "photosailing," is a spaceship powered by the flow of photons, or particles of light, from the Sun. Scientists from NASA's Jet Propulsion Laboratory formed the World Space Foundation to help plan and design this kind of solar-powered spacecraft. By 1990, scientists from six nations had designed "sailing ships" for a race from Earth to Mars. Called the Columbus 500 Space Sail Cup, the race was to have been timed to take place on the 500th anniversary of Columbus's discovery of the Americas.
If all had gone as planned, the spacecraft would have been launched on normal rockets and sent into orbit near the starting line, within 1,000 miles of Earth. Then the craft, which would have been quite small with their sails folded, would have unfurled very thin plastic and aluminum sails and caught the Sun's light. For months they would have gained speed slowly because they would have been "sailing" against the strong pull of Earth's gravity. Eventually they could have reached a speed of 60,000 miles per hour, and arrived at Mars in about 250 days. The first designs were completed, but the backers of the project were unable to raise enough money to build and launch the entries in time for the planned 1992 race.
Human Missions to the Stars
Why couldn't Daedalus be brought to a halt as it neared its target? The superfast spacecraft could not stop because to lose speed is just as hard as to gain it. For Daedalus to brake from one-eighth light speed would take as much fuel as to accelerate to that speed. If fuel for braking were taken along then this would greatly increase the probe's starting mass. As a result, much more fuel would be needed to propel the spacecraft to its cruising speed.
The figures work out like this. Simply to fly by Barnard's star, without slowing down, Daedalus would have to begin its mission with 46,000 tons of fuel. But in order for it to stop when it reached the star, it would need to start with 46,000 times 46,000, or more than 2 billion tons! Carrying that much fuel would pose a tremendous engineering problem for the spacecraft's designers.
Yet, if people are ever to travel to the stars, they will want to stop when they arrive. And unless they intend to stay forever, they will need some means to get back home. It might be possible, for instance, to build a much larger version of the Daedalus probe that would carry a human crew. This starship would have enough fuel on board to slow it down at its destination. Then the crew would refill their fuel tanks for the return journey to Earth, using material obtained from the planetary system they were exploring. Such a mission will still involve billions of tons of fuel that would have to be carried on the journeys both to and from the star. There would also be a risk of the astronauts becoming stranded if they failed to find enough fuel halfway through the mission.
Scientists, however, have proposed a different way to power a starship with a human crew. According to this plan, not only can we avoid carrying huge amounts of fuel aboard a starship, we can avoid carrying any fuel at all!
Have Scoop, Will Travel
The great gaps between stars are, in fact, not completely empty. Spread very thinly throughout interstellar space are particles of hydrogen. If a spacecraft could somehow collect enough of this hydrogen as it moved along, it could use those particles as a fusion fuel to propel it to the stars.
Because some of the hydrogen in space carries an electrical charge, it could be dragged into the spacecraft by a powerful magnetic field. That field could be generated by an enormous, funnel-shaped scoop, made of wire mesh, attached to the front of the ship. After being sucked in, as if by a huge vacuum cleaner, the particles of hydrogen would be fused together to produce a hot, fast exhaust. In this way the starship, called an INTERSTELLAR RAMJET, would be driven forward (see Figure 5).
The faster the ramjet traveled, the more hydrogen it would "ram" into, and so the more fuel it would have to increase its speed. Eventually, this kind of starship would come very close to the speed of light itself.
But what about slowing down? Again, the ramjet has a big advantage over other types of spacecraft. Simply by reversing its magnetic field, the starship could push away the hydrogen in front of it. This would have the effect of gradually cutting the ramjet's speed until it arrived at its destination faraway in space.
Some quite difficult problems will have to be overcome, though, before an interstellar ramjet can be built. First, a ramjet engine could not work effectively at low speeds. A second type of engine, then, such as the one proposed for the Daedalus probe, would be needed to make the starship reach a speed at which the ramjet could take over. Since this engine would add greatly to the ship's mass, it would make the probe harder to accelerate.
A more serious problem is posed by dust particles in space. Although they are tiny and widely scattered, these particles would cause severe damage if they smashed into the starship at tends of thousands of miles per second. The damage would be even greater because the mass of objects increases with increasing speed. From the point of view aboard the starship, the dust would be rushing toward it very quickly. As a result, each dust particle would appear to have the mass of a large boulder. A powerful shield of some kind would be needed to deflect the dust before it slammed into the main body of the spacecraft.
Let us assume, though, that problems such as these can eventually be solved. Imagine that, in time, an interstellar ramjet is built that can travel almost at the speed of light. It would certainly work for taking human crews back and forth between the Sun and the nearest stars. A round trip to Proxima Centauri, for instance, would take less than 10 years.
But most stars in space are much farther away than Proxima Centauri. To go on a round-trip journey to a star that is 50 light-years away, even close to the speed of light, would take 100 years. What is more, that does not allow for speeding up, slowing down, or any time spent exploring.
One idea that was first suggested many years ago is a "space ark." This would be a starship big enough for hundreds or even thousands of men, women, and children to travel in. During its voyage to a distant star, several generations of people aboard might be born, live out their lives, and die. Those who were adults when the ship reached its destination could be the great-great grandchildren of those who set out from Earth. But even though a space ark could be built in the future, it might be very hard to find volunteers for the crew!
In fact, there is a much easier way for people to reach distant stars without growing old and dying before they arrive. This has to do with the strange things that happen close to the speed of light.