Worlds of David Darling > Children's
Encyclopedia of Science > Could You Ever Fly to the Stars? > 2. Interstellar
COULD YOU EVER FLY TO THE STARS?
a book in the Could You Ever? series by David Darling
2. Interstellar Overdrive
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. This is a small, dim star, 5.9 light-years away, that
may have planets circling around it.
Artist's impression of Daedalus arriving
at Barnard's Star
Artwork courtesy of Adrian Mann
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
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 parts of the Daedalus
|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.
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. 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.
|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.
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
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
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
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.
Artist's impression of an interstellar ramjet.
Artwork courtesy of Adrian Mann
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.
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.