160
PHYSICS OF THE IMPOSSIBLE
ramjet engine could scoop hydrogen as it traveled in outer space, essentially giving it an inexhaustible source of rocket fuel. Once the hydrogen was collected it would then be heated to millions of degrees,
hot enough so that the hydrogen would fuse, releasing the energy of a
thermonuclear reaction.
The ramjet fusion engine was proposed by physicist Robert W. Bussard in 1960 and later popularized by Carl Sagan. Bussard calculated
that a ramjet engine weighing about 1,000 tons might theoretically be
able to maintain a steady thrust of 1 g of force, that is, comparable to
standing on the surface of the Earth. If the ramjet engine could maintain a 1 g acceleration for one year, it would reach 77 percent of the velocity of light, sufficient to make interstellar travel a serious possibility.
The requirements for the ramjet fusion engine are easy to compute. First, we know the average density of hydrogen gas throughout
the universe. We also can calculate roughly how much hydrogen gas
must be burned in order to attain 1 g accelerations. That calculation,
in turn, determines how big the "scoop" must be in order to gather hydrogen gas. With a few reasonable assumptions, one can show that you
would need a scoop that is about 160 kilometers in diameter. Although
creating a scoop of this size would be prohibitive on Earth, building it
in outer space poses fewer problems because of weightlessness.
In principle the ramjet engine could propel itself indefinitely, ultimately reaching distant star systems in the galaxy. Since time slows
down inside the rocket, according to Einstein, it might be possible to
reach astronomical distances without resorting to putting the crew
into suspended animation. After accelerating at 1 g for eleven years,
according to clocks inside the starship, the spacecraft would reach the
Pleiades star cluster, which is 400 light-years away. In twenty-three
years it would reach the Andromeda galaxy, which is 2 million lightyears from Earth. In theory, the spacecraft might be able to reach the
limit of the visible universe within the lifetime of a crew member (although billions of years might have passed on the Earth).
One key uncertainty is the fusion reaction. The ITER fusion reactor, scheduled to be built in the south of France, combines two rare
forms of hydrogen (deuterium and tritium) in order to extract energy.
PHYSICS OF THE IMPOSSIBLE
ramjet engine could scoop hydrogen as it traveled in outer space, essentially giving it an inexhaustible source of rocket fuel. Once the hydrogen was collected it would then be heated to millions of degrees,
hot enough so that the hydrogen would fuse, releasing the energy of a
thermonuclear reaction.
The ramjet fusion engine was proposed by physicist Robert W. Bussard in 1960 and later popularized by Carl Sagan. Bussard calculated
that a ramjet engine weighing about 1,000 tons might theoretically be
able to maintain a steady thrust of 1 g of force, that is, comparable to
standing on the surface of the Earth. If the ramjet engine could maintain a 1 g acceleration for one year, it would reach 77 percent of the velocity of light, sufficient to make interstellar travel a serious possibility.
The requirements for the ramjet fusion engine are easy to compute. First, we know the average density of hydrogen gas throughout
the universe. We also can calculate roughly how much hydrogen gas
must be burned in order to attain 1 g accelerations. That calculation,
in turn, determines how big the "scoop" must be in order to gather hydrogen gas. With a few reasonable assumptions, one can show that you
would need a scoop that is about 160 kilometers in diameter. Although
creating a scoop of this size would be prohibitive on Earth, building it
in outer space poses fewer problems because of weightlessness.
In principle the ramjet engine could propel itself indefinitely, ultimately reaching distant star systems in the galaxy. Since time slows
down inside the rocket, according to Einstein, it might be possible to
reach astronomical distances without resorting to putting the crew
into suspended animation. After accelerating at 1 g for eleven years,
according to clocks inside the starship, the spacecraft would reach the
Pleiades star cluster, which is 400 light-years away. In twenty-three
years it would reach the Andromeda galaxy, which is 2 million lightyears from Earth. In theory, the spacecraft might be able to reach the
limit of the visible universe within the lifetime of a crew member (although billions of years might have passed on the Earth).
One key uncertainty is the fusion reaction. The ITER fusion reactor, scheduled to be built in the south of France, combines two rare
forms of hydrogen (deuterium and tritium) in order to extract energy.