The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

206 interstellar space
problem by harvesting hydrogen for use as a propellant
from the interstellar medium. The captured hydrogen is
fed to a nuclear fusion reactor, which supplies the energy
for a high-speed exhaust.
Bussard’s original design envisaged atomic hydrogen
being mechanically scooped up by the spacecraft as it
went along. However, his calculations suggested that, in
order to achieve the ideal acceleration of 1g (see 1g
spacecraft), a 1,000-ton spacecraft would need a frontal
collecting area of nearly 10,000 square km. Even assuming
a knowledge of materials science far in advance of
our own, it seems inconceivable that such a scoop could
be constructed with a mass less than that budgeted for
the entire vehicle. A 10,000-square-km structure made of
0.1-cm-thick Mylar, for example, would weigh about
250,000 tons.
A way around this problem is to ionize the hydrogen
ahead of the spacecraft using a powerful laser. The hydrogen
ions—naked protons—can then be drawn in by a relatively
small Bussard collector that generates a powerful
magnetic field. Since the harvesting process is electromagnetic
rather than mechanical, the scoop does not
have to be solid (it can be a mesh), nor does it have to be
unrealistically large, because the field can be arranged to
extend far beyond the physical structure of the scoop.
However, difficulties remain. One is the enormous
power needed to generate the Bussard collector’s magnetic
field and to operate the ionizing laser. Another
problem concerns the way the ram scoop works. As the
lines of the magnetic field converge at the inlet funnel,
they will tend to bounce away incoming charged particles
rather than draw them in. In effect, the scoop will act like
a magnetic bottle, trapping material in a wide cone in
front of the vehicle and preventing it from being injected
as fuel. A solution might be to pulse the magnetic field,
but the implementation would not be easy. Yet another
problem is that most of the collected matter will be ordinary
hydrogen, which is much harder to induce to fuse
than either deuterium or tritium, hydrogen’s heavier isotopes.
Finally, the Bussard ramjet will work only when
the vehicle is moving fast enough to collect interstellar
mass in usable amounts. Therefore a secondary propulsion
system is needed to boost the spacecraft up to this
critical speed—about 6% of the speed of light.
A modified design known as RAIR (ram-augmented
interstellar rocket), proposed by Alan Bond in 1974, tackles
the fusion-reaction problem by using the scooped-up
interstellar hydrogen not as fuel but simply as reaction
mass. The incoming proton stream is decelerated to
about 1 MeV, then allowed to bombard a target made of
lithium-6 or boron-11. Lithium-proton or boron-proton
fusion is easy to induce and releases more energy than
any other type of fusion reaction. The energy produced
in this way is added to the mass stream, which then exits
the reactor. In the exhaust nozzle, the energy created by
initially braking the mass stream is added back to it.
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The so-called catalyzed-RAIR offers an even more efficient
approach. After the incoming mass stream has been
compressed, a small amount of antimatter is added. The
reaction cross section is not only enormous compared to
fusion, it happens at much lower temperatures. According
to one estimate, the energy release is such that the
drive reactor of a 10,000-ton antimatter catalyzed-RAIR
accelerating at 1g and maintaining 10 18 particles per cubic
cm within the reactor only has to be about 3.5 m in
diameter. The downside is that large amounts of antimatter
would be needed for sustained interstellar flight. 274
interstellar space
Space starting from the edge of the Solar System and
extending to the limit of the Milky Way Galaxy. Beyond
this lies intergalactic space.
ion
Usually, an atom from which one or more electrons have
been stripped away, leaving a positively charged particle.
Positive ions are used in ion propulsion. Negative ions
are atoms that have acquired one or more extra electrons.
ion propulsion
A form of electric propulsion in which ions are accelerated
by an electrostatic field to produce a high-speed
(typically about 30 km/s) exhaust. An ion engine has a
high specific impulse (making it very fuel-efficient) but a
very low thrust. Therefore, it is useless in the atmosphere
or as a launch vehicle but extremely useful in space,
where a small amount of thrust over a long period can
result in a big difference in velocity. This makes an ion
engine particularly useful for two applications: (1) as a
final thruster to nudge a satellite into a higher orbit, or
for orbital maneuvering or station-keeping; and (2) as a
means of propelling deep-space probes by thrusting over
a period of months to provide a high final velocity. The
source of electrical energy for an ion engine can be either
solar (see solar-electric propulsion) or nuclear (see
nuclear-electric propulsion). Two types of ion propulsion
have been investigated in depth over the past few
decades: electron bombardment thrusters and ion contact
thrusters. Of these, the former has already been used
on a number of spacecraft. A particular type of electron
bombardment thruster, known as XIPS (xenon-ion
propulsion system), has proved to be particularly effective
and will be used increasingly to propel satellites and
deep-space probes.