13 interstellar travel and the Fermi Paradox 04/17/2014
Why is interstellar travel so difficult?
THE COSMIC SPEED LIMIT
Einstein’sspecial theory of relativity that it is impossible to travel through space faster than the speed
Another challenge of interstellar travel is the tremendous amount of energy it would require, particularly if
we wanted to send people and not just lightweight robotic probes to the stars.
the energy required to put an object in motion depends on only two things: the object’s mass and the speed
with which you want it to move.
rocket equation, which describes how a vehicle’s final speed depends on the propellant’s velocity
escape velocity, the speed necessary to overcome gravity and leave Earth behind (about 11 km/s, or
in 1926, Goddard, launched his first liquidfueled rocket
builditinagarage phase of rocketry
The rapid development of rocketry that followed the war was driven largely by German scientists who had
been recruited by both the Russians and the Americans. The space age truly began in October 1957 with
the launch of the Soviet Union’Sputnik I, an 84kilogram beeping metal ball—the world’s first artificial
the mass ratio
is defined as the mass of the fully fueled rocket (including any spacecraft it is carrying) divided by the rocket
(and spacecraft) mass after all the fuel is burned. chemical rockets simply are not powerful enough to deliver large payloads to the stars in a reasonable
length of time.
The first uses “conventional” technology— that is, technology that seems within our grasp (at least if we
disregard cost), even if we don’t yet have it. The second group involves technologies that are theoretically
possible but far beyond our present capabilities.
Two basic types of nuclear reactions can be used to generate power: fission and fusion.
Nuclear fission involves the splitting of large nuclei such as uranium or plutonium.
Nuclear fusion, the power source of the Sun and other stars, is about ten times as efficient as fission.
Fusion of hydrogen into helium converts about 0.7% of the hydrogen fuel mass into energy.
atomic Energy Commission and the U.S Air Force (and later NASA) embarked on an experiment called
Project Rover to develop nuclear fission reactors that could be flown in a rocket.
At its peak, Project Rover employed 1800 people and ultimately tested six fission engines. The program
made substantial progress and showed that fissionpowered rockets could achieve speeds at least two to
three times those of similarsize chemical rockets.
Another experimental approach, dubbed Project Orion, was more radical. Physicists at Nevada’s Los
Alamos Scientific Laboratory realized that one way to get a rocket up to much higher speed would be to
toss small nuclear (fusion) bombs out the rear and let the resulting explosions push the craft forward.
Project Orion represented the first true “starship” design to be fashioned by humans. No actual construction
ever began, although in principle we could build a Project Orion–type starship with existing technology.
However, this kind of starship would be very expensive and would require an exception to the international
treaty banning nuclear detonations in space. Project Orion ended in 1965, because of both budget cuts and
the nuclear test ban treaty.
Another nuclear rocket design was developed in the 1970s by the British Interplanetary Society under the
name Project Daedalus
The idea was to shoot frozen fuel pellets of deuterium and helium3 into a reaction chamber where they
would undergo nuclear fusion. Because we cannot yet build nuclear fusion reactors, this design remains beyond our current technological
Nuclearpowered rockets are undoubtedly feasible in some form. Still, at best they would achieve speeds of
about onetenth the speed of light. Interstellar journeys would be possible, but it would take decades for
them to reach even the nearest stars.
IONS, SUNLIGHT, AND LASERS
Another approach is to use a lowpowered rocket whose engines keep firing continuously. The ion
engine is an example of this approach.
Both NASA and the European Space Agency (ESA) have already used lowpower ion engines successfully.
These engines can be started only in space (they don’t have enough thrust to lift off Earth, and they work
best in a vacuum), but they can keep firing for long periods be cause the mass expelled per unit of time is
small. Moreover, the exhaust ions are shot from the craft at tremendous speeds, and so a powerful ion
rocket could in principle reach speeds approaching a percent or so of the speed of light.
Large, highly reflective, very thin (to minimize mass) solar sails could be pushed by the pressure
exerted by sunlight.
Solar sailing might well prove to be a fairly inexpensive way of navigating within the solar system, and it
could even be useful for interstellar travel
The fact that sunlight weakens so much with distance limits the ultimate speed of a solar sailing spacecraft
using a powerful laser on Earth as an energy source
the laser could provide a steady and continuous “push” for the solar sail, all the way to its destination if
The primary drawback to these schemes is the power requirement. For example, accelerating a ship to half
the speed of light within a few years would require a laser that uses 1000 times all current human power
A third suggestion is to build enormous craft that can accommodate a very large crew: in essence, an “ark.”
Many generations would live out their lives aboard this slowmoving vehicle before it finally reached its
Impractical way THE ROLE OF RELATIVITY The fact that we cannot exceed the speed of light might at first make
distant stars seem forever out of reach. How ever, the same theory that imposes the cosmic speed limit—
Einstein’s special theory of relativity—also tells us that time is different for high speed travelers than for
people who stay at home.
time measured aboard proceeds noticeably more slowly than time measured by clocks at rest—a
phenomenon called time dilation.
Astronomer Carl Sagan considered a hypothetical rocket that accelerates at a steady 1 (og “1 gee”)—an
acceleration that would feel comfortably like gravity on Earth—to the halfway point of its voyage.
special relativity offers sufficiently fast travelers only a one way “ticket to the stars.” No place is out of reach
—but you cannot return home to the same people and places you left behind.
In any event, while such incredible trips are allowed by the laws of physics, the energy costs would be
extraordinary. Because special relativity also tells us that an object’s mass increases as the object
approaches the speed of light, the energy cost rises just as much as time slows down.
The mass approaches infinity as the ship’s speed nears the speed of light—and no force in the universe
can give a push to an infinite mass. That is why the ship can never reach the speed of light
But fusion converts only 0.7% of the mass of the f