Enabling Interstellar Voyages
Source: Glenn Research Center
The following essay was adapted from: Millis, M. G.,
"Breaking Through to the Stars", In Ad Astra, The Magazine of the
National Space Society, Vol. 9, N. 1, pp. 36-40, (Jan-Feb. 1997).
Based on projections of current technology, it is
feasible to send a probe to one of our neighboring stars, but it is
still prohibitively expensive. This was the conclusion of a conference
hosted by Ed Belbruno in New York City in September 1994. The
conference, titled "Practical Robotic Interstellar Flight: Are We
Ready?," examined concepts for interstellar propulsion that are based
on firmly established science. It covered many concepts including
light sails, magnetic sails, and nuclear rockets. Although these
methods are technologically feasible, they would still require
enormous investments to bring them to fruition. To bring down the cost
of developing interstellar technologies, conference attendees
suggested that less expensive "pre-stellar" missions should be used as
starting points, missions such as sending probes to explore the Kuiper
Belt or Oort Cloud, or sending a telescope beyond 550 AU to use the
gravitational lensing effect of our own Sun for astronomy.
There is an entirely different approach, however. Rather
than limit ambitions to foreseeable solutions, why not seek the
solutions to the original ambition? In this case the ambition is to
travel comfortably and affordably to our neighboring star systems. As
already stated, this is beyond the ability of our foreseeable
solutions -- solutions based on text book science and projected
technology. To seek the solutions to make interstellar travel
practical and affordable it is necessary to search beyond current
understanding -- to go back to the sciences from which technology
emerges and search for the new science which could lead to propulsion
breakthroughs -- the kind of breakthroughs that would make
interstellar travel practical.
Challenges of Interstellar Propulsion
First, let's look at what breakthroughs are required
before we can travel comfortably to our neighboring stars. Our first
challenge is mass, propellant mass. Today's spacecraft use rockets and
rockets use large quantities of propellant. As propellant blasts out
of the rocket in one direction, it pushes the spacecraft in the
other -- Newton's third law. The farther or faster we wish to travel,
the more propellant we'll need. For long journeys to neighboring
stars, the amount of propellant we would need would be enormous and
prohibitively expensive. For example, to send a vehicle the size of
the Space Shuttle, and equipped with the same chemical rockets, to our
nearest star at a leisurely pace of ten centuries, we would need about
10^119 kg of propellant. Compare that with 10^55 kg, which is an
estimate of all the mass in the universe (estimate based on models for
a closed, finite universe). Even if we used all the mass in the
universe, we would not be able to fuel this journey. With the best
rockets conceivable, say antimatter rockets or ion engines with an
exhaust velocity two hundred times greater than for current rockets,
we would still need over 500 supertanker-sized propellant tanks just
to fly past our nearest star within a century. If we wanted that same
spacecraft to actually stop when it got to its destination, we would
need to use that 500 supertankers for braking and would need another
300 million supertankers of propellant to propel the vehicle toward
the star along with all its braking propellant. Clearly, rockets are
NOT the way to go to the stars. We need to find some fundamentally new
mode of travel that requires little or NO propellant. This implies the
need to find some way to modify gravitational or inertial forces or to
find some means to push against the very structure of spacetime
itself.
Our next challenge is speed. Even though the
breakthrough of eliminating propellant would greatly boost how quickly
we could travel in space, to reach interstellar destinations in
comfortable time frames (say, within a term of congress), would
require another breakthrough in physics. The fastest thing we know of
is light. Yet, even at light speed it would take almost 9 years for a
round trip journey to our nearest star system. The mission's financial
backers might want a quicker return on their investment. And this 9
year time table assumes that we are at light speed. For objects like
people and spacecraft that are built of matter rather than photons,
the journey would be even slower. To travel to our neighboring stars
in comfortable time frames, it is desirable to have the physics
breakthrough that allows us to travel faster than the speed of light.
Most scientists say this is impossible; others are more optimistic.
Our third big challenge is energy. Even if we had a
non-rocket space drive that could convert energy directly into motion
without propellant, it would still require a lot of energy. Sending a
Shuttle-sized vehicle on a 50 year one-way trip to visit our nearest
neighboring star (subrelativistic speed) would take over 7 x 10^19
Joules of energy. This is roughly the same amount of energy that the
Space Shuttle's engines would use if they ran continuously for the
same duration of 50 years. To overcome this difficulty, it is desired
to have a breakthrough where we can take advantage of any energy in
space or a breakthrough in energy production physics.
Fortunately, science and technology continue to evolve.
In just the last few years, there have been new, intriguing
developments in the scientific literature. Although it is still too
soon to know whether any of these developments can lead to the desired
propulsion breakthroughs, they do provide new clues that did not exist
just a few short years ago. This NASA project will determine if and
how these emerging possibilities can be applied to the goal of
creating the desired propulsion breakthroughs
http://www.grc.nasa.gov/WWW/bpp/bpp_INTERSTELLAR.htm