Interstellar Travel
On April 1, 1987 the Greater New York Chapter of the
American Institute of Aeronautics and Astronautics (AIAA), the
Space Frontier Society and the space Studies Institute (SSI)
sponsored a joint meeting aboard the Intrepid Sea, Air Space
Museum moored at a midtown Manhattan Pier in the Hudson River.
The topic was "From the Solar System to Interstellar Flight."
Speakers were Dr. Brice N. Cassenti and Dr. Gregory L. Matloff.
Dr. Cassenti is a Senior Research Engineer at United Technology
Research Center. Dr. Matloff is both professor of Atmospheric
Physics at Baruch College and a NASA consultant at NASA Goddard
Spaceflight Center.
More than 30 people were in attendance. About half (by a
show of hands) were AIAA members. [SEE COMMENT 1]
Larry Maltz of AIAA a played major role in organizing this
meeting, and also made the opening remarks. Maltz set forth the
background as to how and why the symposium came about. He
cautioned, however, that these statements represented his
personal point of view and did not necessarily reflect the views
of AIAA.
Interstellar flight, Maltz stated has been a dream of
mankind for generations. It is clear, moreover, that to the
human species such a voyage will represent far more than the
voyage of Christopher Columbus. According to Maltz, "we exist
here today as a tiny island in space in possession of doomsday
weapons which we have the intelligence to create, but not the
temperament to control. It is, as some would say by the grace of
God that we have not already destroyed this planet. Once the
technological genie has been let out of the bottle there is no
way to put it back. Thus isolated on this planet our species
will be constantly at risk." Developing the capability for
interstellar flight is thus, Maltz implied, necessary to provided
humanity with an insurance policy against self destruction.
In the last ten years, Maltz said, there have been dramatic
theoretical advances concerning potential propulsion systems for
interstellar flight. As a result, it has now become possible to
consider interstellar flight as more than science fiction. It is
therefore important to make NASA and Congress recognize that an
advance planning group in this area is needed regardless of any
necessity to cut back spending elsewhere.
Maltz then introduced the first speaker, Dr. Brice N.
Cassenti. Dr. Cassenti earned his doctorate in physics from
Polytechnic Institute of Brooklyn, specializing in applied
mechanics. He then spent several years working for Bell Labs.
His current position is with United Technologies Research Center.
He is a member of both AIAA and the British Interplanetary
Society. In 1985 Dr. Cassenti and others presented a paper
before a joint conference of the AIAA and the American Society of
Mechanical Engineers (ASME). This dealt with the use of anti-
hydrogen propulsion systems for high delta-V missions. He has
also published an article in the Journal of the British
Interplanetary Society (JBIS) on design considerations for anti-
matter rockets.
Dr. Cassenti's presentation dealt with possible propulsion
systems for interstellar vehicles or "how to get there." He
began by stating that the first thing to consider is the nature
of the problem. Reduced to its essence, this is astronomical
distances.
Manned spacecraft have only gone as far as the Moon. The
unmanned Voyager probe is now on its way to the planet Neptune,
10,000 times as farther away from the Earth than the Moon. It
will take Voyager about ten years to complete this journey. The
nearest star, however, is 10,000 times farther away than Neptune.
At now-achievable speeds it would take 80,000 years to get there.
We must therefore find ways to increase the speed of our space
vehicles.
According to Dr. Cassenti "that really is the primary
question, how fast *** (do we have to) go?" Only by answering
that question is it possible to set up design criteria. In turn,
once design criteria are established there can be consideration
of the propulsion system options available. Several propulsion
concepts, Dr. Cassenti stated, have been suggested for
interstellar flight. These include pulsed nuclear propulsion,
anti-matter rockets, ramjets, and laser sailing.
Dr. Cassenti began by presenting some hypothetical missions
in order to analyze which of these propulsion concepts are likely
to prove possible. The two most important parameters, he
asserted were vehicle size and speed. Interstellar spacecraft,
he contended would not be small. Indeed, they would have to be
many times larger than the current space shuttle.
The first potential mission Dr. Cassenti examined was the
Interstellar Precursor Mission discussed in Aeronautics and
Astronautics in January, 1980. This proposed spacecraft would
travel out past Pluto's orbit to the equivalent of one five-
hundredth the distance from Earth to the nearest star. Projected
launch date of this mission is between 2000 and 2005. The
propulsion system would be a nuclear reactor that would
accelerate charged particles to very high velocity. Such a
spacecraft would take 10,000 years to reach the nearest star.
This, Dr. Cassenti pointed out, was still far too long.
Nevertheless, it represents one-eighth the time the Voyager space
probes would take to cover the same distance. In other words, in
about 20 years the maximum speed of spacecraft will have
increased by a factor of eight.
Dr. Cassenti suggested that such a rate of increase in the
maximum speeds attainable by man-made vehicles is not untypical.
He presented a graph extrapolating the rate at which vehicle
speeds have been increasing and using sailing ships as a base.
The graph implies that it should be possible to send missions out
a distance of 10 light years by the year 2200.
Against this background, Dr. Cassenti then proceeded to
discuss a manned interstellar mission. The spacecraft would have
to attain enough speed to permit completing a round trip within
the crew's working lifetime. (Although, he remarked, it might be
only "a crew of nuts" that would willingly undertake such a
mission.) That sets a practical limit of 45 years on mission
duration. This breaks down to a 20 year outbound journey a five
year stopover at the destination star system and a 20 year return
flight. A related consideration is that in order to attain such
speeds "constant boost" spacecraft would be necessary. Humans
cannot withstand continuous acceleration/deceleration forces
greater than 1G. Assuming a 20-person crew this implies a
spacecraft with a minimum payload mass of 5,000 metric tons.
One hypothetical unmanned mission assumed what Dr. Cassenti
called a "rather large" payload mass of 500 metric tons. This is
still only one-tenth that required for a manned mission.
Furthermore, the only constraint on acceleration for unmanned
missions is the structural strength of on-board equipment.
Nevertheless, although the "crew" remains on Earth, unmanned
interstellar probes are still subject to the "working lifetime"
limitation. The signals from the spacecraft have to arrive while
the people who launched it are still available to evaluate the
data transmitted. Otherwise mission continuity will be lost.
However, since the signals will return at essentially the speed
of light, the spacecraft can take longer to get to its
destination. For a flyby of the Alpha Centauri system (about 4
light-years from Earth) the outbound journey could take as long
as 40 years. [SEE COMMENT 2]
Using the 45 year mission length as a guide, Dr. Cassenti
outlined a hypothetical unmanned stellar flyby mission. Transit
time would by 40 years. Acceleration would be 1G, even though
the probe is unmanned. There would be no deceleration at the
target star. [SEE COMMENT 3]
Under the foregoing scenario, if the target was the nearest
star (i.e-4 light years away), the rocket would have to fire for
approximately one month. By contrast, the Space Shuttle's
rockets fire for a total of about 20 minutes, producing a maximum
acceleration of about 3Gs. This, Dr. Cassenti stated, conveys
some idea of just how energetic an interstellar vehicle has to
be. That in turn gives some indication of how energetic the fuel
has to be and how much fuel the vehicle must carry.
Another parameter of this problem is mass-ratio, the initial
weight or mass of the rocket divided by the final mass. For
example, a mass ratio of 100 would mean that 99% of the vehicle
was fuel and one part was payload. [SEE COMMENT 4]
While a graph Dr. Cassenti displayed plotted mass-ratios as
high as 500, he noted this was unrealistic. The highest mass-
ratio that can be considered reasonable is 50. A more usual
mass-ratio, by current standards, is 20.
These strictures make chemical rockets impractical for
interstellar missions. The same is true for nuclear-fission
rockets, ion rockets and nuclear pulse rockets. Nuclear-pulse
fusion rockets might be practical for an unmanned stellar-flyby
mission. They would not, however, be feasible for a manned
interstellar mission.
Turning to a manned interstellar mission, Dr. Cassenti
remarked on a paradox familiar to science fiction readers: As a
spacecraft approaches the speed of light, time slows down. Thus
a spacecraft capable of accelerating at 1G for an unlimited time
could cross the entire galaxy and return within the lifetime of
the crew. However, he remarked, they would be "really moving,"
since the distance across the galaxy is 75,000 light years.
Returning to more realistic mission profiles, Dr. Cassenti
noted that a manned mission is "much more demanding" than an
unmanned one. He again assumed a mission to the nearest star, a
constant acceleration of 1G and a transit time of 20 years. In
order to achieve this the propulsion system would have to fire
for about 3 months. Moreover, it would have to do this 4 times.
(Acceleration and deceleration are necessary on both the outward
and return legs of the mission.) As result, the only practical
propulsion system for a manned interstellar mission is an anti-
matter rocket. [SEE COMMENT 5]
Dr. Cassenti then proceeded to address 3 propulsion
concepts, "Orion," "Daedalus" and the anti-matter rocket.
The "Orion" rocket was designed in the 1950s and early
1960s. It would have used "nuclear charges" (to borrow a Soviet
phrase) as a means of propulsion. Dr. Cassenti commented tongue-
in-cheek that this would be a good way to dispose of unwanted
atomic weapons. It also could travel through the solar system
very rapidly. However, it could not develop enough velocity for
an interstellar mission. "To do that," Dr. Cassenti stated,
"your really need to look at fusion, not just fission."
This led to the discussion of the second propulsion concept.
About 10 years ago the British Interplanetary Society (BIS)
sponsored a study of a vehicle called "Daedalus" that could visit
Barnard's star. This star is about 6 light years away. It was
selected as the target because it was a more likely candidate to
have planets orbiting it than Alpha Centauri, which is a triple-
star system. [SEE COMMENT 6]
Propulsion for the mission proposed by BIS would be by the
fusion of deuterium and helium-3. (These two materials were
selected because fusing then together generates large amounts of
charged particles.) Helium-3 is relatively rare on Earth. In
fact, according to Dr. Cassenti, Earth's total resources of
helium-3 would not come close to being sufficient for the
proposed mission. In order to obtain enough helium-3 it would be
necessary to mine the atmosphere of Jupiter. Hence spacecraft
construction would be on-orbit around one of the Jovian moons.
The vehicle would be a two stage rocket. The first stage
would have a burn time of about two years and an exhaust velocity
of about 10,000 kilometers per second. It would expend about
40,000 tons of propellant. After the cut-off and jettisoning of
the first stage, the second stage would burn for an additional
one and three-quarter years. This would consume a further 4,000
tons of propellant. The combined acceleration from both stages
would give the vehicle a velocity of about 17% of the speed of
light.
The spacecraft would arrive at Barnard's star in about 45
years. Again, this would be a flyby mission. Hence, due to the
speed of the spacecraft, the time available to examine any
planets orbiting Barnard's star would be extremely limited.
Dr. Cassenti reiterated that "the problem with nuclear-pulse
propulsion **** is that it just can't go fast enough" to carry a
crew to the target star and return them to Earth. "For that, you
need more energy." Hydrogen-fusion converts about 7/10 of 1% of
the propellant mass into energy. However, to make manned
interstellar mission practical, almost all of the propellant mass
must be converted into energy. The only way to do that, he said
is to annihilate matter with anti-matter. We already know how to
make and store anti-matter, he stated, but only in very small
quantities.
In order to visualize how anti-matter propulsion might work,
one must, according to Dr. Cassenti, understand certain aspects
of elementary particle physics. Dr. Cassenti used hydrogen, the
simplest of all atoms as an illustration. Each atom of hydrogen
has a nucleus consisting of a single proton with a positive
electric charge. Surrounding the nucleus, and essentially
orbiting it, is an electron with a negative electric charge.
Anti-hydrogen, on the other hand, has a negatively charged
nucleus and a positively charged electron.
These particles have been known for a long time. Anti-
electrons, sometimes referred to as positrons) were predicted in
the late 1920s and discovered in the early 1930s. By the 1940s
the existence of the antiproton was posited. By the early to
mid-1950s, this particle had been produced in accelerators.
When an electron collides with a positron, all of the mass
of both particles is converted to energy. Specifically, the
collision produces two high energy photons of light. These are
gamma rays, each with an energy of about 500,000 electron volts.
However, gamma rays are very difficult to stop. Hence as a
practical matter it is impossible to capture and use the energy
from electron-positron annihilation.
One the other hand, the collision of protons and antiprotons
yields particles known as pions. (It is theorized that pions are
what holds the nucleus of an atom together.) There are 3 kinds
of pions, positive, negative and neutral. (Neutral pions,
however, are extremely hard to detect.) There are two important
points about charged pions. First, as they travel away from the
locus of a proton-antiproton collision they "dump" their kinetic
energy. This energy is recoverable. Second, the direction of
pions can be altered by means of magnetic fields. In short,
unlike the case of electron-positron annihilation, the energy
resulting from proton-antiproton annihilation is controllable.
"The unfortunate part of this," according to Dr. Cassenti,
is that pions don't last forever." Neutral pions decay almost
immediately into two very high energy (2,000,000 electron volt)
gamma rays. Charged pions last longer, taking about 30
nanoseconds (30 billionths of a second) to decay. The decay of a
charged pion also yields 2 particles. However, these are not
gamma rays but a muon and a neutrino. Neutrinos are even harder
to stop than gamma rays. They have been described "nothing
spinning on its axis and moving at the speed of light." Unlike
gamma rays, however, neutrinos do not produce unwanted radiation.
Nevertheless, it is important to direct the pions so as not to
lose energy to the neutrinos. Muons are actually "heavy" or
"energetic" electrons, and last much longer (about two-millionths
of a second) before they decay. Hence the "secret" of anti-
matter propulsion is to catch and use pions and muons.
The current technique for producing anti-matter entails
raising a beam of protons to very high energy levels in an
accelerator. This proton beam is then caused to collide with a
heavy element such as tungsten. The "debris" that results from
this collision contains a few anti-protons. These anti-protons
are "selected out" with magnets and then focused. The difficulty
is that the anti-protons have to be slowed down to about one-
tenth the speed of light. They then must be cooled very rapidly
without touching the walls of the chamber. When this has been
accomplished the particles are "stored" using one of the
accelerator's magnets. Anti-protons have been stored for as long
as 84 hours without any serious problems developing.
While this technique is adequate for particle physics
experiments, it is not practical for anti-matter propulsion. The
latter requires cooling the anti-matter to a much greater extent
and storing it for a much longer time. (It also, very obviously
requires much greater quantities of anti-matter.) Dr. Cassenti
suggested combining the anti-protons with positrons to make anti-
hydrogen. The anti-hydrogen, as initially produced, would be in
either gaseous on liquid form. For storage, however, it would
have to be solidified and suspended in a chamber without touching
the walls. One way to do this is by making the chamber spherical
and electrically charging the walls. According to Dr. Cassenti,
this has already been done experimentally with ordinary matter.
Here the problem is that the anti-hydrogen must remain in solid
form. If it begins to vaporize, the vapor will collide with the
chamber walls. This collision causes heating in the chamber,
resulting in more anti-hydrogen vaporization, until an explosion
occurs.
Hence the chamber must be kept extremely cold. For example,
to store 1 milligram of anti-hydrogen for 36 years requires a
temperature of 4 degrees Kelvin. On the other hand, reducing
storage temperature to 3 degrees Kelvin (about minus 450 degrees
Fahrenheit) permits 1 milligram of anti-matter to be stored for 3
million years. The good news is that the greater the mass of
anti-hydrogen being stored the longer it will last at the same
temperature. Specifically, an increase in mass by a factor of
1,000 will yield a factor of 10 increase in storage time.
Surprisingly, however, Dr. Cassenti said that storage should not
be a problem. In fact, he stated that it should be possible to
store anti-hydrogen at temperatures as low as .05 degrees Kelvin.
Dr. Cassenti turned next to the question of "how much energy
we can really extract" from anti-matter. This is determined by
using Einstein's famous formula "E=MC squared" as a yardstick.
That is, the total energy contained in any given amount of matter
equals the mass of the matter times the speed of light squared.
The important thing to remember, Dr. Cassenti implied, is that
even a small percentage of MC-squared represents a great deal of
energy.
Two types of propulsion systems are under consideration.
Both assume only low speed collisions between protons and anti-
protons. No accelerators would be involved.
In one case, a magnetic field is used to collect and direct
the particles resulting from proton anti-proton collisions.
Assuming only muons can be collected and directed, studies
indicate the theoretical efficiency of an anti-matter rocket
would be 52% of MC-squared. If pions can be collected and
directed as well, efficiency increases to 67%.
In the second case, the energy produced by the proton-anti-
proton reactions would heat a propellant. Here, efficiencies
would be lower, although large fractions of the total energy
produced would still be recovered. If the medium used was liquid
hydrogen, 45% of the energy could be captured and used. If
gaseous hydrogen were used instead of liquid hydrogen, efficiency
would fall to 35%. Dr. Cassenti indicated, however, that these
percentages might prove impractical. The practical limit is more
likely to be on the order of 15%.
In practical terms this means 19 kilograms of antimatter
would suffice for a "low speed" unmanned mission to the nearest
star. ("Low speed" being 20% of the speed of light.) This also
assumes a 500 ton payload. Dr. Cassenti admitted that, by
today's standards, 19 kilograms represents a great deal of
antimatter. On the other hand, he pointed out, the amount of
antimatter required is very small in proportion to the payload.
Dr. Cassenti presented a proposal for a proof-of-concept
system, which, he said, "could be built long before we build an
interstellar spacecraft." This would be an anti-matter orbital
transfer vehicle. The aim would not be to attain high
performance, but rather to demonstrate the feasibility of
antimatter propulsion. Nevertheless, like Orion rocket, the
antimatter OTV could move payloads around the solar system very
quickly. It could also make repeated trips from low-Earth orbit
(LEO) to geostationary Earth orbit (GEO) or other higher energy
orbits.
The propulsion system would use the "magnetic bottle"
technique. Anti-hydrogen would into a chamber, where it would
collide with normal matter, probably hydrogen, injected from
another source. Propellant would also be introduced into the
chamber. (While Dr. Cassenti did not explicitly so state, the
propellant would probably also consist of hydrogen.) The
reaction chamber would be about 2 meters (80 inches) in diameter.
By contrast, the throat of the rocket nozzle would only be less
than a foot, about 28.4 centimeters. across. Surrounding the
reaction chamber would be magnetic coils. They would confine the
particles generated by the matter-antimatter collisions long
enough for these particles to "dump" their kinetic energy into
the propellant. The strongest of these magnetic coils would have
to generate a force of 500,000 gauss. This, Dr. Cassenti
admitted, is "a large magnet by today's standards." Indeed, when
he first looked into this concept "they couldn't be made."
However, recent breakthroughs in superconductors may make
possible magnetic coils with a strength of 2,000,000 gauss, or
four times that required.
As to vehicle cost, Dr. Cassenti noted it now costs about $5
million to lift a ton of liquid hydrogen into LEO. Even assuming
a cost of $5 Million a milligram (i.e $600 Billion an ounce) to
produce antimatter, the proposed system would be more economical
than chemical rockets. It would also be more economical than
nuclear-fission rockets for missions requiring high speeds. The
reason, according to Dr. Cassenti, is the tremendous amount to
energy per unit of volume produced by matter-antimatter
annihilation. [SEE COMMENT 7]
Concluding his presentation on antimatter propulsion, Dr.
Cassenti predicted "these (antimatter) rockets will work, and
they will get people to the stars ***." He also stated that in
his opinion the technical problems associated with manufacturing
and storing antimatter were manageable. The antimatter OTV which
he proposed would require only about 4 milligrams of antimatter.
Current production of antimatter, Dr. Cassenti admitted, amounts
to only ten billionths of a gram a year. However, this figure is
increasing by a factor of ten every two and one half years. The
antimatter OTV which he proposed would require only about 4
milligrams of antimatter. Hence "by the year 2010 *** (producing
4 milligrams of antimatter) should be easy ***." The strength of
the magnetic fields strength required is "high, but *** no longer
out of sight." The cost appears to be competitive, at least in
cases where mission velocity requirements are high enough.
In this last connection, Dr. Cassenti suggested that the
Strategic Defense Initiative (SDI) might be an early application
for an antimatter OTV. This is because many SDI scenarios
contemplate very high speed orbit changes, presumably to evade
enemy anti-satellite (ASAT) vehicles.
However, Dr. Cassenti did admit that one particularly
serious problem is radiation shielding. If the antimatter OTV
were to be man-rated, the amount of shielding required to protect
the crew would be significant. He did not put forth any
proposals for addressing this problem. However, he implied that
he did not consider it a "show stopper." [SEE COMMENTS 8 and 9]
Dr. Cassenti also briefly discussed two of what he called
the "more unconventional ways to get to the stars, ramjets and
laser sailing." The latter involves illuminating a very large,
very low-mass "sail" with a very powerful laser. The pressure of
the light photons will push the sail away, towing the spacecraft
behind it. A laser generating 65 gigawatts (65 billion watts) of
power would be sufficient for an unmanned Alpha Centauri flyby
mission. The laser would be in orbit around the Sun and would be
focused through a 1,000 kilometer lens. The laser sail itself
would be 3.6 kilometers across and weigh about 1 ton. Although
65 gigawatts represents a great deal of power, such a concept,
Dr. Cassenti stated, is not out of the question.
Theoretically, Dr. Cassenti said, it would be possible to
stop the spacecraft at its destination and even return it to
Earth. In each case, this would entail sending out another sail
ahead of the main spacecraft. The second sail would reflect
laser light back on the main sail and thus decelerate the vehicle
so it would go into orbit around the destination star. For the
return trip, a third sail would be deployed and used in the same
manner as the second. This time the sail would re-accelerate the
spacecraft.
The problem here is that energy requirements become almost
incomprehensible. To slow the spacecraft at its destination
requires a laser with an output of 26 trillion watts. For a two
way mission, 75,000 trillion watts is necessary. By way of
comparison, the total power output of the entire Earth is about 4
trillion watts. However, according to Dr. Cassenti, the size of
the lens remains stable at 1,000 kilometers. He went on to say
he believed such a lens could be built, although "I'm a little
suspicious about the laser." [SEE COMMENT 10]
The other "unconventional" propulsion system mentioned by
Dr. Cassenti was the interstellar ramjet. This concept has been
around since 1960 when Bussard first proposed using the gas in
interstellar space as a means of propulsion. The interstellar
medium consists mostly of hydrogen. Theoretically, it is
possible to collect this hydrogen, react it in a fusion power
system and use it to accelerate a spacecraft. The obvious
advantage would be that the vehicle need not carry any fuel. The
problem is that the spacecraft would have to be enormous.
Furthermore, if enough gas could not be collected, the power
plant would not function at all.
Dr. Cassenti displayed an artists conception of what a
"Bussard ramjet" might look like. This particular vehicle
involved a combination of an interstellar ramjet and laser
sailing. The hydrogen-collecting scoop of would, he noted be at
least 650 kilometers across. This concept would also require a
solar collector 500 by 500 kilometers in area to power the laser.
(However, this could be broken down into several smaller arrays.)
[SEE COMMENT 11]
Dr. Cassenti concluded his talk by summarizing "where we
are." He noted that he had focused primarily on "relativistic
vehicles," that is, "vehicles that can move at sizeable fractions
of the speed of light." This in turn implies mission durations
of 20 to 40 years, which Dr. Cassenti feels are reasonable.
Nuclear pulse propulsion, in his opinion, can send an unmanned
probe to the nearest star. A manned mission, however, requires
something more energetic, such as antimatter propulsion.
Antimatter rockets, in his opinion "will work." Interstellar
ramjets and other "exotic" vehicles are very difficult to build.
In Dr. Cassenti's view they are at least 200 years in the future.
Antimatter propulsion, on the other hand, might become practical
in 50 to 100 years. Nuclear pulse propulsion will be available
even before then. However they are powered, interstellar
spacecraft will be very large, dwarfing anything we can conceive
of today. That in itself is not a problem, since "what looks big
today is not big tomorrow."
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