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."