POSSIBLE PROPULSION METHODS FOR INTERSTELLAR TRAVEL.

There is a wonderful book about interstellar space travel titled The Starflight Handbook - A Pioneer’s Guide to Interstellar Travel, by Eugene Mallove and Gregory Matloff (John Wiley & Sons Inc., 1989). It’s a very readable description of the problems of interstellar space flight and some possible technologies for doing it, and I highly recommend it to anyone seriously interested in prospective and speculative technology for interstellar flight. The following is an overview of interstellar travel methods described in The Starflight Handbook. It is relevant here in UFOFORUM because UFOs, if they really are alien spacecraft, have to get here from somewhere else. The Starflight Handbook describes some possible methods by which the trips could be accomplished. While interstellar travel would not be easy, it’s certainly not impossible, either.

As an example of some of the problems of interstellar space flight, consider a flight from Earth to Zeta Reticullum, where the Greys supposedly come from (or vice versa). Zeta Ret is 40 light years away. At one percent of the speed of light © that trip would take at least 4000 years (I say at least because I haven’t figured in the time it takes to accelerate up to the final cruise velocity). A voyage of such duration would not be very convenient, to say the least! Furthermore, to accelerate 1 kilogram of payload up to the speed of 0.01C using chemical rocket propulsion would take 4x10^265 (4 times 10 to the 265th power) kilograms of rocket fuel. That’s not practical, considering that the mass of the entire solar system is only about 2x10^30 kilograms!

A faster method of travel would be better, obviously. If you could get up to 0.5C, then the trip would only take 80 years from the point of view of the people at home, and even shorter from the perspective of the crew because of relativistic time dilation. If you can get going as fast as 0.8C, the voyage would take only 50 years from the point of view of the Earth observer, and the crew would experience a duration of only 30 years. At 0.99C, the crew would experience a trip duration of only 5.6 years, while it would take about 40 years as observed from the Earth.

There’s two basic ways of getting around the speed and fuel requirement problems posed by rocket propulsion. One is to use more powerful engines, and the other is to use a method that doesn’t require carrying fuel with you. The power of a rocket engine depends upon a quantity called the "specific impulse". A typical liquid-fuelled space shuttle engine has a specific impulse of 500. There are exotic chemical fuels being researched which would allow a specific impulse of up to 3000. If you could get a specific impulse of 3000, then your fuel load only has to be equal to the mass of the solar system to get to 0.01C. This is still not practical, of course, but at least we’re on the right track.

Experimental electric ion engines have achieved specific impulses of 10,000. With such an engine, the required fuel load to achieve 0.01C is now almost credible, at 2x10^13 kilograms.

There are several types of nuclear fission or fusion powered rocket engine designs, some of which have already been built and tested. Fission engines, whether using solid, liquid, or gaseous fissionable material in the reactor cores, are not very good in terms of specific impulse. The best estimates are in the range of 7000; consequently, better results probably would be obtained using the fission reactor to power an ion engine.

Fusion engines are much more efficient than fission engines, and theoretically are capable of a specific impulse of 200,000. This is a practical engine power for interstellar travel, although still only at small factions of the speed of light. To accelerate 1 kilogram of payload to 1% of C only requires 5 kilograms of fuel, and you’d have some chance of getting to higher speeds, perhaps up to 10% of the speed of light.

The most powerful rocket engines would be matter-antimatter rockets, usually referred to simply as antimatter rockets. Annihilation of matter and antimatter is 100% efficient in converting fuel mass to propulsive energy, compared to 0.4% efficiency for fusion, 0.08% for fission, and about 15 trillionths of a percent efficiency of chemical rockets. Furthermore, the exhaust velocity of a matter-antimatter rocket would be equal to the speed of light. Theoretically, specific impulses of 1,000,000 or even higher would be possible, and would allow for a real chance of getting a useful payload up to a significant fraction of the speed of light, thereby making short-duration interstellar travel practical (from the point of view of the crew, that is).

Another approach to practical interstellar space flight, rather than increasing the power of the engines, is to reduce the need to carry fuel. Then, even a very weak engine can have a large cumulative effect in accelerating a spaceship because it does not have to also accelerate a large amount of fuel for later use.

One method to dispense with the need to carry fuel would be to collect the fuel enroute, for example, from interstellar gas. Such schemes include the interstellar fusion ramjet and variations of that theme, in which hydrogen would be collected from the interstellar medium and used as fuel in a fusion rocket. Some fuel would have to be carried, however, to get the craft up to a speed at which the ramscoop can operate adequately. This method combines the high specific impulse possible with fusion engines as noted above, with the elimination of most of the fuel load.

Solar sail starships, propelled by the pressure of starlight against a large reflective sail, obviate the need to carry fuel, or even a main propulsion engine at all. A principal disadvantage is that you have virtually no propulsive power unless you’re very near a star. A related concept would be a solar-sail type of craft, but propelled by an Earth-based high-power laser beam instead of sunlight.

Other possible interstellar propulsion concepts include nuclear pulse propulsion. The way that would work is that the craft would drop nuclear bombs behind it, and the explosions push the craft forward. There’s even a concept that would involve shooting small pellets from a gun stationed on Earth, at a starship, that push it forward when they hit. A variation on that idea would be to shoot pellets of fusion fuel at a starship to re supply a fusion rocket engine.

Of the various schemes, the fastest relativistic interstellar travel would make use of the matter-antimatter rocket. Lower-speed, longer duration interstellar flight could work best, at least conceptually, using a fusion ramjet. Some additional propulsive boosts near stars could employ solar sail propulsion.

The bottom line is that we can even now conceptualise ways to make interstellar travel work, and it seems reasonable to believe that if we can at least imagine it now, then maybe we’ll be able to actually do it in the future. Perhaps even someone else in the universe might actually be able to do it now. Interstellar travel may not be as ridiculous as some would have us think.