The second in a series of articles on science and science fiction by Charles Ivie

Carl Sagan once said, with characteristic understatement, "No matter how you look at it interstellar travel will be a rather grand undertaking." His observation addressed both the appeal of the concept and an awareness of the extraordinary technical difficulties that would have to be overcome.

In order for an author to weave this combination of appeal and difficulty into a story with exciting results the writer must have some concept of how the technology works and what its capabilities and limitations might be. Frequently flaws or limitations of a design or technology are an important part of the tale and the way the protagonist deals with the problem can be the soul of the story.

The first thing you have to think about is the tremendous distances involved. For example, the nearest star system, The Centauri triple star group, is about 4.3 light years or over twenty five trillion miles away, thousands of times further than Jupiter in our own solar system. Now consider that our galaxy, the Milky Way, is estimated to be well over 100,000 light years across. So, if you want to travel to even the nearer star systems in a human lifetime you must go fast, very very fast.

For humans to venture to the nearest star system travel velocities must be on the order of tens of thousands of miles per second. So what technologies can we envision that can provide the enormous energies required to move at these speeds?

Chemical rockets, barely adequate for travel within the solar system fall short by factors of several thousand when interstellar distances are considered. To be sure, we have several space probes that are headed into interstellar space after exploring the outer solar system but at their current speeds it will be thousands of years before even the fastest one reaches the distance of the nearest star.

Atomic fission is the next technology to consider and when coupled with very efficient electric propulsion systems such as ion or proton plasma drives it could provide a viable mission to Alpha Centauri with a travel time on the order of 40 to 50 years. A bit long for human missions but still achievable. In fact, such a mission was studied in the mid 1970's by Cal Tech's Jet Propulsion Laboratory in Pasadena.

The unmanned spacecraft was to be powered by a breeder reactor that was automatically maintained by robotic systems during the entire trip. Peak speeds approaching three percent of the speed of light were considered possible.

Next comes hydrogen fusion with nearly one thousand times the energy potential of fission for systems of the same size and weight. Fusion powered exotic propulsion systems such as the VASIMR rocket may provide the speeds necessary for human travel to the nearer stars begin to look possible, long but possible.

Then comes the SCIFI favorite: Antimatter. With energy potential approaching 1/2 MC squared this technology, if it can be made to work, finally makes reasonable trip times for voyages to the nearer stars possible, at least in terms of the energy requirements.

And beyond antimatter come some truly super exotic energy systems whose existence is theoretical at best. Vacuum energy, quantum fluctuation, dark matter, and relativistically degenerate quantum black hole reactors are some of the exciting but very far out concepts that are beginning to get the attention of physicists, astrophysicists, and cosmologists. Could one of these ideas hold the key to interstellar travel? Time alone will tell.

But now comes the hard part. Assuming you have access to the energy required to power an interstellar vessel what sort of propulsion system do you use?

AN INTRODUCTION TO ROCKET SCIENCE

If we stick, for the time being, to Newtonian reaction systems, e.g. rockets of one sort or another, the question of propellant efficiency becomes critical. (I will get to Near Light and FTL drives and non-Newtonian systems, I promise.)

Rockets need reaction mass to function. In chemical rockets both the energy and the reaction mass are provided by the propellants but in electrically or atomically powered systems, the reaction mass and the energy come from two different sources.

Regardless of what kind of rocket we are talking about there is a simple relationship that describes the efficiency of any rocket system, it's called "Specific Impulse" or Isp and it's expressed in seconds. Here is how it works.

If I have a rocket engine that produces one pound force of thrust and I have one pound mass of reactant with which to fuel it the length of time required to consume the fuel while producing the thrust is measured in seconds. This is a measure of the efficiency of the engine and is similar to fuel economy in miles per gallon for an automobile. For example if one pound of reaction mass will produce one pound of thrust for 100 seconds the specific impulse is then 100 seconds. It turns out that specific impulse is determined by exhaust velocity. The faster the exhaust gas leaves the rocket the higher the specific impulse. Why is this important? The higher the specific impulse the longer the fuel will last.

The specific impulse of the main engines of the Space Shuttle is a bit over 400 seconds. There are three engines each producing about one and a half million pounds of thrust for a total of four and one half million pounds. If there were four and one half million pounds of propellant in the external tank the engines could run at full power for over 400 seconds. However, as the fuel is burned the vehicle gets lighter so one of two things must happen, either the vehicle will accelerate at higher "g" levels as it gets lighter or the engines must be throttled back to keep the acceleration forces from becoming too high. Typically, the strategy is the latter and the force is limited to about three times the force of gravity or 3 g's. At this acceleration level the velocity of the vehicle increases by about 100 feet per second every second.

For inter planetary rocket systems specific impulse values of 300 to 400 seconds are acceptable and until recently all of the lunar and planetary missions have operated with chemical fueled rockets in this range of efficiency.

THE NEXT STEP, ION DRIVE AND BEYOND

The NASA Smart 1 spacecraft used electric ion drive to make the trip from near Earth orbit to the moon using only a few pounds of propellant. Powered by solar panels the ion rocket motor operated with an Isp efficiency of nearly 10,000 seconds. While the thrust was but a fraction of a pound the constant force made the mission possible with no chemical propulsion systems at all. This technique can also be used for interplanetary and even interstellar vehicles. For example, a Mars mission accelerating constantly at .01 g can make it from the Earth to the red planet in less than 35 days. Currently, Ion drive engines produce very low thrust but new technologies such as the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) promises to revolutionize space travel, at least within the Solar System.

NASA physicist and astronaut Franklin Chang-Diaz proposed a new generation of propulsion system architectures. This extraordinary concept, the VASIMR engine, provides a combination of high thrust and reaction mass efficiency in a single system. With chemical rockets well over 90 percent of the initial vehicle weight is fuel and the rocket system must accelerate this fuel as well as the payload to interplanetary trajectory velocities if trips to other worlds are to be performed. As a result highly convoluted orbit strategies must be employed to get large payloads to other planets. It is a tribute to celestial mechanics (equipped with appropriate wrenches and screwdrivers) that use of gravitational slingshot effects make the delivery of large payloads to Jupiter and Saturn possible. Trips to Jupiter and Saturn require a sojourn to Venus and a revisit to Earth to steal some of the kinetic energy from these bodies to speed the craft on to its final destination. Transferring some of the kinetic energy of a large body like Venus or Earth to a spacecraft may sound like science fiction but it is a very real technique that only requires the spacecraft to be in the right place at the right time. The penalty is very long travel times, especially to the outer planets. The benefit is drastically reduced fuel requirements. Chang-Diaz's engine may change all that. With an appropriate power source, his rocket system may reduce the time required for visits to other planets from years to weeks.

Interestingly, the gravitational slingshot is closer to a warp drive than might be realized. Enormous velocity changes are achieved and yet no acceleration forces are felt on the vehicle. This is because the moving gravitational field of the planet that provides the boost acts on every atom of the spacecraft equally. In principal, there is no real limit to the velocity change that can be obtained by this technique. If a gravitational exchange were to be employed using a neutron star or even a black hole velocities approaching the speed of light could be achieved in a few minutes with no acceleration forces being experienced by the passengers on board. With conventional propulsion systems, acceleration of thousands of gravities would crush the vehicle and its occupants. So, in a way, the dream of using gravity itself to propel a spacecraft is with us now.

So how do we use this technique for interstellar travel? Here is one way that we could think about.

Let's assume that we have a really advanced civilization with some very impressive engineering capabilities. We construct a pair of neutron stars or better yet black holes that orbit one another at very high velocities. We locate this binary pair in solar orbit in a remote part of the solar system, preferably well out beyond Pluto and Neptune, and this becomes our launching platform. We locate this thing well away from the habitable planets because the gravitational disturbances caused by its operation would disrupt the orbits of occupied worlds if it were too close.

We travel to the departure station by means of a fusion powered Chang-Diaz rocket and once there board our interstellar ship. Using only small maneuvering thrusters we put the starship on a course that takes it to the appropriate location and time in the rapidly rotating gravitational field produced by our black holes and zap! We are traveling at nearly the speed of light toward our destination and we didn't feel a thing. If a similar system is in place at our destination, we can use it to slow down. The beauty of this system is that the interstellar ship only caries enough fuel for life support and maneuvering. There is no interstellar drive on board, all the work is done by the black hole launcher.

If we assume that we are traveling to the nearest star group, the Centauri system, and the black hole slingshot gives us a velocity of 0.9999999 of the speed of light; the trip in earth time will be 4.3 years but on board the ship only 5 days will pass before we reach our destination. Einstein still rules.

Far-fetched? Maybe not. There is currently a program to study the feasibility of creating synthetic quantum black holes in the laboratory. If successful, this program could lead to the technology necessary to create very large black holes that would have a number of applications. For our gravitational slingshot the material to feed small black holes and turn them into useful monsters could be found in the asteroid belt and in the Oort cloud out beyond the planets. Once the concept is shown to be possible the problem can be turned over to engineers who are frequently comfortable solving difficult problems that involve very large numbers.

Next time we will talk more about how a black hole slingshot might be built. near light drives, faster than light (FTL) systems, and what else you have to worry about when you go that fast.