Marc G. Millis of NASA’s Glenn Research Center manages the Breakthrough Propulsion Physics Program.
Oct. 22, 1998 — Have you ever wondered when we’ll be able to travel to distant stars as easily as in science fiction? Believe it or not, scientists are seriously looking at concepts such as wormholes, space-time distortions and space drives.
But transforming these flights of fancy into reality will require scientific breakthroughs on three fronts: propulsion, speed and energy. Although we do not yet know if these breakthroughs can be achieved, we at least know how to begin making the progress to find out.
The real question is not whether interstellar travel can be done, but when it will be fast and easy enough to send the first mission.
In a sense, interstellar travel is already happening. The Pioneer 10 and Voyager 1 spacecraft, both launched in the 1970s, have traveled more than 6.5 billion miles from
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Earth and are on their way out of our solar system. But at the speed they’re going, it would take tens of thousands of years for a probe to reach our nearest neighboring star. That’s longer than all of recorded human history. Further technological developments can significantly reduce this time, but further scientific breakthroughs are needed before interstellar travel becomes practical.
The propellant problem
The first challenge is propulsion, specifically propellant mass. Unlike aircraft that can use the air as their reaction mass, rockets need to bring along their own reaction mass, propellant, with them. By blasting propellant out the back, rockets push spacecraft. The problem is quantity. Propellant needs rise exponentially with increases in payload, destinations, or speed.
For interstellar voyages the numbers get, well, astronomical. For example, to send a payload the size of a school bus to the nearest star within 900 years, you’d need ... well, more mass than there is in the entire universe. This assumes that you’re using chemical engines like those on the space shuttle. With nuclear fission rockets the situation gets better, but not by much — the propellant required would fill a billion supertankers.
Although the situation gets much better with ion propulsion or antimatter concepts, the numbers get astronomical again if you want to get there in less time than 900 years, or if you actually want to stop when you reach your destination.
Ideally, we would want to use a space drive that doesn’t need any propellant. A few researchers have begun studying how to achieve this, searching for something else in space to push against, perhaps even by pushing against the very structure of space-time itself, or by finding a way to modify gravitational or inertial forces.
The need for speed
The next and more obvious challenge is speed. Our nearest neighboring star is about 26 trillion miles away. That’s more than four years away at the speed of light, and light-speed is about 17,000 times faster than the Voyager spacecraft.
Although the search for a non-propellant space drive would dramatically improve this speed situation, some researchers have even contemplated circumventing the light speed limit for interstellar travel.
Break the light speed limit? No. The trick is to circumvent the light speed limit by distorting the fabric of space-time itself to create “wormholes,” which are shortcuts in space-time, or by using “warp drives,” which are moving segments of space-time.
The warp drive idea is something like a moving sidewalk, similar to what you find at many airports. By expanding space-time behind the starship and contracting it in front, a segment of space-time moves and carries the ship with it. The starship itself still moves slower than light within its space-time, but when you add the “moving sidewalk” effect; the apparent motion exceeds the speed of light. There are numerous difficulties with these concepts, however.
Looking for energy
The last challenge is energy. Even if we had a space drive that could convert energy directly into motion, it would still require a lot of energy. Sending a shuttle-sized vehicle on a 50-year, one-way trip to the nearest star would require 70 quintillion joules of energy — the equivalent of running the space shuttle’s engines continuously for that same 50 years. This amount is roughly the same as the output of a nuclear power plant.
For our warp drives and wormholes, the energy situation is much, much worse. To create a 3-foot-wide wormhole, you would need to convert something with the mass of Jupiter into negative energy. To overcome these difficulties, a few breakthroughs in energy production would help.
To find out if we can actually begin making progress toward these grand ambitions, NASA established the Breakthrough Propulsion Physics Program in 1996. The program has supported conference sessions, workshops and Internet sites to foster collaborations and to identify affordable research.
The next step is to sponsor a few, small research tasks. After two years of supported research, we’ll ask if the progress gained is worth sustaining the program. If the answer is “yes,” increased support will be sought. If the answer is “no,” then the program will be put on hold until further significant developments emerge from general science.
Why bother with these seemingly impossible goals? Well, progress is not made by conceding defeat. History is replete with conquered impossibilities — flying machines, moon landings, and tapping the power of the atom, to name but a few. It took four decades to go from the first liquid rocket to the first landing on the moon, and three decades to go from the confirmation of radioactive decay to the first nuclear reactor.
Physics continues to uncover new possibilities — possibilities that might someday solve the challenges of interstellar flight. Even if we don’t achieve a propulsion breakthrough during my lifetime or my children’s lifetime, and even if such a breakthrough is impossible — I am firmly convinced that we as a society will gain far more from trying than if we didn’t.
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