Breakthrough Propulsion Physics Project


Enabling Interstellar Voyages

Source: Glenn Research Center

The following essay was adapted from: Millis, M. G., "Breaking Through to the Stars", In Ad Astra, The Magazine of the National Space Society, Vol. 9, N. 1, pp. 36-40, (Jan-Feb. 1997).

Based on projections of current technology, it is feasible to send a probe to one of our neighboring stars, but it is still prohibitively expensive. This was the conclusion of a conference hosted by Ed Belbruno in New York City in September 1994. The conference, titled "Practical Robotic Interstellar Flight: Are We Ready?," examined concepts for interstellar propulsion that are based on firmly established science. It covered many concepts including light sails, magnetic sails, and nuclear rockets. Although these methods are technologically feasible, they would still require enormous investments to bring them to fruition. To bring down the cost of developing interstellar technologies, conference attendees suggested that less expensive "pre-stellar" missions should be used as starting points, missions such as sending probes to explore the Kuiper Belt or Oort Cloud, or sending a telescope beyond 550 AU to use the gravitational lensing effect of our own Sun for astronomy.

There is an entirely different approach, however. Rather than limit ambitions to foreseeable solutions, why not seek the solutions to the original ambition? In this case the ambition is to travel comfortably and affordably to our neighboring star systems. As already stated, this is beyond the ability of our foreseeable solutions -- solutions based on text book science and projected technology. To seek the solutions to make interstellar travel practical and affordable it is necessary to search beyond current understanding -- to go back to the sciences from which technology emerges and search for the new science which could lead to propulsion breakthroughs -- the kind of breakthroughs that would make interstellar travel practical.

Challenges of Interstellar Propulsion

First, let's look at what breakthroughs are required before we can travel comfortably to our neighboring stars. Our first challenge is mass, propellant mass. Today's spacecraft use rockets and rockets use large quantities of propellant. As propellant blasts out of the rocket in one direction, it pushes the spacecraft in the other -- Newton's third law. The farther or faster we wish to travel, the more propellant we'll need. For long journeys to neighboring stars, the amount of propellant we would need would be enormous and prohibitively expensive. For example, to send a vehicle the size of the Space Shuttle, and equipped with the same chemical rockets, to our nearest star at a leisurely pace of ten centuries, we would need about 10^119 kg of propellant. Compare that with 10^55 kg, which is an estimate of all the mass in the universe (estimate based on models for a closed, finite universe). Even if we used all the mass in the universe, we would not be able to fuel this journey. With the best rockets conceivable, say antimatter rockets or ion engines with an exhaust velocity two hundred times greater than for current rockets, we would still need over 500 supertanker-sized propellant tanks just to fly past our nearest star within a century. If we wanted that same spacecraft to actually stop when it got to its destination, we would need to use that 500 supertankers for braking and would need another 300 million supertankers of propellant to propel the vehicle toward the star along with all its braking propellant. Clearly, rockets are NOT the way to go to the stars. We need to find some fundamentally new mode of travel that requires little or NO propellant. This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself.

Our next challenge is speed. Even though the breakthrough of eliminating propellant would greatly boost how quickly we could travel in space, to reach interstellar destinations in comfortable time frames (say, within a term of congress), would require another breakthrough in physics. The fastest thing we know of is light. Yet, even at light speed it would take almost 9 years for a round trip journey to our nearest star system. The mission's financial backers might want a quicker return on their investment. And this 9 year time table assumes that we are at light speed. For objects like people and spacecraft that are built of matter rather than photons, the journey would be even slower. To travel to our neighboring stars in comfortable time frames, it is desirable to have the physics breakthrough that allows us to travel faster than the speed of light. Most scientists say this is impossible; others are more optimistic.

Our third big challenge is energy. Even if we had a non-rocket space drive that could convert energy directly into motion without propellant, it would still require a lot of energy. Sending a Shuttle-sized vehicle on a 50 year one-way trip to visit our nearest neighboring star (subrelativistic speed) would take over 7 x 10^19 Joules of energy. This is roughly the same amount of energy that the Space Shuttle's engines would use if they ran continuously for the same duration of 50 years. To overcome this difficulty, it is desired to have a breakthrough where we can take advantage of any energy in space or a breakthrough in energy production physics.

Fortunately, science and technology continue to evolve. In just the last few years, there have been new, intriguing developments in the scientific literature. Although it is still too soon to know whether any of these developments can lead to the desired propulsion breakthroughs, they do provide new clues that did not exist just a few short years ago. This NASA project will determine if and how these emerging possibilities can be applied to the goal of creating the desired propulsion breakthroughs

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