By Dr. Gary L. Deel, Ph.D., J.D.
Faculty Director, School of Business, American Military University
This is the third article in a five-part series on the dynamics of human spaceflight systems for interplanetary and deep space missions.
In part II, we looked at the need for gravity aboard manned spacecraft, and how we might go about achieving artificial gravity in deep space. In this part, we’ll discuss different propulsion systems that might be used on future spacecraft to cross vast distances in reasonable periods of time.
Get started on your Space Studies Degree at American Military University.
As discussed in part II, one way of achieving artificial gravity is through constant acceleration and deceleration at a rate of 1G. But the energy required is substantial. To make matters worse, it turns out that the laws of physics make it particularly difficult to travel at those kinds of speeds for convenient interplanetary travel.
Chemically Powered Rockets Are the Tried and True Method of Space Travel
Scientists are currently looking into a number of technologies for the first interplanetary transport vehicles. Chemically powered rockets are of course the tried and true method of space travel. However, the problem with this method is that the spacecraft must carry all necessary fuel for the entire trip from the start, so this naturally leads to massive inefficiencies for long voyages.
One potential idea to work around this problem is to harvest hydrogen atoms from outer space as a spacecraft moves along, via a hydrogen ”ramjet” collector. However, based on the fuel consumption of chemical rocket engines and the known density of hydrogen in interplanetary space, a collector large enough to capture sufficient hydrogen for chemical propulsion would be almost as large as the diameter of the Earth itself. So unless we’re planning on building Death Star-size space stations, this idea won’t work.
Another option is solar sail propulsion, which would use the kinetic energy of photons from the Sun’s solar winds to propel a spacecraft. We have already tested this technology and know that it works in principle: the Sun’s rays really do “push” against objects with which they collide. The advantage here is obvious: A space station propelled by light needn’t carry any fuel. However, the amount of force that photons apply is so small that even a trip to our nearest planetary neighbors aboard a decent size space station fitted with solar sails would go so slowly before reaching its destination that no astronaut crew could afford to wait that long.
Shockwaves from Carefully Timed Nuclear Detonations Might Propel a Spacecraft
There is no shortage of creative suggestions for moving a space station from A to B. For example, the ion engine has been the subject of intense NASA research for years. Engineers have also proposed the use of shockwaves from carefully timed nuclear detonations to push a spacecraft along its path. But there is a bigger problem.
Even if our most brilliant scientists and engineers could devise a workable propulsion system, Einstein’s work in relativity demonstrated that as an object’s speed approaches the speed of light (the universe’s speed boundary of 186,000 miles per second), the object’s mass actually increases exponentially. So a theoretical object moving at the speed of light would be infinitely massive. What this means in simple terms is that, as an object speeds up, it becomes heavier and heavier, and thus, harder and harder to accelerate.
This increasing mass effect is totally negligible with respect to the miniscule speeds at which we’re used to traveling on Earth. But the distances between planets mean that a spacecraft could potentially travel at significant fractions of light speed to make trip times reasonable for transients. For example, Mars is on average about 13 light minutes away from Earth. So if a spacecraft could travel at perhaps 25 percent of the speed of light, it could potentially make an Earth to Mars trip in under an hour.
However, to make such a quick trip the spacecraft would require lots of energy, in part because its mass would slowly increase as it accelerated toward light speed. So if we can’t develop propulsion technologies capable of servicing these energy needs, then we must accept the fact that planetary voyages will always take months or years to complete.
In the next part, we’ll look at the need to protect spacecraft from the harsh outer space environment and maintain temperatures that are comfortable for human crews.
About the Author
Dr. Gary Deel is a Faculty Director with the School of Business at American Military University. He holds a J.D. in Law and a Ph.D. in Hospitality/Business Management. Gary teaches human resources and employment law classes for American Military University, the University of Central Florida, Colorado State University and others.