Expert: Daniel Mazur - 7/16/2005

Question
I am 12 years old, but I know far more than you think I do.  I have been researching ionic propulsion and was wondering your take on it.  I believe it is possible, and I generally know how it works.  Do you think its for real, and do you know of any resources that would be helpful for testing it as I hope to do? Thanks. -Admiral_N

Answer
Hello, Admiral_N,

I will not make any assumptions about your current knowledge based on your age, I promise. In fact I am going to write you an answer that I would give a twenty-year-old, so I hope it is fine with you.

The ionic propulsion, if it was made to work, would be a VERY good one. NASA and other laboratories have put quite some effort in it from the beginning. So far people have been unable to mount this horse, despite huge amounts of money spent. I do not know all the details, as it is not quite my area, but I will give you an example of problems connected to it.

Imagine an ion-propelled space shuttle in the open space, say, on an Earth's orbit. It starts off as electrically neutral, naturally. Now you start the engine and the spaceship starts emitting ions. Let's suppose you have electrically balanced engines, so that each jet emitting positive ions is paired with a jet emitting negative ions. This way you keep the electric neutrality of your shuttle and it might seem that you effectively get a stream of neutral compounds (made of your ions, after a neutralizing reaction) reactively propelling a neutral shuttle.

Take note that you have the jets under a respective high voltage. The dynamical result will be that behind every jet (biased '+' or '-') there is a spreading stream (or cloud) of ions of opposite charge ('+' jet emits negative ions, '-' jet positive ones). Again, this is a 'reactive engine' similar to the ones currently used in rockets. With them the maximux acceleration gain is obtained, when the spreading of the ion beams is at minimum. On the other hand, with no spreading, the ions cannot mix and neutralize behind your shuttle properly. You will then observe, that the ionic clouds have so much instantaneous charge on them that it remarkably slows down the shuttle's progress. Simple Coulomb attraction between the (+,-) clouds and their respective (-,+) jets will give you the basic idea. The reality is more complicated due to the effective multipole interaction (you can find details of it in all college electrostatic study texts, or in the 2nd volume of Feynmann-Leighton-Sands "Feynmann Lectures in Physics"), but the basic idea of attraction between the shuttle and the emitted ions stays the same.

The second complication lies in the fact that charges in motion create a magnetic field that in turn curls up the paths of the other charged particles. It is a complicated business to predict the motion of the real-life ions, but we are most likely to end up with a shuttle emitting energetic ions on one hand (so there WILL be some observable acceleration), but being heavily bombarded by the back-flowing ions. These impacts will gradually corrode and bring destruction to your ship.

I am a slight optimist in this matter, I believe it will be made to work one day. In science usually predictions that something "cannot be done" are those proven wrong later. I am not sure, what literature to recommend to you, but let me try.

My advice would be to observe NASA research closely as they have gone a long way already. The publication "21st Century and Beyond - Future Space Rockets and Breakthrough Propulsion" should be written in a more accessible language than raw scientific articles, most of which are difficult to read even for a professional researcher.

In my experience, one can trust internet resources hosted on ".edu" domains but should be extremely careful about inventions presented on people's personal pages. There are quite a few people out there claiming they are ingenious and the whole scientific community are deliberately oppressing them. :-) As far as the printed books go, you will be safe - they may not tell you much new but they are unlikely to mislead you by telling an outright nonsense.

At last, about an experiment to test the ion propulsion. The biggest design problem would be to build two (at least) vacuum chambers, each with a plasma discharge and each accelerating and emitting ions - all this mounted so that you can measure any accelerating force obtained. In order to evaluate the total propulsion efficiency, you'd need the whole room to be under vacuum and shield everything from high-intensity radiation that will emanate along the path of the reacting ions and many other dangers. This is the reason why individuals are unlikely to solve this problem. Should you be already an accomplished programmer, familiar with all the necessary equations of motion and familiar with computer modeling methods, you could try to run a virtual experiment inside your computer. I assure you, however, that such a thing is a big task even for a whole group of physics graduates!

I wish you the best luck in your research, we really need to make some of the advanced propulsion ideas work. I hope my response has been understandable enough and that it helps you at least a bit.

Good Luck!
Daniel