The Futurist Interviews Space Expert Edgar Choueri

Traveling through the Solar System on a Budget
Rick Docksai interviews Edgar Choueri

Launching spacecraft is extremely expensive now, but Princeton University physicist Edgar Choueri believes that some up-and-coming propulsion technologies could make it much more affordable. On Feb. 4, he visited the Philosophical Society in Washington and discussed several concept launch systems that space agencies are now exploring:

A space elevator. A cable would run from a base on earth to a tether somewhere up in low-earth orbit. An elevator pod would ferry people and payloads from the base to the tether and back. Carbon nanotubes, which are more than 100 times as strong as steel, would be an optimal building material. Once the elevator is up and running, the cost savings would be massive: It costs $10,000 to $20,000 per kilogram to lift a payload into space using conventional rocket thrusters, but it might cost no more than $5,000 per kilogram via a space elevator. The idea has been gaining traction in recent years.

“It’s moved from science fiction to there being conferences every year where scientists talk about how to build space elevators,” Choueri said. “It would make sending a payload to space as inexpensive as sending a package via Federal Express.”

Plasma rockets. The earth’s surface is a horribly inefficient place for launching spacecraft. Instead, we could build our spacecraft in low-earth orbit— with the materials and the human crews transiting up on the space elevator. Upon completion, the craft would propel through the vacuum of space by plasma rocket engines, which use electromagnetic currents to ionize plasma gas and create powerful propellant forces.

Choueri said that there are there are 139 spacecraft in space that now use electromagnetic propulsion. It is used on many spacecraft from the United States, Europe, and Japan. A Japanese spacecraft used one last year to land on an asteroid. Since plasma rockets don’t involve any combustion, they consume just a fraction of the fuel of conventional chemical rockets, while producing much more thrust: The best chemical rockets will reach velocities of 4 kilometers per second, but some propulsion rockets reach 50 or 60 kilometers per second.

Plasma rockets would be our best bet for flying to the moon, according to Choueri.

Magnetoplasma dynamic thruster (MPD). An exceptionally powerful plasma rocket system, it uses lithium as a propellant. Princeton University has built several lithium MPD thrusters. Lithium is very reactive; an MPD that runs on it can reach velocities of 60 kilometers per second and 100 kilometers per second.

An MPD would be our transit of choice to get to Mars, Choueri said. A spacecraft with MPD thrusters could get a human crew to the red planet in a few months while using 20 to 30 times less propellant than one with comparably sized chemical rocket thrusters.

Solar sails. To send a spacecraft to the solar system’s outer edges, we might outfit it with solar sails that would capture radiation energy from the sun. These “sails” would span one to 10 kilometers in length but would be just millimeters thick. The spacecraft would fly toward the sun and, once it is in the sun’s vicinity, receive a massive burst of radiation through its sails.

This burst of solar energy would might hurtle the spacecraft as far as the Oort Cloud, the outer realm of comets that encircles our solar system. Theoretically, the spacecraft could even keep going and reach another start system in 50 years’ time. Whether it’s headed for comets or for the stars, though, it had best be an unmanned probe: Flying toward the sun and receiving large doses of solar radiation would probably fry any human beings on board.

If these four concepts attain fruition, according to Choueri, humanity could be sending humans to the farthest ends of the solar system in a mere 30 years. At that point, we could then begin seriously exploring our options for taking the next step: traveling to other star systems and beyond.

Choueri is director of Princeton’s Program in Engineering Physics, as well as the chief scientist and director of the university’s Electric Propulsion and Plasma Dynamics Laboratory, a physics professor in the Mechanical and Aerospace Engineering Department’s Applied Physics Group, and an associated professor in the Astrophysical Sciences Department/Program in Plasma Physics. He graduated from Princeton in 1991 with a PhD in Aerospace Engineering/Plasma Science.

Off campus, he has worked as a Principle Investigator (PI) in charge of directing and managing more than 25 competitively selected research projects funded by NASA, AFOSR, the National Science Foundation, and other governmental and private institutions. From 2004 until 2007, he led a team of researchers from NASA and several academic institutions to develop a high-power plasma rocket system intended for the robotic and human exploration of the Moon and Mars.

He is the author of more than 140 journal articles, conference papers and encyclopedia articles on plasma propulsion for spacecraft, plasma physics, instabilities and turbulence in collisional plasmas, plasma accelerator modeling, space physics and applied mathematics. He has been an invited speaker at symposia and leading institutions in more than 20 countries and has advised more than a hundred graduate and undergraduate students at Princeton University. He also serves as Associate Editor of the Journal of Propulsion and Power.

He is the recipient of a number of awards and honors, including the Howard B. Wentz Award for Excellence in Teaching and Scholarship and the Medal of the Order of the Cedars (rank of Knight), and was recently elected President of the Electric Rocket Propulsion Society, whose members include hundreds of scientists working on plasma propulsion for spacecraft in more than 15 countries. He served as the elected Chair of the American Institute of Aeronautics and Astronautics' Electric Propulsion Technical Committee (EPTC) from 2002 to 2004, the leading professional society in his field, was awarded the AIAA's distinguished service award in 2004 elected Fellow of the AIAA in 2009.

Following his Feb. 4 presentation, he spoke a bit about his research on advanced propulsion. In the interview, Choueri laid out how NASA and other space agencies could use the technologies he was describing to make human exploration of the solar system feasible.

Edgar Choueri: What would happen in 30 years? What would space travel ideally be like? Today we do it with very inefficient technology: the chemical rocket. This structure is basically an entire structure holding fuel the payload is a very small percentage of it. I propose to you a vision for 30 years from now.

Let’s go back to the moon. I think China would be going back to the moon over the next decade. I have a strong feeling that they’re working on colonizing the moon over the next decade.

It’s ridiculous to conquer the moon over the next 40 years with the same technology we used 40 years ago, however. Going to the moon with a plasma rocket you can cut down the propellant mass by a factor of 20 or 30.

Rick Docksai: The U.S. government cancelled the Constellation Project and directed NASA to focus more on satellites and earth sciences, less on humans in space. How come things like the space elevator or solar sail didn’t enter into the equation? Where are the discussions about possible future technologies like these taking place, if not in government space agencies?

Choueri: If you go deep down you will find people. A lot of the projects are being researched at NASA centers. The big question is why NASA cancelled the constellation program. The answer has to do with appropriation and funding. The priorities today are not there.

There was a commission the Augustine Commission that studied for a few months the viability of what NASA should be doing in space. Headed by Norm Augustine, the chair of Lockheed Martin. His committee realized it was impossible to increase the funding to a level that space programs need to be effective.

I believe the solution is to trim down NASA, though I admit that it would be very hard to trim down NASA. The $18 billion that it now receives yearly, that is not much money by some standards: We spend that much in Afghanistan in a few weeks. Augustine and his group conclude that with that money, there is not much we can do in terms of human exploration in space. I would agree but I would say we could do a lot more with it if NASA were not so loaded with fat.

But I know that that isn’t politically viable. So unfortunately, $18 billion dollars can go a long way, but only if you are willing to reshape the agency.

Rick Docksai: What level of R&D into lithium and plasma rockets would we need to be investing year to year in order to meet your goal of travel to Mars in 30 years? By what magnitude would it have to increase from what we’re investing today?

Choueri: It’s cyclical. From 2002 to 2005, there was a peak in these propulsion ideas. By 2005, they had pulled the plug on it. Now we’re hearing that, depending on how appropriations happen, there is a significant interest in increasing the budget on these technologies again.

But there have been many such cycles like these over the last 30 years. So it looks like these levels of funding could happen in NASA. My ideal would be to fund it by NASA for a few years at universities and NASA centers, with the intent of making it as soon as possible mature enough to be picked up by industry.

Rick Docksai: You say you see a role for industry. So commercial space enterprise would work on the projects alongside NASA and other government space agencies?

Choueri: I think that one of the good things the government can do is take research that doesn’t offer give a clear enough incentive for industry to be involved. It has been proven that if a project stays under NASA’s wing for a while, it becomes habitually expensive and needlessly expensive because there is no competition.

Lithium and ion thrusters do have potential for development in private industry because they would be used in low earth orbit. Some of them are being used now on commercial spacecraft. NASA can help on the research part.

Propulsion shows a clear example of how government research can fund a new idea. It was sponsored by the government and the research was done at NASA centers and universities. The research matured the concept to where it was taken up by private companies and used. Once the technology becomes very mature, the government should not be involved in it.

Rick Docksai:Your forecast on China colonizing the moon makes me wonder – to what extent is the momentum on space exploration moving away from the United States? To what extent are other countries taking up the initiative while the United States slacks off?

Choueri: Since the 1960s, there has been a very clear shift in the center of gravity outside the United States. Now there are many players. The European Union is a very important player in terms of launching abilities. You have Europe, the United States, Russia, China, India, and Israel. All are countries that can put spacecraft in space.

The big question is how serious the investments in America in space are. NASA is in my opinion a behemoth with a lot of layers of fat. It still has some wonderful centers, but other aspects of NASA are layered in fat. I believe NASA should be reshaped from the ground up. NASA still does research on gas turbines: That’s a subsidy for research that industry can make.

The government really should only be funding things that industry does not have the time or the will to fund. The government doing everything is not the most efficient way. Having industry involved in space is an important thing involved in my opinion.

Rick Docksai: What country do you imagine would host the base of the space elevator? What geopolitical considerations would enter into it?

Choueri: I’m not in the field of geopolitics, and so I can’t predict how it would happen. But from a technical standpoint, it would be best if it was in some place on the equator, like equatorial Africa. To be geo-stationery, it has to be on the equator. If you are off the equator, you will start dipping. You won’t be at a fixed point in the sky.

Rick Docksai: A solar sail – how durable is it? What is the risk of debris in its path hitting it and causing damage?

Choueri: A solar sail being punctured by a pebble or meteoroid is not necessarily a huge thing, as long as it’s a small area and it doesn’t cause mechanical failure it’s a small area. If you have a one-kilometer sail, depending on how many punctures you get, it might be an insignificant defect. Small holes in a big structure don’t amount to much. It might not be a show stopper.

There is a project that uses a solar wind by creating a magnetic field on the spacecraft using special magnets. This would theoretically cause the solar wind to push against it. So imagine a spacecraft unfurling this big magnetic field. It's like a sail. The wind pushes against it. It’s called M2P2 and its being studied at the University of Seattle. But there are some yet-to-be-resolved problematic issues with how to couple the plasma with the magnetic field and how to produce thrust.

If you travel at the speeds necessary for interstellar travel, however, then collision with objects as small as a pebble becomes very problematic. The nearest star system known to have planets is 10 light years away. We can possibly do this trip in 50 years, but we’d have to travel at 20% of the speed of light. If you travel at one tenth of the speed of light and you collide with a pebble at that speed, it would be like colliding with an atom bomb.

Rick Docksai: A magnetic field exerting force… Maybe a spacecraft could create a magnetic field that would act as a shield and block debris in its path. Are there any experiments in “force fields” of that kind right now?

Choueri: No, there are no such projects right now. At those speeds, there is very little you can do. It's something we cannot think of right now. These things are practically impossible to deal with.

Rick Docksai: I’ve been told that as an object’s velocity increases, its mass increases. So if a spacecraft accelerates to a fifth of the speed of light, it would surely take on a very, very large mass. What effect would this have on fuel requirements? What options do we have for a fuel that would be adequate to keep it running even as the mass goes up?

Choueri: Close to speed-of-light travel is very problematic. Right now we see of no way to reach stars that is feasible in a period of 50 years. It is conceivable we could do it using antimatter as a reactor. It is somewhat conceivable that we could use a solar sail as I mentioned. But going to further stars light years away, we have no idea to do it now.

There is a proposal that we could do an interstellar precursor mission. Instead of the nearest start we go 10,000 astronomical units. We will go past the solar system into the Oort cloud and even past that. It’s a good practice. Conceivably we could do that with solar cells. It won’t be a round trip. But it could be a one-way trip.

I would go to maybe Titan. I would think about exploring the moons of Jupiter. I think the next big adventure in space will be exploring the outer planets using nuclear power and plasma propulsion. And also possibly nuclear thermal propulsion.