Blasting off, with the help of plasma

In honor of Israel Space Week, we talk to a prominent physicist and learn more about plasma rockets.

Dr. Samuel Cohen (photo credit: ELEANOR STARKMAN)
Dr. Samuel Cohen
(photo credit: ELEANOR STARKMAN)
Israel dedicated this week to exploring the wonders of outer space – an opportune time to spotlight this writer’s interview last October with Dr. Samuel Cohen, who was attending the International Aeronautical Congress held in Jerusalem.
We met in the display booths area near the entrance to the city’s International Convention Center, and sat at the NASA booth, since it was not in use for any particular missions at the time.
Cohen was on his first visit to Israel. His home is in New Jersey, where he is director of the Plasma Science and Technology Program at Princeton.
Are you attending many of the sessions here? I tend to only go to sessions related to my work.
Which is? We research plasma; the main application I am involved with is plasma rocket engines. Most now are small and are used to steer spacecraft such as satellites. This is a thriving, relatively new industry – 20 years ago, no one would pay attention if you spoke of putting plasma engines on spacecraft. Today it is a multibillion-dollar industry; virtually every satellite which goes up has five or so small plasma engines on it. About one hundred satellites are launched worldwide annually.
How did you get interested in science? Since I was a young kid, I was fascinated by technology, such as a tape player I got as a child. Later, I liked playing with toy rockets, all home-made, an activity that would be frowned upon today.
You have been at the Princeton Plasma Physics Lab for your whole career, since you finished your PhD at MIT.
What is Jewish life like at Princeton? There is an active Jewish student community there, with a kosher cafe, daily prayers, etc. I participate in the prayers there. Just last year, the university approved the construction of an eruv [a way to permit carrying on Shabbat] for campus and somewhat beyond.
Really? Yes, and from what I understand, it was encouraged by the dean of the graduate school, who said: “We are losing observant students to other universities because they cannot carry [things] here on Shabbat.”
Wow! An Ivy League eruv! I see you wear a kippa. How does that go over at work? Work is the one place I don’t wear a kippa. The reason is that the people there have known me for so long – since before I became what is called a ba’al teshuva [one who has become observant].
When was that? About 20 years ago. I grew up in a kosher home and attended one year of yeshivot in elementary school in Brooklyn. Later, we moved to the Catskills, where my mom was a science teacher. My first marriage was out of the faith.
How does Judaism intersect with your professional life? Extremely well. It defines how I work. I am always out of the lab in time for Shabbat and holidays. The Jewish approach to study and to Halacha is very similar to my scientific work practices. Go back to the original sources; strictly obey the established protocols and minhagim [customs] Respect the mesora [tradition]. Argue in the spirit of Hillel and Shamai.
Tell me more about the plasma rockets.
As I said, almost every satellite which goes up today has several small plasma rockets on it. These require electrical power to heat plasma and provide impulses for steering. The electrical power ordinarily comes from solar cells or batteries.
What we are doing differently is trying to develop a fusion power source that could make lots of electrical power in space, even far from the sun, where solar cells fail, and at levels far above those that batteries can produce. An important additional advantage of our rocket design is that the fusion power could directly produce the thrust the rocket needs to provide without going through the possibly inefficient step of making electricity first.
The rocket engine design we are studying, both theoretically and experimentally, is expected to make 10,000 times more electrical power than currently available to satellites. This would greatly increase their communication capabilities. For example, the recent New Horizons mission to Pluto took days to send back a single photograph.
Our rocket engine could send back 1,000 pictures a day. Moreover, the rocket could have gotten to Pluto three times faster, orbited the [former] planet for years, and even placed a lander/rover on its surface for detailed exploration.
Our big challenge is to develop the science and technology required for a steadystate fusion engine, which will create the electromagnetic energy needed to run a plasma propulsion engine. With such an engine, longer exploratory flights would be possible.
To get a spacecraft out of Earth’s atmosphere, extremely powerful chemical rockets are used, but their energy, their fuel, is depleted quickly, most in 10 minutes! Plasma rockets would take over once the craft reached low-Earth orbit. They could deliver thrust for months or years, even decades.
One of the goals of the space community now is a manned orbital trip to Mars.
Such a trip with our rocket would take about three months each way, with two months there to orbit and observe. Astronauts need to be back within a year because after that, people start to get sick in space.
A third use of our fusion engine might be for rockets on missions to deflect large asteroids or comets which might endanger the planet. Though thousands of small, ~10-cm. diameter meteoroids hit the Earth each year, they are harmless. No death has ever been reported. In contrast, kilometer- size asteroids, though extremely rare – perhaps once every million or 10 million years – would cause mass extinctions and destroy cultures.
An actuarial analysis of comet impacts shows that tens of billions of dollars should be spent each year to develop a protection system for the Earth from these rare events. Our rocket engine could get to an approaching comet quickly enough to explode a bomb near its surface and deflect it from Earth impact.