Imagine a solar powered sail that could propel a space craft through the vacuum of space like a wind that drives a sail here on Earth. The energy of photons steaming from the Sun alone would provide the thrust. NASA and other space agencies are taking the idea seriously and are working on various prototype technologies. Edward Montgomory is the Technology Area Manager of Solar Sail Propulsion at NASA. They just tested a 20-meter (66 foot) sail at the Glenn research center’s Plum Brook facility in Sandusky, Ohio.
Listen to the interview: NASA Tests a Solar Sail (3.7 mb)
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Fraser Cain – Can you give me some background on solar sails in general?
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Edward Montgomery – This is a technology that our agency has been interested in for some time, but the history goes back several hundred years to Fredrick Sander at the turn of the century (19th). In more recent times, we have found that advances in a couple of particular areas have made it something that we really have to look into. The composite materials that have been coming out in the last couple of years, such as in sports equipment that is made out of ultra, lightweight rods, and film technology which in some ways is related to the materials industry and integrated circuits fields for instance and paint additives. These fields have made it possible to build structures in space that are gossamer-like and we have never really been able to do that till a couple of decades before (now) and once you can get the kind of mass down really low, then it doesn’t take a lot for force to get some acceleration and some good propulsion out of it.
How can light provide propulsion to aluminum foil in space?
That’s a very fascinating property that light has; it doesn’t really have mass, so it can’t bounce off of something, but in fact it does interact with obstructions; it does impart momentum to it and this was theorized by Einstein and it has been proven in a number of laboratory experiments.
What is the technology that you are testing at NASA right now?
We are taking one particular solar sail concept which is a square sail; it has 4 booms that come out and in between the booms are triangular sails and that system is designed to carry payloads that are relatively modest in size: the Robotic Science payload. We’re looking at several missions to the inner solar system to study the physics of the Sun and how it interacts with the Earth.
So you would be sending your solar sail in from our position; the Earth’s orbit, closer into the Sun? Sounds kind of backwards to me.
Well, the thrust that the sail can produce is proportional to the strength of the sunlight and as you go closer to the Sun, the strength of that propulsion goes up as the square of the distance as you get closer so actually, it works much more efficiently close to the Sun. The missions that have been planned to look at the outer solar system; almost all of them have involved first going to the inner solar system flying close to the Sun and getting a good boost and then going out. But the near term missions that we are looking at are missions that hover; they don’t go really fast. There is a balance point between the Earth’s gravitational pull and the Sun’s gravitational pull called the Lagrange point, and we have satellites that site there now. That doesn’t require any particular propulsion, but if you want to sit and hover at some point closer to the Sun (to get to) that particular point in space, then you have to have some propulsion capabilities and our scientists have an intense interest at wanting to be at that point. You can imagine how that might be an advantageous place to put some instruments in between the Earth and the Sun to understand how that physical property is.
Ok, so I understand; it would be as if the Sun was a fan and you had your sail and you let it drift down towards the Sun to the point that the force of the Sun’s energy coming off of it is perfectly balanced to hold the solar sail at the point. It wouldn’t go any further or go any closer.
Right. That is correct.
What kind of experiments would you be interested in doing if you could get that close and be able to station keep?
I’m a propulsion engineer, not a research scientist; they can do a much better job of explaining what exactly they’re studying, but some of the instruments that they plan to put on it measure the magnetosphere, they measure high energy particles as they go by. Of particular interest is sensing coronal mass ejections; these are the large flare events that happen on the Sun, that once they reach Earth can really disrupt our communications and they actually can harm and destroy sensitive electronic equipment. Such a flare in 1986 caused several million dollars of damage in North America alone so we want to be able to predict those events when they are happening and if we have enough warning time, we can turn our equipment off or in particular conditions, keep them from getting hurt so it is important to know when a coronal mass ejection is coming.
What could the future hold for this technology, with being able to explore the outer solar system?
Well, that’s a good point. As I just mentioned, these coronal mass ejections also can be very harmful to our astronauts so NASA is looking in the near future to going back to the Moon and Mars which there has been a lot of discussion of. We’ll need to be able to predict when these events (coronal mass ejections) happen so that our astronauts can get to safe havens from those events, so we will probably need to have these warning satellites positioned near the moon and mars and possibly around the solar system for a warning in doing that. (After that) eventually in the future there is an intense interest in wanting to understand the structure of our solar system outside the orbit of Pluto, particularly the Heliopause, now the Voyager space craft has just entered that region; there’s been some interesting results coming back in there; and there is a lot that we’d like to know about in that region of space. Just beyond that is something called the Oort Cloud which is supposedly the area of space where a lot of the comets that we see live most of their lives, but occasionally they come into the Sun. So there’s quite a bit of science to be done; observing and exploration just beyond the edges of the solar system.
Would anything be different in building a solar sail to that could travel out into the outer solar system then what you are working on right now?
It doesn’t have to be. You could take the technology that we are pursuing now to do these coronal mass ejection signals and you could send that sail on a mission. The problem is that it would take or more to get to those Oort Clouds and out into the Heliopause. If we can build a sail that is a order of magnitude or a tenth of the weight for the same amount of area; that is performs 10 times better if you will, then we can make that same mission in half the time, so to really start considering that mission, we will want to build higher performing sails to really do it and to do it within our lifetime, if you will.
What is the time frame now on forward with the prototype you are testing and your future plans?
That’s something that there is a lot of studying going on in the agency right now; particularly, there is a science advisory committee that’s meeting and determining what their science priorities are and that will set the need date for when sails need to be ready. When it can be ready…, well what we’ve been doing over the last 3 years that has culminated in these tests at Plumbrook is to do the best we can on the ground to design and operate a solar sail in a simulated space environment. The next step is to go up into space and that’s going to be an important step. We really have to have a flight of the solar sail and see how it operates in space: the loads on the structure of the sail are much, much less than they are here on the ground. Gravity puts a load on the sails 4000 times higher than what the Sun will do. So a really true environment is in space and we have to take it (the sail) up to test it out. That’s another 3-5 years to do that sort of thing, and then it will be ready to be infused into a science mission; 3-5 years nominal space mission planning and development phase. So, within the next decade, certainly, I expect to see a solar sail flying.