Interstellar Sails and Their Precursors

by Paul Gilster on November 27, 2007

Lou Friedman’s work on solar sails dates back to his days at the Jet Propulsion Laboratory where, in the 1970s, his team began work on a rendezvous mission with Halley’s Comet. It was a mission that never flew, but you can read about its planning stages in Friedman’s book Starsailing: Solar Sails and Interstellar Travel (Wiley, 1988). That title is, as far as I know, the first book-length study of this technology, though it has since been joined by Colin McInnes’ key text Solar Sailing: Technology, Dynamics and Mission Applications (Springer/Praxis, 1999).

Now executive director of The Planetary Society, Friedman’s interest in solar sails led to his work on the Society’s Cosmos I mission, unfortunately lost during the launch attempt in 2005. His interest in interstellar issues remains keen as well, as evidenced by an article he recently wrote for Professional Pilot magazine. “Making Light Work” runs through solar sail basics for an audience that may seem surprising, but I can tell you from my own flying days that as we used to wait in the pilot’s lounge for students to arrive, we would often kick around outlandish concepts like deep space missions (and there were always a few dog-eared copies of Professional Pilot scattered around the room, out of date and thoroughly read).

Friedman speculates about sails kilometers wide in the area of 0.1 microns in thickness, ultralight films that would, when the photons from sunlight lost their punch, take advantage of huge laser installations that could be focused for interstellar distances. Now we’re into Robert Forward territory and also in range of feasible interstellar missions. For as Friedman notes, solar sails are the only technology we currently have that could complete such missions in a single human lifetime:

What is exciting is that we know the way forward. We don’t have to invent some new physics (like matter/antimatter engines) and we don’t have to conjure up new technologies from science fiction (such as interstellar ramjets scooping up and using interstellar hydrogen molecules). Rather, it’s all a matter of engineering—make the light sail materials thinner, the spacecraft lighter and the lasers more powerful.

Of course, the demands are still huge, power on the order of 100 gigawatts, which means power stations located in space, assembled in the inner solar system where solar radiation is much higher than here on Earth (presumably sails would be involved in ferrying the needed materials). And then there’s the problem of sail construction, conceivably handled by making the sail out of plastics whose evaporation would leave only the needed molecules to reflect sunlight and laser photons. Imagine a square kilometer sail weighing just a few kilograms, its electronics sprayed onto the sail rather than flying as a separate payload.

Solar sail technology is no idle dream. After extensive study at Marshall Space Flight Center, NASA’s basic sail design has reached the point where space testing is the logical next step even as research continues in European venues like Germany’s DLR. When we begin a serious push into solar sail technologies, we’ll need to test these designs in near-Earth orbit, and then move out into the Solar System. A logical mission for early sails will be, as Friedman notes, a replacement for the Advanced Composition Explorer (ACE), a mission nearing the end of its lifetime.

ACE operates at a libration point where the gravitational forces of Sun and Earth balance, some 1.5 million kilometers from Earth. A sail mission that could monitor solar weather (and warn us of solar storms) could offer a new kind of station-keeping, one that uses the momentum imparted by photons to stay in position closer to the Sun without the need of remaining at the libration point. Such a position would, among other things, allow greater early warning of potential ionospheric disruptions.

The range of sail missions available in coming decades will be huge, but if we keep at it, we may get to the point where building the kind of laser we’ll need for an interstellar mission becomes possible. Solar sailing is the kind of next-step technology that moves us from one-shot mission spectaculars like Apollo into the realm of a stable and long-term human presence expanding into the Solar System. For the short term, we need to keep doing what Friedman and sail advocate Gregory Matloff are doing, explaining and arguing for the needed steps to get sails into nearby space where their value for more complex missions will be obvious.


Tim November 27, 2007 at 14:29

The solar sail concept is intriguing however; I see a possible technical problem with laser powered solar sails. Assuming that the laser is operated from some ‘off-earth’ position enabling it to take advantage of enhanced solar energy in some form to power the laser, the laser itself will experience an acceleration in the opposite direction to the beam it emits (the laser acting as a ‘photon propulsion system’) will it not? Assuming, instead, that the laser is anchored to some ‘air-less’ planet or moon; and fired frequently to propel a variety of spacecraft, eventually the lasers accelerations so produced may alter the orbit of its base. I wonder, has anyone done investigated this ‘reaction’ problem?

Christopher L. Bennett November 27, 2007 at 19:35

Well, the laser installation would be far more massive than the sail, and thus would be subjected to proportionately less acceleration. I imagine it could compensate for any slight change in orbit using attitude thrusters.

ljk November 28, 2007 at 0:42

Looks like The Planetary Society is trying to make Cosmos 2
a reality:

Adam November 28, 2007 at 3:49

Hi Guys

Tim, if the lasers are attached to the power collector system – solar, of course – then the net thrust of the laser will always be less or equal to the energy received by the collectors. Thus the thrust will always be compensated for.

Of course the problem then becomes how do we stop the collector from being carried away by the incoming reaction force of the sun’s energy? All sorts of tricky configurations of counterweights, extra solar sails and so forth can be used and the reaction force can even be used to turn the facility into a “statite” which is suspended by the reaction force against the sun’s gravity – i.e. it’s not in orbit anymore. This can have advantages for aiming the lasers and so on.

dad2059 November 28, 2007 at 8:41

How about putting the launching lasers in the asteroid belt? Close enough to the inner system to use solar energy possibly, if not, plant the lasers where the asteroid has uranium or H3 to power an advance nuclear reactor for power.

For long term station keeping, use ion thrusters.

Ronald November 28, 2007 at 10:22

I would rather plant the lasers on the moon and use Helium-3 fusion instead. Of course, nuclear fusion and in particular the He-3 variant, first have to be mastered. But the advantages are 1) that you won’t need a huge solar power collecting area and 2) a solid base (surely laser pulse reaction won’t be a problem on a body as large as the moon), maybe even 3): He-3 mining on the moon (though not nearly as abundant as on Saturn and Uranus).

Yes, laser sails are a technology for interstellar flight that ia already (almost) available, but as I have stated before, interstellar flight (and a lot more) probably hinges on our mastering of (Helium-3) fusion.

James Nicoll November 28, 2007 at 12:24

If the ability to build large collection areas comes with the ability to build very large phased array emitters, then the range at which we can provide power to interstellar missions becomes surprisingly large, mainly limited by our ability to keep track of where the target will be when the beam arrives:

The limit is somewhere past the Leo I Dwarf Galaxy if we use blue light to transmit the energy.

This also comes with the useful ability (at least for KII civilizations) to turn an otherwise useless Earth-massed rocky world into a cloud of fine particles over the course of about a week. These could then be gathered to construct useful items, like another Dyson swarm.

Lubo December 2, 2007 at 8:42

Solar Sails are not protected against deadly radiation in space! An advanced fusion driven VASIMR craft should be used as primary interplanetary space vehicle, maybe later, as interstellar craft too.
Ion rockets exist as well, they are becoming more and more powerful with time.

Here is DS4G —>

It’s far better to build a Bussard ramjet, rather a huge Solar Sail. And most important – the size of both crafts is equivalent :)

Lee December 2, 2007 at 10:58

This article says that sails are the only present day technology that could be used for interstellar travel. What about nuclear fission using a fuel like americium? I found a Discover article that says this:

Nuclear Fission
Pros: near-term feasibility
Cons: very heavy; needs processed fuel; requires massive radiation shielding; has limited top speed and range

Nuclear fission engineers have 60 years of experience working with fission, the process that powers atomic bombs and nuclear reactors. When the center of a radioactive atom is split apart, the resulting charged atomic fragments fly away at 3 percent of the speed of light, about 5,000 miles per second. Researchers led by George Chapline of Lawrence Livermore National Laboratory have designed a conceptual “fission fragment” reactor to harness those high-speed particles. Their reactor resembles a stack of vinyl records rotating into a cylindrical tower. Each “record” consists of graphite covered with radioactive fuel, such as plutonium or americium. When the fuel spins into the tower, it encounters additional radioactive material inside and triggers a controlled fission chain reaction. Powerful magnets around the reactor corral the resulting nuclear fragments so that they fly away in one direction, producing an exhaust that could accelerate a rocket to 6 percent of the speed of light.
To surpass 10 percent of the speed of light, Frisbee proposes building two fission rockets and staging them one on top of the other. The second stage effectively doubles the rocket’s top speed, so the expanded version could zip along at 12 percent of the speed of light. Add another two stages to slow everything down by the end of the trip and you could pull into an orbit around a sister Earth in the Alpha Centauri system in 46 years. More-distant voyages would take more than a human lifetime, however, even using additional stages. To keep weight to a minimum, the fission rocket would require a fast-decaying nuclear fuel such as americium. Americium is not a naturally occurring element, so it would have to be processed from spent nuclear fuel. A mission to the next star would require roughly 2 million tons of americium, not to mention a considerable amount of radiation shielding. Using cheaper uranium or plutonium would drive the fuel mass even higher. But the fundamental technology is ready to go.

The whole article can be found at

Administrator December 2, 2007 at 11:15

Unfortunately, the Bussard ramjet has serious problems, not the least of which is that it seems to generate more drag than thrust. Bussard’s concepts, as Robert Zubrin and Dana Andrews have shown, actually translate rather well into an interstellar braking technology for slowing down upon entering a destination system. And the size is definitely not comparable to a solar sail, Assuming 0.1 hydrogen atoms per cubic centimeter in interstellar space, you need a scoop at least 20,000 kilometers in diameter — some estimates go as high as 60,000 — for a Bussard-style mission, assuming it would work in the first place.

Lubo December 2, 2007 at 17:27

20 000 – 60 000 km in diameter? :shock:
And how long is the biggest theoretical Solar Sail?

Administrator December 2, 2007 at 21:37

Lubo, one of Robert Forward’s solar sail concepts for an interstellar mission reached 300 kilometers in diameter. He envisioned this as a laser-driven lightsail that would be capable of a manned mission to Epsilon Eridani. In addition to his scientific papers on sails, he wrote this one up in his novel Rocheworld, which is quite an entertaining look not only at getting a mission to a distant star but also slowing down to explore the system there.

johnF December 3, 2007 at 10:53

Has anyone ever considered more exotic proposals for interstellar probes? I may not be putting this idea over very well! Plasma crystals, which are basically particles of dust suspended in a plasma, can be very well organised structures. If a plasma crystal could be made stable in deep space could it serve as the basis for a spacecraft? Information could be stored in the structure of the crystal, perhaps recording the electromagnetic or gravitational enviroment around another star. If dust particles from that solar system could be pursuaded to incorperate into the crystal it could even perform a sample return mission. As it would mass very little and be made largely of empty space both the energy requirements and collision problems would be much releived. I dont know if a plasma crystal could be manufactured that would remain stable in space, but has anyone heard of anything comparable or equally exotic?

Lee December 4, 2007 at 13:13

Thanks for the info about the Ramjet, but that wasn’t what I was referring to. I was talking about using americium for nuclear fission propulsion. Both this idea and the ramjet (as well as many other ideas) are in the Discover article I referenced.

Lubo December 5, 2007 at 9:32

I’ve got a question about the performance of the Ion drive. Can a spaceship liftoff with ion? In Star Wars movie, vehicles do exactly this. Is it possible to be done in the next few decades?

Administrator December 5, 2007 at 15:42

It’s a good question, Lubo. Current ion technology can’t get a payload off a planetary surface. An ion engine like the one on Deep Space One produces only one-fiftieth of a pound of thrust at full throttle, but its specific impulse is high, making it valuable for deep space missions. Just where ion propulsion will be in twenty years is worth considering, and I hope readers with more background in ion methods than I have will jump in here with some thoughts.

Lubo December 5, 2007 at 18:14

What about GDM and VASIMR? Their thrust capabilities must be far better than this of Deep Space 1 or Dawn spacecraft, right?

Adam December 5, 2007 at 18:30

Hi Guys

Ion drives and all electric propulsion systems suffer from heat loss issues because the power source is separate to the rocket. Chemical, nuclear thermal and pulse drives all lose their waste heat in their exhaust stream and have enough mass flow of propellant to use that for cooling if needed.

Ion drives are pretty inefficient losing about 40% of the electrical power as heat in the ion making & accelerating process. Plasma drives are pretty efficient depending on the propellant heating system – VASIMR and helicon thrusters both use RF heating that can be 90% efficient in converting electricity into heat. But the magnets in the VASIMR system do need cooling.

But the real heat loss problem is the power source – nuclear reactors, for example, require massive cooling systems and about 60-70% of the energy they make has to be dumped as waste heat. If super-efficient thermoelectrics could be invented the system is still limited by how well it can be cooled to create a thermal differential to extract power from.

What’s really needed for ion or plasma drives is a compact and highly efficient electrical power source. Robert Bussard’s fusor using p+B11 can get 95% of the ion energy converted into electrical power. He designed a 16 gigawatt system using two fusors to power a hybrid scramjet/rocket system for launching from Earth. To convert electricity into thrust high-powered electron guns would blast reaction mass into plasma and the exhaust channelled using magnetic fields.

Alternatively ultra-power density batteries – say using room-temperature superconductors – could power an all electric hybrid, but the trick is whether such high-temperature superconductors can be made. The mass of batteries would be pretty heavy – say a 100 ton vehicle is flying into orbit. To get there about 9 km/s total delta-vee is needed – the equivalent of 40 megaJoules energy per kilogram of propellant for an air-breather. Experimental batteries can manage maybe 1000 Watt-hours per kilogram (most are a lot less) – 3.6 megaJoules per kilogram. A hundred-fold improvement would be needed to get a hyrbid vehicle into space, double that for a pure rocket.

Lubo December 6, 2007 at 11:59

Thanks for the info, Adam :)
And, Adam, one more thing – is there any plan to test the Robert Bussard’s fusor on VASIMR soon?

Plasma or Ion drive will be the primary interplanetary propulsion system in the next 30 years?

Adam December 7, 2007 at 6:53

Hi Lubo

There’s no specific plans, but…

A Bussard fusor could power a VASIMR quite effectively, but only once a working power-generating fusor is demonstrated. The next two fusors, WB-7 and WB-8, are being funded by the US Navy and it’s hoped they will prove Bussard’s scaling relations right, thus allowing a power-producing fusor to be produced.

If high efficiency direct-conversion power is available then several different plasma rockets become viable, VASIMR being the best researched. VASIMR is basically a fusion rocket, but without any actual fusion. Hooked-up to a Bussard fusor and that might change – direct fusion product propulsion being the most energetic rocket design, exhausting the fusion plasma directly and getting up to 1,000,000 seconds Isp, or more. The magnetic nozzle of a VASIMR would then be ideal, but the helicon heater would be redundant.

The first demo flight of a VASIMR will be a small one attached to the ISS, perhaps to be used for reboosting the Station against orbital decay. The next demo would be on a planetary probe using a few kilowatts from solar collectors. Manned missions will need fission or fusion reactors to produce megawatts/gigawatts of power, or large laser collectors to catch beamed power.

Tim September 6, 2008 at 10:12

those who think that improved ramjet designs can not eliminate drag are

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