What happens to a solar sail as it flies through space? Made of the most diaphanous materials possible, the sail gradually begins to degrade. Roman Kezerashvili and Gregory Matloff (CUNY) have looked closely at problems like these and have considered everything from sail thickness and performance to the merits of different metallic films. The sail material of choice seems to be beryllium, three times lighter than aluminum, and with a usefully high melting point. One interesting configuration is a twin-walled, hydrogen-inflated sail with walls ten to twenty nanometers thick. But build such a craft carefully. If solar radiation causes the constituent beryllium to become degraded, the structural integrity of the sail is at risk and hydrogen begins to escape.

Solar sails need flight testing (and The Planetary Society’s plans for a solar sail seem to be developing nicely), but we’ll doubtless learn huge lessons from those early tests that will substantially revise our thinking. What happens, for example, when ultraviolet radiation ionizes the sail, creating a positive charge? The ionized sail then begins to deflect protons in the solar wind, increasing its speed. That sounds like a welcome effect, but too much electrostatic pressure carries the potential for rupturing a hollow-body sail. We need to experiment with these things in space as well as applying our best minds to the problem of deployment and performance.

**Working Close to the Sun**

It’s no surprise to the *Centauri Dreams* community that both Matloff and Kezerashvili have been actively at work on solar sail questions. We’ve examined many of Matloff’s papers and books in these pages, and Kezerashvili’s presentations at the summer confeence in Aosta, covering the effects of General Relativity on sail operations, were also discussed here. But how welcome to see them given publicity outside the space community. The colleagues made the cover of CUNY’s *Salute to Scholars*, a glossy publication for the university community featuring the two men, along with relativist and string theorist Justin F. Vázquez-Poritz and fiber-optics researcher Lufeng Leng.

The article is available online. That’s Matloff at top left, Vázquez-Poritz at top right, Kezerashvili bottom left and Lufeng Leng bottom right.

Vázquez-Poritz joined Kezerashvili at Aosta in examining sails from a unique perspective. Newtonian physics works fine for most current space operations, but things change dramatically when you start talking about bringing a solar sail in close to the Sun, as would occur in ‘sundiver’ mission concepts that seek to maximize the effect of solar photons and drive up the acceleration of the sail. Kezerashvili and Matloff calculate that a sail deployed at 0.5 AU could reach the heliopause in a mere 2 1/2 years — compare that to the thirty years the trip took the Voyager probes.

CUNY’s Neill Rosenfeld quotes Kezerashvili:

“You ask me why do I want to come so close to the sun with a solar sail?” says Kezerashvili. “I want to know what happens beyond the solar system, and if I launch something, I should know the result during my lifetime.”

Absolutely right, if we can find a way to do it, and a close solar pass seems to be the ticket, provided we can make the needed advances in materials science to withstand the environment. The article goes on:

He and Vázquez-Poritz considered Kepler’s third law, a mathematical formula conceived in the early 1600s that describes the relationship between the longer orbital periods of planets far from the sun and the shorter orbital periods of planets close to the sun. In two papers, they argue that deviations from Kepler’s law occur when the curvature of spacetime and solar radiation pressure act simultaneously on a solar sail-propelled satellite. In short, if you open the sail close to the sun and don’t account for Einstein, you might miss your target by more than 1 million miles.

“This could be an ideal way to test the effects of general relativity,” Vázquez-Poritz says. “With a solar sail, we could measure effects that otherwise would be too small to measure.”

Yes, and another way besides GPS to get the effects of General Relativity into an actual technological application.

**The Asteroid Deflection Question**

As to Matloff, the CUNY article homes in on his work on deflecting potentially threatening asteroids like Apophis. Here’s one way to move an asteroid: Use a large, parabolic collector sail to reflect sunlight onto a smaller, ‘thruster’ sail that would concentrate a hot beam of light on the asteroid’s surface. Eventually you get vaporized rock and a controllable jet that can be used to nudge the asteroid onto a new path. Lufeng Leng’s work at CUNY’s photonics lab aims to build a mathematical model of how the process would work, beaming differently colored laser beams at meteorite samples.

A crucial question is how far the light penetrates below the surface, for a beam that penetrates too deeply will simply heat the asteroid, while a beam that penetrates just the right amount — perhaps a thousandth of a millimeter — would produce a steerable jet. It all depends on better understanding the penetration depth of electromagnetic radiation (like light) in NEO regolith.

The CUNY story goes on to discuss the matter with Robert B. Adams, who headed a NASA team that performed an asteroid deflection study in 2007:

“The solar collector is definitely on the table,” says Adams. So are ideas including a nuclear explosion away from the asteroid, a kinetic impactor that would ram into it and a gravity tractor, which would hover near the asteroid and use the gravity that naturally occurs between them to pull the asteroid slowly off its course.

“The solar sail hasn’t received as much attention, but it’s a good application with NEOs because it gives you more control over which way your thrust is generated,” Adams says.

**Pushing Past the Heliosheath**

Where does all this take us? A manned mission to a nearby asteroid seems a reachable goal, one that would deploy and test solar sail methodologies to provide data needed to achieve workable designs. That’s an insurance policy we need to have no matter which technology is ultimately deployed for any future deflection mission. Asteroids aside, the CUNY researchers are also showing us a realistic option for a sail mission into nearby interstellar space. Ponder a sail 1800 meters in diameter carrying a 150-kilogram scientific payload, capable of reaching the Oort Cloud in thirty years.

Writer Neill Rosenfeld’s work on this story is excellent. He covers not only the researchers’ ongoing studies but looks at the development of the sail concept in history and runs through asteroid deflection models ranging from sails to kinetic energy impactors and nuclear interceptors. We’re at that point in funding where we’re technologically ready to move sail work into space for shakedown and development, but funding constraints almost guarantee that sails like The Planetary Society’s LightSail and other private or commercial ventures will carry the concept forward.

I have a personal prejudice about this. Since I’m 53 I have a better chance of living to see the results of a sundiver mission to near-interstellar space (even if it launches 20+ years from now) than I do of a more conventional mission (e.g. the Innovative Interstellar Explorer) that launches tomorrow.

Purely on that basis I say go for the solar sail!

Hello, NS,

We’re about the same age, and you may be right. There are many propulsion methods, and sails seem to have surmountable problems: expected issues are cited, anticipated, and likely remedies are suggested. Sails look like one of the more practical methods which certainly ought to be tried. The success of a prototype might cause other players to develop more models and technologies.

Any successful sail designs could also contribute to transportation systems; if the degradation problems are solved, the Earth-Mars-Earth loop can be attempted.

The sail’s utility in true interstellar transit can be assessed from its performance as an interplanetary propulsion method. Like the development of jet aircraft, the designs and refinements came with actual use. LightSail will be a good thing to start these ventures.

Like many on us here, I was brought up with the sci-fi notion of a general purpose spacecraft that could be routed and rerouted from planet to planet, from (Jupiter) moon to moon, and then back again. All as dictated by the scheduling whims of its Captain. Well, as dictated by the whims of the story’s plot, to be honest. But every spacecraft we currently launch is ultimately single-purpose, one-off, expendable. I’ve been heartened that some gravity-assist and ion-drive missions *do* have a new lease of life as their primary mission draws to a close, i.e. Pioneer, Voyager, etc; but that’s a long way short of the vision that Arthur C. Clarke sold me so many years ago.

So it’s nail-bitingly frustrating that solar sails remain untested by any of the more “orthodox” space agencies, as they show a lot of promise as the kind of general purpose spacecraft that’s capable of travelling to every nook and cranny in the solar system, and back again. And on and on. Imagine a spacecraft capable of repeatable trips, that doesn’t have to be redesigned around every mission, that is *production manufactured* with the durability, longevity and flexibility for whatever mission (within reason) that its “captain” may have in mind. And not just one of ’em. Dozens and dozens of ’em, plying the solar winds. Oh let’s make that hundreds of ’em, why not.

To allude to the exploration of the oceans: yer European keeled sailing boats were as capable of sailing the Southern Seas as those in the northern Atlantic. They could just as easily turn a profit sailing around Britain as travelling to the Antipodes and back. They were basically multi-purpose, durable transports. Now *that’s* the kind of spacecraft that My Clarke promise me…

Solar sails: bring ’em on.

Ric

Hi Paul;

I am interested in the concept of using carbon graphene for solar sail materials. Perhaps graphene can by laminated with a highly reflective and refractive metalic material. One would ideally like the refractive material to have an optical skin depth of a nanometer or less for ambient sun light so that most of the Sun light is reflected by the material.

Graphene seems to be much talked about in scientific circles as of late in part because of its cool and exotic properties. The idea of a one carbon atom thick membrane is too cool not to investigate its properties for solar sails. The caveat is ramping up production of large area bulk graphene membranes and then stroring them in a folded manner without tearing the graphene membrane(s).

Hi Folks, while we are on the subject of high performance solar sails, one can imagine a sail made of graphene that is 99 percent empty space in the form of a cross woven net where the nets strings are 1 nanometer wide strips of graphene seperated by 200 nm. Note that graphene is a one atom thick membranous sheet of carbon and as such is about 0.1 nanometers thick with a density roughly equal to that of water. Graphene has also been measured to be 200 times stronger that structural steel.

Te equation of motion for a dive and fry relativistic solar sail is:

B[(1 + (B EXP 2)]dB/[(1 ? B)EXP 2] {[1 ? (B EXP2)] EXP 3/2} = p [(R0/x) EXP 2](dx/Ro), where B = v/c, v is the speed of the sail, x is the distance from the star, R0 is the initial distance from the star,

P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, m0 is its rest mass, and u0 is the energy density of starlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2].

Adopting f = 1, a value of M0/A = (10 EXP ?9) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 = .006AU, I find P = 2.3578. Note also that the equation of motion can be integrated analytically to find the terminal speed.

Just integrate from zero to its terminal value and x from R0 to infinity. This yields for the terminal velocity:

{[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 42.37 ~ 7 + 15p = 42.367

With p = 2.3578 .the terminal velocity = 0.6795 C.

Once again, assuming fraction f of the starlight is reflected straight back and the sail moves radially outward, the equation of motion is

B[(1 + (B EXP 2)]dB/[(1 ? B)EXP 2] {[1 ? (B EXP2)] EXP 3/2} = p [(R0/x) EXP 2](dx/Ro) where B = v/c, v is the speed of the sail, x is the distance from the star, R0 is the initial distance from the star.

P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, m0 is its rest mass, and u0 is the energy density of starlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2]. Adopting f = 1, a value of M0/A = (10 EXP ?10) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.006AU, I find P = 23.578

This yields {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 360.67. With p = 23.578 .the terminal velocity = 0.8499 C.

This latter calculational scenario might apply to a solar sail made of a cross woven net of linear carbon atom chains that are only one atom wide or perhaps highly conducting atomic chains made of one or more other elements.

If baryonic periodic table elements would not suffice to form such chains, then perhaps cold dark matter chains, or supersymmetric matter atom like chains would work provided that such chains can reflect solar or stellar energy very effectively. CDM is by definition dark and so the ability to use such is another major caveat.

The above two examples of gamma factors pretty much put half of the currenly observable universe in range of manned galaxy hopping colonization programs even while accounting for the projected rate of the expansion of the universe over the next 13 billion years or so. One we learn how to achieve relativistic velocities, then huge solar or stellar driven colony or world ships can hop from galaxy to galaxy, colonizing each galaxy as we go.

The good old glory days of sailing might come back to us in the form of “Sailing the Ocean Black”

Hi Folks;

I seem to be enamoured with solar sails as of late after reading the latest issue of The Planetary Report from the Planetary Society. They have an excellent story on their work in developing a solar sail space craft to be launched next year. I hope the readership has not grown impatient with these latest three posts of mine, but I must say, I am becoming more and more of an advocate for using in situ ambient energy sources for powering manned interstellar space craft since the resources would otherwise go unused and therefore wasted. Perhaps nature knows best how to creat the necessary thermodynamic conditions that are required for interstellar and intergalactic space travel. Afterall, nature created the Big Bang.

Some additional extreme dive and fry sail craft scenarios are as follows

Adopting a graphene-like sail as an example of extreme dive and fry stellar sails, assuming f = 1, a value of M0/A = (10 EXP ?9) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L being a high end F class star’s luminosity at 10 EXP 28 watts and R0 = .01AU, I find P = 39.2976. Thus, {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 596.464. With p = 39.2976 the terminal velocity = 0.875.

Adopting f = 1, a value of M0/A = (10 EXP ?10) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.01 AU, I find P = 392.976. This yields {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 5,901.64. With p = 392.976.the terminal velocity = 0.947 C. The relativistic gamma factor of this baby will be 3.113.

Adopting a graphene-like net sail as an example of extreme dive and fry stellar sails, assuming f = 1, a value of M0/A = (10 EXP ?9) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L being a high end A class star’s luminosity at 5 x 10 EXP 28 watts and R0 = .015AU, I find P = 130.9918. Thus, {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 1,971.877. With p = 130.9918, the terminal velocity = 0.91975 C.

Adopting f = 1, a value of M0/A = (10 EXP ?10) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.01 AU, I find P = 1309.918. This yields {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 19,655.77. With p = 1309.918 the terminal velocity = 0.966755 C. The relativistic gamma factor of this baby will be 3.9107.

The caveat, How to find and/or manufacture the requisite stellar sail materials (that can withstand BB temperatures of as much as 10,000 K).

It seem a far better plan to launch a 1 million ton spacecraft from the South Pole using an Orion concept with tailored 1 megaton NPU (Nuclear Pulse Units). Once in space with an entire factory almost any concept can be experimented and examined. It’s the first 1,000 miles that is the hard bit. Asteroids converted into cities, an end of any type of material want in terms of raw materials for the Earth in trade for difficult to manufacture items like pharmaceuticals and entertainment. Sails are lovely, but first you need a dock.

Hi Folks;

I ask the reader to not grow tired of this final example I will give on this thread regarding single pass by stellar sail dive and fry craft. The example I give is of high end O class stars such as Eta Carinae which may have a radius as great as 195 times that of the Sun. High end O class stars or Blue Hyper Giants have a luminosity of anywhere from 2 million solar luminosities to perhaps 40 million solar luminosities.

Once again, P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, m0 is its rest mass, and u0 is the energy density of starlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2]. Adopting f = 1, a value of M0/A = (10 EXP ?10) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the stars luminosity and R0 ~ 1.3 AU, I find P = 483,662.

This yields {[(1 ? (B EXP 2)] EXP (1/2)} [7 ? 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 ? B ) EXP3](1 + B) = 7 + 15p = 7,2549,323. With p = 483,662, the terminal velocity = 0. 9967963 C. The relativistic gamma factor of this baby will be 12.5028.

A high end blue hyper giant converts 160 trillion metric tons of mass into energy per second. Capturing an average of only one millionth of one percent of this power out put for 100 seconds and converting the power to space craft kinetic energy would bring a 600,000 metric ton rest mass ship up to a gamma factor of 100.

Columnated beam sails could do much better since the energy could be focused on a smaller sail and wherein the sail might concievably exist as a net like grid made of superconducting nanofibers separated by meters if not tens of meters thus enabling capture of radiofrequency radiation. The net might be negatively charged while the space craft might be positively charged thus resulting in a coupling between the space craft in the net without the need for line connections between the space craft and the net sail which might otherwise be torn from the space craft under high force loadings.

Alternatively, the sail might exists as a super high mass specific area beam footprint sail that exists in the form of a magnetic plasma bottle sail on which powerful microwave or even long wave rf radiation is directed. If the plasma sail was electrodynamically coupled to the space craft in a manner simmilar to that of the proposed magnetic sail plasma bottle that could ride the solar wind, perhaps the plasma bottle could efficiently reflect the microwave and/or long wave rf radiation thus permitting truely extreme gamma factors to be obtained.

I personally like to muse that light sails are a kind of middle ground between less glamorous fission, fusion, and/or matter antimatter powered rockets and the as yet highly speculative forms of space travel such as warp drive, antigravatic propulsion, and wormhole travel. The beautiful thing about light sails is that they require no fuel or onboard generated energy supply.