# Big Sails, Challenging Dreams

by on June 6, 2012

I’ve been thinking about solar sails these past few days, a topic that inevitably invokes Arthur Holly Compton, who first demonstrated that x-rays have particle-like properties. Compton’s experiments in 1923 produced a body of work for which he would receive the Nobel Prize in Physics later that decade. Thanks to him we learned that while photons have no mass, they do have momentum, a useful fact for space exploration in that momentum can be transferred to a thin reflective sail, like the Japanese IKAROS that was successfully launched and tested in space in 2010. No question that the force is tiny — a sail would have to be a square mile in area to feel just five pounds of force at the Earth’s distance from the Sun.

The beauty of the sail, of course, is that it can keep producing thrust as long as it’s in sunlight. But how to increase the thrust? In an essay in his new book Going Interstellar (edited with Jack McDevitt and just out from Baen), Les Johnson notes that if we wanted to equal the thrust produced by one Space Shuttle engine, we would need a sail of one hundred thousand square miles in area, which works out to the surface area of Alabama and Mississippi combined. Working at Marshall Space Flight Center in Huntsville, Johnson knows the terrain of which he speaks. He also knows that we’re not about to start producing solar sails of this magnitude any time soon.

The alternative is to take advantage of the inverse square law, which says that moving the sail twice as far from its light source will result in its receiving four times less illumination. A similar move to four times the distance drops the illumination to one-sixteenth of its former value. The flip side of this is that reducing the distance to one-half its former value results in four times the force, and so on. You can see where this is heading: Johnson is building us up for a close solar pass, what sail experts Gregory and James Benford call a ‘Sundiver’ maneuver. Get your sail to swing close enough to the Sun and you get quite a push.

It’s interesting to speculate on just how big that push could be. Here we can take advantage of Gregory Matloff and Roman Kezerashvili (New York City College of Technology, CUNY), who have been massaging the numbers on a sail one mile in diameter moving to within nine million miles of the Sun. They find that a sail of this class could achieve a Solar System exit velocity of 250 miles per second. Johnson talks about all this in miles per second but let’s switch to kilometers, which is my normal practice here. 250 miles per second works out to about 400 kilometers per second, which we can usefully compare to Voyager 1’s 17 km/sec, as Johnson does:

A craft traveling this fast would pass the Earth in four days, Jupiter in twenty-one days and reach the Alpha Centauri system in just over three thousand years. By comparison, the fastest rocket we’ve ever sent into space won’t cover the distance to the Alpha Centauri system for another seventy-four thousand years! By increasing the sail size and keeping the payload mass the same, we can see an engineering path to building a sail that could cover this immense distance in about a thousand years. For you and me, there isn’t much difference between a thousand years and seventy-four thousand years. But in the lifetime of civilizations, the difference between these numbers is significant.

The difference is history. Go back 74,000 years and you are in the realm of archaeology, looking for the remains of nomadic, pre-literate cultures. Go back a thousand years and you are dealing with recorded human history. Ralph McNutt, who is doing such splendid work on interstellar concepts with the Innovative Interstellar Explorer design, once in a NIAC study worked out a trajectory involving a close solar pass that would get a payload to Epsilon Eridani in 3500 years, which he noted as the lifetime of the Egyptian empire. A 1400 year mission to Alpha Centauri would get the payload there in a time period as long as some buildings — the Hagia Sophia in Constantinople and the Pantheon in Rome — have been maintained.

And what would happen to the crew in that amount of time? One solution is the ‘generation ship’ that sees the birth and death of numerous generations on its way to the stars. But in a short story Johnson wrote for the same Going Interstellar volume, the author speculates about another method. The crewmembers of the long-haul starship in “Choice” are put into suspended animation, their minds kept active through hundreds of years of virtual reality simulations, just as their bodies are occasionally adjusted and massaged by intelligent machinery.

Thus the experience of the story’s protagonist, Peter Goss:

He, and every member of the crew, had programmed into the computer system their general wishes for the type of virtual reality scenarios they’d wanted to experience during the long voyage. The liquigel and the regular neuromuscular stimulation that went with it had kept their bodies alive and in peak condition while they slept. The VR scenarios had done the same for their minds and right now Goss wished he were one of the crew, blissfully unaware of the impending crisis, living out some extended adventure in a dream-like stupor. But it was a fleeting thought. He’d always preferred reality to the VR sims — that was one of the reasons he’d volunteered for the trip to Epsilon Eridani. Goss had to get away from the existential existence that was slowly creeping across the Earth and sapping the lifeblood out of the people there.

Missions of vast duration pose problems we haven’t even thought of, and the one Johnson dreamed up is a beauty. If, having experienced your choice among thousands of available VR simulations for a thousand years, you realized you were approaching your destination, would you really want the journey to end? After all, being awakened to land on a planet would put you back on your normal biological clock. No more simulations stretching out before you with no end in sight, but rather a far shorter and physically demanding existence creating a colony on what might be a difficult and chancy world. Obviously, I’m not going to give away the ending.

Going Interstellar is an intriguing volume. I’ve only read the two Johnson essays and another by Greg Matloff on antimatter propulsion, but you’ll find the major ideas for getting us to the stars here, though the editors are quick to note that they cover only the propulsion options open to us with known physics. You’ll find no warp drives or hyperspace vehicles in these pages. You will get, in addition to the work of scientists, stories by science fiction writers like Ben Bova, Michael Bishop and Mike Resnick. The combination reminds me of Arthur C. Clarke’s Operation Solar Sail (1990), which took on every aspect of sail traveling through both a fictional and non-fictional perspective.

It’s a good combination when you’re offering up a potpourri of current thinking on the intractable issue of starflight. We’ll be coming back to this book as I get deeper into it.

Geoffrey Kidd June 6, 2012 at 11:20

May I also commend the stories “Lesser Beings” and “Twenty Lights to the Land of Snow.” I read an advance reader copy a few months ago, and both of those masterpieces are still with me.

The Cosmist June 6, 2012 at 12:27

To take this idea further, if virtual reality simulation is preferable physical life, then why get on a spaceship in the first place? For that matter, maybe this has already happened and we are all in a simulation right now!

Scott June 6, 2012 at 13:21

Is it possible to shrink the size of the sail/s by employing a magnetic deflector or shield?

Alex Tolley June 6, 2012 at 15:16

I take it that the accelerations and velocities are based purely on light energy momentum transfer. On an earlier post the idea of sublimating the sail surface with microwaves to significantly enhance the sail’s acceleration was proposed.

Suppose the sail used this approach on close approach to the sun, increasing its velocity not just by light pressure, but by emitting some of its own mass under the intense insolation. Could that be used to increase it’s final velocity very significantly?

I would envisage the sail as having reflective and sublimative surfaces. The deorbit to the sun uses the reflective surface. The sail then turns around, exposing its sublimative surface. After that surface has sublimated, the sail can reverse again to maximize its pure solar sailing with the reflective surface.

AdamK June 6, 2012 at 20:14

I would be interested to see the relative gain from a Near Solar Boost maneuver performed absent the Crew / and or their bulky module. The boosted sail could then be ‘caught’ on a pass past Earth alleviating the Radiation risk to the crew, and also maximising the sail boost when close to the sun..

Eniac June 6, 2012 at 22:11

Suppose the sail used this approach on close approach to the sun, increasing its velocity not just by light pressure, but by emitting some of its own mass under the intense insolation. Could that be used to increase it’s final velocity very significantly?

No. The best exhaust velocity you can hope for in thermal sublimation is a few km/s, the thermal velocity of atoms or molecules. Via the rocket equation, the delta/v achievable from this is not much larger. Both are negligible compared with the 400 km/s we hope to achieve here, and thus not worth the considerably trouble.

Gregory Benford June 6, 2012 at 23:29

Alex Tolley:

Yes, these ideas work. One could also drop a sail sunward with a blocking vehicle between it and the sun, to minimize early heating, sublimation and stress problems; a chunk of rock would do.
As a simple example from an earlier paper my brother and I published, consider a sail falling sunward on a parabolic orbit. It will be accelerated by
• the ΔV imparted by desorption at perihelion
• ordinary solar sail acceleration on the outward-bound leg, once the desorped layer is gone, leaving a reflecting sail
We can find an approximate expression for the final velocity VF with respect to the sun, following energy analysis, as in Matloff’s Deep Space Probes (Matloff, 2000). The sail’s parabolic velocity at distance R is
V = 1.4 (GM/R)1/2 = 93 km/s (R/0.1 AU)-1/2
At perihelion of 0.1 A.U. the sail reaches a temperature (for seemingly plausible values of absorption and emissivity)
T = 927 K [(α/0.3)(ε/0.5)-1]1/4 (R/0.1 AU)-1/2
For such temperatures, a considerable ΔV > km/s is plausible for a range of desorption materials. Losing its mass load at perihelion, the sail thereafter works as an ordinary solar sail, attaining a final exit speed from the solar system

VF = 19.5 km/s [(ΔV/2 km/s) + (3σ)-1 ]1/2 = 3.9 AU/year (ΔV/2 km/s)1/2 [1 + 0.33 /(ΔV/2 km/s)(σ’)]1/2 (11)

Here σ is the sail areal mass density in units of 100 gm/m2. In the brackets, the first term comes from acceleration (a), the ΔV imparted by desorption at perihelion and the second from (b), ordinary solar photon acceleration on the outward-bound leg, once the desorped layer is gone, leaving a reflecting sail.

The sail’s speed as it passes through the outer planets will exceed VF. The linear sum of ΔV and the ordinary solar sailing momentum in the square root above means there will be a simple tradeoff in missions between the two effects, which are equal when the last term in brackets above is unity.

This is only a rough calculation, omitting many mission details, such as sail maneuvering near the sun. We assumed a perfectly reflecting sail on the outward leg, and that desorption would occur quickly at perihelion.

Joy June 7, 2012 at 0:53

Thanks for the post. In my opinion, using solar sails on sundiver approaches and passage times measured in a few thousands of years are the best bet for our species, our machines, and any other species. It is the only technology that allows multiple destination journeys, as long as the stars are of solar luminosity.

Even this technology requires a space infrastructure dependent on very cheap access to space. It is a long shot. But, if we can build an orbital elevator, we just might be able to spread our wings to the stars.

Michael June 7, 2012 at 3:40

It could be possible to use a combination of a Solar close approach and a laser system (on Mercury? or the Moon) to accelerate the sail.

Adam Crowl June 7, 2012 at 8:20

Best thing about being virtually alive is one can dial the clock-rate up or down, as a matter of taste. Body storage is an issue. Cryo-storage for whole living bodies might elude us, but I wonder if we couldn’t send an equivalent mass of stem-cells and re-organise them at the destination? All the brain-scanning and plotting efforts which are currently underway *might* reveal that storing our “essence” in neural terms isn’t such an onerous task and uploading will prove to be easier than imagined. Could we engineer sufficiently equivalent robot bodies for scanned human minds? A few more advances and we’ll be able to “print-out” bodies as required on arrival. But all hand-waving until the hard work of mapping a living brain/mind is done.

Interstellar Bill June 7, 2012 at 13:27

You can be sure nobody’s going to embark on a thousand year trip for a mere four light years.

With a sail that big it would be far cheaper and faster to build a giant laser than to develop a generation ship.

A 50-year voyage is entirely plausible,
but 1000 years is so totally implausible
that it’s a waste of time even bringing it up.

Just for one show-stopper,
how many centuries does it take
to develop 1000-year reliability?
Answer: 50 times longer than building the giant laser.

James Benford June 7, 2012 at 14:56

Michael asks isfpower beaming can be coupled to Sundiving to get even higher speeds. Indeed it can. Greg & I proposed that in a 2006 paper (Power-Beaming Concepts for Future Deep Space Exploration, Gregory Benford and James Benford, Proc. Fourth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, J. Brit. Interplanetary. Soc. 59, pg. 104, 2006.).

It is online at

Eniac June 7, 2012 at 23:34

If I understand correctly, Greg Benford’s calculation yields a much more conservative Vf of 20 km/s, 20-fold less than the 400 km/s mentioned previously. In this case, a few more km/s from desorption might be worth the trouble, particularly when you consider the Oberth effect. However, it does depends on the amount of trouble. A pure sail should be a lot easier to engineer than one meant to sublimate.

At 20 km/s we are talking 50,000 years to the next star, not a thousand. On top of all this comes the erosion due to interstellar gas, which is substantial and much more so the faster you go.

I would not put my hopes on sails, solar or otherwise.

Eniac June 7, 2012 at 23:44

Greg Benford: I am not sure I understand this equation:

VF = 19.5 km/s [(ΔV/2 km/s) + (3σ)-1 ]1/2 = 3.9 AU/year (ΔV/2 km/s)1/2 [1 + 0.33 /(ΔV/2 km/s)(σ’)]1/2 (11)
Here σ is the sail areal mass density in units of 100 gm/m2.

It appears σ is in the numerator, i.e. the final velocity increases with areal mass density. This cannot be correct. Did I misread?

kalish June 9, 2012 at 5:43

@adam k I just had the same idea while reading this lines, i like “the flip side” of the square root law. We need a good damping to catch a bullet that goes so fast. There are a lot of limitation to this project, the sail might be able to sustain radiations able to accelerate it that much. I don t think a lot of reflective materials can keep their reflective properties with those intensity. If possible, then we can imagine a mirror orbiting close to the sun and collimating light to here, retrieving the divergence of the beams. ( That can be very low even close to the sun). It should be a strange orbit managing with huge radiation pressure to stay stable. More if the pressure is so high we can imagine the mirror sustaing itself at the same altitude only by deflecting the light in two opposite directions, then we can get the power of the light from a region close to the sun. It could also be useful to take directly some energy by falling to the sun and then spreading the sail to lift up. But this is dream only.

Rob Henry June 12, 2012 at 1:59

Travelling this way still looks just too slow to be of interest to me. I have finally plucked up the courage to ask what may be a very stupid question.

Is there any way to use coronal mass ejections (or even better solar flares) to accelerate anything solid (or that may later solidify) to the 3000km/s that would get a probe to the nearest star within the life time of many children of the senders? Would ablation work as sufficient protection to leave anything.

My idea is not that any useful equipment could survive on this probe, but that nanobots could be launched behind it, adhering to the surface, and having a shield that would then protect them from the interstellar medium.

Ole Burde June 12, 2012 at 15:56

Rob Henry
There is probably not any single effect strong enough to give the acceleration we dream about , but if you combine several kinds it could get closer : first dive close to the sun to get push from mass ejection and after throw away THAT kind of sail ,, then use general radiation pressure on a “normal” sail and after throw away THAT sail , and then open up the magnetic sail to be pushed from Earth by laser or microwave , and then when too far away form the sun there is the galactic magnetic field . If a spacecraft can build up a strong enough electric charge , acceleration is possible . Last of all , there must be some onboard fuel for course corrections if we want to hit the target ..

tom June 13, 2012 at 7:20

I remember the first time I heard about ‘a solar slingshot maneuver’, it was an episode of ‘Star Trek’. 2 more time later with ‘Star Trek IV:The Voyage Home’ & ‘Star Trek:Deep Space 9′ (the last 2 were with ‘Bird of Prey’). And of course, ‘Rendezvous with Rama'; alien technology got their ‘world ship’ moving to 20% lightspeed after swinging by the Sun.
This is all so sexy, but in reality, has there ever been an attempt to break the record for an exotic slingshot around the Sun? The problem seems to end up with either ‘the set up uses more maneuvering fuel than payback in final velocity?’ Or ‘the optimal perigee would ‘incinerate’ any material known to our science!’
I think there is still plenty of room to put this to the test.
Maybe there is a better application that no one has thought of yet? Maybe there is a way to fill a spacecraft’s fuel tank with ‘solar plasma’ that would make the risk worth it? If 80 days later you end up orbiting in one of Jupiter’s la-granges… it would be a wild ride worth taking?

ljk June 29, 2012 at 13:17

Baen Books Teacher’s Guide to Going Interstellar online here:

http://www.baen.com/ya_guides/Going_Interstellar_Teachers_Guide.pdf

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