Solar sail development has surely been a frustrating thing for Sandy Montgomery, who knows that what stands in the way of pushing this technology into space isn’t the need for scientific breakthroughs but adequate funding. Montgomery’s team at Marshall Space Flight Center has been examining the potential of solar sails for a long time, and is well aware that leaving the propellant behind is a way to get more payload to your destination with considerably less overhead all around.
And solar sails, which can ride the momentum imparted by photons from the Sun, are the ideal way to study ‘propellantless’ propulsion with near-term technologies. What a pleasure to see the launch window approaching for a solar sail deployment experiment in space, led by Montgomery’s team and counterparts at NASA Ames. NanoSail-D is to be launched aboard a Space Exploration Technologies (SpaceX) Falcon rocket some time at the end of July or the beginning of August. Montgomery calls it the “…first fully deployed solar sail in space, and the first spacecraft to use solar pressure as a primary means of attitude control or orbital maneuvering.”
Image: NanoSail-D is made of extremely lightweight gossamer fabric designed to glide into space. Image Credit: NASA/MSFC/D. Higginbotham.
It’s also an opportunistic way to reach low-Earth orbit, since NASA Ames was already committed to participation in the Falcon 1 launch, and the Poly Picosatellite Orbital Deployer (P-POD) developed at California Polytechnic State University is ready to deploy the sail. ‘Nano’ may be an overstatement, but it does capture the small size of this mission’s payload, which weighs in at a scant nine pounds and measures only slightly larger than a loaf of bread in length.
Compared to some of the huge designs contemplated for future missions, the sail itself is relatively small. Weighing less than ten pounds (Montgomery notes that the team carries it around in a special suitcase), the sail will deploy to about 100 square feet of surface. This video offers a helpful explanation of the deployment of the sail’s panels.
A realistic technology for future missions? Believe it. Although the push from the Sun is tiny, the effects are cumulative and quickly begin to mount. Says Montgomery:
“It’s not so much about how far a sail will go compared to a rocket; the key is how fast. The Voyagers have escaped the solar system, and they were sent by rockets, but it’s taken more than three decades to do it. A sail launched today would probably catch up with them in a single decade. Sails are slower to get started though. So, for example, between the Earth and the moon, rockets might be preferred for missions with a short timeline. It’s a trip of days for rockets, but months for a solar sail. The rule of thumb, therefore, would be to use rockets for short hops and solar sails for the long hauls.”
Think, too, about how the idea of solar sails is changing. The vast sails described by Cordwainer Smith (“The Lady Who Sailed the Soul”) or Arthur C. Clarke (“The Wind from the Sun”) were envisioned without a functioning nanotechnology to support them. Montgomery notes that today’s microelectronics make it possible to shrink the size of the sail and still perform serious missions. In a few decades, nanotech may have reached the point where smaller sails are sufficient to get assembler-laden research stations to their destinations. As we deploy NanoSail-D, let’s keep an eye on developing sail technologies, including beamed microwave propulsion, as we look to future prospects for even longer missions via laser or particle beam methods.
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Although small in size, this mission gives encouragement to us interstellar mission advocates because of the benefit of “leaving the fuel behind”. Here, any incremental step is a step forward.
However, a sail is just one part of the equation. We need pretty powerful lasers or masers, the power supply, and perhaps the ability to use a multi-bounce approach.
Some fairly powerful solid state lasers are being built such as the following:
Joint High Power Solid State Laser (JHPSSL)
The military has megawatt-class lasers but those are chemical and so we get into an issue of mass. Does anyone know if the materials for a chemical-based laser is available from lunar or asteroid resources?
Finally, Earth right now produces electricity in the low Terrawatt range. That’s a lotta power! Presumably enough to vaporize our thin-skinned craft. And shooting through the atmosphere would create a problem of diffusion. Might there a way to use adaptive optics to make our laser function as though there were no atmosphere?
Interesting idea re adaptive optics, but I suspect that by the time we are serious about laser propulsion for lightsails, we will be talking about space-based installations to supply the needed power. Re power and vaporizing the craft, it’s interesting that when Geoffrey Landis ran the numbers on Robert Forward’s Starwisp proposal, he discovered that is precisely what would have happened — the microwave beam would have essentially fried the delicate spacecraft.
It is fun to consider light sails or beam sails to the near ultimate degree.
As an example, I can imagine using beamed energy to accelerate a beam ship to a gamma factor of 100 to 10,000 wherein a sail or other mechanism that is made of negative index of refraction materials would pull the craft forward into the oncoming CMBR and/or star light for further increase in gamma factor.
One can only speculate how high of a gamma factor a craft could reach with the negative refractive index light pull mechanism, however, if we are able to fabricate negative refractive index materials with active differential volume or surface elements operable on the scale of nanometers, Angstroms, or Pico meters based on pico-technology manipulation of neutron dense matter, perhaps the sky is the limit.
Assuming some sort of craft could be developed and deployed with such extreme short wave negative refractive index materials, why not go a step further and consider femto- scale neutron dense refractive differential volumetric or surface elements.
Since I have dared to speculate about femto-scale materials, I might as well consider a sail craft as such on steroids that would have such a high gamma factor that the craft would interact with the zero point electromagnetic fields so as to be pulled forward by the incident virtual photons made real. The analogy here is remotely similar to the process of Hawking Radiation by which virtual particles become real and radiate mass away from a black hole as a result of the extremely stressed state of the space time near the event horizon of a black hole.
I am not sure what the outcome will be with light sails, but even if light sails have a practical ultimate finite gamma factor capability, I think human ingenuity will either find ways to approach C ever more closely from below or find ways to skirt the light barrier of C. If we could start with a 20 TW beam system that is 50% efficient, then 1/3 of a century, a space craft would gain about 10 EXP 22 joules. For a 10,000 metric ton space craft, a velocity of 0.1 C could be achieved, for a 1,000 metric tons space craft, 0.4 could be achieved, for a 200 metric ton space craft, 0.867 C could be attained or a gamma factor of 2.
Hi John & Paul
Jordin Kare’s sail-beam pellets use the ultra-high transmittance of some materials to allow very high accelerations – a hundred trillion watts per square metre or some such, he aims to get. The trick is that a substantial fraction is reflected, virtually all the rest is transmitted through the sail and a tiny bit is absorbed and dissipated.
I’ve always thought that the problem with using beams to accelerate a solar sail craft leaving the solar system is that there isn’t a beam at the other end (Alpha Centauri?) to slow the craft down once it gets there.
James M. Essig
“a sail or other mechanism that is made of negative index of refraction materials would pull the craft forward into the oncoming CMBR and/or star light for further increase in gamma factor.”
What are these negative index of refraction materials, & how would they pull the craft into the oncoming CMBR? This sounds like it would violate the 2nd law of thermodynamics, which makes me doubt its feasibility (slight understatement ;^)
Good point, Matt, although Robert Forward did work out a way to turn such missions from flybys into rendezvous attempts by using a three-part lightsail. The scheme was complex, but involved separating part of the sail, which would then be used as a reflector to push back against the payload-bearing remainder of the sail (he even wrote a novel, Rocheworld, on this concept). On the other hand, coupling laser-beaming with a magsail for deceleration upon arrival seems a nice combination of technologies to accomplish the same thing.
Hi Jim Baerg;
Thanks for asking. Negative index of refraction materials as they are envisioned and being developed at Duke University of Chapel Hill North Carolina within the U.S. theoretically have the peculiar property that instead of being pushed by an incident beam of light, they are pulled by the incident beam of light. The Duke University website should give you a comprehensive summary of the work that is being done there. The U.S. military is looking into negative index materials for the purpose of the development and production of invisibility cloaks for field equipment and its soldiers.
Regarding the laws of thermodynamics, they are good principles, however, the creation of the universe in the Big Bang may effectively have been the ultimate free lunch. As a result, I kind of expect that laws we now take for granted in the context in which they are interpreted may be overthrown at some point and replaced with either more general rules or completely different principles.
Hi Jim Baerg;
I have a correction to make. Duke University is located in Durham North Carolina. At the following URL is a brief introduction to negative refractive index materials:
Another useful website is:
Note that I did a Google search on Negative Index of Refraction Materials and was overloaded with hits. I am most familiar with the Duke University site, however.
Our colleague and fellow commenter dad2059 here at tau zero brought to my attention some of his ideas related to using neutron beams for fueling interstellar space craft with energy.
His fascinating comments lead to consider the possibility of utilizing a huge on board supply of mass pellets which would be placed within a reaction chamber within as space craft or outboard of a space craft wherein as the space craft was accelerated to high gamma factors such as by matter antimatter rocket propulsion, interstellar ramjet, or beam energy, etc., a precisely timed and spaced pellet stream of lumps of matter that would be laid out in front of the ship would sequentially be oriented such that they would collide with the lumps of matter sequentially placed within a reaction and energy collection chamber. The idea here is to make use of the relative kinetic energy between the ship board mass lumps with that of the incoming lumps laid out ahead of the ship in interstellar space. For really high gamma factors, perhaps the pellet stream could be light years long and perhaps at a very distant future time, the pellet streams could be billions or more light years long.
One can imagine that a one gram ship based pellet colliding with a run way pellet wherein the ship and its deployed pellet are moving at a gamma factor of 1,000 would yield an energy release of the equivalent of 1 kilogram of mass converted into energy. For the pellets of the same rest mass wherein the ship and its deployed pellet are traveling at a gamma factor of a whopping 1,000,000, the collision energy yield would be equal to one metric ton converted into energy.
As one can see, the interaction energy of the ship board mass lumps and that of the pellet stream lumps asymptotically approaches infinity has the speed of the ship approaches C.
The caveat is extraction of a majority of, or the entirely of the collision interaction energy and converting this energy efficiently into ship based KE. The effects of the backward momentum of the collision debris would need to be overcome so that reaction products can be efficiently converted to ship KE. Some means would also need to be worked out to overcome CMBR, starlight, and baryonic matter induced drag and negating its effects.
A Brief History of Solar Sails
NASA Science News for July 31, 2008
Have you ever stared up at the night sky, felt a gentle breeze, and wished you could set sail for the stars? Get in line. Many great thinkers from history have had the same idea. This long-held fancy could soon become reality with one solar sail mission on the drawing board and another already on the launching pad, slated to blast off this summer.
FULL STORY at
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