Nanophotonic light sails may travel at relativistic speeds

September 6, 2018 by Lisa Zyga, Phys.org feature
A light sail requires multiband functionality: high reflectivity in the near-infrared range for propulsion and high emissivity in the thermal (mid-) infrared range for cooling. Credit: Ilic et al. ©2018 American Chemical Society

One day in the not-so-distant future, light sails may hurtle through space at speeds of around 20% of the speed of light (or 60,000 km/sec), propelled not by fuel but rather by the radiation pressure from high-power lasers on Earth. Traveling at these relativistic speeds, laser-powered light sails could reach our nearest neighboring star (other than the Sun), Alpha Centauri, or the nearest known potentially habitable planet, Proxima Centauri b, in about 20 years. Both objects are a little more than four light-years away.

Designing sails is a major engineering challenge, however, requiring conflicting features that sound nearly impossible: an ideal light sail should be several meters wide and mechanically robust enough to withstand intense , yet be merely 100 nanometers or so thick and weigh just a few grams.

Further requirements arise from the mechanism by which light sails work. According to Maxwell's equations, light has momentum and as a result can exert pressure on objects. However, light sails are not simply pushed by pressure like a sailboat is pushed by the wind. Instead, the push results from the light sail reflecting the radiation. As a result, an optimal sail should reflect the majority of the radiation in the laser beam's near-infrared spectrum, while simultaneously emitting radiation in the mid-infrared range for efficient radiative cooling.

Nanophotonic sails

In a new study published in Nano Letters, researchers Ognjen Ilic, Cora Went, and Harry Atwater at the California Institute of Technology, Pasadena, have shown that nanophotonic structures may have the potential to meet the stringent material requirements for light sails capable of traveling at .

Previous light sail designs have used materials such as ultrathin aluminum, various polymers, and carbon fiber. Unlike these materials, nanophotonic structures have the ability to manipulate light at subwavelength scales, giving them an advantage in addressing the simultaneous requirements of efficient propulsion (reflection) and thermal management (emission). As an example, the researchers showed that a two-layer stack of silicon and silica shows promise due to the combined properties of both materials. Whereas silicon has a large refractive index—which corresponds to efficient propulsion—but a poor cooling ability, silica has good radiative cooling properties but a smaller refractive index.

In their paper, the researchers also proposed a new figure of merit that measures the trade-off between achieving a low sail mass and a high reflectivity. In the future, this concept will help to minimize constraints on the laser power and the size of the .

Background on light sails

Although conceptualized for nearly a century, only in the past few decades has technology caught up to scientists' early visions of propelling a spacecraft with the pressure of light. Inspired by the way that the Sun's radiation pushes a comet's tail in the opposite direction, the earliest concepts were of solar sails that use the radiation pressure from sunlight rather than from lasers.

The first solar sail was launched in 2010 by the Japan Aerospace Exploration Agency (JAXA) and successfully reached the orbit of Venus in six months, powered only by the radiation pressure of sunlight. Now researchers are working on designing solar sails capable of greater accelerations that are competitive with rocket acceleration, offering the possibility of launching spacecraft without the billion-dollar cost of conventional propellants.

Although solar sails may achieve rocket-like speeds, sunlight radiation is relatively weak compared to a high-power laser array. As a result, a laser array offers the potential for much faster propulsion, up to relativistic speeds—but more work is needed before such -powered sails are demonstrated.

Explore further: Researchers develop new solar sailing technology for NASA

More information: Ognjen Ilic et al. "Nanophotonic Heterostructures for Efficient Propulsion and Radiative Cooling of Relativistic Light Sails." Nano Letters. DOI: 10.1021/acs.nanolett.8b02035

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23 comments

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Old_C_Code
5 / 5 (4) Sep 06, 2018
Not bad, 20 years to get to Alpha Centauri .This will probably be the only way we 'travel' to other stars.
Gigel
3 / 5 (5) Sep 06, 2018
How do you point a laser on a light sail 100 meters across and 4 light-years away?
barakn
4.2 / 5 (11) Sep 06, 2018
You don't. The laser is only used when the light sail is a lot closer. The sail spends most of its time coasting.
carbon_unit
3.3 / 5 (3) Sep 06, 2018
I think they can eventually make the part about getting to the Centari systems in mere decades work. I just don't see how an object weighing a few grams will be able to report back across 4 LY. Basic transmitter power is one problem. The other is that any signal it sends will be lost in the radiation of the star. Maybe it can use gravity to bend its path as it goes through the system to get it away from the star as viewed from Earth, then report? (At .2 C, how much deflection could there be??) Solve the phone-home problem and this could really be be something.
Old_C_Code
3.3 / 5 (3) Sep 06, 2018
Electronics weighing a few grams can still send a massive signal, using solar from a near by star.
holoman
1.2 / 5 (5) Sep 06, 2018
Totally missed the most mis-understood phenomenon in space for travel, solitons.

Solitons in space plasmas have tremendous possibilities.

https://aip.scita...Code=php
Dark_Solar
3 / 5 (3) Sep 06, 2018
Is anyone accounting for erosion? .2C+interstellar dust x 4LY
TheGhostofOtto1923
2.6 / 5 (5) Sep 06, 2018
Is anyone accounting for erosion? .2C+interstellar dust x 4LY
And you think that's something no one thought of but you? Why not do a little research and find out for yourself instead of asking others to do it for you?
Bryan_Kelly
3 / 5 (2) Sep 06, 2018
IMO, the optimal propellantless propulsion using solar will be less of a sail and more of a photon pump-jet, absorbing and directionally emitting a wide spectrum. This would include heat, which would be pumped for radiation pressure propulsion and not merely dissipated. Stated simply, the photon pump-jet will pull light through mass, ideally including 3K CMB, like a Virginia-class sub pumps water. This might first be achieved by containing hydrogen in a material enabling its stimulation for light absorption and emission, e.g., graphene.
Thorium Boy
1 / 5 (4) Sep 07, 2018
They've been talking about these sails for decades, like "space elevators." Nothing has come of it. On the other hand, Project Orion has had at least sub-sized model proofing of concept. They should have built it. One day, a nation not hamstrung by political timidity will.
TopCat22
2.7 / 5 (3) Sep 07, 2018
I do not believe that a single human body will ever travel to another star system. Not a practical thing to do.it will take many generations.

humans can only expand by colonisation and in several million years of expanding an outpost may eventually settle near one. Six month worth of travel distance is about as long as would be reasonable for a human to go to or come back from.
alexander2468
not rated yet Sep 07, 2018
Not bad, 20 years to get to Alpha Centauri .This will probably be the only way we 'travel' to other stars.

"One day in the not-so-distant future, light sails may hurtle through space at speeds of around 20% of the speed of light"
Not bad, one day + 20 years

Benni
3 / 5 (2) Sep 07, 2018
The sail being proposed will be ripped to shreds by the kinetic energy impacts of interstellar micron-sized dust particles.

It is vaguely estimated within every cubic kilometer of space a dozen such particles are also whizzing through that volume of space at ANY moment in time also at relativistic speeds.

By the time the sail has traversed the distance of 4 lyrs nothing will remain of it, it might survive a fraction of a light year but after that will be so shredded as to be useless to bounce a laser beam off it.
TheGhostofOtto1923
1 / 5 (1) Sep 07, 2018
I do not believe that a single human body will ever travel to another star system
How about a partial human body? A human/machine hybrid designed for the purpose?

From here on we will become progressively more machine and less biology. Humans are already swapping out body parts and outsourcing memory and processing. This is the whole purpose of technology is it not? Externalizing evolution, making knives and spears rather than evolving talons and fangs, riding horses rather than evolving stronger legs, etc.

So now we can finally internalize it again.

Imagine a container designed for high G acceleration, hybernation, high flux radiation. A brain reengineered to tolerate and welcome isolation, extended sensory input and direct interface with the vessel it travels in, with all the parts not necessary for the mission removed.

Is this something that will ever make sense?
rrwillsj
not rated yet Sep 07, 2018
I'm not sure I can see the point of turning a human brain into a machine. Aside from the costs and the costs of the failures. This would delete the monkey-humaness of the condemned victim of "Mad Science".

Do you truly believe that you could ever convince the funding Public of the morality of playing at Dr. Frankenstein?

Wouldn't it be a lot simpler and comparatively less expensive to continue developing our computer-based "Artificial Stupids"?

As a partnership between Earth based Human futurist-planners and innovators. To provide the motivation, the reason, the sentience to guide and direct the purpose of the machines-in-space.
TopCat22
not rated yet Sep 07, 2018
the only way to go is to download a human brain as software onto a silicon chip designed to hold information for a billion years. You then send the chip to another star system and whenever it gets there the local new sun will provide energy to charge up the hardware and run the software brain on the silicon chip. That human consciousnesses will then observe upon waking up. The time that will have passed for that software brain will be zero time. It will be a blink of an eye and it arrives to see what there is to see. Although hundred of millions of year may have passes since its launch.

In fact there may be such hardware software consciousness on the way here from another system launched hundreds of millions of years ago by someone out there, who's life forms may be long ago extinct. It could arrive here tomorrow.
rrwillsj
not rated yet Sep 08, 2018
Wjy is it the "only way"?
What dubious purpose does it serve to go to send a single human recording off to voyage a hundred million years?

You underestimate the relentless damage that would accumulate to any physical machinery over that period of time.

If successful? So what? Data collected then what does your victim do with it? To whom would it report it's findings? By time the message arrives, it will be long obsolete.

As for "downloading" Human consciousness onto a chip? The messy, squishy biological brain is a lot more than a recording of coding.

This is the problem with "curing" senility. They may develop preventive medicines or even palliative methods to stop the progression of the damage?

But our minds are a tangled weave of biochemical chains influenced bu our life events. All our senses are involved. Disrupting those chains destroys the memory that helps to make up your personality.

Each bit of damage reduces your person. And that is irrevocable.
adave
not rated yet Sep 10, 2018
How to use an active light sail? Use the electrons streaming from the sun to make a free electron microwave laser. You could get your six billion watts as the intersteller device is one side of the cavity. Great boost from riding a solar flare. What about navigation?
rbanffy
5 / 5 (1) Sep 10, 2018
While I understand there is a huge limitation on payload for any probe like this (if we want it to be really fast), for solar system exploration it could make a lot of sense. If we have another, previous probe already orbiting the place like, for instance, Juno on Jupiter or the many probes orbiting Mars, we could send additional, cheap, instruments on flyby routes with enough communication capabilities to use the already deployed probe as a relay. Since it'd be guided from Earth, probe mass could be reduced to barely the instrument and PVs so that the same laser that propels it could power the instruments.
Anonym654101
not rated yet Sep 11, 2018
Why would you put it on the earth? It could be on the moon and have 24 hour sunlight for power and no losses from atmosphere. As mentioned above, the material of the sail would have to be self-healing so the inevitable punctures would be repaired as they occur.
Benni
3 / 5 (2) Sep 11, 2018
Why would you put it on the earth? It could be on the moon and have 24 hour sunlight for power and no losses from atmosphere. As mentioned above, the material of the sail would have to be self-healing so the inevitable punctures would be repaired as they occur.


As the SAIL is repeatedly hit/punctured by micron sized particles, it's trajectory will be slightly altered with each hit, thus the payload will need a self correcting navigation system, more mass as part of the payload therefore the longer it will take to get the velocity of the payload to relativistic speed.
granville583762
5 / 5 (2) Sep 11, 2018
What is actually blowing this sailing ship, photons or the solar wind, which has the greater momentum, if the sail has to more tenuous than a photon to sail on the photon wind, the solar wind will blow it to shreds especially as the solar wind is massive particles where as the photon wind are massless particles
Benni
2.3 / 5 (3) Sep 11, 2018
What is actually blowing this sailing ship, photons or the solar wind


".....propelled not by fuel but rather by the radiation pressure from high-power lasers on Earth. Traveling at these relativistic speeds, laser-powered light sails could reach our nearest neighboring star (other than the Sun), Alpha Centauri,"

I guess they propose a continual beam from a location on Earth. They are not forseeing what will happen to that sail once it starts getting hit with micron or even atom sized particles. Every such impact on the sail will impart kinetic energy that will affect trajectory as well as integrity of the sail which is a pretty good sized target. Add to this that the faster this is going the greater will be the kinetic energy impacts & eventually all the sail will become is a thousand fluttering ribbons a few feet long which will scatter the laser beam in every direction causing it to lose trajectory from the original course .

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