NASA-Supported Solar Sail Could Take Science to New Heights:
As NASA's exploration continues to push boundaries, a new solar sail concept selected by the agency for development toward a demonstration mission could carry science to new destinations.
The Diffractive Solar Sailing project was selected for Phase III study under the NASA Innovative Advanced Concepts (NIAC) program. Phase III aims to strategically transition NIAC concepts with the highest potential impact for NASA, other government agencies, or commercial partners.
"As we venture farther out into the cosmos than ever before, we'll need innovative, cutting-edge technologies to drive our missions," said NASA Administrator Bill Nelson. "The NASA Innovative Advanced Concepts program helps to unlock visionary ideas – like novel solar sails – and bring them closer to reality."
Like a sailboat using wind to cross the ocean, solar sails use the pressure exerted by sunlight to propel a craft through space. Existing reflective solar sail designs are typically very large and very thin, and they are limited by the direction of the sunlight, forcing tradeoffs between power and navigation. Diffractive lightsails would use small gratings embedded in thin films to take advantage of a property of light called diffraction, which causes light to spread out when it passes through a narrow opening. This would allow the spacecraft to make more efficient use of sunlight without sacrificing maneuverability.
[...] Work under Phase III will optimize the sail material and perform ground tests in support of this conceptual solar mission. Orbits passing over the Sun's north and south poles are difficult to achieve using conventional spacecraft propulsion. Lightweight diffractive lightsails, propelled by the constant pressure of sunlight, could place a constellation of science spacecraft in orbit around the Sun's poles to advance our understanding of the Sun and improve our space weather forecasting capabilities.
(Score: 2) by Immerman on Tuesday May 31 2022, @02:11PM (5 children)
They offer almost no information in the linked article, so I'm trying to picture how this would work.
The momentum available to any solar sail is directly proportional to the cross-sectional area facing the sun, while the thrust from a "traditional" (as if they've been around that long outside of theory...) solar sail is directly perpendicular to the sail. So the thrust falls to zero as the sail turns perpendicular to the sun.
I would assume the bending of light from diffraction would would mean that the emitted light would be at a larger angle from the sail's axis than the sun is, so that you could actually generate thrust directly perpendicular to the sun, or even towards it, which would dramatically increase the versatility of solar sails.
However, while that would be easy enough to do with traditional prism diffraction, that would come with a huge weight penalty from a much thicker sail. Meanwhile the small opening diffraction they mention would avoid that weight penalty - but my (very limited) understanding is that such diffraction spreads out light symmetrically from the straight-line path, so wouldn't have any net effect on the thrust vector of a solar sail.
Am I missing something? Or would you actually need something like a double-layer sail with the second layer being a mirror that deflects the portion of light bent the wrong way?
Ideas? Links to actual information?
(Score: 1, Informative) by Anonymous Coward on Tuesday May 31 2022, @03:25PM (1 child)
They want to build a diffractive membrane using metamaterial design, so you can design it to bend the light into any angle that you want, so essentially a big thin metalens. As you point out, there is not a lot of detail here, but you have to put it in perspective in that this is an NIAC project. It is selected for Phase III, however, so I think they're supposed to produce some actual sail material and test/verify it.
My understanding of metamaterials is somewhat limited. It does appear that they want to have an electrically active material where you can apply voltage and determine the angle of the diffraction. The metamaterial info that I'm aware is that it is pretty wavelength specific, so you'd pick what wavelengths you want to bend or "cloak." I don't know how much you can generalize it these days. Maybe you can design it so that the wavelength you optimize it at, you have complete control, but other wavelengths outside of that will generally behave the same, so maybe not go off into the same diffractive angle, but maybe in the general neighborhood.
Phase I was "get all the smart guys in the room and think up something clever" and Phase II was "this is the basic approach we want to take, and these are the reasons why." Phase III should be "ok, now I'm going to build a prototype in the lab and see if it does what it is supposed to do."
Here's a couple of earlier links referring to the Phase I and II solicitations (again, not with a whole lot of detail):
NASA to fund research on advanced solar sails [earthsky.org]
Diffractive Lightsails [nasa.gov]
(Score: 2) by hendrikboom on Tuesday May 31 2022, @04:26PM
One of the ways of building a steerable laser beam is to diffract it off a piezoelectric crystal. You adjust the spatial wavelength on the crystal surface to get different angles of diffraction. The spatial wavelength is adjusted by using radio-frequency sound along the surface.
None of this is likely to do on a material light enough to be a solar sail, but it's at least conceivable that such things can be done.
(Score: 0) by Anonymous Coward on Tuesday May 31 2022, @04:28PM
i think what they really need is a keel.
you can only sail "against the wind" or "go faster then the wind" if you have a keel.
but since outer-space is empty, the keel (*) invention will prolly be the break thru, not the sail. *shrug*
i guess, by then, finding a way to add a "outboard motor" will be easy.
(Score: 2) by inertnet on Tuesday May 31 2022, @09:04PM (1 child)
I'm wondering if prism diffraction itself will result in an almost perpendicular force.
(Score: 2) by Immerman on Wednesday June 01 2022, @04:00AM
Well, maybe not the diffraction itself, but the result of it. Ultimately all that matters is the direction the photon enters, and the direction it it leaves - conservation of momentum tells us that the difference in those vectors must be exactly equal to the thrust vector.
It seems like it should be possible - if all the sunlight hitting the prism gets bent by 90*, then you'd get thrust at 45*. Twist the sail 45* as well, and that thrust could be 90* from the sun. The only problem is that for a given mass of prism you could make a vastly larger simple-mirror sail that could generate a much larger perpendicular thrust. It would necessarily be combined with an upward thrust, but with both up-and-faster and up-and-slower in your toolbox, combined with the directional interactions of orbital dynamics, you can still accomplish pretty much any orbital modification you might wish to - it's just a bit less straightforward
(Score: 0) by Anonymous Coward on Tuesday May 31 2022, @10:28PM (4 children)
Science: "Photons have zero mass."
Also Science: "Photons impart momentum onto solar sails."
lol?
(Score: 0) by Anonymous Coward on Wednesday June 01 2022, @04:20AM
Not when you know science.
(Score: 2) by Immerman on Wednesday June 01 2022, @04:21AM (2 children)
Old math, easily verified.
Photons have zero *rest* mass - however like all massless particles they can't actually be at rest, and instead can *only* travel at the speed of causality (aka the speed of light).
It might be helpful in soothing the cognitive dissonance to consider the fact that everything with mass has it's relativistic mass approach infinity its speed approaches c. Apply that to a photon and you get a relativistic mass of zero*infinity, which is undefined - otherwise known as a sign that you need to approach the problem with a different technique in order to get a real answer.
I don't recall all the physics behind it, but use that different technique and the result is that every photon has a momentum equal to Planck's Constant divided by its wavelength.
In an unrelated mind-fark - since photons travel at light speed they experience infinite time dilation, so that from their reference frame they exist for zero duration, being emitted by one atom and absorbed by another in the same instant. Even if those atoms are billions of light-years apart, from the photon's perspective infinite Lorentz contraction of the rest of the universe shortens the distance between those atoms out of existence. Or as an alternative interpretation, it's travel path is its timelike dimension, while it remains stationary in its spacelike dimensions.
(Score: 0) by Anonymous Coward on Wednesday June 01 2022, @07:30PM (1 child)
i suppose one could also mount a big freakin' laser on the tail end of a rocket and once lit, scorch far away developing neolithic planets to a crisp to get some thrust to mars for that selfy?
on a more srsly side note, i have tried releasing a square mirror in a pool under water at different angles.
the resulting movement of the mirror, whilst sinking is not very uniform.
if we replace "water in pool" as parallel light rays, and take earths gravity as the light-force acting on the mirror?
i guess it's not a good analogue, since the mirror creates turbulence and there's no turbulence on the dark side of the solar sail?
on this matter, shadows and all that, i was wondering if there's a opposite to focusing light-rays to a point?
so like with the new space telescope, a parabola dish gathers far away, old light to a point. if we now draw a line from the far away point thru the focus point of the telescope and continue, maybe the line intersects with surface on earth? does this location exhibit some special properties or effects?
(Score: 2) by Immerman on Wednesday June 01 2022, @10:06PM
>mount a big freakin' laser
Absolutely. It's called a photon rocket, and has the advantage that it doesn't require any reaction mass, but the disadvantage that to get the same thrust it takes *radically* more energy than would be in your typical rocket fuel.
Dropping a mirror in water is not remotely analogous to solar sails. First off the thrust from any reflected light is so tiny that the fact that it's a mirror is basically irrelevant to its motion. And more significantly it's moving through water, so drag, etc. is all based on its current direction of motion, which is constantly changing. As for gravity - it works exactly he same in space as on Earth - people in the ISS still experience almost the same gravity as on Earth's surface, they just don't notice it because everything around them is falling at the same speed, and they're moving sideways so fast that the surface of the Earth curves away from them as fast as they're falling, so they fall in a circle forever (a.k.a. orbit).
A better (2D) analogy so a solar sail would be to stand on extremely slippery ice while holding a large rigid plate while somebody shoots you with a powerful water hose. Or just sail in a flat-bottom boat with no keel (the keel lets you get the water in on the action a lot more so that stuff like tacking is possible - but you can't build a vacuum-keel in space - there's nothing for it to push against)
Nope - no fancy properties for focusing light - aside from imaging, etc. Nothing thrust related. They do make both diverging and converging lenses and mirrors that scatter parallel/distant light outwards instead of focussing it to a point - but either way you're getting less thrust than if it bounced off a flat surface - the thrust from the (un)focused "sideways moving" portion of the light cancels out rather than doing anything useful.