Slashdot Mirror
Solar Sail News and Upcoming JPL Missions
abkaiser writes "I had the opportunity to interview a supervisor at NASA's Jet Propulsion Laboratory. The JPL is putting together several missions utilizing solar sail technology. The interview and article detail where NASA and the JPL are in using solar sails for applications and research.You can read the article or skip ahead to the cool pictures of prototype and proposed solar sails. The article addresses NASA's JPL solar sail missions, but not other commercial or private projects like Cosmos 1."
The principle that was actually at work in the window-paddle was not, in fact, the conversion of the photons' momenta into kinetic energy of the paddle, which is the principle behind solar sails, but the black side of the paddle would absorb the photon, increase in temperature and warm the air just above the black surface. The air would then expand, causing the paddle to rotate.
I got the idea after visiting a friends house and seeing a strange ornament displayed in their window. It was a glass dome and inside were 4 paddles mounted cross wise (horizontally) on a vertical support. One side of each paddle was black and the other white (or silver, its hard to remember now).
That was a radiometer. http://images.google.com/images?&q=radiometer
I had been taught that the greater kinetic energy on the black side caused the rotation. I decided to break with local tradition and do some research before calling you an idiot and a Microsoft spy -- apparently we're both wrong.
What I'm listening to now on Pandora...
The real problem isn't that they went with the Russians, but that they went "bargain shopping". There's a reason why they got such a cheap ride (cheaper than the Russians normally offer with Proton or Soyuz): the Volna wasn't designed as a satellite launch vehicle. The USSR wants to get rid of its old ICBMs, and so undertook a program to convert them to satellite launch vehicles.
Soviet ICBM maintenance has been way underfunded (as previously mentioned, they don't want most of them), and so when you modify a poorly maintained launch vehicle, well... it's not too surprising if it fails. More simply, if you launch on any vehicle that doesn't have a very extensive flight record for the type of task that you want to use it for, you're taking a big risk.
You can't change that... by gettin' all... bendy.
Well, the sun is a thermonuclear explosion, so there is a lot of crud flying away from it. Some particles are quite massive and not as fast moving as photons. As I understand it, it is mainly this slower moving smoke that will be used by a sail.
Oh well, what the hell...
Actually, solar sails are pushed by photons. The force generated on a solar sail by the solar wind (all of the "crud") is about an order of magnitude lower than that generated by photon momentum transfer. OTOH, the proposed Mini-Magnetospheric Plasma Propulsion (M2P2) system does use a "magnetic sail" of sorts to obtain thrust from the solar wind.
There is one big detail these amateur scientists have not accounted for : space is full of stars. For any photon coming from the sun, there are millions that come from the exact opposite direction. Why do you think it's dark outside ? because those photons anihilate electromagnetically against each other.
You would be correct if, when you looked outside, the sky were uniformly grey - there would be equal amounts of light of all colors coming from all directions, resulting in zero net solar pressure.
However, if one looks around in space, one will rapidly see that there is a WHOLE LOT more light coming from the sun than from any other direction. Aside from everyday experience, one can prove this just by recalling that there are shadows on the moon's surface caused by sunlight. If all the light from other stars was large in comparison to the light of the sun, the Moon's surface would appear (from Earth and Neil Armstrong's P.O.V.) almost uniformly grey.
As for why the sky is dark, it has nothing to do with the photons of other stars interfering with one another. It has to do with dust and the incomprehensibly vast distances between stars. The light from other stars appear as pinpricks of light because they are so far away. Stars that are really far produce weak light (recall light intensity decrease by 1/r^2 - or do you recall that, since you obviously know nothing about how physics and astronomy really work), and a lot of that light gets intercepted by dust long before it reaches our eyes.
So, in short, the myth that you speak of is itself a fallacy. There is a tangible and harnessable force from the solar wing.
This Wiki Entry has a good explanation.
Solar sail craft are already in orbit, around the Earth or Sun. They don't fly directly toward or away from the sun; instead, the sail can be angled to push the craft's orbit higher or lower, or shift from a terrestrial orbit to a solar one.
It's not too far removed in principle from how sailing ships can sail into the wind.
Live simply, that others may simply live. -Gandhi
Caltech solar sail enthusiast's list of Books about Solar Sails
...
Project: Solar Sail - editor: Arthur C. Clarke, managing editor: David Brin.
Penguin Books, 1990. ISBN: 0-451-45002-7
A collection of essays and short stories about solar sails. This book was part of a fund-raising effort for the World Space Foundation....
Locus describes this as:
Project Solar Sail ed. Arthur C. Clarke (NAL/Roc 0-451-45002-7, Apr '90 [Mar '90], $4.50, 246pp, pb); Anthology of seven stories, three originals, featuring solar sails plus five essays, four poems, and introductions by Arthur C. Clarke and Isaac Asimov. The profits from this anthology are to go to the World Space Foundation to help fund a solar sail project.
1 Foreword: The Winds of Space Arthur C. Clarke fw
3 Introduction: Sailing the Void Isaac Asimov in
9 The Wind from the Sun ["Sunjammer"] Arthur C. Clarke nv Boys' Life Mar '64
33 To Sail Beyond the Sun (A Luminous Collage) Ray Bradbury & Jonathan V. Post pm *
41 The Canvas of the Night K. Eric Drexler ar *
53 Ice Pilot David Brin ss *
67 A Solar Privateer Jonathan Eberhart pm, 1981
69 Sunjammer Poul Anderson nv Analog Apr '64
95 A Rebel Technology Comes Alive Chauncey Uphoff & Jonathan V. Post ar *
105 Argosies of Magic Sails--Excerpts from "Locksley Hall" Alfred Tennyson, Lord pm
107 Ion Propulsion: The Solar Sail's Competition for Access to the Solar System Bryan Palaszewski ar *
115 The Grand Tour Charles Sheffield ss Analog May '87; as "Grand Tour"
137 Lightsail Scott E. Green pm *
139 Rescue at L-5 Kevin J. Anderson & Doug Beason ss *
153 Lightsails to the Stars Dr. Robert L. Forward & Joel Davis ar *
163 The Fourth Profession Larry Niven nv Quark #4, ed. Samuel R. Delany & Marilyn Hacker, Paperback Library, 1971
219 Goodnight, Children Joe Clifford Faust ss *
231 Solar Sails in an Interplanetary Economy Robert L. Staehle & Louis Friedman ar
245 Afterword Arthur C. Clarke aw
Jonathan Vos Post, former Adjunct Professor of Astronomy, Cypress College
Except...from the bottom of the above linked web page:
:-)
By the way. It is possible to measure radiation pressure using a more refined apparatus. To make it work you have to use a much better vacuum, suspend the vanes from fine fibers and coat the vanes with an inert glass to prevent out-gassing. When you succeed the vanes are deflected the other way as predicted by Maxwell. The experiment is very difficult but was first done successfully in 1901 by Pyotr Lebedev and also by Eenest Nichols and Gordon Hull.
So...MOST light mills require an atmosphere.
If a photon bounces off the sail rather than being absorbed by it, twice as much momentum is imparted. Mirrored sails are the way to go.
JPL never really had anything to do with jets as we know them today. My understanding is that when it was founded by some Caltech faculty and students in the 40's to do rocket research, "rocket" was kind of a dirty word due to lingering memories of German rockets in WWII. Rockets were also commonly called jets until the mid to late 40's. One of JPL's first successes was the development of JATO technology, which stands for "Jet-Assisted Take Off", even though it is really the use of rockets to help large planes take off on short runways.
JPL was transferred to NASA in the 50's and built Explorer I, the US's answer to Sputnik. Since then, JPL has been the prime NASA center for unmanned spacecraft, mostly sent beyond Earth orbit. Since most JPL missions travel pretty far into space, solar sails are a natural thing to be researching.
Still trying to think of a clever sig...
Oh for goodness sake!
7 .cfm
"The photons hitting the black side of the vanes will be absorbed transferring their momentum to the vane. Those hitting the white surface will be reflected transferring up to TWICE their momentum to the vanes.
1) In a vacuum: The above concept dominates and the white vanes trail the black vanes.
2) In a poor vacuum: the air on the black side of the vane gets heated and the air molecules give an extra "kick" to the black vane side overriding the photon momentum transfer causing the black vanes to trail. I.e. the air molecules transfer more momentum to the vane than the photons do."
Answered by: Pete Karpius, Physics Grad Student, UNH, Durham
from: http://www.physlink.com/Education/AskExperts/ae67
Now if the vacumm version was in space with zero gravity and NOT fixed (like a giant paddlewheel in space), then the vanes would spin indefinitely and be propelled in a straight Newtonian line away from light sources. If there are a few light sources, (eg The Sun, reflection of the Moon etc), then you can use force vectors to plot its course and modify them with shades
Now, if you apply that to a lightsail ship, it would be possible to spin the craft as well. If it were large enough, then the spinning would create artificial gravity for its occupants.
There are other ways that rotary motion can be employed within the ship itself.
Don't be apathetic. Procrastinate!
Well, I don't believe anyone has tested an actual sail in space yet. The physics is fairly straightforward though, we know how momentum exchange should work with photons and solid particles, of course. We have a pretty good handle on photon and charged particle flux from the sun. Radiation pressure has been demonstrated in the laboratory.
Let's see... the Planetary Society says that solar wind contributes less than 1% (http://www.planetary.org/solarsail/faqs.html).
There have also been laboratory experiments with simulated solar wind and photon pressure. So yes, the solar wind does contribute something, though not much. Which, I believe, is what I said. Note also that the line you quoted, while literally correct, is also taken out of context. The original poster claimed that solar sails fly on the solar wind. I disagreed, but instead of saying "you idiot, you read too much science fiction" (sort of like you did), my reply was more along the lines of "you're right, the solar wind does contribute, but photon pressure provides most of the energy."