This scientist claims that solar sails won't work. Check out the link (http://www.newscientist.com/news/news.jsp?id=ns99993895)

What do you think?

:eek:

The basic idea is right (solar sails can nt be considered to be perfect mirrors, and thermodynamic effects should be considered), but I think the conclusions are flawed.

Yes, the sail will absorb some photons, and heat up. The sail will also re-radiate heat according to its temperature. The sail will quickly reach equilibrium with the heat energy absorbed equalling the heat energy radiated. The sail will gain the momentum of each photon absorbed, it will gain twice the momentum of each photon reflected, and assuming re-radiated photons are radiated equally in all directions, they contribute nothing as far as momentum goes.



The refutation mentioning Crookes' radiometer is also flawed. It is true that your regular toy light-mill works by thermal creep around the edges of the vanes. This is because they don't have a great vaccuum. However, it was demonstrated as early as 1901 that a Crookes Radiometer with a very good vaccuum, virtually frictionless bearings, and coated vanes (to prevent outgassing) will spin as predicted by transfer of momentum from photons.

Go here: http://www.islandone.org/APC/Sails/00.html

Solar sails have been proven to work, solar light pressure has move satellites and has been used to control rotation on Mariner 10. laser experiments have lifted solar foils in a vacuum proving that light can move matter.

Unless I'm not reading the article right, Chuckster, what
Gold is saying is that solar-sails don't work the way some
have hypothesized (via reflection), not that they don't work.

WellCookedFetus ... They have not been proven to work.

:m: :cool: :m:

I'm sorry but the concept has been proven.

The 'concept' =/= A 'solar sail'.

As far as I know, the only attempt to deploy one failed.

:m: :cool: :m:

No the concept: propulsion via light direct… proven.

Either you are confusing 'solar wind' with sunlight, or
you have yet to post anything other than conjecture.

See: http://web.mit.edu/space/www/

:m: :cool: :m:

If one reads the original article (http://www.arxiv.org/html/physics/0306050) one notices that it is full of flaws. I wouldn't really be suprised if someone told me this is a joke, but it would be a really bad one. There is a lot of comments about this in today's slashdot here (http://science.slashdot.org/science/03/07/03/1230209.shtml?tid=134&tid=160).

Chagur,

http://www.space.com/news/lasersail_000301.html

http://highway2space.com/ast/presentations/3g_know.pdf

Having read the NewScientist article and now the 'original
article', I fail to see how you can say 'it is full of flaws'. Do
you have any specific 'flaw' in mind, myhr?

Are you referring to the JPL microwave-beamed experiment?
If so, actual solar sails are far from 'proven', WellCookedFetus.
Note: The JPL bit was done three years ago and nothing has
..........come of it.

See: http://www.jpl.nasa.gov/releases/2000/lasersail.html

:m: :cool: :m:

You're grasping for straws!

:m: :cool: :m:

and how is that so Chagur? I provide proof and you say it not so?
You fail to make a legitimist counter argument by providing nothing against my evidence, making you the one suffering form a straw man fallacy.

Actually, Chagur, in space the impulse imparted by the solar wind on satellites is much less than that of photon pressure from the sun.

The referred to JPL experiment used microwaves to 'lift'
the 'sail' and a high power laser to move it slightly ... That
is not 'proof of concept' by far.

You're still grasping, WellCookedFetus.

:m: :cool: :m:

No it proof entirely, light provided pressure, in space even the smallest pressure is propulsive. That’s total and perfect proof: if a light can push a piece of solar sail material in a lab, then the same thing can be replicated anyway, such as outer space with sunlight.

Hmm ... "first known measurements of laser photon thrust
performance" using "laser powers from 7.9 to 13.9 kilowatts"

Not exactly non-coherent 'sunlight', wouldn't you say?

:m: :cool: :m:

I take it then that measurement has been refined enough to
differentiate between the two. Earth shadow effect?

Last I knew, '98, they still weren't sure how much of the total
interaction (Earth albedo, IR distribution, molecular and photon
interaction) could be attributed solely to photon action.

:m: :cool: :m:

a square meter here at one AU get 1.4 Kwatts of sunlight, estimates are that at one Au a square kilometer of solar sail could provide 9 Newtons for thrust: it’s not much but in space with virtually no frictions you can get anything to accelerate with that much thrust.

This is actually the first time I have read anything that pretends to be serious physics, but which I have been able to spot at first sight that is wrong (normally it is more hidden, and people as me don't find them easily).

The first thing that I noticed is that he is using a concept of the maximum efficiency of a cyclic thermodynamic process (the Caront cycle) on a process which is obviously not cyclic.

Later asuming that the total energy of radiation irradiated on a black body is converted into kinetical energy is something no physicist would do. Most of the energy will of course heat the body.

But the best is still to come:
From a formal point of view, it is clear that one could not equate radiative momentum content with Newtonian momentum. Newtonian momentum is Mv, clearly a vector, while the momentum attributed to radiation is E/c, a scalar, since E is a scalar and c is a universal constant of nature.

The magnitude of the momentum of the radiation is E/c, but it do indeed have a direction. This is basic electromagnetism, which I every physicist should know (and if there is some new revolutionary idea that known electromagnetism, quantum mechanics, etc. is wrong, he should state that clearly).

And in the and he asumes that one can define a temperature in the radiation field of the sun as the temperature a black body will reach in that point. This is only possible if the radiation is isotropic (that is: is equal in all directions). If you place to equal black plates in the radiation field, one with one face towards the sun and the other with the face parallell to the radiation, the latter one will get a lower temperature than the first, and the temperature will be impossible to define.

You can find more comments by people more competent than I among the slashdot comments (http://science.slashdot.org/science/03/07/03/1230209.shtml?tid=134&tid=160). (And even more flawed comments)

I don't claim that I could not be wrong or misunderstand something, but when it is this much and this obvious I think the probability for that is low.

Let's see ... Accepting your 9 Newtons/km^2 guesstimate, where are
you intending to deploy it so as not to have it fall back to Earth?

Or are you intending to lift it to L1? And, if so, why bother?

:m: :cool: :m:

Any stable orbit will do, like the Cosmos test this fall.

why don't you read my link:
http://www.islandone.org/APC/Sails/00.html

Did ... No mention of deployment altitude.

Cosmos 1 intended deployment is 800 Km.
*** An altitude of about 500 miles ***

:m: :cool: :m:

Checked out /. comments. Most interesting one was:
It is obvious that if he is at the Cornell Center for Radiophysics and Space Research, then he is not an actual physicist there. I actually suspect some ulterior motive behind this.
Checked out Cornell: http://people.cornell.edu/pages/tg21/vita.html

Need I say more?

:m: :cool: :m:

I would point out that this is one person's theoretical conjecture. There have been a lot of people that have looked at the theory since it's proposal a few decades ago - and while it doesn't prove his statements wrong, it does set a pretty high hurdle for his assertion to get over.

The cosmos-1 experiment later this year should give definitive data one way or the other anyway. Hopefully.

As far as I know, the test is only to verify deployment.

Or am I mistaken?

:m: :cool: :m:

Chagur,
As stated on the Cosmos-1 website (http://www.planetary.org/solarsail/index2.html):WHAT: A 30-meter diameter sail, configured in 8 triangular blades and deployed by inflatable tubes from a central spacecraft at the hub. The 100-kilogram spacecraft will be launched by Volna, a submarine-launched converted ICBM, into a 800-kilometer circular, near-polar orbit of Earth.

WHERE: The submarine launch will be from the Barents Sea north of Murmansk. The spacecraft will be operated from the Babakin Space Center near Moscow. Telemetry data will be received in Russia and in the United States.

WHEN: The mission is planned for mid 2003.

HOW: The solar sail will use the pressure of sunlight to increase its orbital energy and raise its orbital altitude. The sail is controlled by pitching the blades, thus turning the direction of the solar force. The inflatable tubes keep the sail rigid.

WHY: To conduct the first solar sail flight and demonstrate the technique for traveling between planets -- and someday, to the stars.

http://www.planetary.org/solarsail/mission_design_images/Slide1.jpg

This was done July 20, 2001 - but the capsule didn't seperate from the rocket as planned, and was lost as a result. However, they're not planning on repeating the deployment sub-orbital test and are moving straight to phase two:

http://www.planetary.org/solarsail/mission_design_images/Slide3.jpg

Thanks much.

Figured that due to past failures when attempting to test deployment
they'd be doing a repeat. Should have realized from the altitude that
there'd more to it than just a deployment test.

:m: :cool: :m:

The failures were do to a problem with the missile rather then the project.

Hopefully the same failure won't recur this time...

Solar sails..hmm.
They do work, but have limits on their performance, which may illustrate something. A few points :

1. The sun looks like a bright-ish star around the orbit of Pluto/Charon.
2. The total amount of kinetic energy (from visible photons, radio/x-ray frequencies etc, forget high energy particles for now) absorbed by the earth in one year (I think that’s the time period I remember) from ALL the stars, excluding the sun, is equivalent to one snowflake hitting the ground. I was told this around '99, I am assuming its still about right. This EM radiation, comes from all directions at the same time.
3. Manufacturing matter (quarks etc) from EM frequencies is still considered 'tricky'.. Fermilab may give you some pointers.. I believe you can get their Director's direct-dial somewhere in Paul W. Dixon's post 'Supernova from…' Hehe.

The significant 'force' on the sail around the inner-outer solar system is 'solar wind'. Mass carrying particles (alphas etc, not photons) smack into the sail and drive it in the opposite direction to the sun. Another contribution to this is heating by photons. As the electrons get excited. They may excitedly escape orbit (or however they travel) and fly away, creating thrust. This does not necessarily mean in the opposite direction of the strike (depends on the crystal system of the element composing the sail). Many will go in the wrong direction and slow it down. From this I ascertain (wrongly?) that the 'electrostatic propulsion' effects will be pretty negligible. Crookes radiometer works on mass particles also, one side (black) heats rarefied gases, speeds up and creates thrust. In a Perfect vacuum, it should just sit there. Luckily space in not perfect.

Over time the sail will lose mass, quickly in the inner solar system. The mass carrying particles will make lots of holes in the sail, reducing its efficiency but also making it easier to push. Problem is there is less surface to hit..

Once it passes the outer solar system, its thrust will approach zero. It is now possibly moving at a small, but significant fraction of c. As it moves past the Oort cloud it will be pushed fairly evenly in all directions by celestial radiation. It will have fun getting past the Oort cloud in one piece however. Anything it hits at this speed will go straight through, or, go off like a small nuclear device as the sails atoms undergo nuclear fission upon contact. This kinda depends on how close to c its going. If it hits enough particles (they are pretty rarefied..) it will slow down gradually and float around quadrant like tissue paper in wind. One that is being continually eroded by solar particle strikes.

I'm reaching here, but there is a possibility is could move faster. This depends on the speed of the sail. The minute nuclear explosions, if the timing is right and the atom loses it just behind the sail, it will push it forward (like a ramjet) but not much. This is all pretty irrelevant however as any significant amount of this, say a grain of sand, will spell 'Instant nuclear death' for the probe module… Unless it has an EM-field 'deflector'.

So basically, the sails will have to be released. Which is probably what they plan to do…

It needs a second sail to stop.

'According to the philosopher Ly Tin Wheedle, chaos is found in great abundance wherever order is being sought. It always defeats order, because it is better organised.'
Terry Pratchett.

Lasers have already propeled small objects.

So if high light energy in a small area can propell something. Then low energy on a large area should be able to propell something.

The same should go for solar sails.

Maybe a laser from earth if targeted right could also propell a space craft.:)

Why bother with solar light sails when you can use a laser to accelerate the craft right out to Pluto?

Also worth looking at are magsails (http://www.islandone.org/APC/Sails/03.html)- no physical framework required, although the sail might not work near the magnetosphere of a planet,
and there is the same problem of 'quenching' the superconductors that you get in fusion reactors.

---
As far as Tom Gold is concerned, he seems to be a controversial figure- the Russians certainly don't like him, as they accuse him of stealing their ideas on deep subterranean petrology-

which is ok. because I have been stealing ideas off Tom Gold for deep subterranean alien life :)http://www.orionsarm.com/xenos/Rheolithoids.html
_________________
SF worldbuilding at
http://www.orionsarm.com/main.html

"What we're proposing to do is create a magnetic bubble to deflect the solar wind."
http://science.nasa.gov/newhome/headlines/prop19aug99_1.htm
By far my personal favourite for the last few years.

Sails are mechanically very fragile, cause problems with collisions (unlikely, but I'd prefer to be a smaller target..). The 'magnetic bubble' system is easier to achieve, and may provide some shielding for the crew, similar to the Earth's magnetosphere. It is also fully adjustable, which will be very important during solar storms. There may also be possibilities for generating electrical power as a side effect...

BTW dinokg I did not mean say I think 'photon pressure' does not exist :) The law of conservation of momentum is nicely explained here :

http://science.nasa.gov/newhome/headlines/prop19aug99_1.htm

I just disagree with estimates on how much of the solar wind will pass through the sail. I was hoping for a good argument... I have some concerns about the JPL's experiment (laser pushing lightweight target). This was conducted in 'rarefied atmosphere' not a perfect vacuum. It may well have moved due to photon pressure, just it seems most of the pressure may be due to the 'Crookes radiometer' effect in the rarefied atmosphere. Which may make the results misleading…

I really don't want to see a hulking great laser in orbit… too tempting…anyone seen 'Real Genius'? :eek:


'I get me brain medicines from the National Health' Future Lisa's butler, the Simpsons.

I really don't want to see a hulking great laser in orbit… too tempting…anyone seen 'Real Genius'?
I'll see your 'Real Genius' (great way to make popcorn, btw) and raise you a 'Deep Impact' and an 'Armageddon' and an 'Asteroid' (gach!).

Large space-based lasers are too useful a tool to not have to be honest...

Regardless of Gold's qualifications, his arguments are not very convincing.

His main dispute seems to be that a perfect mirror would not work. This isn't much as a problem, because we don't have 'perfect mirrors'. It is completely possible for some photons in a stream to be absorbed... and impart their energy to the mass.

Photon pressure has been proven... and Gold doesn't do much to dispute this. A solar sail just takes the solar wind and photon pressure on a larger scale to use as propulsion.

Originally posted by EI_Sparks
Large space-based lasers are too useful a tool to not have to be honest...

Yeah.. its just the tool who is directing them worries me more than a bad case of 'Asteroids'.

See your american scijunk and raise you Day of the Triffids. Stevie Wonder as our leader...

Lasers have already propeled small objects. If your refering to the experiments I'm thinking of, it worked by flashing the air into plasma, producing thrust. While this would work in orbit, you would have to carry a fuel to plasmatize, pretty much defeating the purpose of leaving the fuel behind. And th esize and enrgy requirements of the laser...:eek:

Maybe he is referring to the ones I and others posted above, in which light pressure is actually demonstrated!

Laser powered air engines would need a 30-40Gwatt laser to propel as space shuttle massed vehicle into orbit, it would cost only a few hundred dollars in electricity and require only 1/20 the amount of fuel needed by a rocket, since most of it flight it would use air as the propellant, when it needs to finish orbital insertion it would need fuel (because of a lack of air duh) this fuel can be anything, most likely water or compressed/liquefied air. Now getting a 30-40Gwatt laser array that’s is not going to come cheap (say about 15 nuclear power plants!)

http://www.islandone.org/APC/Beamed/06.html

And you have to watch out for induced weather effects (oooh! obscure sci-fi reference :D ).

I still think an elevator is the best solution though. Free interplanetary launchings, minimal energy costs to get something to orbit, and the best rate of launchs going.

ya making a elevator to space will cost several thousand times as much as a laser/maser space launching system, and I already mention how much that would cost and its not peanuts!

Not to mention the catastrophic things that would go wrong if a space elevator fell.

Originally posted by WellCookedFetus
Not to mention the catastrophic things that would go wrong if a space elevator fell.

it would just wrap itself around the globe a couple times... no big deal, some people die, meh, all in the name of human progress, and it'll help with the survival of our species, i think it's worth it

ya making a elevator to space will cost several thousand times as much as a laser/maser space launching system,
Depends on how you make it.

Not to mention the catastrophic things that would go wrong if a space elevator fell.
Yes, true enough. Well, that's it so. Let's cancel the idea because of terrorism. And let's get rid of aircraft for the same reason. And cars (car bombs), bikes (bicycle bombs), litter bins (litter bin bombs), the postal system (mail bombs and postal workers), firearms, explosives, knives, rocks, sticks, ....
:)

woow you two no slippery slope fallacies here.

please tell me of how your going to make a cheap space elevator?

Originally posted by WellCookedFetus
please tell me of how your going to make a cheap space elevator?

there was an article in Wired (http://www.wired.com), i forget when, which a guy who specializes in nano tech said that he could do it for something like 10 billion, using carbon nanotubes, which he has been developing.

Okay, i found the article (http://www.wired.com/news/technology/0,1282,57536,00.html)

Not to mention the catastrophic things that would go wrong if a space elevator fell.

To be honest, I would worry more if it lost its anchor or its attachtment to the earth. You would then have a floating mutli-ton mass going around the earth hitting the high spots. Sooner or later it would create instability and no telling where the thing would come down at.

To make matters worse with the terrorism aspect, it would have to be a supreme target for them. What else would create such a mass of terror short of the sun dying or an equal disaster?

To add to the mess, it would most probably be a source of lightning without a ground to bleed off the charges build by air currents.

As for costs, it will be most prohibitive. Until the cost of carbon nanotubes comes down, until our presence in space is of a more regular and longer duration, and our capabilities extended beyond what is now within our ability to accomplish the space elevator will remain a dream.

I might add that we have no experience in moving asteroids into orbits, how to go about making nanotube structures of any size, or even how to build a factory capable of mass manufacturing nanotube structures on the fly and in opposing directions at the same time.

Originally posted by wet1
To be honest, I would worry more if it lost its anchor or its attachtment to the earth.

It's in geosynchornous orbit

To be honest, I would worry more if it lost its anchor or its attachtment to the earth. You would then have a floating mutli-ton mass going around the earth hitting the high spots. Sooner or later it would create instability and no telling where the thing would come down at.
That's not how the orbital mechanics work. destroy the anchor, and the rest of the cable pretty much just sits there stably.

As to the "falling cable" bit, this I'm guessing, comes from Kim Robinson's Red Mars. Scary images, but remember who brought the elevator down in the book - it wasn't a terrorist attack and it had to be done in a specific manner requiring that the people who built it build in demolition charges and subvert the entire structure and program.

To make matters worse with the terrorism aspect, it would have to be a supreme target for them. What else would create such a mass of terror short of the sun dying or an equal disaster?
Little, if nothing, other than maybe (depending on which terrorists you mean), Tel Aviv, Mecca, Jerusalem, Washington DC, Moscow, Belfast, London, Paris...

It's in geosynchornous orbit

Err, not quite.


The Space Elevator concept is a structure extending from the surface of the Earth to geo-stationary Earth orbit (GEO). Its center of mass is at GEO such that the entire structure orbits the Earth in sync with the Earth’s rotation maintaining a stationary position over its base attachment at the equator. It is envisioned that such a structure would be used as a mass transportation system in the latter part of the 21st century for transporting people, payloads, gasses and power between Earth and space.

http://flightprojects.msfc.nasa.gov/fd02_elev.html

You see the mass, not the structure is in geosynchornous orbit. (remember that this link presented has been "dumbed down" for Joe Blow Public to get the picture) In order to ensure tension, it will actually be a bit beyond and depend on the anchor to hold it fixed to the ground and an asteroid for an anchor at the other end. If you remove the anchor you upset the stablilty which is calculated for the GEO spot. Center then shifts without the anchor to tie the end down. This creates an instablility in the orbit which will most likely at the start be an up and down movement as it amplifies the instability throughout the structure.

The thought of this falling of the elevator did not come from the Red Planet as I never read that. It did come from one of Clarkes earlier works dealing with the setting of the anchor and also what would happen if that anchoring was not successful on the first try. For the life of me I can not remember the title.

The paragraph you posted stated that it would mantain it's position, due to it's orbit... So that implies it needs no anchor

Suggest you read the link then. It will not stay there without an asteriod for a counter weight or some simular mass..

The very next paragraph states...

Large reels with high strength cables work back and forth to provide adjustments to the position of the geo station and an asteroid counter weight above (not shown) to maintain the center of mass for the entire structure at a geo-stationary point above the Earth. Research into the feasibility of space elevators indicates that there are Five Key Technologies for Future Space Elevator Development

Alright, but if you read my link :P you'll see the one I was speaking of, for 10 billion, requires a counterweight in geo, so thats the one I was basing my argument on

wet1,
That's not how you build a space elevator. You put an asteroid in geosynch orbit, then begin manufacturing the cable on the asteroid, and extrude it in both directions - towards the earth and away from it - simultaenously. End result is a long cable with no "counterweight" other than it's own distributed mass. And this means that you can drop cargo off at any point up to geosynch orbit - or let it fly off the far end like a skater playing crack-the-whip and get to explore the rest of the solar system with little more than a fancy stopwatch to time the release.

wow are we all of subject or what?... and no 10 billion $ to make this thing is not going to cut it unless these was the year 3003.

A sky elevator
1. Your going to need to move in an asteroid anchor into a geo-synch. orbit
2. Your going to need to build a 25km tall anchoring tower somewhere on the equator.
3. Your going to need 40,000+ km long carbon nano-fiber wire constructed in space.

It would take more then a day to get up to the top even if you were going at mach 1!

A Laser/Maser luach port
1. 30-40 Gwatt nuclear power plants
2. A equivalent laser or maser array

The laser array can power earth-to-orbit shuttles, or propel solar sails anywhere within the inner solar system.

How much for all that?

10 billion $ to the nano-tubing (the cheapest part) as claimed above, grabbing an asteroid and moving it in to geo-synch orbit will cost much much more, as well as building a 25km height tower. The total cost would be beyond a trillion dollars and construction would take many decades.

laser/maser luanch port system would cost:

40Gwatts in pepple bed nuclear reactors: 40 billion dollars
40Gwatt IR laser array: 5-6Billion dollars
or
40Gwatt Maser array: 1-2 billion dollars

the technology is all proven all that is needed is to make it big enough. It would take less then one decade to make if .

Originally posted by Automan
Crookes radiometer works on mass particles also, one side (black) heats rarefied gases, speeds up and creates thrust.

The mechanism you describe does not produce thrust in a closed system. Crooke's radiometer in a poor vaccuum works by an edge effect where the heated gas on the black side of the vanes 'creeps' around the edge of the vane to the light side. The radiometer turns with the light side of the vanes leading.

In a Perfect vacuum, it should just sit there.

Nope. A Crookes radiometer in a very good vaccuum with very low friction bearings and coated to prevent outgassing works by photon pressure, turning with the dark side of the vanes leading. This was demonstrated as early as 1901.

Pete, you are you are quite right, I did misrepresent the 'Crookes' Radiometer' in my first post.

At the time I was aware it would not work in this manner in a closed system. Space is not a closed system however, and there should be some thrust imparted as described. Unless an electrostatic field built up, when the gases/micro-impact debris etc might well be trapped in a closed system (bleeding around the edges). This would depend on whether the ionised particles from the Sun were balanced in charge. If it were not, the sail might start to curl back in on itself, (like a party blower) if the gas filled tubes deflated due to impacts (probably ways around that). This may become significant as the sail reaches the same relative speed to the solar wind, it may dissipate one the sail has reached its 'terminal velocity'. IF it starts to move faster than the solar wind, driven by photons, there should be pressure on the star facing side. I should have made that clear 'engage brain, before putting mouth in gear…' :D

I did not spot your post (page1), I was bored on the train and was surfing on my iPAQ (has one of the cute folding keyboards.. hmm techie heaven..;) but the screen size makes it easy to miss things. Further to your comment on its rotation, it would also start to spin the other direction as the temperature of the rarefied gases inverted (depending on the starting state).

I did not remember the stringent experiment, physics class was a while back... As I said in my second post, the maths do work for photon pressure. I also am suspicious of the tests to date. A theory (even if backed by convincing maths) is only a theory until it is proven. As far as I can tell from the low quality press data (any good source?), JPL's test had a hell of a lot of other influences available on it.

My main concerns relate to practicality and reliability of the solar sail. It can't comfortably a heavy cargo payload for mars runs etc. The mass of a cargo/crew module would require a VERY big sail for reasonable round trips. You would need shielding for organic material, which is very heavy.

Again shielding would be a natural side effect of the 'magnetic' bubble system. In addition to the much greater 'torque', by manipulating the EM field quickly (I am reaching..) it may be possible to move against the solar winds. It would be expensive on helium though. This would be similar to rowing a boat or turbine if you were being really clever!. While much heat would be generated by induction, it should be manageable. It's interesting that this system was though of years ago (long before '99) and can be done with 1970's technology (superconductors not strictly necessary). With solar panels and possibly an ability to recharge reaction mass gathered from the magnetic poles later??, a small ion engine would be a practical way to manoeuvre out of orbit.
I guess somebody wants an excuse to put big lasers in space, to make sails more practical.. arg! Big Space Lasers! I really must check the numbers, or I might start my first 'conspiracy theory' by accident! :bugeye:

This is too long, as I am on the train again…
I will check out http://www.niac.usra.edu/studies/ when I get some time to tighten up my opinions...

I am on the move, so I am quite slow to respond these days...

Chagur wrote:

Checked out Cornell: http://people.cornell.edu/pages/tg21/vita.html

Need I say more?

Well, yes please!

I had already checked that out before I wrote the fist time. I never doubted that Thomas Gold is actually a professor emeritus and working at "Cornell Center for Radiophysics and Space Research". I have also seen him referenced as the one who first suggested rotating neurton stars as the source of pulsars and he seems to be a well-known physicist (you will also find his name mentioned in Stepen Hawking's "A Brief History of Time").
The /.-comments stating that he is not are simply wrong.


So this leaves us with the question: Why would a well-known and acknowledged physicist write something which is not only wrong, but also demonstrates that the autor is not into basic physics? I can only speculate, and none of the following options seems probable to me:

He didn't write it. Someone else has submitted it to arXiv in his name. Unlikely because it appears as NewScientist actually spoke to him.
He is getting old (born in 1920) and is loosing the grip.
This is a joke on his part.
I and everyone else who finds this arguments don't understand the principles he is trying to communicate.
I (and everyone else critisizing it) don't know what we are talking about (or at least have some big gaps in our knowledge).


I am a bit curious and hope I will find out one day what the explication is.

And just to keep it clear: I am (and Gold is in his article) talking about the physical principles, not about the actual implementation.

http://www.sciforums.com/showthread.php?s=&threadid=25273

A while ago I wrote:

I am a bit curious and hope I will find out one day what the explication is.

(It should of course be explanation, I have spoken more Spanish than English lately.)

Looking at this (http://www.planetary.org/solarsail/ss_and_physics.html) it looks as if the second item in my list is the most likely explanation.

... I called Prof. Gold and discussed it with him. He has forgotten more physics than I ever knew, ...


Well, at 83 a lot of people have forgotten so much physics that they aren't able to write an article like this. At least he gave us a good excuse to discuss the physics behind the solar sails.

Well just to make note, the effect of propulsion by light would be different than solar wind or galaxtic wind, all three would effect the flight of a solar sail. solar wind is basically the same as cosmic or galaxtic wind( cosmic wind is different than galaxtic wind) thay contain particles which are ions that travel at fast speeds, ggenerally noting solar wind has particles that arrive with a energy about 10 to 30 MV, at thousands of miles per sec or hour, take your pick. the different winds are comprised manily of helium and hydrogen, but galaxtic and cosmic winds contain as well cosmic particles.
In our system in addtion to praticles the solar wind also contains light, Light has a directs a force equal to 4.2 lbs.
If the concept of just light is used for a solar sail rather than solar wind. the are pushed by the 4.2 pounds must not exeed the weight of the force of light, this surmizes that each mm or cm ect must be lighter than 4.2 lbs and the capsule equal to the difference in weight across the entire sail minus the weight of the sail.

Therefor a light sail is clearly workable, a solar wind sail even better.

the reaction of light and the sail is generally about 7 to 9 mph or simply equal to the normal reaction of chemistry for most chemicals, here in the reaction the spectrum emmissions of the sail will determine a rate of preformace.

As far as tempture effects a warm or heated body will exspand, and be attracted to the coolest point of space, this means that a heated body is always traveling to the coldest region of space, there for a umbrella type sail is the most effect solar sail.

As a body in space is attracted to the sun by gravity most orbits relate to velocity as a factor of stablity, the object of a sail is to operate without this velocity and to gain motion from the gravity of the sun, this requires a balance of the mass of the capsule, and the the 4.2 lbs directed by the force of light.


Dwayne D.L.Rabon