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Solar Sail Will Work, says Planetary Society
degauss writes "In response to Cornell Physicist Thomas Gold's paper declaring the theroy behind solar sails flawed (previously mentioned in this Slashdot article), Louis Freedman, executive director of the Planetary Society (the organization behind the COSMOS project), has written a brief rebuttal to the claims in Dr. Gold's paper regarding the feasibility of solar sails for use as a method of transportation in space. He does not go in to detail with equations and such, but does give an overview of the reasons he believes Gold's hypothesis is incorrect."
The acceleration to get to 100 mph is what ?
100 years ?
So, rather than going through all the trouble of avoiding air molecules, so that differential thrust due to heating does not propel the spacecraft, why not use the differential thrust to your advantage? After all propulsions is what they want. If differential heating of air molecules is capable of producing motion, so be it. Why not use it instead of trying to avoid it.
An Indian-American Hindu committed to non-violent thought/speech/action alarmed by the global explosion of radical Islam
The interesting thing is that the Mercury orbiter that NASA launched (one of the pioneer series) used the pressure of sunlight on its solar panels, just like a solar sail would on the sail material, to give it a spin. That, IMO, gives the theory supporting solar sails working a whole lot more credibility.
Do you know why the road less traveled by is littered with the bones of the unwary?
who cares about some method of propulsion when it comes to space travel. not until we discover some type of yet to be disovered natural occuring phenomena will be able to travel with any meaning in this universe of ours.
what is the big deal of traveling thirty years to find yet another pile of dull looking rocks and funky barren planets. and find out wow our signals can travel that far.
The rebuttal is pretty interesting, it rests on a fairly simple principle:
When the photons are travelling towards the sail an observer at the light source will see a red shift (doppler effect at work here).
When the photons are travelling away from the sail an observer at the light source will see a blue shift.
Because the observer hasn't changed position the shift can be attributed to a change in energy, which must have gone into the sail (as the only thing the photons encountered, assuming a perfect vaccume) meaning that the increased KE of the sail breaks no laws of physics.
Beep beep.
a presigious US astronomer, wrote a paper in 1902 in which he concluded:
p 16 7y1977-78.pdf
"Flight by machines heavier than air," Simon Newcomb declared one day in 1902, "is unpractical and insignificant, if not utterly impossible."
His arguments were quite reasonable on the surface - Imagine a bird as a model. If you increase the size of the bird, the mass increases proportionally to the third power of its wingspan. But the surface area of the wings only increases proportionally to the square of its wingspan. Thus something much larger than a bird would never be able to fly, and all attempts to build heavier than air flying machines capable of carrying a human would prove futile.
Fortunately, the Wright brothers never read his paper, or at least never took him seriously.
About 40 years later it was argued by learned men that manned supersonic flight would never be achievable.
http://www.garfield.library.upenn.edu/essays/v3
Marconi wasn't formally educated, and he was laughed at for spending vast sums of money to send a radio signal across the Atlantic ocean. Any fool knows that radio waves couldn't penetrate the earth, and was limited to line of sight communication! Yet despite all logic, the damned fool contraption eventually worked. It was only later that they discovered the ionosphere could reflect certain frequencies back to earth.
Even great men of science make mistakes sometimes.
My rights don't need management.
in more speed then any conventional propelled rocket that you send in space.
The main practical difficulty I see is stopping. You can't slap propelled rockets on the ship to do the job; if you did, I would want to know you didn't propell you ship with that to begin with.
Maybe some fancy gravity trickery... deaccelerate as a star's gravity starts to whip you around. Other than that, I don't see how you could do it. I still don't see a use for these other than minute corrections in satellite trajectories. They're not directional, and the methods by which, say, sailboats can sail against the wind won't work here. Only way to slow yourself down is to stop sailing and let gravity do it's thing... a big problem when you're somewhere in space in which gravity isn't acting against you. Moreover, we wouldn't get very far away, because the force provided by sunlight diminishes exponentially as you move further away. And going towards another star wouldn't help, because you can't sail against the "wind" in this case (ship sails can because of how the wind will curve and press on the sail in a different direction than what it was originally travelling, which won't happen with light).
We're getting to the point where it will just take too long to go where we want to go, and eventually it's going to make us ask if we really can go there. I mean, hundreds of years later, who's going to care that a probe, unable to communicate with us, is careening somewhere past Neptune? As for people, don't hold your breath on this transporting us; it just takes too long. I don't know about you, but going to another planet wouldn't be worth most of my life, if not the whole thing and part of my children's. And I don't even want to hear this whole "Once we figure out how to go faster than light" garbage. You've all been taking Star Trek too seriously. Granted, people didn't believe we could fly, or the earth was round, blah blah blah. As we progress we do realize that things were wrong, but some things become more compellingly right as well. The speed of light, the fact that we can't exceed it and its correlation with time are what defines our reality. Sending something faster than light, AND slowing it back down without the obvious logical and physical laws getting in the way is impossible. Sending something as complicated and sensetive as an organism is absurd. Sending an organism as intricate as a person should be grounds for insanity.
Sorry, that just bothers me when geeks worldwide sit back after watching something on the Sci-Fi channel and think "Man, once we learn to warp... that'll be good times my friend", to which his friend replies "Affirmative" and starts taking readings on his platic light-up box... er, tricorder.
It's always the turtle that wins...
Unless there's a dog running after you. I got 5 bucks on the hare.
Now that I think of it, it really looks like it shouldn't work.
Assume there are two ideal parallel mirrors. If you send a ray exactly perpendicular to the mirrors, it would bounce between them indefinitely, depositing an infinite amount of energy!
And if you think of red shift, then I can construct a scenario in which the ray will bounce on an infinite number of (initially static) mirrors-just send it a bit diagonally.
OK.
The "equilibrium temperature" of any object in space, where the only dominant source of heat transfer is radiation, depends completely on the thermal surfaces of this object.
Earth's "equilibrium" temperature is about 300k, that's true. But for other spacecraft, its equilibrium tempreature depends on what kind of thermal surfaces the engineer stick on it. Most spacecraft aims for 300k, since the spacecraft's instruments are from Earth, and most "normal" components have their optimum operating temperature at 300k.
However, this does nto have to be so. You can put a big black box in space, with zero emissitivity, and it will reach a temperature between 6000k and 3k as an equilibrium.
IN fact for the solar sail, if it is a perfect mirror, it will actually reach equilibrium near 3k, since it does not absorb any energy from the Sun. THus it will radiate away as a blackbody until it equililbriates with the rest of the Universe, i.e. 3k.
hope this helps.
Mode (3) smart-aleck mode. Press * to return to main menu.
>The main practical difficulty I see is stopping. You can't slap propelled rockets on the ship to do the job; if you did, I would want to know you didn't propell you ship with that to begin with.
1. Angle the sail to oppose your orbital motion. In the extreme case, stop relative to the sun and fall inward.
2. Carry the rockets. At least you won't be incurring the cost of rockets to push the braking rockets out to where they're needed.
3. Aerobrake, as one of the sibling replies suggested.
4. Send a disposable mirror in front of you to concentrate and reflect light backwards onto your sail. This is the technique proposed for decelerating a laser-propelled sail.
And, I discovered that this reponse is also incorrect. Even Britannica still gives the incorrect explanation that I wrote above.
This is the right answer.
Something interesting you may try with your radiometer to prove to yourself that it is not light radiation propelling it is to cool the device. You'll discover that it turns the opposite direction.
If tits were wings it'd be flying around.