Solar Super-Sail Could Reach Mars in a Month

ti-coune sent us a story running on newscientist describing solar super sails and how they could one day get us to Mars in a month. The key is a special new paint. The cast of Trading Spaces is unavailable for comment.

Comments

And how does it slow down when its there?

[Viol8]

Or get back to earth for that matter? Nice idea as long as you don't mind a one way trip into deep space.

Re:And how does it slow down when its there?

[Angstroem]

Slow down: Rocket thrusters (mainly for maneuvering) and athmosphere.

Coming back: Send robotic missions do deliver necessary parts and prefabricated modules, then send human heroes to put everything together. If some part fails, they at least are heroes.

Mind you, exploration never included the guarantee of a safe way back. It always took some people to take the risk of losing their lives.

Because you can now safely travel over the Pacific Ocean in 5 hours doesn't mean it always was like that.

Re:And how does it slow down when its there?

[MagPulse]

They might be able to get the gas to emit from the other side, but some of the push is just from the momentum of the microwaves, so you'd actually want to ignore that.

Re:And how does it slow down when its there?

[tgd]

Not sure why that was modded insightful.

If you could carry enough rocket power to stop that speed, you could carry enough rocket power to build up that speed, and we wouldn't need exotic ground-based propulsion systems.

You can't use rockets to stop. In fact, the suggestion that you could use another array at Mars to stop seems unlikely because of the unlikely possibility that you could vaporize just the paint on the back side of the sail, and not cause the heat to vaporize the bottom layer of paint on the opposite side of the sail surface, dislodging it.

Re:And how does it slow down when its there?

[tgd]

That doesn't make sense.

Normally you need less fuel to stop a spacecraft than to get it moving because you have burned up half the fuel getting the craft moving. Since fuel is the majority of the weight, stopping a probe is a lot easier than starting it. In reality the fuel to stop a probe is still significant enough, NASA uses aerobraking or orbital tricks when possible to save that weight.

The problem becomes enormously worse if you use ground propulsion, because you need to store enough energy in the form of fuel to counteract the amount of energy you are pumping into it from the ground. Because you only need to carry half the fuel for a given speed (only needing fuel to stop, not start), your craft will weigh something aproximating half what it otherwise would've (since most of the weight would be fuel, the actual probe itself isn't meaningful). Given the craft weights half as much, you're probably going to get it going quite a bit faster, requiring more fuel to stop it.

The point is, you can't pump more energy into the craft from the ground than you happen to be carrying along with it to stop it, and that means there's a maximum amount to be gained using an overly complex system on the ground. And the speeds they're talking about are so far beyond what any chemically-propelled spacecraft move at, it doesn't make sense that you could reasonably use a chemical system to stop it. You may get a doubling in speed out of a spacecraft and still be able to stop it, but you won't get an order of magnitude increase in speed and still be able to stop it.

Re:Cast? What cast?

[Arimus]

You're in front of it - you'd be pushed along so it would make sense to put the sail at the back. I'd be more worried though about the effects if the craft turned unexpectedly and dropped your capsule into the microwave beam.

Why pipe microwaves from the surface?

[FreeUser]

From TFA:

Gregory Benford of the University of California, Irvine, and his brother James, who runs aerospace research firm Microwave Sciences in Lafayette, California, envisage beaming microwave energy up from Earth to boil off volatile molecules from a specially formulated paint applied to the sail. The recoil of the molecules as they streamed off the sail would give it a significant kick that would help the craft on its way.

Why does this seem incredibly wasteful of energy?

Wouldn't it be far wiser to build solar panels in orbit, use them to power Microwaves, and avoid the attenuation in the atmosphere? This would have the added advantage of not draining power from the Earth to power the spacecraft: we would get our power from the Sun and pipe it directly to the spacecraft as Microwaves, without involving the planet at all (except, of course, as controlling entity).

Re:Why pipe microwaves from the surface?

[PIPBoy3000]

Take a look at this article for way too much information about orbital solar power.

Basically microwaves can be transmitted through the atmosphere without too much trouble, while building stuff in orbit is incredibly expensive. It might be possible to create an orbital relay station to deal with issues like focus, assuming that's a problem.

Re:Why pipe microwaves from the surface?

[Waffle Iron]

Why does this seem incredibly wasteful of energy?

Wouldn't it be far wiser to build solar panels in orbit, use them to power Microwaves, and avoid the attenuation in the atmosphere?

Talk about penny wise, pound foolish. A 60 MW solar power station in orbit would be far larger than the International Space Station. It cost dozens of $Billions just to launch the space station; designing, launching and maintaining a 60MW station would probably cost hundreds of $Billions.

When you spend money on something, you're allocating a certain fraction of the economy towards a that purpose. That comes with a roughly similar fraction of the world's energy consumption. Dedicating huge teams of people designing, building and launching an orbital power platform will consume a commensurate amount of energy down here on this planet as they go about their tasks. Building the station and the massive rockets to launch it will consume vast energy resources before it even gets off the ground; vastly more energy than the station could ever produce.

For example, assume the station costs $100 Billion. That's about 1% of one year of the ~$10 Trillion US economy. The US consumes about 1e20 joules of energy per year, so if the money spent on the station is associated with a proportional amount of energy, that's 1e18 joules. That's more power than a 60MW power plant would produce in 500 years.

Then?

[intercodes]

powered by a solar sail get from Earth to Mars in just one month

Then what....!?

Re:Fuel

[will_die]

Once you get it into inital orbit payload weight would not be a problem, size would matter size it could not block the sails. Whatever the payload is I hope that you don't mind it smashing into the planet. They don't discuss slowing down or landing.
Going at 60 kilometres per second it is going to take a good amount of fuel and time to slow that thing down.

Does this take into account slowing down?

[Gorath99]

Not an astronomer or space engineer, but does this one month timeframe take the required slowing down into account? You can't just point a spaceship at something, shoot it away at 60 km/s and expect it to both stop at its destination and survive arrival.

And while we're at it: how does one slow down a craft like this? Without destroying it or tugging along a rediculous amount of fuel, that is.

Re:New Slashdot record!

[unitron]

Stupid means lacking in intelligence. Dumb does not mean lacking in intelligence, it means mute, i.e., incapable of speech. Not knowing this is not an indication of a lack of intelligence but of ignorance.

Re:Fuel

[clausiam]

Once you get it into inital orbit payload weight would not be a problem

2 words - Mass and Momentum. Sure the payload matters. Size wouldn't matter as the payload carrying part would be facing away from earth and would need to be protected from 6MW of microwawes anyway.

Amazing what you can do with math

[Ancient_Hacker]

One of their minor obstacles is going to be finding some way of heating up their reaction mass to fantastic temperatures while not simultaneously heating whatever is containing it. And forget about nano-tech. The basic laws of scale are working really hard against them. The volume being heated is miniscule, while the surface area is much larger in proportion, so it's effectively impossible to heat anything very small. Think of the smallest flame you've ever seen. You just can't make them any smaller.

Getting back

[nairolF]

I think the term "solar sail" is a bit of a misnomer here. If I understood correctly, almost all the thrust comes from the recoil of particles boiling off the surface, because the surface is heated by a microwave beam. This thrust is therefore perpendicular to the surface of the "sail", which is (largely) independent of the direction towards the microwave source. The exception is that, if the sail is parallel to the beam, then the microwaves don't hit the sail at all, and the system doesn't work.

But it would work perfectly well for travelling towards the microwave source (i.e. Earth), or, equivalently, for slowing down on the way to Mars: just have the paint on the other side of the sail, which gets heated from behind.

So one can imagine a craft which has two sails. The first is unfurled in Earth's orbit, with paint facing the Earth, which is used to kick it in a suitable direction to get it to Mars. After the microwave beam is turned off, the sail is discarded. Once the craft gets close to Mars, it unfurls a second sail, this time with paint on the other side, pointing towards Mars. Again a (extremely well focussed!) microwave beam from the Earth heats this sail from behind and the craft can slow down to safe speeds to land on Mars.

Of course, if your beam is so well focussed that you can use the sail near Mars, then you can use a much weaker beam for much longer to get up to the same (or greater) speed. This means you don't need a 60 megawatt beam at all - just use a 1 megawatt beam for 60 hours or whatever.

... It's not really ...

[ninjagin]

... a solar sail if you're beaming microwaves at a film with a coating that releases a gas, right?

For some reason I thought that solar sails captured photon pressure to accelerate an object by very very teeny tiny amounts over a long period of time.

As I read the article, they're still using the idea of a sail, but the acceleration comes from the release of gas. So isn't this a "gas sail"?

If it is a gas sail, then don't you have to worry about holes in the sail fabric/material? You're back to fluid pressure on a sail surface, aren't you?

It seems (admittedly, in my own uneducated, poorly-informed estimation) like the "gas sail" material would have to be more robust than with a solar sail.

Can someone clarify for me?

Re:Cast? What cast?

[Catbeller]

Nope. I believe they said that it would require ONE hour of focused microwave energy on the sail while still in low earth orbit to achieve Ludicrous Speed.
Then it coasts.

So, basically you build 20 2 MW transmitters and focus their output on a point a few hundred or thousand miles away -- I assume after an hour the craft will be moving away pretty damned quick, so a few thousand miles then.

How does this thing STOP? You make Mars, but what's slowing it down from 150 miles per second so that it'll achieve orbit? Atmospheric braking? Um, no, let that go - no airbraking, it'd vaporize. Even if it could withstand a 150 mps entry without puffing out, it'd punch out of the atmosphere in seconds, with no time to kill much speed. No rockets either -- can't carry enough fuel to kill 150 mps.

You'd need another microwave array in a high Martian orbit to fire at the solar sail as it came streaking in from Earth, if you want it to downspeed to make orbit. I'd assume the sail reverses somehow, so the craft comes in tail first.

Now. If you want a FAST vehicle, build a solar powered multi-megawatt laser at an LaGrange point, and use the nicely focused red laser on a solar sail. The craft'll be at Mars in, what, two weeks?

There's a couple of points that occur to me: the mass of the object being towed by the sail is irrelevant, mostly; you could tow the Sears Tower if you want. You'd just have to fire the lasers/microwaves for a longer time. A laser/purely reflective sail would be used for really heavy objects, and the gas-outing microwave system for smaller payloads, because the amount of paint on the sail is limited and will be exhausted, while a pure mirror-sail is static and can be used indefinitely.

Photons have zero mass

[FreeUser]

The laws of reaction still apply to the emiiter, but at these levels it should be negligable (most of the thrust on the other end is coming from the boiling gas).

Okay, I am not a physiscist, and its been 13 years or more since my last physics course, but ...

The thrust which isn't is due to electromagetic radiation, i.e. photons, which have no mass. The microwave transmitter won't have any back thrust, any more than a flashlight hanging in a perfect vacuum is going to produce "thrust" opposite the direction of the flashlight's beam.

The photons hit the sail, experience redshift as they reverse direction (thereby imparting some energy on the sail, which pushes it forward). They aren't particles with mass hitting the sail the way ions would be, or molecules of air in the wind against a sailboat. The exchange of energy is reletavistic (red shifting) IIRC, not Newtonian.

I don't believe a space based microwave transmitter will experience any thrust due to the emission of electromagnetic radiation, any more than a laser would if we were using Dr. Forward's solar sail design.

You're missing

[Kickasso]

something called "stored energy density". For rocket fuel it's X J/kg (look up a suitable value for X, I'm lazy). For microwaves it's exactly infinity J/kg because microwaves are not stored on board. There's some difference, eh?

people should read the important part.

[xutopia]

*could*

Re:Then what?

[MullerMn]

Uh, I'm no expert, but I'm pretty sure there's some rules about conservation of energy that would like to have a word with you.

If we have no propulsion system capable of accellerating a craft to ludicrous speed, exactly which 'any type of propulsion system' are you going to use to provide the same amount of energy for decelleration?

(BTW, I'm ignoring the answers in other comments about how this propulsion system can provide thrust in any direction. Just commenting on what I believe is a fallacy in the parent post)

Re:Cast? What cast?

[TigerNut]

Sure, but any rigid truss arrangement that can transfer the thrust generated by the sail in compression or bending loads will still be far heavier than a set of cables that can handle the same load in tension.
The microwave radiation thing wouldn't be an issue to the occupants of the ship - they're going to be sitting inside a metal enclosure, and will be shielded from the radiation the same way you are if you're standing in front of your countertop microwave waiting for the water to boil. Similarly, the metal enclosure can be designed as a deflector for the microwaves, so that it won't melt.

How do you make a construction like this?

[Hank the Lion]

Simple energy calculation:
1/2*m*v^2=P*t.
P=60MW, t=3600s, v=60km/s.
At an efficiency of 100%(!), the maximum mass you can give this speed is 120kg.
The sail will be 100m across, this is 10,000 m2.
The maximum mass per square meter, including structural integrity (there will be quite a bit of force on the sail to make it accelerate to 60 km/s in just one hour, about 2000N!) is 12g/m2.
Then, I think, you will want to have some payload to reach Mars to do the actual experiments with... This needs to be subtracted from the mass of the sail.

OK, some of the mass of the sail will evaporate to enhance propulsion, so acceleration at the end (when the construction is lighter) will be higher than in the beginning, but a lage part of the energy will be taken away by the evaporating gas as well, so efficiency will be quite abit lower than 100%.

All in all, how do they think to make this construction?

Re:Cast? What cast?

[TigerNut]

I'll bite. 100m diameter gives 7850 square meters area, so the power density is 7600 watts per square meter - toasty warm. I'd say that's between five and ten times what the inside of your microwave sees. You'd definitely want anything that's exposed to that intensity to reflect or deflect, rather than absorb, the radiation.
Since the beam was intended to be parallel, the distance doesn't matter. You could do this I suppose if you built a 100m diameter maser, although it's not immediately apparent to me how exactly that would be done.