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

Cast? What cast?

[numbski]

I don't see any cast.

BTW. The sail emits carbon monoxide to get its speed boost. You know, the stuff the kills humans almost as fast as dihydrogen monoxide.

You really want to be behind that thing for a whole month?

What is this...Fark?

[Uncle Eazy]

The cast of Trading Spaces is unavailable for comment. TSIA.

Re: what is this...fark?

[Rude Turnip]

Who cares, I submitted this with a funnier headline.

Re:Cast? What cast?

[the_mad_poster]

Yea, I imagine carbon monoxide poisoning is probably the biggest issue facing unprotected free-floating humans in space.

Fuel

[MyLongNickName]

It is very cost benefitial to not have to take fuel with you... or at least not as much.

My question is, what kind of payload is practical with this kind of thing? I've always read that to get any kind of larger payload, you cannot use solar sails. Do they get around this by using the microwave beam they talked about (ie higher energy per square meter)? I wish there were more numbers in the article...

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.

Re:Fuel

[nine-times]

Yeah, numbers would be nice to know, for example, how big the sail would need to be. I sure hope this picture isn't supposed to be to scale.

Re:Fuel

[ThosLives]

Indeed. 60 kps is way past escape velocity for the solar system; from Earth mean orbit, solar escape velocity is only 42kps (give or take)!

Incidentally, I want to see their 60 kps calculation ... that's a huge momentum change over an hour! The average acceleration for 60000 m/s in 3600 seconds is: ~16.7 m/s2. I don't know about you, but methinks they got a decimal point wrong. Or their entire ship was made of aerogel and has very little mass...

Here's some fun math: 60 MW of (1mm/300GHz) microwaves will carry a momentum of 0.2 N; if the sail absorbs all of the photons, the force would be 0.2 N; if it reflects them completely the force would be 0.4 N. To get an acceleration of 16.7 m/s2 you need a force of 16.7 N per kilogram. All this says is that they're getting a lot of force from breaking the chemical bonds in the paint. Kind of like burning fuel.

Show my work: Energy per photon is h*f where h is plank's constant and f is the frequency. For 300 GHz microwaves, f = 3e11 Hz and h is always 6.626e-34 J.s; each photon has ~2e-22 J. 60 MW means you have ~3e29 photons per second. Momentum per photon is p = h/w, where w is the wavelength (1e-3 m), so each photon has a momentum of ~6.6e-31 N.s. 3e29 photons per second of 1 mm microwaves have a momentum flux of 0.2 N.

Then what?

[FroBugg]

So you're going faster than any interplanetary craft to date, and your only propulsion system requires you to be moving away from the sun (or the Earth, if they're using a laser to push you).

How do you slow down? Orbital insertion at that speed would be seriously difficult, if not impossible.

Re:Then what?

[merlin_jim]

So you're going faster than any interplanetary craft to date, and your only propulsion system requires you to be moving away from the sun (or the Earth, if they're using a laser to push you)

As I've previously discussed on slashdot, you do not need to be moving outward from your energy source in a solar sail, you can achieve thrust vectors in any direction from full away to orthogonal (perpendicular for the 2D vector peeps)

And orbital mechanics isn't of the form of "thrust straight at where you want to go" it's more like "thrust in the direction of orbit to move away from primary, thrust against the direction of orbit to move towards primary"

The only time a solar sail would even find it efficient to thrust directly away from the inner solar system is if it was an interstellar sail, after it reached escape velocity... before then thrust away from the primary doesn't change the mean orbit distance, it changes the eccentricity of the orbit.

Re:Then what?

[CmdrGravy]

They would drop the solar anchor

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.

Jumping to conclusions!

[ArsSineArtificio]

The cast of Trading Spaces is unavailable for comment.

Did you even bother to ask them?

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?

[revscat]

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

Well, from TFA:

The feat would require a 60-megawatt microwave beam with a similar diameter to the sail.

Now, I'm no EE, so somebody please correct me if I'm wrong, but I'm thinking that generating that much power from solar cells would be an undertaking in and of itself. You'd be hard pressed to generate that much energy in space.

Re:Why pipe microwaves from the surface?

[saigon_from_europe]

Now, I'm no EE, so somebody please correct me if I'm wrong, but I'm thinking that generating that much power from solar cells would be an undertaking in and of itself. You'd be hard pressed to generate that much energy in space.

It seems feasable.

I am EE, but I don't know what is the value of Sun's radiation. IIRC, it is about 400W/m^2 on Earth, so it is in worst case the same in orbit. With cell's efficiency of 25%, we get about 100W/m2, so for 60,000,000W we need only 600,000m2. It is sqare with side of 775m - not something too complicated to build even on our current technological level.

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.

Re:Cast? What cast?

[Anonymous Custard]

BTW. The sail emits carbon monoxide to get its speed boost. You know, the stuff the kills humans almost as fast as dihydrogen monoxide.

You really want to be behind that thing for a whole month?

Right, like they're going to be flying along to Mars with the windows open.

Re:Cast? What cast?

[paranode]

The idea is actually pretty good, but the problem is the technology is not there to support it yet. The most notable obstacle from the article:

"The feat would require a 60-megawatt microwave beam with a similar diameter to the sail. It would also have to be capable of tracking the craft as it accelerated away. But this power level could not be delivered by any existing microwave transmission system. The deep-space communications network that NASA uses to communicate with Mars rovers and the Cassini probe now orbiting Saturn can only manage half a megawatt. The Benfords say the power could be ramped up in future and hope to persuade NASA to consider doing this as part of a future upgrade to the network.

So basically NASA's currently-used equipment is 1/120th of the power needed to get this sail to Mars. I would say this idea is not in our near future for sure.

60 MW ...

[matth1jd]

at least it's not 1.21 Gigawatts...

Re:In related news

[Frymaster]

The invention of a warp drive... could get us there immediately

man, if you have a time machine, words like "immediately" don't really mean anything.

Re:Holes in the sail?

[merlin_jim]

I wonder how susceptible this sail would be to space dust, meteorites and space junk?

Not that susceptible. You design it to tear on impact, leaving an impact hole only marginally larger than the impact object.

This sail isn't like a wind sail; wind sails work off of a pressurized fluid, which will tend to flow through holes and tears, meaning even a small tear can greatly effect efficiency.

This sail works off of photon pressure, which does not flow like a fluid, so a small hole means you only lose thrust in proportion to the area of the hole...

Re:Cast? What cast?

[Jonathan_S]

"The feat would require a 60-megawatt microwave beam with a similar diameter to the sail. It would also have to be capable of tracking the craft as it accelerated away. But this power level could not be delivered by any existing microwave transmission system. The deep-space communications network that NASA uses to communicate with Mars rovers and the Cassini probe now orbiting Saturn can only manage half a megawatt. The Benfords say the power could be ramped up in future and hope to persuade NASA to consider doing this as part of a future upgrade to the network.

So basically NASA's currently-used equipment is 1/120th of the power needed to get this sail to Mars. I would say this idea is not in our near future for sure.

Um, NASA's current communications system puts out 1/120th of the power needed for this sail.

That's like saying since your cell phone can only put out 1/2 a watt it's impossible to heat things in your microwave.

A couple of points of reference, the radar mounted on US Aegis cruisers can put out 4 MWs and the stationary Cobra Dane early warning radar that went online in 1977 puts out 15.4 MW.

I don't think we are that far away from building a 60 MW transmitter now that we have a reason to.

Re:Cast? What cast?

[GileadGreene]

The sail emits carbon monoxide to get its speed boost. You know, the stuff the kills humans almost as fast as dihydrogen monoxide.

Actually, if you RTFA you'll see that they discovered the effect as a result of inadvertently boiling off carbon monoxide, but the paint that the article is about would actually use something like hydrogen (or perhaps methane).

You know, the stuff that burns much faster than dihydrogen monoxide ;)

Re:Cast? What cast?

[Tassach]

A couple of points of reference, the radar mounted on US Aegis cruisers can put out 4 MWs and the stationary Cobra Dane early warning radar that went online in 1977 puts out 15.4 MW.

I don't think we are that far away from building a 60 MW transmitter now that we have a reason to.

Unfortunately, RADAR, like all other forms of non-coherent EM radiation, spreads out over distance. In order for this to work, you need to have the power actually hitting the sail, which basically means you're going to need a battery of MASERS which will still be sufficiently focused at 35 million miles to deliver most of their power to the sail.

Monoxide Warning

[Supercarps]

Warning: Not for terrestrial escape. Solar sail for outdoor use only. Do not deploy in poorly ventilated regions of space. Emissions sticker must be displayed on windshield of personnel module at all times. Solar System law prohibits tampering with or disabling of solar sail emission control systems. (Additional requirements apply above Earth's California.) It is illegal to use 60 MW microwave source in any manner inconsistent with labeling (e.g. popcorn, "phone home", mind control of homeless.) Not safe for children under 6 (toxic if eaten, asphyxiation hazard). May be fatal if used as shelter in bright sunlight or under lightning conditions. Improper disposal threatens wildlife and environment - ask local authorities about stellar propulsion system recycling programs.

"Yeah, they found the poor bastard in his space garage with the door closed and the solar sail running. Damn shame."

Why this is not helpful; other useful technologies

[Dr. Zowie]

The sail is being used as a rocket engine. That is not too helpful, as the exhaust speed of the paint is still limited by chemical bonds (it's hard to make individual paint molecules leave with more kinetic energy than is contained in a chemical bond). That means you would need just as much propellant (in the form of paint) for this scheme as you would need for a normal chemical rocket engine.

These guys are definitely on an interesting track, though. The problem with rocket engines in general is that they have a tradeoff between mass efficiency (you want to put as much momentum on each piece your propellant as possible, so that you get as much push from it as possible) and energy efficiency (it costs energy to push propellant, and you have to supply the energy).

Chemical rockets can't get much more efficient than the Space Shuttle Main Engines, because the amount of energy available for each molecule of exhaust gas is whatever you can get by chemically reacting your fuel to make the propellant molecule. The SSMEs use one of the most energetic-per-unit-mass chemical reactions around: hydrogen and oxygen (fuels) combining to make water (propellant).

Electric ion rockets do better because each molecule of propellant gets much more energy than would be available from chemical reactions. The problem there is that you still have to produce the energy. Nuclear electric propulsion uses plutonium to generate heat, which is converted to electricity and then used to run the ion rocket. Solar electric propulsion uses solar panels to generate electricity that runs the ion rocket. The problem is that both of those schemes are limited by the power available: it's hard to make energy rapidly with either a conventional radiothermal (noncritical) generator or solar panels, so while the rocket is extremely fuel efficient it is also quite slow.

Pure solar sails use the best/worst propellant in the Universe: photons. Best, because photons are disposable -- "use all you want, we'll make more!". Worst, because photons use the most energy per unit delivered momentum of any propellant in the universe. So a sail transduces huge amounts of power (at least in the inner solar system) but uses a very inefficient process to convert that energy to momentum.

Making the sail into a hybrid rocket is a Good thing, but using this paint scheme doesn't help, because the ejected molecules don't ever get much more energy than their own chemical binding energy into the paint -- that means they're being more or less wasted as propellant, because you want to put as much kinetic energy on the propellant as possible.

A better scheme is to use a curved solar sail as a concentrator to heat up a high power electrical generator, and then use the electricity to drive an ion rocket. In 2000 or 2001 I and a colleague worked up the numbers for such a scheme (there are technical problems with making high-power ion rockets; but we considered just energy flow). A smallish curved solar sail (say, 120m in diameter) can concentrate 10 megawatts of heat onto a heat collector. At 10% conversion efficiency to propellant power (15% for conversion to electricity, times 67% efficiency in the rocket engine) that would still be a megawatt of power, enough to provide hundreds or thousands of Newtons of thrust. In several scenarios we considered, the acceleration of the whole craft is higher than the unloaded self-acceleration of the sail, so it would be necessary to repel the sail electrostatically or something like that to keep its shape correct.

Ion rockets can be 100 to 1000 times more propellant-efficient than chemical rockets, provided that there is enough energy available.

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.