Relativistic Navigation Needed For Solar Sails 185
KentuckyFC writes "Last year, physicists calculated that a solar sail about a kilometer across with a mass of 300 kg (including 150 kg of payload) would have a peak acceleration of roughly 0.6g if released about 0.1AU from the Sun, where the radiation pressure is highest. That kind of acceleration could take it to the heliopause — the boundary between the Solar System and interstellar space — in only 2.5 years; a distance of 200 AU. In 30 years, it could travel 2500AU, far enough to explore the Oort Cloud. But the team has discovered a problem. Ordinary Newtonian physics just doesn't cut it for the kind of navigational calculations needed for this journey. Because the sail has to be released so close to the Sun, it becomes subject to the effects of general relativity. And although the errors these introduce are small, they become magnified over the course of a long journey, sending the sail roughly 1 million kilometers off course by the time it reaches the Oort Cloud. What these guys are saying is that if ever such a sail is launched (and the earliest estimate is 2040), the navigators will have to be proficient in a new discipline of relativistic navigation."
Computers? (Score:4, Insightful)
the navigators will have to be proficient in a new discipline of relativistic navigation.
Probably you are trying to say that the computers will have to be proficient in this new discipline.
Re:Computers? (Score:5, Insightful)
It's not that hard, either. Just math. We have the equations. They're well-understood. Some physics grad students could probably write the basic engine for such an endeavour. I'd worry more about $UNKNOWN_EXOTIC_EFFECT pushing something off-course.
Re:Computers? (Score:5, Insightful)
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The ideal time to make that discovery is not when you're kissing the Sun from 0.1 AU away.
Let's start with unmanned probes, hey?
Re:Computers? (Score:4, Interesting)
Pfft, and give up the chance to have an unknown exotic effect named after you? Small price to pay my friend! Did you think all those crazy radiations and particles from Star Trek were named after unmanned probes???
Re:Computers? (Score:4, Insightful)
I'd be more worried about simple things like...
Kilometers-to-AU translation errors (nobody would be using "miles" in their calculations, now would they?)
cumulative floating point rounding errors
antenna positioning failure
There are more than enough problems that could re-occur, before you start looking for new ones.
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The article sounds like it's extrapolating the peak 0.6g acceleration for the entire length of the flight. Seems to me that acceleration is proportional to the light flux trapped and/or reflected by the sail, which will fall off with the square of the distance from the sun. So you can't get to the Oort cloud in just a couple of years.
What am I missing?
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That's what I get for posting before whipping out the calculator. The acceleration needed to go 200 AUs in 2.5 years is only 9.5 E-3 meters/second. Or around .001g. I don't trust my calculus any more, but integrating the acceleration over that time is in the ballpark.
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a = F/m
F = (the force with which sunlight pushes the sail) - (the force with which the sun pulls the sail back)
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And who's going to stop you once you've got to 0.6g ? last time I've check there was no friction in vacuum.
"Once you've got to 0.6g"?
g is acceleration! c is velocity!
1g is the rate of acceleration due to gravity on Earth 9.8 m/s^2. 1c is the speed of light, 299,792,458 m/s.
If you don't put in more energy, you're gonna stop accelerating, friction or no friction!
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(Damn summary.) G is acceleration. g is grams.
1G is acceleration due to gravity at the surface of the Earth, 9.8 m/s^2.
An object in motion will stay in motion unless acted on by an outside force.
An object in acceleration will cease to be in acceleration when the outside force is removed. As the force reduces, the acceleration reduces.
0.6G means if you stand with your head in the direction of acceleration, you'll weigh 3/5ths your weight on Earth.
0.6c means you're moving 3/5ths the speed of light.
Re:Computers? (Score:4, Informative)
It's a solar sail. Without significant solar thrust, it _will_ drag against the interstellar gas, and it's likely to gain mass as it does so.
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Kilometers-to-AU translation errors
How about just g to c translation errors?
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It's not that hard, either. Just math. We have the equations. They're well-understood. Some physics grad students could probably write the basic engine for such an endeavour. I'd worry more about $UNKNOWN_EXOTIC_EFFECT pushing something off-course.
You mean, something like the Pioneer anomaly? [wikipedia.org]
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A "new discipline" that is 100 years old.
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Computers can't do anything they haven't been programmed to to.
You knew what this mission was when you signed up (Score:2, Offtopic)
The navigators? Correct me if I'm wrong, but isn't this a guaranteed one-way trip? For what possible reason would we use human pilots?
Re:You knew what this mission was when you signed (Score:3, Funny)
Ping... pong (Score:2)
After 2.5 years, it will be a 2 day affair.
We might need something that can think on its own to have any useful input.
One part in 37 million... (Score:5, Insightful)
One million kilometers sounds like a big number, until you realize that 2,500 AU is 3.7 * 10^11 kilometers. So that error is one part in thirty seven million. I suspect that accumulated errors from variations in light intensity due to sunspots and flares will be a bigger problem.
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You mean one part in 370,000, but on the whole you're right. The unfolding speed of the solar sail, or its random deformation during travel will have a higher impact. What a stupid article.
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You mean one part in 370,000
Doh.
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So, basically, we shouldn't expect Cassini style precision with this?
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That's why you have in-flight course corrections. I think Cassini made several burns on the way to Saturn, as well as the circularizing burn once it got to the system.
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My thought exactly. Of course if we want this new probe to end up near a neighbouring star in 30,000 years, it really starts to add up. But over that time, chances are we'll have lots of other unknown and unexpected effects to correct for as well, so we're going to need to ability to adjust course during flight anyway.
On the other hand, I don't think keeping track of relativistic effects is really going to be that big of an issue. It's not as intuitive as Newtonian physics, but we've got all the necessary m
Re:One part in 37 million... (Score:4, Insightful)
Unless we have some specific target in the Oort Cloud that we aim for at the beginning of the trip, with no course-corrections, this is pretty much meaningless.
And with essentially unlimited ability to maneuver, course-corrections aren't going to be an issue, really.
Re:One part in 37 million... (Score:4, Funny)
I would think that the Oort cloud itself would be the destination. Theoretically, the distribution of rocks is pretty even, so we should be able to get data no matter where in the cloud the probe goes. If it gets to that random point and finds either nothing, or a whole lot, we need to change the theory, don't we?
Remember, Columbus set out to sail to the Indies, not land in Mumbai harbor. Of course, if we follow that example the probe will crash into Neptune and we'll declare it a new comet, but the general principle is the same.
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I agree with the posts which note that the relative magnitude of the navigational error is trivial (a million kilometers in 2500 AU is the same relative error as one kilometer on a trip to the Moon). I also fully agree that any expedition to the Oort would be a random crapshoot anyway.
I do have to quibble with the notion of 'essentially unlimited ability to maneuver', however. The amount of thrust
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Well, you could use a moon-based laser system to deliver the power for a mid-course correction. And laser power while also falling off as the square of the distance, can still fall off a lot more slowly. You'd need a hefty laser beyond appreciable atmosphere, which is why I said moon based (to allow for cooling). Asteroid based might be even better, but harder to build.
Besides, if you build it on the far side of the moon no rogue faction can use it to threaten it's home country. But it could still shoot
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great idea. now if only we could fix the moon in place, so that the dark side is always pointing towards the probe...
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Unless we have some specific target in the Oort Cloud that we aim for at the beginning of the trip, with no course-corrections, this is pretty much meaningless.
Why call it the "Oort Cloud" if there's nothing in it? My view is that such solar sails would be first used for Kuiper Belt targets and the heliopause (the latter not needing trajectory accuracy aside from making sure the probe heads away from the Sun). Later as we discover targets in the Oort cloud to investigate, probes could be sent out in this way. It's also good for interstellar missions. These velocities provide a good first stage boost. Accurate trajectories might greatly reduce the propellant consum
Need to think relatively (Score:5, Funny)
As always.... (Score:2)
xkcd to the rescue...
http://xkcd.com/265/ [xkcd.com]
Shouldn't this be irrelavent... (Score:3, Funny)
Wouldn't this be completely besides the point as long as we keep enough spice in their tanks? They can always just think their way back on course.
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Should be some sandtrout around someplace we can genetically modify a bit to live in oceans...
What else is new? (Score:5, Interesting)
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Should have taken that left turn at Albuquerque.
Is this that important? (Score:5, Insightful)
sending the sail roughly 1 million kilometers off course by the time it reaches the Oort Cloud.
Is there a specific part of the Oort Cloud they want to go to?
If this ability is needed to travel to other planets accurately, then it seems important. For the Oort cloud, not as much.
Will this solar sail be going at a speed that will allow it to do any useful observations, or are we just going to watch for the flash when it 'finds' something at that speed?
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Space Craft [feeddistiller.com] Feed @ Feed Distiller [feeddistiller.com]
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The only force acting on a solar sail is outwards from the sun. If the sail is angled the force would be reduced (less cross-sectional area), but the direction of the force would remain the same. A sailboat can only turn because it has a keel that exerts force against the relatively-motionless water normal to its direction of motion. There is nothing to push against in space.
The only way to move in a direction other than away from the sun is to employ alternate propulsion, or to somehow find another sour
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Wrong.
The solar wind force is essentially outward (in the solar wind direction) only. (The particles initially stick to the sail and then are released, if at all, by a different mechanism such as electrostatic repulsion.) And the portion of the light that is absorbed by the sail also produces an outward force.
But for a mirror-finished solar sail the portion of the light that is reflected (most of it) gives the vector sum of the momenta of its arrival and the recoil of its departure. So tilting the sail t
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The solar wind force is essentially outward (in the solar wind direction) only. (The particles initially stick to the sail and then are released, if at all, by a different mechanism such as electrostatic repulsion.)
Or penetrate it and deposit momentum from drag averaging out to be along their incoming direction of travel.
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Takes just as much energy as a solid capsual, actually. Just don't deploy the sail til you get there...
Manuvering system for a sail..... (Score:4, Interesting)
Would that be an RTG powered ion thruster? or do you make holes in the sail that are opened and closed by tiny articulated motors?
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Mid-course corrections? (Score:5, Insightful)
No space craft has ever been aimed accurately. At various times during the mission, you look at where you are and where you're supposed to be, and make a correction to your trajectory. Is there some reason why this won't work with a solar sail?
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And isn't starting at the Sun and aiming for a point in the Oort cloud complicated by the N-body problem anyway? Course corrections will have to be done for the entire trip because of all of those large chunks of rock and gas floating around. Gravity's a bitch, man.
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And isn't starting at the Sun and aiming for a point in the Oort cloud complicated by the N-body problem anyway?
Not really. The Oort cloud isn't exactly small. Just keep flying away from the sun and you'll get there eventually.
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You missed the point. Hitting any part of the Oort cloud is easy. Trying to hit any particular point in space without course corrections is unbelievably hard, unless it's a really deep and small gravity well....
How are you making your course corrections? (Score:3, Insightful)
2 thoughts come to mind:
1) If the solar sail is your means of propulsion, do you include some sort of 'conventional' rockets to make your course adjustments? Can course adjustments somehow be made with the sail itself? It's not like a ship with a solar sail has a rudder. If not with the sail, how are you making those corrections.
2) Efficiency - getting the correct path to start with means you'll get there sooner. Perhaps a LOT sooner, because making course corrections might have the effect of slowing down s
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On a "good" sail the surface is very reflective. The force that propels the spacecraft is the sum of two vectors; one pointing from the sun to the spacecraft, and a second for the reflected radiation leaving the sail. So you can steer the spacecraft by shortening one side and lengthening the other side of the says attaching the sail to the spacecraft, redirecting the outgoing vector. Or do something similar (e.g. reorient segments rather than the whole sail).
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If the solar sail is your means of propulsion, do you include some sort of 'conventional' rockets to make your course adjustments?
Just like you do on a sailing ship: you turn the sail.
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Re:Mid-course corrections? (Score:4, Informative)
It can't work like a sailboat does... steering partly into the wind, or changing the sail angle to alter the thrust exerted. There's no resistive force to work against, so it just kind of goes where it is taken.
However, tacking [caltech.edu] with the solar sail is still possible.
Re:Mid-course corrections? (Score:4, Insightful)
Do not forget for one instant that your solar-sailship is in orbit around something. You aren't using your solar sail to overcome the sun's gravity and drift off into the outer reaches of the solar system... there's a term what happens when a star is generating enough radiation pressure to overcome its own gravity: a supernova. Travel by solar sail (and any other modern propulsion system) is based on giving a gentle nudge to your orbit so that eventually you swing by where you want to be.
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Of course there's resistive force. It's called gravity and most people, when they think about space travel, vastly underestimate it's strength.
Gravity is not a resistive force. A resistive force is a force that acts opposite the motion of a moving object. Gravity is an attractive force between masses independent (in the Newtonian model in which it exists) of the motion of the masses. I think GR has an extremely weak resistive force in that gravity waves carry away some energy from masses moving near one another.
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With most probes they're pretty compact, small thruster bursts will do a lot.
How do you tack a solar sail though?
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Just how big is the Oort Cloud? (Score:5, Funny)
Re:Just how big is the Oort Cloud? (Score:4, Informative)
I think that watching The Empire Strikes Back may have given you the wrong idea about just how densely packed objects like asteroids and comets are in our solar system.
Consider this. Get your own envelope and pencil if you want to follow along at home. The inner boundary of the Oort cloud is at about 5,000 AU, or 750 billion km from the Sun. The outer boundary is expected to be somewhere around 100,000 AU or 1500 billion km. Inside that volume are an estimated twelve billion objects. Nobody has been able to count them, but Jan Oort guessed that there would be that many and no astronomer has been able to contradict him yet.
That gives us a total volume on the order of 10^28 km^3, with just 12,000,000,000 objects in it. That's 10^18 km^3 for each object, giving you an average distance between objects of at least a million kilometers. A million km is three times the distance from the Earth to the Moon, and the size of a cometary nucleus is on the order of ten km. You'd be lucky just to see a 10 km object at that distance, let alone see it well enough to justify the trip out there.
That means that if you're aiming for an object in the Oort cloud but miss by up to a million km, you're going to sail right through empty space. You won't narrowly dodge between densely packed cometary bodies, rolling and weaving to avoid laser blasts, and then have to hide inside the belly of a giant space worm while the Empire searches for you. You'll just pass on by and miss everything.
Real astronomy isn't nearly as exciting as Star Wars, but that's probably good news for everyone who lives in our galaxy.
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Actually, if they're roughly a million km apart, and you miss one by a million km... you're in the vicinity of another one.
Now missing by *half* a million km... that'd suck.
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Much ado about nothing (Score:2, Informative)
It's more likely that the flight engineers would just add course corrections in (i.e. change the sail orientation to redirect the force) if they had a specific target in the Oort cloud in mind.
Just as small errors due to GR get magnified over the long trajectory, so do small corrections get magnified if made early enough. And, as one earlier commenter noted, a million km isn't much of anything at these distances.
My pet peeve about big numbers (Score:2)
TFS says that it will set the sail 1 million kilometers off course. I have no idea if that's a lot or a little. Don't switch units (from AU to km) mid paragraph (it's a smelly hint of wool coming over eyes).
I beg you please don't just put big numbers without context or feel for what they mean.
\begin{rant}
I especially hate it when the government publishes such big numbers. Is a pork barrel item of $1,000,000 big or small.
\end{rant}
one more stat (Score:2, Interesting)
Dream on, space sailors. It's an idiotic idea and always will be.
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The probability of it getting all the way there without one single part of the 1 KM sail getting hit by any single piece of space rock or other debris: 0%
And the consequence of a blowing a few 1cm^2 holes in a 1km^2 sail is...? It's not like a perforation will let all the sun leak out.
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<deep sigh/>
What happens if you shoot a bullet at a sailboat's sail? You get a tiny hole in the sail. Now imagine the bullet is a million times less massive, and traveling a thousand times as fast. Same kinetic energy, but it's going to punch a much smaller and neater hole.
Any space debris at the relative velocity of a solar sail will punch right through any imaginable sail material, vaporizing the tiny bit that it contacts. The surrounding material won't even feel a tug.
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The probability of it getting all the way there without one single part of the 1 KM sail getting hit by any single piece of space rock or other debris: 0%
Dream on, space sailors. It's an idiotic idea and always will be.
That, plus the fact that they're talking about in 30 years being able to have the technology to make the trip take only 30 years.
Great.
Here's an alternative mission profile for you: we use a VASIMR drive, with an estimated specific impulse of around 30,000s. We have one large enough to produ
Pardon the pun, but the scale tilts both ways. (Score:2)
From Earth's vantage point, an error of a million miles at 2500 A.U. would amount to a pointing error of about
As for the environment at the Oort cloud, it
Move along. Nothing to see here. (Score:4, Informative)
The JPL ODP (Orbit Determination Program) has incorporated relativity since the 1960's and uses the proper Einstein Infeld Hoffmann (EIH) equations of motion for the harmonic gauge.
0.1 AU? (Score:3, Interesting)
I can understand why it would be nice to start off a solar-sail-based craft at one-tenth AU from the Sun; more light pressure = more acceleration. Thing is, it will almost certainly be starting out from Earth. You'd need to accelerate it just to drop it down to 0.1 AU. Wouldn't it be more efficient to use that acceleration to throw it outward instead of inward? Anyone care to calculate this?
GPS must correct for special & general realtiv (Score:3, Informative)
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There is slowing of the clock onboard GPS satellites both due to the orbital speed (special relativity) and lower gravity (general relativity).
A colleague who used to teach a "Modern Physics for Engineers" course took great delight in detailing the history of the GPS system, and how they had to bring in some hard-core theoretical physicists to work out the GR corrections.
Engineers have a tendency to think theory is irrelevant and stupid, and this is a nice example of how the GPS system would have either fai
I already use relativistic navigation (Score:3, Funny)
What is the top speed? (Score:2)
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I don't get it... Why don't they just drop it, &am (Score:2)
wait until when it gets about say 8 Light minutes away 8') send it new coordinates, giving it an "update" to it's trajectory. This should be able to be done w/ small rockets & such, and it's way earlier, the speed will be high, but not it's peak speed. We should be able to give it a Garmin (tm)navigational update.
but how is it going to navigate around ojects in it's path? My guess is that any "Solar Sail" application will be torn to shreds by space dust at those high speeds by the time it get's betwee
I'm Confused (Score:3, Insightful)
> it could travel 2500AU, far enough to explore the Oort Cloud... sending the sail roughly 1 million kilometers off course by the time it reaches the Oort Cloud
How could you possibly miss the Oort Cloud, a spherical region, when you start inside it. Considering that we don't know jack, or even 10% of jack, about the Oort Cloud, what the hell are we aiming at? Fling the sucker out there at random and see what we find. The unaimed arrow never misses.
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I think the issue with knowing the course accurately has to do with being able to communicate with it. Knowing its course means knowing how it should be oriented to keep an antenna pointed at the Earth.
The math does not look promising (Score:2)
When I do the math, a square kilometer sail weighing 150 kilograms can only weigh 0.15 grams per square meter. If the material is only 0.0025 cm thick, it would have to have a density of 0.006. It's hard to find anything solid that is that light.
And that's ignoring the non-negligible weight of whatever lashes the 150Kg payload to the square kilometer of sail.
And if this thing is going to pull 0.6G, you need some kind of structure that can transfer the force to the payload without collapsing the sail.
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0.025mm? Why use something that thick? McMaster sells 0.0125mm PET film for $0.25 per square foot.
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oookay, so the stuff can be as dense as 0.012.
PET's density is 1.35 so it's still a bit over 100 times too heavy. And I doubt if PET can stand being 0.1 AU from the Sun for very long.
The Spice must flow (Score:2)
Its easy, just take a massive overdose of Spice, float in your tank, and visualize the spaceship getting there. Presto! There it is.
neat (Score:2)
Something Missing Here (Score:2)
Haven't these guys ever heard of the mid-course correction? I mean, really...
Maths required for space navigation? (Score:4, Informative)
And Magellan had to weigh the threads in his sails (Score:2)
What truly amazing twaddle. The concept of a solar sail that cannot _steer_ to correct any errors in its original launch is simply amazing. This would be a very expensive spacecraft, not an arrow. It's going to need some control in order to keep its sail aligned for maximum effective thrust, lest it twist very slightly and get pushed slightly wrong for days or years. Even the slightest control of the sails, very slightly pulling in one corner or even two, could be used over a voyage to avvect its course.
Re:Wont the accleration decrease with distance (Score:4, Insightful)
The reall question will be: how does it stops? I doubt it can use the gravitational slingshot trick at these speeds using only comets.
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There are ways [wikipedia.org].
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It's a solar sail, right? It can just luff up.
(kidding)
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Power problem? I guess those mars rovers are doing ok with solar.
Except solar isn't going to help you much at 2500 AU.
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Unless you can arrange for that 1km^2 solar sail to also be a solar cell, in which case you'd still be intercepting around a hundred watts. Good luck making a 1km^2 solar cell that masses less than 100kg, though.
Solar at that distance? (Score:2)
I wonder, how practical is Solar power past pluto?
I seem to recall from one of my physics classes, a discussion about point sources of light and other EM fields, that as your distance from an object doubles, the 'density' of the field becomes 1/4. My point is, that outside the Solar System, you're so far from the Sun, that wouldn't the density of light at that distance (and thus, the amount of light/power that is hitting your solar cells) be very very very small?
Re:Solar at that distance? (Score:4, Informative)
Right. There's (almost) no friction in space, so your craft isn't going to slow down just because it's no longer receiving enough power from the sun to accelerate. But after a certain point it won't receive enough solar power to power onboard navigation and communications systems. Those would likely be powered by a wee bit o' radioactive power like today's deep space probes.