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Relativistic Navigation Needed For Solar Sails

Soulskill posted more than 4 years ago | from the i'd-rather-be-sailing dept.

Space 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."

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185 comments

Computers? (4, Insightful)

Extremus (1043274) | more than 4 years ago | (#29119923)

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? (4, Insightful)

FooAtWFU (699187) | more than 4 years ago | (#29120071)

Probably you are trying to say that the computers will have to be proficient in this new discipline.

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? (1, Funny)

drseuk (824707) | more than 4 years ago | (#29120191)

Tough choice for NASA: Use MS Excel 2040 and end up a million km off course or use Linux and have the sail get where it's supposed to be for 1st January 1972. Simples!

Re:Computers? (3, Insightful)

sabt-pestnu (967671) | more than 4 years ago | (#29120647)

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.

Re:Computers? (1)

OwnedByTwoCats (124103) | more than 4 years ago | (#29121583)

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?

Re:Computers? (2, Informative)

OwnedByTwoCats (124103) | more than 4 years ago | (#29122091)

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.

Re:Computers? (0)

Anonymous Coward | more than 4 years ago | (#29122397)

First, let me preface this by saying it has been 5-10 years since I've done any kind of serious study into solar sail tech... Because of the frictionless vacuum, the initial acceleration does not dissipate. I believe that what they're referring to as a peak 0.6g acceleration is the accumulated acceleration from the initial, tiny acceleration added together with the ever decreasing, but still positive acceleration as the sail moves farther away. The eventual acceleration for the entire trip would reach a peak of 0.6g, but would start at only a fraction thereof.

Re:Computers? (0)

Anonymous Coward | more than 4 years ago | (#29122515)

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.

Re:Computers? (1)

Pentium100 (1240090) | more than 4 years ago | (#29122967)

a = F/m

F = (the force with which sunlight pushes the sail) - (the force with which the sun pulls the sail back)

Re:Computers? (3, Informative)

maxwell demon (590494) | more than 4 years ago | (#29120763)

Probably you are trying to say that the computers will have to be proficient in this new discipline.

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]

You knew what this mission was when you signed up (1, Offtopic)

Ryvar (122400) | more than 4 years ago | (#29119939)

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 (2, Funny)

SBrach (1073190) | more than 4 years ago | (#29120005)

They are relatively better at the calculations??

Re:You knew what this mission was when you signed (0)

Anonymous Coward | more than 4 years ago | (#29120117)

It's called ground control, major Tom.

Ping... pong (1)

autocracy (192714) | more than 4 years ago | (#29120353)

Passing the orbit of Pluto will give a round-trip time of over 12 hours for the speed of light.

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... (5, Insightful)

argent (18001) | more than 4 years ago | (#29119977)

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.

Re:One part in 37 million... (3, Informative)

Anonymous Coward | more than 4 years ago | (#29120209)

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.

Re:One part in 37 million... (1, Insightful)

Anonymous Coward | more than 4 years ago | (#29121473)

I definitely agree. This article makes it look like relativity is some sort of thing that has just been discovered. This does not qualify as news and is a waste of my time. Seriously, I'm sure that the course will be set and run by computers, if it runs, and those will take into consideration something like that. Furthermore, if it were ever built, there would be control of it from earth. So if there are small corrections that need to be made or adjustments, I'm sure we can compensate...

Re:One part in 37 million... (1)

Volante3192 (953645) | more than 4 years ago | (#29120323)

So, basically, we shouldn't expect Cassini style precision with this?

Re:One part in 37 million... (1)

argent (18001) | more than 4 years ago | (#29120633)

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.

Re:One part in 37 million... (1)

mcvos (645701) | more than 4 years ago | (#29120379)

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 math, right?

Re:One part in 37 million... (3, Insightful)

CrimsonAvenger (580665) | more than 4 years ago | (#29120397)

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... (4, Funny)

R2.0 (532027) | more than 4 years ago | (#29121601)

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.

Need to think relatively (5, Funny)

Lev13than (581686) | more than 4 years ago | (#29121101)

Course correcting a small ship is easy - I'm more worried about everything else. In a relativistic navigation model, the ship is going to be in exactly the right place. However, the energy required to course correct the entire universe by one million km will be prohibitive.

Wont the accleration decrease with distance (0)

Anonymous Coward | more than 4 years ago | (#29120001)

I may be being daft here but wont that acceleration slow with distance from the sun? Does the math stack up?

Re:Wont the accleration decrease with distance (3, Insightful)

UnHolier than ever (803328) | more than 4 years ago | (#29120091)

The acceleration, yes, but not the speed. That is why the sail has to start so close ot the sun, it needs it to accumulate most of its speed.

The reall question will be: how does it stops? I doubt it can use the gravitational slingshot trick at these speeds using only comets.

Re:Wont the accleration decrease with distance (1)

HoboCop (987492) | more than 4 years ago | (#29120301)

Maybe it won't stop.. after it passes whatever it was aimed at it will just keep sending data and traveling until it dies. Power problem? I guess those mars rovers are doing ok with solar.

Re:Wont the accleration decrease with distance (1)

mcvos (645701) | more than 4 years ago | (#29120413)

Power problem? I guess those mars rovers are doing ok with solar.

Except solar isn't going to help you much at 2500 AU.

Re:Wont the accleration decrease with distance (1)

jeffb (2.718) (1189693) | more than 4 years ago | (#29120771)

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? (1)

JSBiff (87824) | more than 4 years ago | (#29120503)

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? (3, Informative)

John_Booty (149925) | more than 4 years ago | (#29120641)

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.

Re:Solar at that distance? (1)

HoboCop (987492) | more than 4 years ago | (#29123329)

Certainly from our sun.. might pass close enough to other stars to get power periodically (for very large values of period).

Re:Wont the accleration decrease with distance (1)

GlobalEcho (26240) | more than 4 years ago | (#29122157)

The reall question will be: how does it stops? I doubt it can use the gravitational slingshot trick at these speeds using only comets.

It's a solar sail, right? It can just luff up.

(kidding)

Shouldn't this be irrelavent... (2, Funny)

Churla (936633) | more than 4 years ago | (#29120015)

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.

Re:Shouldn't this be irrelavent... (1)

el_tedward (1612093) | more than 4 years ago | (#29120187)

What if the terrorists decide to poison all of our sandworms with water of life? We'll run out of spice and be totally fucked. Nothing against using spice, but I think we should develop the navigational computers just in case.

What else is new? (5, Interesting)

jarocho (1617799) | more than 4 years ago | (#29120045)

Pioneer 10 has been off-course [wikipedia.org] for a while now. Maybe the trick for reaching the Oort Cloud is to aim for 1 million kilometers to the left.

Re:What else is new? (0)

Anonymous Coward | more than 4 years ago | (#29120165)

Or throw the probe at the sun ... and miss.

Re:What else is new? (1)

drseuk (824707) | more than 4 years ago | (#29120319)

Oblig:

Commander Pavel Chekov: Course heading, Captain?

Captain James T. Kirk: Second star to the right and straight on 'til morning.

http://www.imdb.com/title/tt0102975/quotes [imdb.com]

Re:What else is new? (0)

Anonymous Coward | more than 4 years ago | (#29121855)

Followed shortly by, the shields can't take this kind of heat cap'n you'll have to go 'round the freakin' ball of gas...

Is this that important? (5, Insightful)

MiniMike (234881) | more than 4 years ago | (#29120109)

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?

Re:Is this that important? (1)

physburn (1095481) | more than 4 years ago | (#29120919)

I doubt it will be easy to get the solar sail within 0.1 AU of the Sun, that is very close, and will need a lot of energy to begin will. Apart from the relativistic course correction need to nagivate (which isn't that numerically difficult), the sail will have to deal with variation in the amount of Sun light and Solar wind, coming towards the Sail, which may vary at random, and be much more difficult to nagivate with. Good luck to Solar Sailors.

---

Space Craft [feeddistiller.com] Feed @ Feed Distiller [feeddistiller.com]

Re:Is this that important? (1)

4D6963 (933028) | more than 4 years ago | (#29123225)

Well, if it's a sail, maybe it can just slow itself down from an Earth-level Sun orbit until it gets lower, then flip 90Â to face the Sun and then get pushed away?

Manuvering system for a sail..... (3, Interesting)

jameskojiro (705701) | more than 4 years ago | (#29120123)

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?

Mid-course corrections? (4, Insightful)

vrmlguy (120854) | more than 4 years ago | (#29120145)

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?

Re:Mid-course corrections? (2, Informative)

clintp (5169) | more than 4 years ago | (#29120261)

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.

Re:Mid-course corrections? (1)

mcvos (645701) | more than 4 years ago | (#29120459)

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.

Re:Mid-course corrections? (1)

clintp (5169) | more than 4 years ago | (#29123269)

And isn't starting at the Sun and aiming for a point in the Oort cloud

Not really.

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? (2, Insightful)

JSBiff (87824) | more than 4 years ago | (#29120281)

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 spacecraft some, and even if you don't have to slow down the craft, making course corrections implies you are not taking the most optimal route. But, hey, what's a few extra AU between friends? Oh yeah, that's right, it's the difference between getting the craft to the correct place, and having it shoot by a few hundred million kilometers off to the side. I'm sure no one will mind if that multi-billion dollar space mission gets lost in space having missed it's objective.

Re:How are you making your course corrections? (2, Informative)

djcinsb (169909) | more than 4 years ago | (#29120495)

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).

Re:How are you making your course corrections? (1)

CrimsonAvenger (580665) | more than 4 years ago | (#29120571)

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 spacecraft some, and even if you don't have to slow down the craft, making course corrections implies you are not taking the most optimal route. But, hey, what's a few extra AU between friends? Oh yeah, that's right, it's the difference between getting the craft to the correct place, and having it shoot by a few hundred million kilometers off to the side. I'm sure no one will mind if that multi-billion dollar space mission gets lost in space having missed it's objective.

Umm, no. Getting the correct path doesn't actually make you more likely to get there sooner, much less a lot sooner. You're just as likely to be going too fast as too slow, so you may get there sooner with a bad course than with a good course.

What's a few extra AU between friends? At the distances that obtain in the Oort Cloud, it's rounding error. Even if we were talking a few extra AU (remember, the number mentioned in the article was 1/150th of an AU.

Re:How are you making your course corrections? (1)

speedtux (1307149) | more than 4 years ago | (#29121873)

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.

Re:Mid-course corrections? (2, Interesting)

autocracy (192714) | more than 4 years ago | (#29120419)

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.

Re:Mid-course corrections? (2, Interesting)

onkelonkel (560274) | more than 4 years ago | (#29120831)

Why not? Reel in the shrouds on one side and lengthen them on the other and the whole sail is tilted with respect to the capsule, and you start to change course. You can't tack (I think) but a broad reach should work.

Re:Mid-course corrections? (3, Informative)

WhiteDragon (4556) | more than 4 years ago | (#29121325)

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? (3, Insightful)

jpmorgan (517966) | more than 4 years ago | (#29122837)

Of course there's resistive force. It's called gravity and most people, when they think about space travel, vastly underestimate it's strength.

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.

Re:Mid-course corrections? (1)

Volante3192 (953645) | more than 4 years ago | (#29120439)

With most probes they're pretty compact, small thruster bursts will do a lot.

How do you tack a solar sail though?

Re:Mid-course corrections? (1)

SBrach (1073190) | more than 4 years ago | (#29120697)

You can call a U-turn a mid-course correction but it is still best to start off pointed in the correct direction.

Just how big is the Oort Cloud? (4, Funny)

Eevee (535658) | more than 4 years ago | (#29120185)

From Wikipedia, "The Oort cloud is a hypothetical spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun." So...um...how do you miss it? You go straight out in any direction. When you see a lot of icy chunks floating around, you're there.

Re:Just how big is the Oort Cloud? (3, Informative)

Minwee (522556) | more than 4 years ago | (#29121051)

So...um...how do you miss it? You go straight out in any direction. When you see a lot of icy chunks floating around, you're there.

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.

Re:Just how big is the Oort Cloud? (2, Interesting)

Anonymous Coward | more than 4 years ago | (#29121151)

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.

Re:Just how big is the Oort Cloud? (3, Insightful)

larry bagina (561269) | more than 4 years ago | (#29122065)

No one has contradicted him? More like no one has any evidence whatsoever that there is an oort cloud, much less that his guess is right or wrong.

Re:Just how big is the Oort Cloud? (0)

Anonymous Coward | more than 4 years ago | (#29122747)

But our telescopes can't see the Oort cloud objects anyway and hence it is hypothetical. So we don't have anything to aim at.

Much ado about nothing (2, Informative)

djcinsb (169909) | more than 4 years ago | (#29120259)

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 (1)

line-bundle (235965) | more than 4 years ago | (#29120331)

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}

Re:My pet peeve about big numbers (0)

Anonymous Coward | more than 4 years ago | (#29122533)

Think you forgot to
\usepackage{rant}

one more stat (2, Interesting)

ILuvRamen (1026668) | more than 4 years ago | (#29120405)

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.

Newsflash (0)

Anonymous Coward | more than 4 years ago | (#29120617)

Space really is rather empty, the chances are not all that bad. Not like you need that sail anyways after initial acceleration. And it might be possible to turn the sail in such a position, that minimizes the risk of debris impact.

By the way, it's 1 km (kilometer), not 1 megakelvin (K = kelvin, M = mega), get your units right. Physics is not about guessing.

Re:one more stat (1)

jeffb (2.718) (1189693) | more than 4 years ago | (#29120869)

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.

Re:one more stat (1)

R2.0 (532027) | more than 4 years ago | (#29121733)

"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% "

Math or GTFO.

Already done for GPS (0)

Anonymous Coward | more than 4 years ago | (#29120513)

Read the wikipedia article on GPS - relativity is already taken into account for general navigation on Earth. If it wasn't, nobody would get anywhere.

Pardon the pun, but the scale tilts both ways. (1)

Tsar (536185) | more than 4 years ago | (#29120523)

Yes, the vast scale of the distances involved does mean that the relatively small influence of relativistic effects will be magnified over the course of the mission, but they also dictate that large variances will have a commensurately smaller effect.

From Earth's vantage point, an error of a million miles at 2500 A.U. would amount to a pointing error of about .55 arcseconds, not significant enough to bother correcting where we (or the probe) point our antennas.

As for the environment at the Oort cloud, it will probably be just as interesting as the one a million kilometers to the left or right. This would be analogous to a shift of 18 meters left or right in low Earth orbit.

Finally, a correction can be easily applied if pinpoint accuracy becomes important. A delta vee of only 1.6 m/sec applied as late as 10 years after launch would do the trick.

Why single stage? (1)

NYFreddie (84863) | more than 4 years ago | (#29120821)

Why only use the solar sail? Use the sail to get up to speed, then disengage it and switch over to using a Bussard ramjet. Use of the ramjet should provide ample power for guidance corrections using a vectored thrust system.

Of course, the article may be leaving out a point like using gravity boosts for the sail on its way out, in which case a thousand kilometers would be a very drastic course deviance.

That sucks! (1)

cowdung (702933) | more than 4 years ago | (#29121031)

Damn, now I'll have to put my solar sail project on hold till I can find a relitivistic navigator! I wish they'd said something before!

Move along. Nothing to see here. (3, Informative)

mbone (558574) | more than 4 years ago | (#29121119)

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? (2, Interesting)

TheSHAD0W (258774) | more than 4 years ago | (#29121459)

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 (2, Informative)

peter303 (12292) | more than 4 years ago | (#29121593)

There is slowing of the clock onboard GPS satellites both due to the orbital speed (special relativity) and lower gravity (general relativity). This paper [google.com] says special relativity errors accumlate about 7 microseconds a day and general relativity 46 microseconds. Radio signals move a thousand feet per microsecond, so the effect significant.

Re:GPS must correct for special & general real (1)

radtea (464814) | more than 4 years ago | (#29122121)

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 failed or been full of inelegant hacks if we didn't have an esoteric but exact theory of gravity on large scales.

Re:GPS must correct for special & general real (0)

Anonymous Coward | more than 4 years ago | (#29122893)

My physici friend says that early GPS devices came with a relativity switch, so you could disable the corrections just in case the theory was wrong.

Seems totally legit -- it's the kind of anecdote you'd hear in our intro-to-relativity subjects.

I already use relativistic navigation (2, Funny)

140Mandak262Jamuna (970587) | more than 4 years ago | (#29121679)

I don't know what the big fuss is about relativistic navigation. Almost every day my close relative sits on the passenger seat tells me where to go. Some times my other close relative sits in the back seat and tells me where to go. Being used to that kind of relativistic navigation, I wonder why NASA is so puzzled.

I don't get it... Why don't they just drop it, &am (1)

DRAGONWEEZEL (125809) | more than 4 years ago | (#29121767)

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 between Venus and Jupiter.

I'm Confused (2, Insightful)

Kozar_The_Malignant (738483) | more than 4 years ago | (#29122725)

> 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.

The math does not look promising (1)

Ancient_Hacker (751168) | more than 4 years ago | (#29122863)

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. Quite a trick.

Also at 0.1 AU from the Sun, the thing is going to get mighty hot.

The Spice must flow (1)

Gothmolly (148874) | more than 4 years ago | (#29123169)

Its easy, just take a massive overdose of Spice, float in your tank, and visualize the spaceship getting there. Presto! There it is.

neat (1)

jjeffries (17675) | more than 4 years ago | (#29123215)

I hope I live to see this, and that it looks exactly like the one in Tron. C'mon reality, don't let me down again!
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