Slashdot Mirror
Relativistic Navigation Needed For Solar Sails
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."
one word: computers.
hurrrr.
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.
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?
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.
I may be being daft here but wont that acceleration slow with distance from the sun? Does the math stack up?
They are relatively better at the calculations??
Maybe NASA should start recruiting some America's Cup skippers...
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.
I'm a fiscal conservative, it's a pity we don't have a political party anymore
Pioneer 10 has been off-course for a while now. Maybe the trick for reaching the Oort Cloud is to aim for 1 million kilometers to the left.
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?
It's called ground control, major Tom.
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?
Tsukasa: All I really want, is to be left alone...
can't let these danged computers do everything for us!
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?
Nothing for 6-digit uids?
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.
Colonization?
I put on my robe and wizard hat..
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.
A signature always reveals a man's character - and sometimes even his name. -- Evan Esar
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.
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}
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.
SIG: HUP
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.
Google's Super Secret Search Algorithm: SELECT @search_results FROM internet WHERE @search_results = 'good'
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?
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.
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.
.55 arcseconds, not significant enough to bother correcting where we (or the probe) point our antennas.
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 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.
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.
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.
Barbie of Borg - She doesn't just Assimilate, She Accessorizes too!
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!
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.
When you have nothing left to burn you must set yourself on fire
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.
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?
There is slowing of the clock onboard GPS satellites both due to the orbital speed (special relativity) and lower gravity (general relativity). This paper 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.
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.
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
How close to C does this probe reach during the journey?
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.
How much is your data worth? Back it up now.
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.
Blasphemy is a human right. Blasphemophobia kills.
> 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.
Some mornings it's hardly worth chewing through the restraints to get out of bed.
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.
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.
Its easy, just take a massive overdose of Spice, float in your tank, and visualize the spaceship getting there. Presto! There it is.
I want to delete my account but Slashdot doesn't allow it.
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!
The Infinite Improbability Drive is a fictional faster-than-light drive. The most prominent usage of the drive is in the starship Heart of Gold. It is based on a particular perception of quantum theory: a subatomic particle is most likely to be in a particular place, such as near the nucleus of an atom, but there is also a small probability of it being found very far from its point of origin (for example close to a distant star). Thus, a body could travel from place to place without passing through the intervening space (or hyperspace, for that matter), if you had sufficient control of probability.[1]
Haven't these guys ever heard of the mid-course correction? I mean, really...
"It's the height of ridiculousness to say for those 9 lines you get hundreds of millions."
xkcd to the rescue...
http://xkcd.com/265/
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
Why is this a suprise at all? GPS satellites have to include relativistic calculations. This isn't difficult for anyone involved. It's hardly rocket science.....
and no Dune reference, at least not at +5?
THE SPICE MUST FLOW
--
$tar -xvf
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.
it's not about steering, it's about navigating, which is about knowing where you are, and relativity is a tricky thing to compensate for (how do you compensate for phenomena you don't completely understand?). It's like the difference btw your steering wheel and your tomtom. I can see how you might get those confused, they do have some overlapping goals, but really, they perform very different functions when you get right down to it.
I've been trying to think about this problem over the past couple days, and your response about turning the sail. The problem which keeps occurring to me, and which doesn't seem like your 'solution' addresses, is that when you are no longer close to the sun, the force from the sun will be very small, and may not be sufficient to make course corrections? I'm not really sure about that, but it really seems like, with a Solar Sail, you gotta get it right early, because the closer you get to the 'destination', the less power there is available to make course corrections (with the sail, at least).