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# Relativistic Navigation Needed For Solar Sails

#### Soulskill posted more than 4 years ago | from KentuckyFC

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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#### Anonymous Coward 1 minute ago

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### One word: (1, Insightful)

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#### Anonymous Coward | more than 4 years ago

one word: computers.

hurrrr.

### Re:One word: (0)

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hurrrr.

Brains?

### Re:One word: (1)

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#### Whalou | more than 4 years ago

One word: Pilgrims.

### Computers? (4, Insightful)

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#### Extremus | more than 4 years ago

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)

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#### FooAtWFU | more than 4 years ago

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

### Re:Computers? (3, Insightful)

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#### sabt-pestnu | more than 4 years ago

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)

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#### OwnedByTwoCats | more than 4 years ago

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)

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#### OwnedByTwoCats | more than 4 years ago

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)

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#### Anonymous Coward | more than 4 years ago

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)

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#### Anonymous Coward | more than 4 years ago

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)

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#### Pentium100 | more than 4 years ago

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)

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#### maxwell demon | more than 4 years ago

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

\end{rant}

### Re:My pet peeve about big numbers (0)

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#### Anonymous Coward | more than 4 years ago

Think you forgot to
\usepackage{rant}

### one more stat (2, Interesting)

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#### ILuvRamen | more than 4 years ago

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)

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#### Anonymous Coward | more than 4 years ago

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)

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

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)

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#### R2.0 | more than 4 years ago

"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)

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#### Anonymous Coward | more than 4 years ago

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)

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#### Tsar | more than 4 years ago

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)

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#### NYFreddie | more than 4 years ago

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)

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#### cowdung | more than 4 years ago

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)

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#### mbone | more than 4 years ago

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)

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#### TheSHAD0W | more than 4 years ago

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)

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#### peter303 | more than 4 years ago

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.

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

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#### radtea | more than 4 years ago

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)

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#### Anonymous Coward | more than 4 years ago

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.

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#### 140Mandak262Jamuna | more than 4 years ago

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.

### What is the top speed? (1)

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#### AttillaTheNun | more than 4 years ago

How close to C does this probe reach during the journey?

### Re:What is the top speed? (1)

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#### careysub | more than 4 years ago

It reaches 0.13% of the speed of light. It will reach the nearest star in 3300 years or so.

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

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#### DRAGONWEEZEL | more than 4 years ago

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)

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#### Kozar_The_Malignant | more than 4 years ago

> 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)

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#### Ancient_Hacker | more than 4 years ago

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)

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#### Gothmolly | more than 4 years ago

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)

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#### jjeffries | more than 4 years ago

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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