typodupeerror

## Relativistic Navigation Needed For Solar Sails185

Posted by Soulskill
from the i'd-rather-be-sailing dept.
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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## Relativistic Navigation Needed For Solar Sails

• #### Computers? (Score:4, Insightful)

on Wednesday August 19, 2009 @11:05AM (#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? (Score:5, Insightful)

on Wednesday August 19, 2009 @11:15AM (#29120071) Homepage

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

• #### Re: (Score:2, Insightful)

The ideal time to make that discovery is not when you're kissing the Sun from 0.1 AU away.

• #### Re:Computers? (Score:4, Interesting)

on Wednesday August 19, 2009 @06:35PM (#29126987)

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)

on Wednesday August 19, 2009 @11:52AM (#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: (Score:2)

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: (Score:3, Informative)

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: (Score:2)

Kilometers-to-AU translation errors

How about just g to c translation errors?

• #### Re: (Score:3, Informative)

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

\end{rant}

• #### one more stat (Score:2, Interesting)

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.
• #### Re: (Score:2)

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: (Score:2)

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)

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
• #### Move along. Nothing to see here. (Score:4, Informative)

on Wednesday August 19, 2009 @12:23PM (#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? (Score:3, Interesting)

on Wednesday August 19, 2009 @12:37PM (#29121459) Homepage

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)

on Wednesday August 19, 2009 @12:44PM (#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: (Score:2)

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

on Wednesday August 19, 2009 @12:49PM (#29121679) Journal
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? (Score:2)

How close to C does this probe reach during the journey?
• #### Re: (Score:2)

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 (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)

on Wednesday August 19, 2009 @01:46PM (#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.

• #### Re: (Score:2)

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.

• #### Re: (Score:2)

0.025mm? Why use something that thick? McMaster sells 0.0125mm PET film for \$0.25 per square foot.

• #### Re: (Score:2)

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)

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!
• #### Something Missing Here (Score:2)

sending the sail roughly 1 million kilometers off course by the time it reaches the Oort Cloud.

Haven't these guys ever heard of the mid-course correction? I mean, really...

• #### Maths required for space navigation? (Score:4, Informative)

on Wednesday August 19, 2009 @03:36PM (#29124789)
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 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.

I tell them to turn to the study of mathematics, for it is only there that they might escape the lusts of the flesh. -- Thomas Mann, "The Magic Mountain"

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