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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."
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.
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.
They are relatively better at the calculations??
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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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.
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.
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?
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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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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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.
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.
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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.
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.
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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.
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There are ways.
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.
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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.
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> 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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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.....