Relativistic Navigation Needed For Solar Sails 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."
One word: (Score:1, Insightful)
one word: computers.
hurrrr.
Computers? (Score:4, Insightful)
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 part in 37 million... (Score:5, Insightful)
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:Computers? (Score:5, Insightful)
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:Wont the accleration decrease with distance (Score:4, Insightful)
The reall question will be: how does it stops? I doubt it can use the gravitational slingshot trick at these speeds using only comets.
Is this that important? (Score:5, Insightful)
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?
Mid-course corrections? (Score:5, Insightful)
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?
How are you making your course corrections? (Score:3, Insightful)
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:One part in 37 million... (Score:4, Insightful)
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:Computers? (Score:5, Insightful)
Re:Computers? (Score:4, Insightful)
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:One part in 37 million... (Score:1, Insightful)
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:Just how big is the Oort Cloud? (Score:3, Insightful)
I'm Confused (Score:3, Insightful)
> 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:Mid-course corrections? (Score:4, Insightful)
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:Computers? (Score:3, Insightful)
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.
"Once you've got to 0.6g"?
g is acceleration! c is velocity!
1g is the rate of acceleration due to gravity on Earth 9.8 m/s^2. 1c is the speed of light, 299,792,458 m/s.
If you don't put in more energy, you're gonna stop accelerating, friction or no friction!
Re:Computers? (Score:2, Insightful)
The ideal time to make that discovery is not when you're kissing the Sun from 0.1 AU away.
Let's start with unmanned probes, hey?