typodupeerror

• #### Re:Computers? (Score:5, Insightful)

on Wednesday August 19, 2009 @12:39PM (#29120443)
Even if you get pushed off course, you get to discover $UNKNOWN_EXOTIC_EFFECT =) • #### Re: (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? • #### Re:Computers? (Score:4, Interesting) on Wednesday August 19, 2009 @07:35PM (#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 @12:52PM (#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. 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.

You mean, something like the Pioneer anomaly? [wikipedia.org]

• #### Re: (Score:2)

A "new discipline" that is 100 years old.

• #### Re: (Score:2)

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

Computers can't do anything they haven't been programmed to to.

• #### You knew what this mission was when you signed up (Score:2, Offtopic)

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?

• #### Re:You knew what this mission was when you signed (Score:3, Funny)

They are relatively better at the calculations??
• #### One part in 37 million... (Score:5, Insightful)

<peter AT slashdo ... taronga DOT com> on Wednesday August 19, 2009 @12:08PM (#29119977) Homepage Journal

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: (Score:3, Informative)

by Anonymous Coward

You mean one part in 370,000, but on the whole you're right. The unfolding speed of the solar sail, or its random deformation during travel will have a higher impact. What a stupid article.

• #### Re: (Score:2)

You mean one part in 370,000

Doh.

• #### Re: (Score:2)

So, basically, we shouldn't expect Cassini style precision with this?

• #### Re: (Score:2)

That's why you have in-flight course corrections. I think Cassini made several burns on the way to Saturn, as well as the circularizing burn once it got to the system.

• #### Re: (Score:2)

My thought exactly. Of course if we want this new probe to end up near a neighbouring star in 30,000 years, it really starts to add up. But over that time, chances are we'll have lots of other unknown and unexpected effects to correct for as well, so we're going to need to ability to adjust course during flight anyway.

On the other hand, I don't think keeping track of relativistic effects is really going to be that big of an issue. It's not as intuitive as Newtonian physics, but we've got all the necessary m

• #### Re:One part in 37 million... (Score:4, Insightful)

on Wednesday August 19, 2009 @12:36PM (#29120397)

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:One part in 37 million... (Score:4, Funny)

on Wednesday August 19, 2009 @01:44PM (#29121601)

I would think that the Oort cloud itself would be the destination. Theoretically, the distribution of rocks is pretty even, so we should be able to get data no matter where in the cloud the probe goes. If it gets to that random point and finds either nothing, or a whole lot, we need to change the theory, don't we?

Remember, Columbus set out to sail to the Indies, not land in Mumbai harbor. Of course, if we follow that example the probe will crash into Neptune and we'll declare it a new comet, but the general principle is the same.

• #### Re: (Score:3, Informative)

And with essentially unlimited ability to maneuver, course-corrections aren't going to be an issue, really.

I agree with the posts which note that the relative magnitude of the navigational error is trivial (a million kilometers in 2500 AU is the same relative error as one kilometer on a trip to the Moon). I also fully agree that any expedition to the Oort would be a random crapshoot anyway.

I do have to quibble with the notion of 'essentially unlimited ability to maneuver', however. The amount of thrust

• #### Re: (Score:2)

Well, you could use a moon-based laser system to deliver the power for a mid-course correction. And laser power while also falling off as the square of the distance, can still fall off a lot more slowly. You'd need a hefty laser beyond appreciable atmosphere, which is why I said moon based (to allow for cooling). Asteroid based might be even better, but harder to build.

Besides, if you build it on the far side of the moon no rogue faction can use it to threaten it's home country. But it could still shoot

• #### Re: (Score:2)

great idea. now if only we could fix the moon in place, so that the dark side is always pointing towards the probe...

• #### Re: (Score:3, Informative)

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.

Why call it the "Oort Cloud" if there's nothing in it? My view is that such solar sails would be first used for Kuiper Belt targets and the heliopause (the latter not needing trajectory accuracy aside from making sure the probe heads away from the Sun). Later as we discover targets in the Oort cloud to investigate, probes could be sent out in this way. It's also good for interstellar missions. These velocities provide a good first stage boost. Accurate trajectories might greatly reduce the propellant consum

• #### Need to think relatively (Score:5, Funny)

on Wednesday August 19, 2009 @01:22PM (#29121101) Homepage
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.
• #### As always.... (Score:2)

xkcd to the rescue...

http://xkcd.com/265/ [xkcd.com]

• #### Shouldn't this be irrelavent... (Score:3, Funny)

on Wednesday August 19, 2009 @12:11PM (#29120015)

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.

• #### Re: (Score:2)

Until we find out that someone has parked No-ships in the cloud...
• #### What else is new? (Score:5, Interesting)

on Wednesday August 19, 2009 @12:13PM (#29120045)
Pioneer 10 has been off-course [wikipedia.org] for a while now. Maybe the trick for reaching the Oort Cloud is to aim for 1 million kilometers to the left.
• #### Re: (Score:2)

Should have taken that left turn at Albuquerque.

• #### Is this that important? (Score:5, Insightful)

on Wednesday August 19, 2009 @12:17PM (#29120109)

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?

• #### Re: (Score:2)

I doubt it will be easy to get the solar sail within 0.1 AU of the Sun, that is very close, and will need a lot of energy to begin will. Apart from the relativistic course correction need to nagivate (which isn't that numerically difficult), the sail will have to deal with variation in the amount of Sun light and Solar wind, coming towards the Sail, which may vary at random, and be much more difficult to nagivate with. Good luck to Solar Sailors.

---

Space Craft [feeddistiller.com] Feed @ Feed Distiller [feeddistiller.com]

• #### Re: (Score:2)

Well, if it's a sail, maybe it can just slow itself down from an Earth-level Sun orbit until it gets lower, then flip 90Â to face the Sun and then get pushed away?
• #### Re: (Score:2, Interesting)

The only force acting on a solar sail is outwards from the sun. If the sail is angled the force would be reduced (less cross-sectional area), but the direction of the force would remain the same. A sailboat can only turn because it has a keel that exerts force against the relatively-motionless water normal to its direction of motion. There is nothing to push against in space.

The only way to move in a direction other than away from the sun is to employ alternate propulsion, or to somehow find another sour

• #### Re: (Score:3, Informative)

Wrong.

The solar wind force is essentially outward (in the solar wind direction) only. (The particles initially stick to the sail and then are released, if at all, by a different mechanism such as electrostatic repulsion.) And the portion of the light that is absorbed by the sail also produces an outward force.

But for a mirror-finished solar sail the portion of the light that is reflected (most of it) gives the vector sum of the momenta of its arrival and the recoil of its departure. So tilting the sail t

• #### Re: (Score:2)

The solar wind force is essentially outward (in the solar wind direction) only. (The particles initially stick to the sail and then are released, if at all, by a different mechanism such as electrostatic repulsion.)

Or penetrate it and deposit momentum from drag averaging out to be along their incoming direction of travel.

• #### Re: (Score:2)

No I'm not sure you get what I was proposing. The idea was to orient the sail so it offers virtually no cross section to the solar wind, and let the atmospheric drag do the job of slowing down its orbit so that it can descend.
• #### Re: (Score:2)

I doubt it will be easy to get the solar sail within 0.1 AU of the Sun, that is very close, and will need a lot of energy to begin will.

Takes just as much energy as a solid capsual, actually. Just don't deploy the sail til you get there...

• #### Manuvering system for a sail..... (Score:4, Interesting)

on Wednesday August 19, 2009 @12:18PM (#29120123) Journal

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?

• #### Re: (Score:2)

No need for holes in the sail, you just articulate the sail itself.
• #### Mid-course corrections? (Score:5, Insightful)

<samwyse@gm a i l.com> on Wednesday August 19, 2009 @12:20PM (#29120145) Homepage Journal

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?

• #### Re: (Score:3, Informative)

And isn't starting at the Sun and aiming for a point in the Oort cloud complicated by the N-body problem anyway? Course corrections will have to be done for the entire trip because of all of those large chunks of rock and gas floating around. Gravity's a bitch, man.

• #### Re: (Score:2)

And isn't starting at the Sun and aiming for a point in the Oort cloud complicated by the N-body problem anyway?

Not really. The Oort cloud isn't exactly small. Just keep flying away from the sun and you'll get there eventually.

• #### Re: (Score:2)

And isn't starting at the Sun and aiming for a point in the Oort cloud

Not really.

You missed the point. Hitting any part of the Oort cloud is easy. Trying to hit any particular point in space without course corrections is unbelievably hard, unless it's a really deep and small gravity well....

• #### 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 s

• #### Re: (Score:2, Informative)

On a "good" sail the surface is very reflective. The force that propels the spacecraft is the sum of two vectors; one pointing from the sun to the spacecraft, and a second for the reflected radiation leaving the sail. So you can steer the spacecraft by shortening one side and lengthening the other side of the says attaching the sail to the spacecraft, redirecting the outgoing vector. Or do something similar (e.g. reorient segments rather than the whole sail).

• #### Re: (Score:2)

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 ki

• #### Re: (Score:2)

If the solar sail is your means of propulsion, do you include some sort of 'conventional' rockets to make your course adjustments?

Just like you do on a sailing ship: you turn the sail.

• #### Re: (Score:3, Interesting)

It can't work like a sailboat does... steering partly into the wind, or changing the sail angle to alter the thrust exerted. There's no resistive force to work against, so it just kind of goes where it is taken.
• #### Re: (Score:3, Interesting)

Why not? Reel in the shrouds on one side and lengthen them on the other and the whole sail is tilted with respect to the capsule, and you start to change course. You can't tack (I think) but a broad reach should work.
• #### Re:Mid-course corrections? (Score:4, Informative)

on Wednesday August 19, 2009 @01:32PM (#29121325) Homepage Journal

It can't work like a sailboat does... steering partly into the wind, or changing the sail angle to alter the thrust exerted. There's no resistive force to work against, so it just kind of goes where it is taken.

However, tacking [caltech.edu] with the solar sail is still possible.

• #### Re:Mid-course corrections? (Score:4, Insightful)

on Wednesday August 19, 2009 @02:52PM (#29122837) Homepage
Of course there's resistive force. It's called gravity and most people, when they think about space travel, vastly underestimate it's strength.

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: (Score:3, Informative)

Of course there's resistive force. It's called gravity and most people, when they think about space travel, vastly underestimate it's strength.

Gravity is not a resistive force. A resistive force is a force that acts opposite the motion of a moving object. Gravity is an attractive force between masses independent (in the Newtonian model in which it exists) of the motion of the masses. I think GR has an extremely weak resistive force in that gravity waves carry away some energy from masses moving near one another.

• #### Re: (Score:2)

With most probes they're pretty compact, small thruster bursts will do a lot.

How do you tack a solar sail though?

• #### Re: (Score:2)

You can call a U-turn a mid-course correction but it is still best to start off pointed in the correct direction.
• #### Just how big is the Oort Cloud? (Score:5, Funny)

on Wednesday August 19, 2009 @12:22PM (#29120185)
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.
• #### Re:Just how big is the Oort Cloud? (Score:4, Informative)

<dcr@neverwhen.org> on Wednesday August 19, 2009 @01:20PM (#29121051) Homepage

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.

I think that watching The Empire Strikes Back may have given you the wrong idea about just how densely packed objects like asteroids and comets are in our solar system.

Consider this. Get your own envelope and pencil if you want to follow along at home. The inner boundary of the Oort cloud is at about 5,000 AU, or 750 billion km from the Sun. The outer boundary is expected to be somewhere around 100,000 AU or 1500 billion km. Inside that volume are an estimated twelve billion objects. Nobody has been able to count them, but Jan Oort guessed that there would be that many and no astronomer has been able to contradict him yet.

That gives us a total volume on the order of 10^28 km^3, with just 12,000,000,000 objects in it. That's 10^18 km^3 for each object, giving you an average distance between objects of at least a million kilometers. A million km is three times the distance from the Earth to the Moon, and the size of a cometary nucleus is on the order of ten km. You'd be lucky just to see a 10 km object at that distance, let alone see it well enough to justify the trip out there.

That means that if you're aiming for an object in the Oort cloud but miss by up to a million km, you're going to sail right through empty space. You won't narrowly dodge between densely packed cometary bodies, rolling and weaving to avoid laser blasts, and then have to hide inside the belly of a giant space worm while the Empire searches for you. You'll just pass on by and miss everything.

Real astronomy isn't nearly as exciting as Star Wars, but that's probably good news for everyone who lives in our galaxy.

• #### Re: (Score:2, Interesting)

by Anonymous Coward

Actually, if they're roughly a million km apart, and you miss one by a million km... you're in the vicinity of another one.

Now missing by *half* a million km... that'd suck.

• #### Re: (Score:3, Funny)

Asteroids do not concern me, Minwee. I want that ship, not excuses.
• #### Re: (Score:3, Insightful)

No one has contradicted him? More like no one has any evidence whatsoever that there is an oort cloud, much less that his guess is right or wrong.

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.

• #### My pet peeve about big numbers (Score:2)

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}

• #### 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 @04:36PM (#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.

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