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Relativistic Navigation Needed For Solar Sails

Posted by Soulskill on 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."

25 of 185 comments (clear)

  1. Computers? by Extremus · · 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.

    1. Re:Computers? by FooAtWFU · · Score: 4, Insightful

      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.

      --
      The World Wide Web is dying. Soon, we shall have only the Internet.
    2. Re:Computers? by TooMuchToDo · · Score: 4, Insightful

      Even if you get pushed off course, you get to discover $UNKNOWN_EXOTIC_EFFECT =)

    3. Re:Computers? by sabt-pestnu · · Score: 3, 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.

    4. Re:Computers? by maxwell+demon · · 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?

      --
      The Tao of math: The numbers you can count are not the real numbers.
    5. Re:Computers? by Antique+Geekmeister · · Score: 3, Informative

      It's a solar sail. Without significant solar thrust, it _will_ drag against the interstellar gas, and it's likely to gain mass as it does so.

    6. Re:Computers? by ByTor-2112 · · Score: 3, Interesting

      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???

  2. One part in 37 million... by argent · · 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.

    1. Re:One part in 37 million... by Anonymous Coward · · Score: 3, Informative

      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.

    2. Re:One part in 37 million... by CrimsonAvenger · · Score: 3, 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.

      --

      "I do not agree with what you say, but I will defend to the death your right to say it"
    3. Re:One part in 37 million... by R2.0 · · Score: 4, Funny

      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.

      --
      "As God is my witness, I thought turkeys could fly." A. Carlson
  3. What else is new? by jarocho · · Score: 5, Interesting

    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.

  4. Re:Wont the accleration decrease with distance by UnHolier+than+ever · · Score: 3, Insightful

    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.

  5. Is this that important? by MiniMike · · 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?

  6. Manuvering system for a sail..... by jameskojiro · · Score: 3, Interesting

    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?

    --
    Tsukasa: All I really want, is to be left alone...
  7. Mid-course corrections? by vrmlguy · · Score: 4, 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?

    --
    Nothing for 6-digit uids?
    1. Re:Mid-course corrections? by WhiteDragon · · Score: 3, Informative

      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 with the solar sail is still possible.

      --
      Did you mount a military-grade, variable-focus MASER on an unlicensed artificial intelligence?
    2. Re:Mid-course corrections? by jpmorgan · · Score: 3, Insightful

      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.

  8. Just how big is the Oort Cloud? by Eevee · · Score: 4, Funny

    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.

    1. Re:Just how big is the Oort Cloud? by Minwee · · Score: 3, Informative

      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.

    2. Re:Just how big is the Oort Cloud? by larry+bagina · · 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.

      --
      Do you even lift?

      These aren't the 'roids you're looking for.

  9. Re:Solar at that distance? by John_Booty · · Score: 3, Informative

    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.

    --

    OtakuBooty.com: Smart, funny, sexy nerds.
  10. Need to think relatively by Lev13than · · Score: 5, Funny

    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.

    --
    When you have nothing left to burn you must set yourself on fire
  11. Move along. Nothing to see here. by mbone · · Score: 3, Informative

    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.

     

  12. Maths required for space navigation? by Crookdotter · · Score: 3, Informative

    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.....