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Shining a Light on Interplanetary Communication 84

An anonymous reader writes "Researchers at the MIT have developed a new device that they claim could one day boost interplanetary communication to broadband speeds. From the article: 'The new light detector improves detection efficiency to 57 percent at a wavelength of 1,550 nanometers--the same wavelength used by optical fibers on Earth to carry broadband signals to homes and offices. Currently, light detectors only absorb about 20 percent of the light they receive. "It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth," said study team member Karl Berggren from the Massachusetts Institute of Technology. "But an optical link can do that thousands of times faster."'"
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Shining a Light on Interplanetary Communication

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  • Latency (Score:2, Informative)

    Currently, the maximum data rate between Earth and Mars is about 128,000 bits per second.

    They keep harping on data rate, but what about latency? Given that Mars are Earth are anywhere from 40 to 160 million miles apart [csmonitor.com], perhaps it suffice merely to:

    100 * 10^6 mi / 186 * 10^3 mi/sec = 538 sec

    to estimate its order of magnitude.

    • There isn't much you one can do about latency, so they're improving what they can.
    • Latency is only important for interactive stuff. For bulk data transport is doesn't matter much.
      • It matters a little bit. Not much use transmitting data for 10 minutes only to have the other end respond with "sorry... did you say something? I wasn't listening".

        Of course if you are going to be transmitting data for a few hours, 15 minutes of handshake and setup time doesn't matter so much.

        • with forward error correction, latency is less of a problem. Just send the data and if something goes wrong, just fix what is broken from what you have. handshakeing is still a killer, but there are better protocols to get around that.. queue to send and very large buffers.
    • Re:Latency (Score:2, Interesting)

      by PieSquared ( 867490 )
      Latency isn't going to be fixed, as the speed of light is a constant. The problem, really, is the rate of transfer. If it does take 538 seconds for light to get from mars to earth, there is no reason that this should do anything other then add a 1076 second delay from the first request to the start of data flow... after that there is no reason an un-interupted flow of data shouldn't flow. There is no reason that we shouldn't fix it so that after the delay the data coming in is more dense, with more arriv
      • Maybe quantum communication using entanglement [wikipedia.org] will help.
        Aye, by now we will stay with the 10 minutes latency for *any* communication to Mars because there is no option!
        • Re:Latency (Score:3, Informative)

          No, it will not help. Entanglement cannot be used to communicate faster than light. In quantum teleportation, until you get the classical bits (through the classical, speed of light bound channel), you have a random choice of one of several quantum states, ensuring that any measurement you do will get completely random classical results (or more exactly, just as random as if the other side had done nothing). It's only after you got the classical bits that you can "decode" the quantum state.
        • by TexVex ( 669445 ) on Tuesday March 21, 2006 @01:18PM (#14965022)
          Here is how so-called photon "teleportation" works. This should explain why nothing happens faster than light, and why otherwise this isn't causality-destroying voodoo.

          You have a source that spits out pairs of polarization-entangled photons. Each particular photon is random -- it'll either be "horizontal" or "vertical" with 50% probability each. But, entanglement means that when one member of each pair is vertical, the opposite member of that pair is horizontal, and vice-versa.

          Because of the way QM works, we can't know the polarization of any particular photon pair in advance -- we only know when we pass the photon through a filter and then try to detect it. Both the filter and the detector change the photon, though, so any photon that we measure becomes completely worthless to us thereafter.

          So, we know that our photon pairs always have opposite polarization, but we don't know the exact state for each pair in advance. Now, let's cheat a bit and peek behind the veil, pretending we know the state of each photon in a sample stream. I'll use a 0 to encode one polarization state and 1 to encode its opposite:

          Stream 1: 0010110101
          Stream 2: 1101001010

          Now, right off the bat, suppose we read Stream 1 here and send Stream 2 to Mars. By looking at the values we read locally, and flipping each bit, we know what data Mars will receive. But, there's no way we can inform Mars of the contents of the bitstream ahead of time, because nothing travels faster than light.

          So, what's all this quantum teleportation stuff about? Well, it's like this. Our Stream 1 and Stream 2 above are random, so they're useless to us for transmitting anything but white noise. But, we can do a cool trick and transmit information in that white noise. We can't exceed lightspeed with it, but we can guarantee that the information can't be undetectably intercepted.

          Let's add in Stream 3, which contains data we want to transmit. I pick an arbitrary message -- suppose I want to send alternating bits, like so:

          Stream 3: 101010101010

          Now, I want to send Stream 3 to Mars, but I want it encrypted in the randomness of Streams 1 and 2. To do this, I read in Stream 1 and perform an operation on each result based on the contents of the corresponding bit in Stream 3: whenever a bit in Stream 3 is a 1, then I flip the result that I read in from Stream 1. Otherwise, I keep the Stream 1 bit unmodified:

          Stream 1: 0010110101
          Stream 3: 1010101010
          Stream 4: 1000011111

          So, Stream 4 now contains the data I want to send, mixed with the randomness in one of the two entangled streams. By itself, Stream 4 is meaningless. Also, Stream 1 has been destroyed by reading it. So, I can only decrypt Stream 4 using the data I have from reading Stream 1 -- or by using Stream 2.

          Now, I send Stream 2 to Mars unmodified. Anyone reading that stream destroys it and gets random data out of it. Using a separate beam, I send Stream 4 to Mars. Anyone can intercept this and get the data out of it, but it's useless without Stream 2. At the receiving station, they can combine Stream 2 and Stream 4 using a variation on the rule used to encrypt the data, to learn the contents of Stream 3, and they can be guaranteed that the data wasn't intercepted without them knowing about it:

          If someone intercepts Stream 2, reads it, and substitutes in another random photon stream, then the decryption on Mars will fail, and so the interception will be detected. If someone intercepts Stream 2, reads it, and manages to make a passable copy to beam to Mars, the time delay will be detected. (Not only that, but QM "no cloning" says you can't make a good enough copy anyway.).

          In all of this, nothing at all is happening faster than light. The veil of QM simply says that we can't know the contents of Stream 1 and Stream 2 until we measure them. When we do our encryption operation, we are putting useful data behind that veil, and when we "teleport" the data to the destination, we are getting it back out from behind that veil. But we still have to send everything at light speed.
    • Re:Latency (Score:2, Insightful)

      Don't forget about data outages, for example when the Sun is blocking the line of sight from Mars to Earth. Waiting for the line to clear is going to cause some major latency. The good news is that these outages should be forseeable.
      • Re:Latency (Score:3, Interesting)

        Don't forget about data outages, for example when the Sun is blocking the line of sight from Mars to Earth.

        This problem could be solved with "interplanetary routers" which just route the signal around the sun (i.e. there's some relay station e.g. at the same orbit around sun as Mars, but at a large enough distance so that if the line of sight to mars is blocked, the line of sight to that station isn't; whenever Mars is behind the sun, the signal is relayed through that station; this gives an additional late

        • We just need to build a Ring Collapsiter [amazon.com] around the Sun!
        • More importantly, it would allow for constant downloading of info. As it is now, we get small windows of opportunties during which we send. But if we had small routers that we could put in a solar orbit, it would enable us to pick up more than just a few sats. While I doubt that it will occur anytime soon, it is possible that the micro sats may enable just that in a very cheap fashion.
      • Considering that we are talking about communication between 2 satellites orbiting 2 differents planets.
        The line of sight will be blocked much more often than that by Earth and Mars. there are no stable orbits that would allow permanent line of sight between 2 spacecrafts.

        A system to have almost permanent conection would need 2 or 3 geostationary s/c around each body and 1 at the Earth or Mars lagrange point 4 or 5.

        For those who praised the superiority of optical communications : The signal would then need t
    • by maynard ( 3337 ) <j.maynard.gelinas@NOSpAm.gmail.com> on Tuesday March 21, 2006 @10:20AM (#14963744) Journal
      if we spin it around the beam will be going many order of magnitude faster than light when it reaches mars. That way the long latency is irrelevent. Also, it looks cool. Sorta like a fire engine.

      I'm gonna go to the sandbox and play with my Tonka Firetruck now.
      • will be going many order of magnitude faster than light when it reaches mars.

        um, no.
        • by maynard ( 3337 ) <j.maynard.gelinas@NOSpAm.gmail.com> on Tuesday March 21, 2006 @11:38AM (#14964250) Journal
          "um, no"

          No what? That the spinning laser light won't be going faster than light when it reaches Mars? Of course it will! Duh. If you spin light around the farther out it goes the faster it goes until infinity. Then it starts going slower. That's relativity. Einstein said so.

          "um, no"

          I hear you. You're just not thinking nonlinearly. Stop walking straight and following the cracks in the sidewalk. Jump over them! That's thinking nonlinearly.

          "um, no"

          Look. What is Gravity? It's just the dents in the felt of a pool table caused by a ball. Sort of like a reverse nipple. Therefore, antigravity is the nipple!

          Physics is a snap, dude. You just gotta think.
          • I see that you're joking, but just in case someone else comes along and doesn't understand, I'll say this very clearly, there is no way to get information from one place to another faster than the speed of light. Sure, you can point a laser at a far away wall and move the laser up and down rapidly. ok, so you have a spot that is moving faster than light. So what? The information - the movement of your hand, still propagates to the wall at the speed of light.

            You can't get past that.
          • First, before I continue, I do encourage you to continue with your studies regarding pool tables and nipples. The benefits to society could be enormous.
            That said, I was in complete and total agreement up until your last conclusion here.
            ...antigravity is the nipple...
            Unfortunately, tests have shown that nipples, over time, are very much affected by gravity...
    • Mars' orbit is at around 1.5 AU: 1.5 times further from the sun than earth.

      When earth and Mars are closest, their distance will be about 0.5 AU, when they are furthest (on opposite sides of the sun), about 2.5 AU.

      Light travels about 8 minutes per AU, so it's 4 to 20 minutes one way. Ping times would vary between 8 and 40 minutes.

      As Mars runs a full circle about every two years, earth only catches up with it (i.e. the "excellent" ping times of just 8 minutes) once every two years.
  • Yes Yes... (Score:1, Insightful)

    by Anonymous Coward
    You won't want to be getting into any Shotgun duels while playing Duke Nuke'em Forever on some Earthly server when you're orbiting Mars. Fine. But for uploading instructions and downloading science the speedboost is pretty freaking handy.
  • SETI? (Score:3, Interesting)

    by maxwell demon ( 590494 ) on Tuesday March 21, 2006 @07:29AM (#14962888) Journal
    If optical interplanetary communication is not only possible, but actually more efficient than radio wave communication, what does that mean for the SETI project which analyses radio waves? Shouldn't we expect extraterrestrical civilisations to switch to optical as soon as possible? Did we perhaps look at the wrong frequency all the time?
    • That would have been mentioned and linked at the bottom of the article;

      http://www.space.com/searchforlife/optical_seti_01 0724.html [space.com]
      • The Optical SETI project has efforts in a number of places. The article refers to the Link Observatory but there is also an effort in Boston at the Harvard Observatory. Here is a link which also points to some Whitepapers:

        Beam me over Scotty [harvard.edu]

        Enjoy!

        PS. Keep Watching the Skys!
    • Shorter wavelengths are more energetic, so I would expect it takes more energy to transmit light than radio. The energy required to transmit a signal from another star that we can pick up with our radio telescopes is enormous, so you can expect them to use the more efficient method. Also, there are key frequencies in the radio spectrum, like the 21 cm line and multiples of it, that an ET might think to use and we can look at more closely.

      The improvement with interplanetary communications is caused by the
      • Re:SETI? (Score:3, Informative)

        Shorter wavelengths are more energetic, so I would expect it takes more energy to transmit light than radio.

        You are mixing up the energy of a single photon with the total energy of your transmission. Yes, a single light photon has much more energy than a single microwave photon. But a 100W light source and a 100W radio source both emit exactly the same energy, 100 Joule per second.
        • Also, light is almost infinitely more focusable than radio frequencies (well, at least until we figure out how to built a laser equivalent at all frequencies...). That means (for us now, at least) light uses LESS energy to deliver a set wattage at a given target. Not to mention that a beam of light is inherently more secure than an omnidirectional beacon of radio emmision.
          • Re:SETI? (Score:3, Informative)

            by AKAImBatman ( 238306 ) *
            Masers [wikipedia.org] (microwave amplification by stimulated emission of radiation) predate the invention of the laser. When the laser was invented, it was mostly ignored as nothing more than a special case of the maser.
          • To send but not to receive. It is easier to build large radio telescope than optical ones. Also it is easier to use interferometry in radio to locate precisely the signal source.
          • IMO, that's a double-edged sword. A laser needs more accurate aiming (and frequent correction to compensate for movement) than a radio antenna.
        • But is ease of reception the same per watt at each wavelength? I was thinking that number of photons is what is important for recieving rather than the power.
    • No, due to redshift. We could look pretty much at any wavelength, and have a chance of getting something.
    • Why stop at optical? The higher the frequency of the carrier, the more data can be packed in, so you really want to go as high up the EM spectrum as possible - ie: x-rays or gamma waves.

      Since these waves are propagating through the vacuum of space, you won't have to worry about any frequency-dependant attenuation issues you might get when trying to squirt them through a medium.

      Additionally, if you go to really high-frequencies, there's probably less background noise from stars and stuff.
      • Unfortunately, -very- high frequencies tend to start reacting with the interstellar medium and CMB (indirectly).
  • Now all they have to do is develop the technology to create fiber-optic cable millions of miles long!
  • we don't even have broadband coverage in like 60% of the united states, and we are seriously talking about broadband access for martians.. c'mon..
  • Ok, so they boosted the sensitivity of their detectors. That is good thing, for sure. I would, however, be interested what this technology does to the signal to noise ratio of the transmission. Large bandwidth won't be of much use if you have to repeat every packet one hundred times, because most of it is lost in the noise.
    • Re:S/N? (Score:3, Informative)

      by kirinyaga ( 652081 )
      Redundancy code is something perfectly mastered today. In fact, interplanetary communication drove the technology to where it is now (and is one of the factor leading to the last increases of hard disk bit density). The modern algorithms are able to automatically update the Signal+RedundancyCode output depending of the signal/noise ratio to obtain an optimal transmission rate. Thus "increasing the bandwith" means "increasing the S/N ratio", since this is today the only limitation. So the whole point of this
      • Interesting. Thanks for the info. However, there has to be a point where an increase in sensitivity is eaten up by an increased need for redundancy, especially when you come into the realm of single photon detection. I am not arguing that this is the case here - just interested in the basic theory.
  • A ridiculous article. Even if they boosted the efficiency by a factor of three, without hurting any other specs, you could accomplish the same thing, or better, just by increasing the diameter of the light-collecting lens. To get three times the light in you just need to increase the diameter by the square root of three-- 1.63 something IIRC. Not a big deal. And lenses and mirrors can be increased in size almost indefinitely, while you can only increase the efficiency of the detector another 30-some per
    • Most interplanetary vehicles are absolutely tiny. They are also lightweight. Increasing the diameter of the receiver and adding mirrors and lenses would drastically increase the mass of the craft, let alone the size. Doing that when a solution like this is available would be an almost criminal waste of space/weight on the spacecraft.
    • by Xaositecte ( 897197 ) on Tuesday March 21, 2006 @09:22AM (#14963387) Journal
      A Ridiculous Comment. Over the past few decades, we've boosted the efficiency of recepters an incredible amount, while simultaniously reducing the dish size on Satellite and space communications terminals. Take military Satellite communications Terminals (My Job) - we've gone from a 20' Antenna to a 6' with comparable data rates and greater reliability off the 6' dish in just the time I've been in (almost two years now). This is possible largely due to increased effiency in Dish coatings and designs.

      Meanwhile, the size of a dish is often a limiting factor in space vehicle design, making every advance in getting better reception out of a same-sized or smaller dish that much more important.

      Besides, it's a LOT easier to transport and set up the 6' dish compared to the 20'.
      • >A Ridiculous Comment. Over the past few decades, we've boosted the efficiency of recepters an incredible amount, while simultaniously reducing the dish size on Satellite and space communications terminals. Take military Satellite communications Terminals (My Job) - we've gone from a 20' Antenna to a 6' with comparable data rates and greater reliability off the 6' dish in just the time I've been in (almost two years now).

        I think the improvement you've seen is due to going to higher frequecies, lower n

        • Speeds are the same for all three frequency bands we operate at, and both terminals I've used operate on Single Sideband suppressed Modulation - meanwhile data compression just increases the need for accurate reception (more data transmitted means more data lost every time you lose a bit...)

          The LNAs probably contribute to it a lot though.
    • by farnsaw ( 252018 ) on Tuesday March 21, 2006 @09:23AM (#14963392) Homepage
      A couple of things here.

      1) Cost - creating a mirror that much bigger becomes very very expensive over a certain size. Even if this technology to improve the sensitivity makes the sensor twice as expensive you are still saving money. Remember, these are not bathroom mirrors, we are talking about optically perfect mirrors of great size.

      2) Size and Weight - If we are using the satelites to capture this information rather than ground based devices then size and weight are a critical factor. This technology would weigh nothing more (or minimally more) while a 1/3 bigger mirrow would weigh 1/3 more.

      3) Currently, I believe, we are using radio waves and so therefore we would not be using mirrors. If we were to go to light transmittion, we would probably need to have detectors in space, and I would bet that at least one of them would be in orbit around mars. That said, a bigger mirror again means more size and weight that would have to be transported all the way to Mars.

      Also, your math is wrong, 1.63 is not the square root of 3, 1.732050808 is the square root of 3.

      For the latency crowd out there, use UDP packets rather than TCP packets and then re-request the sending of any missing packets over time. This emulates TCP over UDP but at a higher level that allows transmittion to continue while waiting for acknowledgement of packets received..
      • Your sig, at the moment:
        "Computer Scientists can count to 1024 on their fingers" (non-mutant, non-mutilatated, human computer scientists)

        I've taught my kids how to count on their fingers in binary, though they're happy enough to count to 0x0F (using just four of the 10 available bits). 0x02, of course, is their favorite number, where the pointer finger is the LSB.

        But even using all the available bits, I can only count up to 1023 (1+2+4+8+16+32+64+128+256+512). Although I guess I could use my tongue as an
  • "A Fire on the Deep", by Verner Vinge.

    Except that it looked an awful lot like Galactic Usenet, complete with inter-species flame wars.
  • With a nice broadband speed from Mars to Earth, I can picture more Rovers on Mars doing tasks in prep for humans landing. Just imagine a nice 3d virtual world of Mars with the construction worker on earth. Robots building everything controled from Earth and then waiting for humans to land. Once the humans bounce to landing the robots could go gather them from wherever they bounce to and connect them up to the structure they built. Once on the planet, humans there could take over control of the robots un
    • bandwidth != latency.

      bandwidth we can improve fairly easilly by using more power, better antennas etc. Latency is pretty much fixed for radio and free space optical communications unless there is some REALLY radical discovery (e.g. on a similar level to perpetual motion).
  • Assuming we had the technology to generate gravity wave beams, It should be possible to create a tiny tunnel through space between earth and mars where the effective distance is far less than the 40 to 160 millon mile distance stated earlier, due to the effects of gravity bending space to reduce the effective distance. Radio waves could pass through this tunnel with reduced latency. It may even be possible to modulate the gravity waves themselves with broadband data

    The bigger question is does this "Roswe

  • I'm surprised that nobody picked up on this article in Wired on Delay Tolerant networks [wired.com].

    Basically people are considering how to design protocols such that they will survive communications over networks with very large delays, for example between here and Mars. TCP/IP won't cut it as it depends on interacting in real time.

    Both light and radio waves will get from here to Mars in the same time, and it is only the sensitivity and selectivity of the receivers that will differentiate them. From reading this art
  • I would think that pointing (or aiming for lay-folk) would be the limiting factor in laser communication. It is a fairly difficult task to point a spacecraft accurately and to have the pointing accurate enough for millions of miles would be very difficult. The nice thing about radio waves is they don't require super accurate pointing, even for a high gain antenna. I would imagine that the whole benefit of better bandwidth is achieved by drastically increasing the gain on axis. IANARS (rocket scientist)
    • not a problem, laser beams have divergence (or spread) - in the Apollo 14 lser beam retroreflector experiments, the laser beam, aimed through a telescope, was 7 kilometers wide at the moon, and over 20 kilometers wide when it returned to earth. That was just a quarter million miles, now do a proportion and figure the rough spread for earth/mars distance!
      • The problem is in how many photons make it to the destination. I was watching an interview on television with one of the experimenters for the ALSEP lunar ranging experiment, even with a high-power laser and a bunch of expensive equipment, they were doing well to detect a few photons from each pulse of laser light.
        • that's true, but for for comm applications when sender and receiver are both in space rather than reflecting experiments with two trips through the atmosphere you get quite a few orders of magnitude more photons. the only point I was making was aiming is the easy part of the problem.
  • More details (Score:4, Informative)

    by Steve525 ( 236741 ) on Tuesday March 21, 2006 @10:50AM (#14963945)
    The device in the article is a photon counting device. That means that each individual photon causes an event that can be measured. (Actually, the 57% efficiency means that 57% of the photons cause as event, the other 43% are missed for one reason or another). There are other types of photon counting devices such as avalanche photodiodes, proportional counters (for xrays), and maybe photomultiplier tubes under the right conditions. The problem with avalanche photodiodes is that they take some time to reset after an event, which limits their speed for communications. This is being improved, but this new type of device may offer an intrinsically faster reset time, as well as high efficiency.

    How this new device works is that a thin (9 nm?) superconductor wire (100 nm wide) is patterned into a serpentine path. A current, just below the critical current is driven though the superconduncting wire. (The critical current is the current at which the superconductor is no longer superconducting). Any photon that is aborbed by the wire causes local heating, and the wire can no longer be a superconductor with the amount of current going through it. This causes a sudden increase in resistance which can be measured.

  • Well, we now know one way to block a light-link: Put a spaceship in the path of the link.

    I can't wait until they use quantum tunneling.
  • Here's the problem I see with this concept. Right now we use radio waves to communicate. Suppose you're communicating with a probe on Mars. Mars emits essentially no RF energy. So almost all the signal you get is coming from your probe.

    But with light waves it is another matter. The sun radiates enormous quantities of light, and substantial amounts are reflected from Mars. Imagine trying to see a light shining from Mars to Earth using a telescope. It would be impossible, the light from your probe would be to
    • Assuming Mars would swamp whatever light bands you would use, simply place a transmitter/receiver repeater sufficiently far enough from Mars so as to be able to resolve the light source as distinct. This could be done with a high polar orbit that always is 90 degrees Sun-ward, or perhaps better yet a pair of halo orbit repeaters to serve both the Sun-ward Side and dark side so all of Mars in in constant communication 24/7. If these orbits prove inadequate to be resolvable from Earth then put a repeater at
    • Helium-Cadmium lasers lase in the UV, as do Nitrogen lasers. Since there are violet laser diodes, I imagine UV is not too far off. The Military even has X-Ray lasers, but they require a Nuclear bomb as a pump.
      See also, Free Electron Lasers for info on possible UV lasers.

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