Laser Communications for Deep Space

by Paul Gilster on December 30, 2013

A recent email from Centauri Dreams regular Carl Keller reminded me about the laser communications tests conducted aboard a NASA satellite. The Lunar Atmosphere and Dust Environment Explorer satellite (LADEE) carried a laser package that demonstrated excellent download and upload rates and successful transmission of two simultaneous channels carrying high-definition video streams to and from the Moon. The fast transmission of large data files shows how useful laser methods will become.

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Image: NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) observatory launches aboard the Minotaur V rocket from the Mid-Atlantic Regional Spaceport (MARS) at NASA’s Wallops Flight Facility, Friday, Sept. 6, 2013, in Virginia. Image Credit: NASA/Clara Cioffi.

All this is heartening because we need better communications as we begin to build a true infrastructure in the Solar System, while the demands of interstellar communication we’ll eventually need for probes of other stars are even more immense. The easy comparison is sitting right on our desktops in the form of the PCs we use everyday to communicate with the Net. Cable connections make website loading relatively painless, but most of us remember the frustration of early graphics coming in over painfully slow modem connections. Can lasers put the same kind of zip into communications from spacecraft at the edge of the Solar System?

Let’s hope so. I’m remembering the overloads that plague the Deep Space Network, extending decades back. In 1993, the Galileo spacecraft had a chance to take a close look at the asteroid 243 Ida, well worth viewing because the cratered rock was orbited by a ‘moon’. But the DSN also had to handle the load from controllers trying to revive the Mars Observer probe, so that important traffic that would have supported Galileo’s asteroid flyby was diverted. Galileo snapped a photo of Ida anyway, but the problem of overcrowded resources has only gotten worse.

In any case, when we’re talking truly long-distance communications, we have to reckon with the fact that our radio signals drop in intensity with the square of their distance, so a spacecraft ten times farther out than its twin sends a signal that’s fully one hundred times weaker. The numbers on actual missions staggered me when I first ran into them: The signal received from Voyager’s 23 watts was twenty billion times weaker than the power needed to operate a digital wristwatch when the Neptune encounter occurred back in 1989. Put that same signal around Alpha Centauri and it would arrive 81 million times weaker still, as I learned from James Lesh at JPL.

No wonder early starship designers leaned on massive dishes — consider the 40-meter second stage engine bell which, when burned out, the Daedalus craft would employ as a massive communications dish. And in order to process the signals from the starship, the British Interplanetary Society team assumed an Earth-based asset called Project Cyclops, one that would have been armed with a thousand 64-meter antennae. Like Robert Forward, the Daedalus designers as well as the SETI community was thinking big back in the 1970s.

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Image: What might have been. The gigantic Cyclops antenna array as envisioned in the 1970s. Credit: Columbus Optical SETI Observatory.

But Daedalus also was conceived as having laser capability that would be used while the craft was under power, and so was the US Navy student project called Project Longshot, which the class that came up with it equipped with six 250-kilowatt lasers, three for communications during the acceleration of the vehicle, and three for communications as Longshot arrived in the Alpha Centauri system. Lasers change the dynamic, but the point is we’re only now testing out the systems that will eventually make them commonplace in space communications.

Radio beams, after all, spread out at a diffraction rate determined by the wavelength of the signal divided by the diameter of the antenna. When we start pushing into higher and higher frequencies, the resulting signal becomes much more narrow. The advantages in reducing spectrum-crowding are supplemented by the laser signal’s ability to carry much more data, as the recent tests aboard LADEE demonstrate. Moreover, the optical telescopes needed aboard a spacecraft can be significantly smaller than the large radio dishes in use today.

Extend all those ideas into the far future and you wind up with an optical installation about the size of the Hubble Space Telescope capable of beaming useful data back to Earth from Alpha Centauri. That’s the 20-watt laser signal that would be beamed back to space-based telescopes in the Solar System, according to JPL’s Lesh in a well-known paper in JBIS. Remember that Voyager signal — it’s now puffed up to well over a thousand times the diameter of the Earth because of beam diffraction. The tight beam of the Centauri laser would get the message through. Of course, a way to propel a communications system as big as Hubble to another star has to be discovered first.

Can we get around all this with gravitational lensing and much smaller equipment? Conceivably, and I’ll have some interesting news about Claudio Maccone’s FOCAL mission to the Sun’s gravitational lens in the next few weeks. I also want to talk a bit more about the LADEE experiments. I’ve mentioned the Lesh paper in these pages before, but here’s the reference again: Lesh, C. J. Ruggier, and R. J. Cesarone, “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7–14.

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{ 22 comments… read them below or add one }

Allen Taylor December 31, 2013 at 1:40

Paul,

It is incorrect to say that the strength of a radio signal drops off as the square of the distance. That is only true if the transmitting antenna is omnidirectional. There are numerous antenna designs that direct the radiated energy along preferred paths rather than omnidirectionally. For these antennas, signal strength drops off at a rate less than the square of the distance. The extreme case of this is the maser, which sends its signal in only one direction.

Ken Murray December 31, 2013 at 1:44

I’m a novice, but first question comes to mind is what if a large object blocks the transmission? Since this laser comm is being considered for longer distance comm then I think the chances of this happening might increase? Any thoughts? Thanks!

Skotch Vail December 31, 2013 at 2:51

Can lasers put the same kind of zip into communications from spacecraft at the edge of the Solar System?

Because of the speed of light, transmission back and forth in deep space will never be zippy. I think what the laser brings us is not speed, but a big fat pipe, so that we can do lots of things at once all over the just, just like your example of the asteroid flyby. A fatter pipe would have solved that one.

Michael Spencer December 31, 2013 at 8:35

Nice to see a new post here, Paul…you were missed! You daily essays are my second daily read, after APOD, of course…

You know, I thought as I read your piece you might talk a bit more specifically about historical and future rates: where we were, are, will be? I read (somewhere) that the actual interplanetary rates now available are piteous to say the least. Perhaps you could point me in the right direction.

Now, take a bit more time off, Paul, as we look for another great year from you! :-)

Paul Gilster December 31, 2013 at 9:32

Allen Taylor writes:

It is incorrect to say that the strength of a radio signal drops off as the square of the distance. That is only true if the transmitting antenna is omnidirectional. There are numerous antenna designs that direct the radiated energy along preferred paths rather than omnidirectionally. For these antennas, signal strength drops off at a rate less than the square of the distance.

Thanks for the clarification! Much appreciated.

Paul Gilster December 31, 2013 at 9:36

Ken Murray writes:

I’m a novice, but first question comes to mind is what if a large object blocks the transmission? Since this laser comm is being considered for longer distance comm then I think the chances of this happening might increase? Any thoughts? Thanks!

Occulting objects are a problem for communications, to be sure, though as we move toward a network model like the one Vint Cerf’s team has been developing there will be many routes for traffic to take. The post following this one gives encouraging news about how optical communications fare even in challenging circumstances — we’re learning more with each demonstrator mission.

Paul Gilster December 31, 2013 at 9:41

Michael Spencer asks:

You know, I thought as I read your piece you might talk a bit more specifically about historical and future rates: where we were, are, will be?

Somewhere around here I’ve got some figures. I believe the X-band (8.40-8.45 GHz for deep space work, a bit higher for near-Earth) can support 35 times the data of the older L-band (between 1 and 2 GHz). Higher frequencies certainly give you more pop. I’ll dig up some more information for a future post with regard to older and future rates. We’ve also got some readers far more knowledgeable than I am on radio communications — they may want to weigh in here.

ljk December 31, 2013 at 10:04

If we ever learn to communicate effectively with neutrinos, obstacles in the path of the signal will not be a problem. :^) A neutrino can pass through a block of lead one billion miles long (yes, billion) and not hit a single atom along the way.

Does anyone know if those neutrino observatories are searching for artificial signals in their data? I will presume it is during off hours.

The official book about Project Cyclops is online here:

http://seti.berkeley.edu/sites/default/files/19730010095_1973010095.pdf

One thing Project Cyclops did contribute towards was the further derailment of Optical SETI for over two more decades after the study was released.

See here for the details:

http://www.coseti.org/cyclops.htm

Had Cyclops actually been built, what are the odds that it would still be functioning today? Look at how the ATA, a very scaled down version of Cyclops, is just limping along doing more work for the USAF monitoring space debris than SETI.

Bryan December 31, 2013 at 12:56

Allen Taylor writes:

It is incorrect to say that the strength of a radio signal drops off as the square of the distance. That is only true if the transmitting antenna is omnidirectional. There are numerous antenna designs that direct the radiated energy along preferred paths rather than omnidirectionally. For these antennas, signal strength drops off at a rate less than the square of the distance.

This is incorrect. Paul’s original statement was correct. It is true that numerous antenna designs offer gains over omnidirectional antennas. But, they still transmit in some solid angle (this applies even to masers and lasers). The “gain” of a directional antenna is due to the fact that it transmits power in a smaller angle than the 4-pi steradians of a perfect omnidirectional antenna (think of a small cone as opposed to the sphere produced by the omni…). The “spot size” still increases proportional to the square of the distance, though. One of the advantages of lasers (and high-frequency radio transmitters) is that for an equivalent size transmit antenna, they transmit in a much smaller solid angle than a lower frequency transmitter, thereby delivering higher intensities to the receiver.

Paul Gilster December 31, 2013 at 13:36

Bryan, thanks very much — I’m learning from this discussion, that’s for sure.

Michael December 31, 2013 at 14:08

ljk said

‘If we ever learn to communicate effectively with neutrinos, obstacles in the path of the signal will not be a problem. :^) A neutrino can pass through a block of lead one billion miles long (yes, billion) and not hit a single atom along the way.’
The big problem with this type of communication is firstly creating a coherent beam of them for transmission, focusing them and then catching them to read the message which is difficult when they can pass through a billion miles of lead!
I had an idea once of using the Sun to focus neutrinos and other radiation to study the Black hole at the centre of our Galaxy in more detail, I suppose the idea could also be used focus a neutrino transmission but that still leaves the problem of creating and reading them.

Michael December 31, 2013 at 14:47

Forgot to add

Have a Happy New Year to yourself Paul and all your readers, as ever I look forward to many more great articles and posts.

Mick

Michael December 31, 2013 at 16:31

I wonder if we could instead of using light use particles of matter accelerated to approaching light speed as a form of communication. It would use a lot more energy but a lot more information could be transmitted as the particles are much smaller than the wavelength of light. Different atoms could be coded information as well, for instance a tungsten atom could mean a whole word or instruction. Dispersion may be an issue though.

Just a thought

Happy New Year and may wisdom and good health fall upon you all

Eniac January 1, 2014 at 17:48

Bryan is correct, all far-field radiation power diminishes with the square of distance.

ljk January 2, 2014 at 10:47

Michael said on December 31, 2013 at 16:31:

“I wonder if we could instead of using light use particles of matter accelerated to approaching light speed as a form of communication. It would use a lot more energy but a lot more information could be transmitted as the particles are much smaller than the wavelength of light. Different atoms could be coded information as well, for instance a tungsten atom could mean a whole word or instruction. Dispersion may be an issue though.”

Not a bad idea. After all there has been the thought of encoding DNA with messages as well:

http://news.discovery.com/space/alien-life-exoplanets/could-an-alien-message-be-embedded-in-our-genetic-code-130401.htm

and…

http://www.coseti.org/lemarch1.htm

My question is, how would we get the attention of the recipients if the messages are on atoms or molecules?

Ron S January 2, 2014 at 14:17

ljk: “My question is, how would we get the attention of the recipients if the messages are on atoms or molecules?”

Send them a virus or nano-device that, on landing in the ocean, coopts local biology or minerals into leaving messages on the planet’s beaches that can be read when the tide goes out. Or high-energy protons that code for messages as they cause particle showers in the upper atmosphere.

ljk January 2, 2014 at 15:37

Ron S said on January 2, 2014 at 14:17:

ljk: “My question is, how would we get the attention of the recipients if the messages are on atoms or molecules?”

Ron S: “Send them a virus or nano-device that, on landing in the ocean, coopts local biology or minerals into leaving messages on the planet’s beaches that can be read when the tide goes out. Or high-energy protons that code for messages as they cause particle showers in the upper atmosphere.”

What kind of messages would we use to get their attention? I also have to wonder if leaving messages on the ground or in their sky would have unintended consequences, such as freaking them out or having them think their deities or spirits are trying to communicate with them.

Look at our culture where people see stains on a wall or burn patterns in their waffles and think they are signs from Jesus or Elvis.

Michael January 3, 2014 at 4:21

ljk: “My question is, how would we get the attention of the recipients if the messages are on atoms or molecules?”

I was thinking more along the lines of communication with space probes, as the velocity of an atom increases the atom is also reduced in length which should thoeretically allow a higher transmission rate but at the cost of energy usage. Maybe two processes could be carried out at the same time, one for communication (one way) with the probe and after the information has been read the energy of the particle is used to drive internal systems (laser return) or even propel the probes physical sail or mag sail.

Ron S: ‘Or high-energy protons that code for messages as they cause particle showers in the upper atmosphere.’

Nice idea, I think high rate information will degrade significantly but it may be enough to bring their attention to an odd occurance which would warrant further investigation, a sort of pointer to the source of transmission. A slight change may be the use of heavier atoms which would firstly be rare so they would see a difference and they would carry a lot more punch.

Michael January 3, 2014 at 11:34

Paul I have just spotted my error

‘…as the velocity of an atom increases the atom is also reduced in length’

This is to the observer (length dilation) at a side observation of the atoms, but not between the atoms, their length remains the same with respect to each other, so the next part of my statement I made would not be valid.

‘which should thoeretically allow a higher transmission rate but at the cost of energy usage’

opps thinking before typing comes to mind…..

Steve Bowers January 5, 2014 at 19:08

“Bryan is correct, all far-field radiation power diminishes with the square of distance.”

This is true past a certain distance, but if the aperture is big enough then the drop off is reduced. A laser with an aperture the size of the Sun would have very little diminution at the distance of the nearest stars; a laser with an aperture the size of VY Canis minoris would be able to put a spot of light at the far end of the galaxy. The size of the spot of light is also affected by the wavelength.
Luke Campbell uses this rule of thumb;
” The smallest possible spot size to which a beam can be focused can be calculated; if the initial beam width is D, the wavelength of the light is L, and the distance to the target is R, the smallest spot size (S) is given by S = 1.2 R L / D.” Notice that if D is very large, the focussing distance can be very far, especially for short wavelengths.
What we need is some way to make the Sun lase in gamma rays, or something similar.

ljk January 7, 2014 at 10:26

A letter to Nature from 2004 on inscribed matter as an energy-efficient means of CETI:

http://frank.harvard.edu/~howard/papers/inscribed_matter.pdf

Michael January 8, 2014 at 10:53

ljk,

Thanks for the link and paper. Perhaps we could use a hydro-carbon chain with deuterium, hydrogen and side chain atoms onto the carbon backbone to encode information instead. Carbon and hydrogen atoms are quite small and the flexible molecule could be wrapped up quite tightly allowing very high information densities and be read like a thread. The tech would require atomic assembiers and readers though.

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