Laser Communications for Deep Space

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.


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.


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.


Finding the Galactic Internet

Greg Egan, a jewel in Australia’s science fiction crown, writes in his 1997 novel Diaspora about a mind-bending far future scenario for interstellar travel. The human race has split into those still in biological bodies, those embedded in humanoid robots, and those who choose to live as software running on central computers. I won’t get into the rich details of the novel this morning, but suffice it to say that the diaspora portrayed here involves a thousand clones of a future Earth community sent to explore nearby stars. Different digitized copies of the same characters spin out their own story lines over a background that spans hundreds of light years.

This is one way to get to the stars, reminiscent of Robert Freitas’ nanotech probes that house thousands of human intelligences in spacecraft no larger than needles. It’s a reminder that highly advanced future cultures may have means at their disposal for star travel even if we find no way of getting up to more than a small percentage of the speed of light. It’s also a memo to SETI theorists about what we might look for as we ponder the shape of extraterrestrial civilizations. If biological life is gradually replaced in favor of software and AI, what signatures will we find?


Image: A classic spiral, the Whirlpool Galaxy (M51) is 30 million light years distant and 60 thousand light years across. What sort of communications network might link civilizations here? Credit: N. Scoville (Caltech), T. Rector (U. Alaska, NOAO) et al., Hubble Heritage Team, NASA.

Egan’s work came to mind as I read this Washington Post story about Geoff Marcy’s recent grant from the Templeton Foundation. The grant is one we’ve discussed before (see Finding ET in the Data), and it’s noteworthy that Marcy’s hunt for Dyson spheres is paralleled by grants to Jason Wright (Pennsylvania State) and colleagues Steinn Sigurðsson and Matthew Povich, who will be using data from the Wide-Field Infrared Explorer satellite (WISE), as well as Lucianne Walkowicz (Princeton University), whose team will be looking through Kepler data for unusual light curves that might flag artificial constructions of enormous size.

I’ll naturally track these projects with great interest. But for today, I want to focus on the rest of Marcy’s search. For while Dyson spheres invariably seem to draw the most attention, only a fraction of the $200,000 Templeton grant will be devoted to their discovery. While some of that money will fund the work of a grad student developing code to search the Kepler data for unusual signatures, the rest buys Marcy time at the Keck Observatory at Mauna Kea, where the game shifts to optical SETI as Marcy tries to spot the laser signaling of an interstellar network.

Enter the Galactic Internet

Like Robert Freitas, Timothy Ferris has speculated about self-reproducing probes that could be sent in small packages to neighboring star systems. Here the idea is that the probe mines local resources, perhaps in an asteroid belt around the target star, and builds an observatory that can send information back to Earth. Over the course of time, the probe reproduces and sends a clone of itself still further out. Ferris’ own thoughts on this go back for more than three decades — see his 1992 title The Mind’s Sky: Human Intelligence in a Cosmic Context for more — and include the emergence of a potentially galaxy-spanning network. The probes become communications stations that constantly monitor other such stations, transmitting and receiving data. Each station becomes a library as galactic information is stored and forwarded:

The interstellar network functions independently of any one world. It has a master program, akin to a set of genetic instructions, originally composed by intelligent biological beings or by another computer. This program gives it its charter — to handle traffic efficiently, to store and organize a copy of everything it conveys… to keep expanding the network as the traffic requires, to search for new communicative worlds, and to keep querying worlds that have gone off line to see whether someone may still be there. How, exactly, it goes about doing these things is its own affair; once set in motion the network has a life of its own.

A galactic network begins with the realization that sending short, conversational messages to other systems is not the way to proceed. Instead, messages will be long and content-laden simply because of the amount of time it takes to communicate. We don’t know how long civilizations last, but Ferris notes that even if a culture managed to stay in communications mode for ten million years, that would still represent no more than one tenth of one percent of the age of the galaxy. And that would mean that only one in 1000 of all civilizations that have inhabited the Milky Way is still in existence, formidable odds for those who want to communicate with distant cultures.

Although Ferris has been thinking about interstellar networking since 1975, the emergence of the Internet spurred the notion of what he calls a ‘galactic central nervous system.’ Node connects to node and, potentially, to the nodes of other discovered civilizations, with an efficient network flow that means knowledge of other cultures spreads gradually and without the need for point-to-point contact between each of the discovered civilizations. Each node keeps and distributes the data it collects. In a 1999 essay called “Interstellar Spaceflight: Can We Travel to Other Stars?” Ferris mused on the consequences of such a network for what we see around us:

If there were any truth in this fancy, what would our galaxy look like? Well, we would find that interstellar voyages by starships of the Enterprise class would be rare, because most intelligent beings would prefer to explore the galaxy and to plumb its long history through the more efficient method of cruising the Net. When interstellar travel did occur, it would usually take the form of small, inconspicuous probes, designed to expand the network, quietly conduct research and seed infertile planets. Radio traffic on the Net would be difficult for technologically emerging worlds to intercept, because nearly all of it would be locked into high-bandwidth, pencil-thin beams linking established planets with automated nodes. Our hopes for SETI would rest principally on the extent to which the Net bothers to maintain omnidirectional broadcast antennae, which are economically draining but could from time to time bring in a fresh, naive species – perhaps even one way out here beyond the Milky Way’s Sagittarius Arm. The galaxy would look quiet and serene, although in fact it would be alive with thought.

In short, it would look just as it does.

Can such a network, one that maintains ‘omnidirectional broadcast antennae,’ survive the analysis of work like Jim and Greg Benford’s on economically viable beacons? A more likely scenario seems to be what Marcy is suggesting, that what we might detect is an errant beam rather than a targeted, beacon-like signal. It’s good to have the hunt proceeding for just such an event (and we can’t rule out the possibility that the famous Ohio State WOW! reception was itself an errant signal at radio frequencies). It’s also good to remember that where our own culture may be most visible to any outside civilization is in the occasional, non-repeating sweeps of our planetary radars, engaged in the effort of studying the heavens for potentially dangerous objects.

We’re a long way from building a Ferris-style network ourselves, but it’s worth pondering what the planet might look like when we do get to these levels of technology. And if someone else has built one? Maintaining a watchful attitude at the most likely parts of the electromagnetic spectrum is good practice, and the emerging methods of transit study and data mining for unusual signatures — from the galaxy level down to individual stars — will set new directions for the overall SETI effort.


Testing Out Pulsar Navigation

Tracking spacecraft from Earth is an increasingly cumbersome issue as we continue to add new vehicles into the mix. The Deep Space Network can track a Voyager at the edge of the Solar System, but using round-trip times and the Doppler shift of the signal is a less than optimal solution for accurate tracking. What we’d like is a method that would allow the spacecraft to calculate its position on its own, taking precise readings from some system of celestial markers.

Pulsars have been in the mix in this thinking for some time. After all, these remnants of stars rotate at high speed and put out radiation beams that blink on and off at regular intervals. They’ve been called ‘celestial lighthouses’ because of this effect, and they’re usefully consistent, producing their pulses in intervals that vary from milliseconds to seconds. The easiest analogy is with the global positioning system, and in this recent article in IEEE Spectrum (thanks to Frank Smith for the pointer), that’s exactly how their use is described:

A craft heading into space would carry a detector that, similarly to a GPS receiver, would accept X-rays from multiple pulsars and use them to resolve its location. These detectors—called XNAV receivers—would sense X-ray photons in the pulsars’ sweeping light. For each of four or more pulsars, the receiver would collect multiple X-ray photons and build a “light curve.” The peak in each light curve would be tagged with a precise time. The timing of these peaks with respect to one another would change as you traveled through the solar system, drawing nearer to the source of some and farther from others. From this pattern of peaks, the spacecraft could calculate its position.


Image: The magnetic poles on a neutron star act like fountains, an escape valve for charged particles that get trapped in the star’s enormously strong magnetic field. As a neutron star spins, its polar fountains turn with it, like an interstellar lighthouse beam. From Earth, we see the beam as it quickly sweeps past us — there, gone, there, gone — many times a second. That looks like a pulse from here. Hence the name, “pulsar.” Credit: National Radio Astronomy Observatory.

Pushing the pulsar navigation idea forward is a table-top device known at NASA Goddard as the Goddard X-ray Navigation Laboratory Testbed (GXNLT), which has been developed to test out a navigation experiment that will be flown on the International Space Station as early as 2017. The ‘pulsar-on-a-table’ can mimic pulsar spin rates and model pulsar locations in the sky, simulating the environment that the upcoming ISS experiment will encounter. The X-ray photons it produces are detected and their arrival times processed by algorithms to extract orbital position.

These technologies are being validated for the Neutron-star Interior Composition Explorer/Station Explorer for X-ray Timing and Navigation Technology mission (NICER/SEXTANT), which will, in addition to demonstrating navigation by pulsars, study the interior composition of neutron stars (for more on NICER/SEXTANT, see this GSFC news release). The assumption is that like the 26-satellite GPS system, pulsar navigation will be able to use onboard software to calculate a position, but without the obvious, Earth-centric limitations of GPS signals.

Luke Winternitz, a co-developer of the Goddard testbed, thinks navigating with X-rays can open up deep space for autonomous navigation, and the test equipment is a vital step in shaking out the system:

“X-ray navigation has the potential to become an enabling technology for very deep space exploration and an important augmentation to NASA’s Deep Space Network. We had to have a way to test the technology. We have GPS constellation simulators that make our GPS receivers think they are in orbit; we needed something analogous for an XNAV receiver.”

So far the ground tests using the pulsar-on-a-table show that the system will be accurate within one kilometer in low-Earth orbit, but the goal is to reach accuracies in the hundreds of meters even in deep space. By the time we do get NICER/SEXTANT into place on the International Space Station, we should have a high degree of confidence that the system will work. Ultimately, basing navigation within the Solar System and beyond on Earth-based equipment will give way to autonomous navigation, a necessity for accuracy as well as practicality as we push outward.


Talking Back from Alpha Centauri

Back when I was working on my Centauri Dreams book, JPL’s James Lesh told me that the right way to do communications from Alpha Centauri was to use a laser. The problem is simple enough: Radio signals fall off in intensity with the square of their distance, so that a spacecraft twice as far from Earth as another sends back a signal with four times less the strength. Translate that into deep space terms and you’ve got a problem. Voyager puts out a 23-watt signal that has now spread to over one thousand times the diameter of the Earth. And we’re talking about a signal 20 billion times less powerful than the power to run a digital wristwatch.

Now imagine being in Alpha Centauri space and radiating back a radio signal that is 81,000,000 times weaker than what Voyager 2 sent back from Neptune. But lasers can help in a major way. Dispersion of the signal is negligible compared to radio, and optical signals can carry more information. Lesh is not a propulsion man so he leaves the problem of getting to Alpha Centauri to others. But his point was that if you could get a laser installation about the size of the Hubble Space Telescope into Centauri space, you could send back a useful datastream to Earth.

The probe would do that using a 20-watt laser system that would lock onto the Sun as its reference point and beam its signals to a 10-meter telescope in Earth orbit (placed there to avoid absorption effects in the atmosphere). It’s still a tough catch, because you’d have to use optical filters to remove the bright light of the Alpha Centauri system while retaining the laser signal.

But while the propulsion conundrum continues to bedevil us, progress on the laser front is heartening, as witness this news release from Goddard Space Flight Center. Scientists working with the Lunar Reconnaissance Orbiter have successfully beamed an image of the Mona Lisa to the spacecraft, sending the image embedded in laser pulses that normally track the spacecraft. It’s a matter of simultaneous laser communication and tracking, says David Smith (MIT), principal investigator on the LRA’s Lunar Orbiter Laser Altimeter instrument:

“This is the first time anyone has achieved one-way laser communication at planetary distances. In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use. In the more distant future, it may allow communication at higher data rates than present radio links can provide.”

The Lunar Reconnaissance Orbiter, I was surprised to find, is the only satellite in orbit around a body other than Earth that is being tracked by laser, making it the ideal tool for demonstrating at least one-way laser communications. The work involved breaking the Mona Lisa into a 152 x 200 pixel array, with each pixel converted into a shade of gray represented by a number between 0 and 4095. According to the news release: “Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bits per second.”

MATLAB Handle Graphics

Image: NASA Goddard scientists transmitted an image of the Mona Lisa from Earth to the Lunar Reconnaissance Orbiter at the moon by piggybacking on laser pulses that routinely track the spacecraft. Credit: NASA Goddard Space Flight Center

The image was then returned to Earth using the spacecraft’s radio telemetry system. We’ll soon see where this leads, for NASA’s Lunar Atmosphere and Dust Environment Explorer mission will include further laser communications demonstrations. The robotic mission is scheduled for launch this year, and will in turn be followed by the Laser Communications Relay Demonstration (LCRD), scheduled for a 2017 launch aboard a Loral commercial satellite. LCRD will be NASA’s first long-duration optical communications mission, one that the agency considers part of the roadmap for construction of a space communications system based on lasers.

If we can make this work, data rates ten to one hundred times higher than available through traditional radio frequency systems can emerge using the same mass and power. Or you can go the other route (especially given payload constraints for deep space missions) and get the same data rate using much less mass and power. The LCRD demonstrator will help us see what’s ahead.

In any case, it’s clear that something has to give when we think about leaving the Solar System. Claudio Maccone has gone to work on bit error rate, that essential measure of signal quality that takes the erroneous bits received divided by the total number of bits transmitted. Suppose you tried to monitor a probe in Alpha Centauri space using one of the Deep Space Network’s 70-meter dishes. Maccone assumes a 12-meter inflatable antenna aboard the spacecraft, a link frequency in the Ka band (32 GHz), a bit rate of 32 kbps, and forty watts of transmitting power.

The result: A 50 percent probability of errors. We discussed all this in these pages a couple of years back in The Gravitational Lens and Communications, so I won’t rehash the whole thing other than to say that using the same parameters but working with a FOCAL probe using the Sun’s gravitational lens at 550 AU and beyond, Maccone shows that forty watts of transmitting power produce entirely acceptable bit error rates. Here again you have to have a probe in place before this kind of data return can begin, but getting a FOCAL probe into position could pay off in lowering the mass of the communications package aboard the interstellar probe.

Whether using radio frequencies or lasers, communicating with a probe around another star presents us with huge challenges. James Lesh’s paper on laser communications around Alpha Centauri is Lesh, C. J. Ruggier, and R. J. Cesarone, “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7-14. Claudio Maccone’s paper is “Interstellar radio links enhanced by exploiting the Sun as a Gravitational Lens,” Acta Astronautica Vol. 68, Issues 1-2 (2011), pp. 76-84.


Pulsar Navigation: Beacons in the Darkness

In a world of search engines, GPS and always-on connectivity, I sometimes wonder what’s happening to serendipity. Over the years, I’ve made some of my best library finds by browsing the stacks, just taking some time off and walking around scanning the book titles. Odd ideas show up, mental connections get forged, and new insights emerge. Targeted searching is generally what we do (think Google), but never forget the value of the odd juxtaposition that comes from random wanderings. Too much targeting can produce tunnel vision.

For that matter, have you noticed how hard it is to get lost these days? I’m just back from Oakland, where Marc Millis and I went for interviews with the History Channel in the gorgeous setting of Chabot Space & Science Center in the hills above the city. The view on the drive up was spectacular, and my guide used an iPad to continually update our position on the map, so getting lost was impossible. My son Miles drove up from his home south of San Francisco and after the interview he drove me back to the hotel, where we met Marc for dinner at a nearby restaurant. All the way down from Chabot, he was keeping one eye on the smartphone he was using for navigation, flawlessly threading his way through streets that were new to him.

Maybe someday the whole idea of getting lost and running into the unexpected will seem quaint — we’ll know where we are at every moment. I can see the value in that even though I enjoy occasionally taking random streets just to see where they lead and surprising myself. Watching city lights under flawless night skies from my window on a Southwest flight last night, I was musing about navigation and stars and remembering being taught the now antiquated art of celestial navigation by a gruff flight instructor who used to do it for real back in the 1930s, when he was flying biplanes and knew how to read the stars like most of us read a roadmap.

Of course, navigating among the stars is going to demand much more precision than this when we’re talking about actual interstellar missions. This morning, pre-coffee and still jet lagged, I was looking through some saved links and ran across a BBC story called Dead Stars to Guide Spacecraft, recounting the work of Werner Becker (Max-Planck Institute for Extraterrestrial Physics). Becker’s team has been studying positioning methods for spacecraft using the X-ray signals sent by pulsars, rapidly rotating and extremely precise sources of emissions.


Pulsar beams are tightly focused and sweep around the sky as the pulsar spins. What we’re looking for is a way to place a spacecraft in a three-dimensional frame, taking advantage of the sheer regularity of pulsars by measuring the time of arrival of their pulses, which offer a stability akin to that of atomic clocks. Carrying the right equipment, our space voyagers should be able, in Becker’s view, to position themselves within five kilometers anywhere in the galaxy. As Becker tells the BBC’s Jonathan Amos, “These pulsars are everywhere in the Universe and their flashing is so predictable that it makes such an approach really straightforward.”

Image: Artist’s impression of ESA’s Rosetta spacecraft, imagined as if it navigated in deep space using pulsar signals. Credit: ESA/MPE.

The accuracy of pulsars and their characteristic time signatures amount to a method of navigation that some are likening to GPS satellites and their signals here on Earth. The needed miniaturization for making X-ray detection practical as a navigation tool is on the way, and Becker believes that within fifteen to twenty years, lightweight X-ray mirrors for navigation devices based on pulsar methods will be available for testing. Their advantages may become quickly apparent if the technology is sound. Right now the positioning errors for the Voyager probes amount to several hundred kilometers, using Earth-based antennae and communications travel times to make the call.

I see that the UK’s National Physical Laboratory and the University of Leicester are working with the European Space Agency to investigate pulsar methods, noting that traditional ground-based space navigation is over-taxed, only able to support a limited number of spacecraft at a time. A future pulsar technology would allow spacecraft to handle navigation chores onboard.

So the benefits are near-term but could reach deep into the future. I like Werner Becker’s enthusiasm, as found in this Royal Astronomical Society news release: “Looking forward, it’s incredibly exciting to think that we have now the technology to chart our route to other stars and may even be able to help our descendants take their first steps into interstellar space.” Indeed. While I will always extoll the pleasures of serendipity, I wouldn’t want to be traveling at random on an interstellar journey, just as I was glad last night that Southwest’s crew was keeping an eye on their gauges while I mused on dark landscapes and distant lights as I crossed the continent.


Neutrino Communications: An Interstellar Future?

The news that a message has been sent using a beam of neutrinos awakened a flood of memories. Back in the late 1970s I was involved with the Society for Amateur Radio Astronomers, mostly as an interested onlooker rather than as an active equipment builder. Through SARA’s journal I learned about Cosmic Search, a magazine that ran from 1979 through 1982 specializing in SETI and related issues. I acquired the entire set, and went through all 13 issues again and again. I was writing sporadically about SETI then for Glenn Hauser’s Review of International Broadcasting and later, for the SARA journal itself.

Cosmic Search is a wonderful SETI resource despite its age, and the recent neutrino news out of Fermilab took me right back to a piece in its third issue by Jay Pasachoff and Marc Kutner on the question of using neutrinos for interstellar communications. Neutrinos are hard to manipulate because they hardly ever interact with other matter. On the average, neutrinos can penetrate four light years of lead before being stopped, which means that detecting them means snaring a tiny fraction out of a vast number of incoming neutrinos. Pasachoff and Kutner noted that this was how Frederick Reines and Clyde Cowan, Jr. detected antineutrinos in 1956, using a stream of particles emerging from the Savannah River reactor.

The Problem of Detection

In his work at Brookhaven National Laboratory, Raymond Davis, Jr. was using a 400,000 liter tank of perchloroethylene to detect solar neutrinos, and that’s an interesting story in itself. The tank had to be shielded from other particles that could cause reactions, and thus it was buried underground in a gold mine in South Dakota, where Davis was getting a neutrino interaction about once every six days out of the trillions of neutrinos passing through the tank. We’ve had a number of other neutrino detectors since, from the Sudbury Neutrino Observatory in Ontario to the Super Kamiokande experiments near the city of Hida, Japan and MINERvA (Main Injector Experiment for ?-A), the detector used in the Fermilab communications experiment.

The point is, these are major installations. Sudbury, for example, involves 1000 tonnes of heavy water contained in an acrylic vessel some 6 meters in radius, the detector being surrounded by normal water and some 9600 photomultiplier tubes mounted on the apparatus’ geodesic sphere. Super Kamiokande is 1000 meters underground in a mine, involving a cylindrical stainless steel tank 41 meters tall and almost 40 meters in diameter, containing 50,000 tons of water. You get the idea: Neutrino detectors are serious business requiring many tons of matter, and even with the advantages of these huge installations, our detection methods are still relatively insensitive.

Image: Scientists used Fermilab’s MINERvA neutrino detector to decode a message in a neutrino beam. Credit: Fermilab.

But Pasachoff and Kutner had an eye on neutrino possibilities for SETI detection. The idea has a certain resonance as we consider that even now, our terrestrial civilization is growing darker in many frequency bands as we resort to cable television and other non-broadcast technologies. If we had a lively century in radio and television broadcast terms just behind us, it’s worth considering that 100 years is a vanishingly short window when weighed against the development of a technological civilization. Thus the growing interest in optical SETI and other ways of detecting signs of an advanced civilization, one that may be going about its business but not necessarily building beacons at obvious wavelengths for us to investigate.

Neutrinos might fit the bill as a communications tool of the future. From the Cosmic Search article:

Much discussion of SETI has been taken up with finding a suitable frequency for radio communication. Interesting arguments have been advanced for 21 centimeters, the water hole, and other wavelengths. It is hard to reason satisfactorily on this subject; only the detection of a signal will tell us whether or not we are right. Neutrino detection schemes, on the other hand, are broad band, that is, the apparatus is sensitive to neutrinos of a wide energy range. The fact that neutrinos pass through the earth would also be an advantage, because detectors would be omnidirectional. Thus, the whole sky can be covered by a single detector. It is perhaps reasonable to search for messages from extraterrestrial civilizations by looking for the neutrinos they are transmitting, and then switch to electromagnetic means for further conversations.

The First Message Using a Neutrino Beam

Making this possible will be advances in our ability to detect neutrinos, and it’s clear how tricky this will be. The recent neutrino message at Fermilab, created by researchers from North Carolina State University and the University of Rochester, is a case in point. Fermilab’s NuMI beam (Neutrinos at the Main Injector) fired pulses at MINERvA, a 170-ton detector in a cavern some 100 meters underground. The team had encoded the word ‘neutrino’ into binary form, with the presence of a pulse standing for a ‘1’ and the absence of a pulse standing for a ‘0’.

3454 repeats of the 25-pulse message over a span of 142 minutes delivered the information, corresponding to a transmission rate of 0.1 bits per second with an error rate of 1 percent. Out of trillions of neutrinos, an average of just 0.81 neutrinos were detected for each pulse, but that was enough to deliver the message. Thus Fermilab’s NuMI neutrino beam and the MINERvA detector have demonstrated digital communications using neutrinos, pushing the signal through several hundred meters of rock. It’s also clear that neutrino communications are in their infancy.

From the paper on the Fermilab work:

…long-distance communication using neutrinos will favor detectors optimized for identifying interactions in a larger mass of target material than is visible to MINERvA and beams that are more intense and with higher energy neutrinos than NuMI because the beam becomes narrower and the neutrino interaction rate increases with neutrino energy. Of particular interest are the largest detectors, e.g., IceCube, that uses the Antarctic icepack to detect events, along with muon storage rings to produce directed neutrino beams.

Thinking about future applications, I asked Daniel Stancil (NCSU), lead author of the paper on this work, about the possibilities for communications in space. Stancil said that such systems were decades away at the earliest and noted the problem of detector size — you couldn’t pack a neutrino detector into any reasonably sized spacecraft, for example. But get to a larger scale and more things become possible. Stancil added “Communication to another planet or moon may be more feasible, if local material could be used to make the detector, e.g., water or ice on Europa.”

A Neutrino-Enabled SETI

Still pondering the implications of the first beamed neutrino message, I returned to Pasachoff and Kutner, who similarly looked to future improvements to the technology in their 1979 article. What kind of detector would be needed, they had asked, to repeat the results Raymond Davis, Jr. was getting from solar neutrinos at Brookhaven (one interaction every six days) if spread out to interstellar distances? The authors calculated that a 1 trillion electron volt proton beam would demand a detector ten times the mass of the Earth if located at the distance of Tau Ceti (11.88 light years). That’s one vast detector but improvements in proton beam energy can help us reduce detector mass dramatically. I wrote to Dr. Pasachoff yesterday to ask for a comment on the resurgence of his interstellar neutrino thinking. His response:

We are such novices in communication, with even radio communications not much different from 100 years old, as we learned from the Titanic’s difficulties with wireless in 1912. Now that we have taken baby steps with neutrino communication, and checked neutrino oscillations between distant sites on Earth, it is time to think eons into the future when we can imagine that the advantages of narrow-beam neutrinos overwhelm the disadvantages of generating them. As Yogi Berra, Yankee catcher of my youth, is supposed to have said, “Prediction is hard, especially about the future.” Still, neutrino beams may already be established in interstellar conversations. I once examined Raymond Davis’s solar-neutrino records to see if a signal was embedded; though I didn’t find one, who knows when our Earth may pass through some random neutrino message being beamed from one star to another–or from a star to an interstellar spaceship.

Neutrino communications, as Pasachoff and Kutner remarked in their Cosmic Search article, have lagged radio communications by about 100 years, and we can look forward to improvements in neutrino methods considering near-term advantages like communicating with submerged submarines, a tricky task with current technologies. From a SETI perspective, reception of a strong modulated neutrino signal would flag an advanced civilization. The prospect the authors suggest, of an initial neutrino detection followed by a dialogue developed through electromagnetic signals, is one that continues to resonate.

The Pasachoff and Kutner paper is “Neutrinos for Interstellar Communication,” Cosmic Search Vol. 1, No. 3 (Summer, 1979), available online. The Fermilab work is described in Stancil et al., “Demonstration of Communication using Neutrinos,” submitted to Modern Physics Letters A 27 (2012) 1250077 (preprint)