by Paul Gilster | Aug 16, 2022 | Communications and Navigation |
My last look at laser communications inside the NASA playbook was a year ago, and for a variety of reasons it’s time to catch up with the Laser Communications Relay Demonstration (LCRD), which launched in late 2021, and the projects that will follow. LCRD has now been certified for its mission of shaking out laser systems in terms of effectiveness and potential for relay operations. Ideally, we’d like to receive data from other missions and relay to the ground in a seamless optical network. How close are we to such a result?
Image: The Laser Communications Relay Demonstration payload. Credit: NASA Goddard Space Flight Center.
LCRD is now in geosynchronous orbit almost 36,000 kilometers above the equator, poised for its two year mission, but before we proceed, note this. The voice is that of Rick Butler, project lead for the LCRD experimenters program at NASA GSFC:
“We will start receiving some experiment results almost immediately, while others are long-term and will take time for trends to emerge during LCRD’s two-year experiment period. LCRD will answer the aerospace industry’s questions about laser communications as an operational option for high bandwidth applications.
“The program is still looking for new experiments, and anyone who is interested should reach out. We are tapping into the laser communications community and these experiments will show how optical will work for international organizations, industry, and academia.”
The Opportunities for Experiments page at GSFC offers the overview for anyone looking to join this effort with ideas for experiments to test optical communications links. Contact information for proposals is provided, and I also note that NASA intends to use LCRD to relay New Year’s resolutions submitted by the public through social media accounts as a demonstration of laser communications capabilities. Sure, it’s a bit of a stunt, but it makes optical communications visible to a general audience as we move into the era of laser networking for space missions near and far.
TeraByte InfraRed Delivery (TBIRD) is to follow, having launched on May 25 of this year. Here scientists are pushing the data downlink, going to 200 gigabits per second, which will represent the highest optical rate NASA has yet achieved. A single 7-minute pass of this CubeSat in low-Earth orbit will return terabytes of data. TBIRD, build by MIT, is integrated into the PTD-3 CubeSat as part of a technology demonstrator mission.
This is exciting stuff in its own right: The Pathfinder Technology Demonstrator program emphasizes using the same spacecraft bus and avionics platform designs across various missions, which moves toward modular spacecraft that are more efficient and easier to produce.
Image: Illustration of TBIRD downlinking data over lasers links to Optical Ground Station 1 in California (not drawn to scale). Credit: NASA/Dave Ryan.
The plan for TBIRD is to demonstrate the stability of laser pointing, with the spacecraft directed toward the ground station at Table Mountain, California. Without moving parts, the laser communications testing will rely on the pointing ability of the entire spacecraft. Beth Keer (NASA GSFC) is TBIRD project manager:
“In the past, we’ve designed our instruments and spacecraft around the constraint of how much data we can get down or back from space to Earth. With optical communications, we’re blowing that out of the water as far as the amount of data we can bring back. It is truly a game-changing capability.”
I’ll also mention a component of laser testing that will go to the International Space Station in the form of ILLUMA-T, which stands for Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal. Sending data at 1.2 gigabits per second, the device will communicate with LCRD, which will then relay data on ISS experiments and other information to ground stations at Haleakal?, Hawaii or Table Mountain.
Image: Illustration of LCRD relaying data from ILLUMA-T on the International Space Station to a ground station on Earth. Credit: NASA’s Goddard Space Flight Center/Dave Ryan
While NASA has been using communication relay satellites since 1983, the ability of LCRD to send and receive data from both missions and ground stations from its geosynchronous orbit means we will have achieved the agency’s first two-way, end-to-end optical relay. ILLUMA-T will shake out this system, demonstrating low-Earth orbit to geosynchronous orbit to ground station links in an end-to-end system.
SpaceX uses laser links to move Internet traffic from spacecraft to spacecraft in its Starlink system, and the European Space Agency does the same for its system of environmental monitoring satellites, but both of these use conventional radio to return data to Earth, and a direct link of optical data to Earth is the logical next step.
Extending further from the Earth, the Artemis II mission will carry its own Optical Communications System aboard the Orion spacecraft, making it the first crewed lunar flight demonstrating laser communications. With a downlink rate as high as 260 megabits per second, the system will be able to send high-resolution images and video.
While we wait to see when the Psyche mission will fly, I note that the Deep Space Optical Communications package is aboard, an attempt to increase communications performance by up to 100 times over conventional deep space missions. Now we take laser technologies outside the Earth-Moon system, with the Hale Telescope at Palomar receiving high-speed data from the transceiver aboard the spacecraft. The uplink will be from a laser transmitter at the JPL Table Mountain facility. This experimental effort is scheduled to begin not long after launch and will extend for at least a year and perhaps longer depending on results.
Can we look at laser communications from an interstellar perspective? Early work on this points to the potential as well as the difficulties. According to one JPL study, it would take an installation the size of the Hubble Space Telescope, beaming a 20-watt laser signal, to reach us from Alpha Centauri, so we have a long way to go before we can contemplate such methods between stars.
We can work wonders up to a point: The Deep Space Network can pick up Voyager’s 23-watt radio signal even though it is billions of times weaker than the power it would take to operate a digital wristwatch. But going interstellar will require moving to lasers to narrow beam diffraction (the Voyager signal is now over 1000 times Earth’s diameter). We know how to communicate if we can put the equipment where we need it, but getting payloads of any size – even a microchip – to another star continues to challenge our best scientists.
Exploring that gravitational lens communications relay described by Claudio Maccone may be one way around the problem. We already have a mission under study at JPL to reach 550 AU and beyond with the express purpose of imaging a planet around a nearby star. One step at a time, then, both for exoplanet observation using the Sun’s gravitational lens and, in some latter mission, possibly exploiting its magnification for communications. And one step at a time for lasers. Let’s get Psyche launched and see what DSOC can do.
by Paul Gilster | Sep 2, 2021 | Communications and Navigation |
Recent updates to the Deep Space Network have me thinking about the data capabilities of laser communications, and how they will change the way missions operate. In late October, a payload called the Laser Communications Relay Demonstration (LCRD) is scheduled for launch aboard an Atlas V from Cape Canaveral. LCRD will begin its work by receiving radio frequency test signals from the mission operations center and responding with optical signals. Ultimately, the mission should be able to receive data from other missions and relay to the ground.
What we have here is NASA’s first technology demonstration of a two-way laser relay system, one that will test laser capabilities to find out, for example, about the potentially disruptive effect of clouds. Because optical signals cannot penetrate them, plans are for LCRD to transmit data from missions to separate ground stations, one in Table Mountain, California and the other at Haleakal? in Hawaii, both chosen because of their low degree of annual cloud coverage.
Also slated for late 2021 is the Terabyte Infrared Delivery (TBIRD) mission, which will demonstrate laser downlinks of 200 gigabits per second, again enlarging the agency’s capabilities at designing laser systems for small satellites.
Image: The Laser Communications Relay Demonstration payload is attached to the LCRD Support Assembly Flight (LSAF), which can be seen in this image. The LSAF serves as the backbone for the LCRD components. Attached to the LSAF are the two optical modules, which generate the infrared lasers that transmit data to and from Earth. A star tracker is also attached here. These components are visible on the left side of this image. Other LCRD components, such as the modems that encode data into laser signals, are attached to the back of the LSAF. Credit: Goddard Space Flight Center.
All of this is by way of looking at how communications are evolving, and how capacity must grow with technology, for the 39 missions the Deep Space Network regularly supports are scheduled to be joined by another 30 NASA missions in development. The network’s tracking antennas are found at Goldstone (near Barstow, CA), in Robledo de Chavela, Spain; and in Canberra, Australia. Two new antennas have added capacity, taking the DSN from 12 to 14. You can track ongoing operations on the DSN on this mesmerizing page.
You’ll recall the issues with DSS-43, the 70-meter DSN antenna at Canberra (see Voyager 2: Back in Two-Way Communication). This is the only southern hemisphere dish with a transmitter in the needed S-band frequency range and powerful enough to send commands to Voyager 2, and it took 11 months of upgrades to resolve problems with its aging equipment. Voyager 1 is able to communicate through the two northern hemisphere DSN stations, but Voyager 2’s course following the Neptune encounter in 1989 was pushed well south of the ecliptic.
JPL’s Brad Arnold is manager of the Deep Space Network:
“The refresh of DSS-43 was a huge accomplishment, and we’re on our way to take care of the next two 70-meter antennas in Goldstone and Madrid. And we’ve continued to deliver new antennas to address growing demand – all during COVID-19.”
Image: DSS43 is a 70-meter-wide (230-feet-wide) radio antenna at the Deep Space Network’s Canberra facility in Australia. It is the only antenna that can send commands to the Voyager 2 spacecraft. Credit: NASA/Canberra Deep Space Communication Complex.
The upgrades are occurring at a time when data flowing through the network has grown by a factor of 10 since the 1960s, with the prospect of much higher data volumes to come. Hence the interest in optical strategies to enable higher-bandwidth communication. Improvements in automation allow operators to oversee multiple links to spacecraft simultaneously, so the sequencing and execution of tracking passes can be fully automated. While waiting for optical methods to mature, the network is also using new protocols for the reception of multiple signals from a single antenna, splitting them in a digital receiver as a way to boost network efficiency.
When it comes to the data overload problem, laser communication is the next step, and the groundwork has continued during the past decade. In 2013, the Lunar Laser Communications Demonstration (LLCD) used a laser signal to enable fast upload and download rates (600 megabits per second) on two simultaneous high-definition video channels. LLCD was followed in 2014 by the Optical Payload for Lasercomm Science (OPALS) experiment, a demonstration onboard the International Space Station. 2017 saw the Optical Communications and Sensor Demonstration mission (OCSD), in which high-speed laser communications were demonstrated via downlink from a CubeSat to ground stations.
Meanwhile, we can look forward to the Psyche mission, scheduled for launch in 2022, in which the onboard Deep Space Optical Communications (DSOC) payload will test laser communications in a mission to an asteroid 240 million kilometers away.
We get serious advantages not just in terms of bandwidth but also in transmission and reception of signals by going this route. The diffraction rate of a radio signal is determined by the wavelength of the signal divided by the diameter of the antenna. Push into higher frequency ranges and the signal becomes narrower, offering advantages in a crowded spectrum. The DSS-43 communications with Voyager 2 make the issues stark. Because of beam diffraction, the Voyager signal now swells to over a thousand times the diameter of the Earth.
Putting this into more futuristic terms: A 20-watt laser signal beamed back to Earth from Alpha Centauri via an installation about the size of the Hubble Space Telescope would reach us. Our current capabilities extend out into the Kuiper Belt, but star-to-star is out of the question. Back in 1989, the signal we received from Voyager 2’s 23 watts was twenty billion times weaker than the power it would take to operate a digital wristwatch, yet the DSN could pluck the signal out of deep space to deliver the data. If we had a Voyager 2 entering Alpha Centauri space, its signal would be 81 million times weaker than that. Going interstellar means going to lasers.
And beyond that? Claudio Maccone has demonstrated mathematically what might be done with a communications relay at the Sun’s gravitational focus beyond 550 AU. Going more futuristic still, a network of interstellar communications could one day grow from similarly placed relays around nearby stars. The efficiencies of a network like that — if we can find a way to put one in place — are breathtaking. See The FOCAL Radio Bridge for more.
On the laser signal at Alpha Centauri, see Lesh, C. J. Ruggier, and R. J. Cesarone, “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7–14. For more on gravitational lensing and communications, see Maccone, “Interstellar Radio Links Enhanced by Exploiting the Sun as a Gravitational Lens,” Acta Astronautica Vol. 68, Issues 1-2 (January-February 2011), pp. 76-84 (abstract/full text).
by Paul Gilster | Jun 5, 2019 | Communications and Navigation |
NASA’s Orbital Test Bed satellite is scheduled for launch via a SpaceX Falcon Heavy on June 22, with live streaming here. Although two dozen satellites from various institutions will be aboard the launch vehicle, the NASA OTB satellite itself houses multiple payloads on a single platform, including a modular solar array and a programmable satellite receiver. The component that’s caught my eye, though, is the Deep Space Atomic Clock, a technology demonstrator that points to better navigation in deep space without reliance on Earth-based atomic clocks.
Consider current methods of navigation. An accurate reading on a spacecraft’s position depends on a measurement of the time it takes for a transmission to flow between a ground station and the vehicle. Collect enough time measurements, converting them to distance, and the spacecraft’s trajectory is established. We know how to do atomic clocks well — consider the US Naval Observatory’s use of clocks reliant on the oscillation of atoms in its cesium and hydrogen maser clocks. Atomic clocks at Deep Space Network ground stations make possible navigational readings on spacecraft at the expense of bulk and communications lag.
While GPS and other Global Navigation Satellite Systems (GNSS) use onboard atomic clocks, the technologies currently in play are too heavy for operations on spacecraft designed for exploration far from Earth. That puts the burden on communications, as distant spacecraft process a signal from an atomic clock on the ground. What the spacecraft lacks is autonomy.
A better methodology is something we have to develop as we look toward a future deep space infrastructure. Testing the miniaturization of atomic clocks and methods to harden them for operations elsewhere in the Solar System is the goal of the DSAC demonstrator mission, which points to a clock architecture that is considerably more efficient and also scalable.
Image: JPL’s Deep Space Atomic Clock will fly aboard the General Atomics Electromagnetic Systems Orbital Test Bed satellite as a hosted payload and launched in June as part of the U.S. Air Force’s Space Technology Program 2. Credit: NASA.
The Deep Space Atomic Clock will be the first atomic clock designed to fly aboard a spacecraft going beyond Earth orbit and, with a stability of better than one microsecond in a decade, it is also the most precise clock ever flown. Ground-based testing has shown that the DSAC is up to 50 times more stable than the atomic clocks on GPS satellites, losing only one second in nine million years. NASA considers it an enabling device for future on-board radio navigation.
It is the length of a second as measured by the frequency of light released by specific atoms that makes an atomic clock so precise as it records the vibrations induced in a quartz crystal. Key to the DSAC clock’s stability is the use of mercury ion trap technology. Contained within electromagnetic traps within the device, these ions are rendered less vulnerable to external forces like changing magnetic fields and variations in temperature than atoms currently in use.
Image: DSAC mercury ion trap housing with electric field trapping rods seen in the cutouts. This is where DSAC interrogates and measures the mercury ion resonance that is used to discipline a quartz crystal clock. Credit: NASA.
The distances involved in deep space operations force new technologies upon us, for communicating with atomic clocks on Earth to determine a spacecraft’s location not only takes time but also places an increased burden on our communication resources, another reason why we’re moving toward networking multiple spacecraft in orbital operations at places like Mars.
The plan is to test DSAC in Earth orbit for one year, with the goal of adapting it for future missions to deep space. Developed at the Jet Propulsion Laboratory, the device has been under development for 20 years, reducing the size of atomic clocks from those used at Deep Space Network ground stations — about the size of a refrigerator — to an object the size of a four-slice toaster, and one that can be further miniaturized depending on the needs of future missions.
You can see that we’re gradually upping the navigation service volume, considering that spacecraft near Earth don’t require an integrated atomic clock, being able to use existing global navigation services like GPS. These technologies, using multiple GNSS constellations, can get us out as far as geosynchronous orbit, and DSAC promises accurate navigation deep into the Solar System. Ahead lies X-ray navigation that keys off the oscillations of remote pulsars, a galactic positioning system that points to missions moving, one day, far beyond our Sun.
by Paul Gilster | Apr 28, 2015 | Communications and Navigation |
We need to improve the way we handle data tracking and deep space navigation. While the near term is always uncertain because of budgetary issues, we can still take the long view and hope that we’re going to see a steadily increasing number of robotic and human spacecraft in the Solar System. That puts a strain on our existing facilities, and a premium on any methods we can find to make data return more precise and navigation more autonomous.
With these ideas in mind, keep your eye on the Deep Space Atomic Clock (DSAC). It’s a NASA technology demonstrator mission being built to validate a miniaturized, ultra-high precision mercury-ion atomic clock that researchers believe will be 100 times more stable than today’s best navigation clocks. Managed at the Jet Propulsion Laboratory, the DSAC has been tweaked and improved to the point where it allows drift of no more than a single nanosecond in ten days.
Image: Drawing of the DSAC mercury-ion trap showing the traps and the titanium vacuum tube that confine the ions. Credit: NASA/JPL.
We need improved atomic clocks in space to take spacecraft navigation to the next level and permit the next generation of studies of distant targets like Europa. Assuming Europa does have a subsurface ocean, this body of water will clearly be affected by tidal effects from Jupiter. Atomic clock measurements of DSAC’s caliber will be needed to provide the tracking data we’ll use to estimate Europa’s gravitational tide, helping to confirm the characteristics of its putative ocean. More on this thought and on the background of the DSAC in this JPL news release.
Europa, of course, is but one target whose investigation will be enhanced by projects like DSAC. Beyond this, improving the accuracy and stability of atomic clocks can change the way deep space navigation is done. Right now, we use a two-way paradigm for radiometric tracking, meaning that the same ground-based frequency standard is used as a reference for an uplink signal and a downlink detector. In other words, we track a spacecraft with our network on Earth and a ground-based team performs the necessary navigation. Improving the DSAC to allow it to operate in deep space will create a one-way tracking paradigm and autonomous navigation.
Think of the GPS unit you probably use to navigate with when you drive. GPS offers a one-way signal requiring no return signal from your car. The goal is to create the same one-way capability in deep space navigation. The smaller clock error (and DSAC is expected to be stable to less than 3 X 10-15 at one day, as measured by its Allan Deviation, a measure of frequency stability) enables one-way tracking with accuracies equal to or better than the two-way methods we currently use, a more flexible and efficient space navigation system.
The benefits of such a system will be striking, particularly in scenarios where we have several spacecraft either in orbit around or on the surface of a planet (Mars is the obvious reference for now). Let me pull a quote from Thornton and Border’s Radiometric Tracking Techniques for Deep-Space Navigation (Wiley, 2003):
…a single deep space antenna can acquire one-way Doppler and telemetry simultaneously from all spacecraft. Multiple uplink signals are not required. Consequently, this configuration results in more efficient use of ground-based resources and enhances orbit solutions and lander position estimates through the use of differential measurements.
Moreover, we get better signal-to-noise ratios for receiving spacecraft telemetry. Thornton and Border go on to explain the two reasons for this:
…one-way transmissions provide better short-term (< 1 s) stability, resulting in less signal loss in the detection process. This is because the short-term stability of two-way transmissions is degraded by solar plasma scintillations of the uplink signal and, for more distant spacecraft, by thermal noise in the spacecraft receiver. Second, the ground antennas are configured in a listen-only mode for one-way tracking, whereas the more complicated diplexer mode, required for simultaneous uplinking and downlinking, increases the effective system noise temperature of the ground receiver.
So DSAC technology can be a game-changer for deep space navigation, assuming the system checks out in flight. The plan is for the demonstration unit to be launched in 2016 aboard a SpaceX Falcon 9 Heavy booster, hosted on a spacecraft provided by Surrey Satellite Technologies. The equipment will be operated for at least one year, making use of GPS satellite signals to demonstrate precision orbital determination. Todd Ely (JPL), principal technologist for the DSAC Technology Demonstration Mission, describes the testing:
“Our in-orbit investigation has several phases beginning with commissioning, where we start up the clock and bring it to its normal operating state. After that we’ll spend the first few months confirming and updating our modeling assumptions, which we will use to validate the clock’s space-based performance. With these updates and our observation data, we’ll spend the next few months determining DSAC’s performance over many time scales…from seconds to days.”
Image: Overview of the mission architecture. Credit: NASA/JPL.
Following that period, the team will monitor clock telemetry to characterize its potential for long-term operations. The initial DSAC flight aims at producing the data that will help to make future units smaller and more efficient, readying them for the lengthy mission times that exploring deep space will demand. The kind of tracking data such a refined atomic clock will make available will improve spacecraft navigation and allow the precise tracking data we’ll need as we explore the moons of the gas giants and prepare for future targets even further out.
by Paul Gilster | Dec 31, 2013 | Communications and Navigation |
Best wishes for the New Year! I got a resigned chuckle — not a very mirthful one, to be sure — out of a recent email from Adam Crowl, who wrote: “Look at that date! Who imagined we’d still be stuck in LEO in 2014???” Indeed. It’s hard to imagine there really was a time when the ‘schedule’ set by 2001: A Space Odyssey seemed about right. Mars at some point in the 80’s, and Jupiter by the turn of the century, a steady progression outward that, of course, never happened. The interstellar community hopes eventually to reawaken those dreams.
Yesterday’s post on laser communications makes the point as well as any that incremental progress is being made, even if at an often frustrating pace. We need laser capabilities to take the burden off a highly overloaded Deep Space Network and drastically improve our data transfer and networking capabilities in space. The Lunar Laser Communication Demonstration (LLCD) equipment aboard the LADEE spacecraft transmitted data from lunar orbit to Earth at a 622 megabits per second (Mbps) rate in October, a download rate six times faster than any radio systems that had been flown to the Moon. It was an extremely encouraging outcome.
“These first results have far exceeded our expectation,” said Don Cornwell, LLCD manager. “Just imagine the ability to transmit huge amounts of data that would take days in a matter of minutes. We believe laser-based communications is the next paradigm shift in future space communications.”
LLCD is actually an overall name for the ground- and space-based components of this laser experiment. What’s aboard the LADEE spacecraft is the Lunar Laser Space Terminal (LLST), which communicates with a Lunar Laser Ground Terminal (LLGT) located in White Sands, New Mexico, a joint project developed between MIT and NASA. There are also two secondary terminals, one at the European Space Agency’s La Teide Observatory (Tenerife), the other at JPL’s Table Mountain Facility in California, where previous laser experiments like GOLD — the Ground-to-Orbit Laser Communication Demonstration — have taken place.
The laser communication between LLCD and ground stations on Earth is the longest two-way laser communication ever demonstrated and a step in the direction of building the next generation of communications capability we’ll need as we explore the Solar System. Imagine data rates a hundred times faster than radio frequencies can provide operating at just half the power of radio and taking up far less space aboard the vehicle. Improvements in image resolution and true moving video would radically improve our view of planetary targets.
Laser methods are proving as workable as we had hoped. The LLCD demonstrated error-free communications during daylight, and could operate when the Moon was within three degrees of the Sun as seen from Earth. Communications were also possible when the Moon was less than four degrees from the horizon as seen from the ground station, and were successful even through thin layers of cloud, which NASA describes as ‘an unexpected bonus.’ A final plus: The demonstrated ability to hand off the laser connection from one ground station to another.
The scientific benefits of lasers are tangible but they’re matched by what could be a rise in public engagement with space if we can produce a networked infrastructure in which video plays a major role. Immersive gaming systems give way to the thought of rovers sending back high-resolution video from exotic places like Titan or Callisto — is this one way to rekindle the passion for exploration that sometimes seems to have died with Apollo? Robotic missions lack the immediacy and glamor of human crews but high bandwidth may help make up the slack, perhaps building momentum for later crewed missions to many of the same targets.
Up next is the Laser Communications Relay Demonstration (LCRD), which recently passed a preliminary design review. LRCD is to be a long-duration optical mission that will tweak optical relay services over a two-year period onboard a commercial satellite built by Space Systems Loral. We’re in the transitional period between demonstrators and reliable flight hardware. After its 2017 launch, LCRD will be positioned above the equator to carry that process forward.
by Paul Gilster | Dec 30, 2013 | Communications and Navigation |
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.
