Icarus: An Early Look at Communications

by Paul Gilster on January 11, 2010

The Project Icarus weblog is up and running in the capable hands of Richard Obousy (Baylor University). The notion is to re-examine the classic Project Daedalus final report, the first detailed study of a starship, and consider where these technologies stand today. Icarus is a joint initiative between the Tau Zero Foundation and the British Interplanetary Society, the latter being the spark behind the original Daedalus study, and we’ll follow its fortunes closely in these pages. For today, I want to draw your attention to Pat Galea’s recent article on the Icarus blog on communications.

‘High latency, high bandwidth’ is an interesting way to consider interstellar signaling. Suppose, for example, that we do something that on the face of it seems absurd. We send a probe to a nearby star and, as one method of data return, we send another probe back carrying all the acquired data. Disregard the obvious propulsion problem for a moment — from a communications standpoint, the idea makes sense. ‘High latency, high bandwidth’ translates into huge amounts of data delivered over long periods of time. I remember Vint Cerf, the guru of TCP/IP, reminding a small group of researchers at JPL ‘Never underestimate the bandwidth of a pickup truck carrying a full load of DVDs,’ a reference commonly used in descriptions of the latency issue.

I was sitting in on that meeting, taking furious notes and anxious to learn more about how the basic Internet protocols would have to be juggled to cope with the demands of deep space communications. And the point was clear: If the wait time is not an issue, then low tech, high bandwidth makes a lot of sense. From the interstellar perspective, alas, we don’t have the propulsion technologies or the patience for this kind of communication. Radio is infinitely better, but we face the problem of beam spread over distance, even with higher and higher radio frequencies being employed. The Daedalus team had two approaches, summed up here by Pat:

a. Make the engine’s reaction chamber be a parabolic dish shape. When the boost phase has ended, use this dish as an enormous reflector to focus the radio transmissions back to Earth.

b. On Earth (or in near-Earth space), set up a huge array of parabolic dish receivers. (An array of receivers is almost as effective as a single receiver of the same size as the array.) This allows much more of the signal to be picked up than would be possible with just a single large dish. (The design was based on the proposed Project Cyclops, which was to be used to search for signals from extra-terrestrial intelligence.)

When we turn to laser communications, things get a good deal better. Extensive testing at JPL and other research centers has shown that much higher bandwidth can return data from deep space for the same amount of power as would be used in more conventional radio systems. Indeed, this is the approach JPL’s James Lesh uses in his study of communications from a Centauri probe. Lesh knows all about propulsion issues, but he’s straightforward in saying that if we surmounted those problems and did reach Centauri with a substantial payload, a laser communications system would be practicable.

Not that it would be easy. Galea again:

Over interstellar distances, despite the fact that lasers create a very tight beam, the beam spreading does cause a problem. The laser also has to be aimed very accurately, and this aim has to be maintained. The tiniest amount of jitter in the craft could cause the beam to miss the target completely. This would be a very tough engineering challenge, combining navigation (so that the craft knows exactly how it is oriented, and exactly where the target is) and control (so that it is actually able to point the laser accurately at the target).

Pat also gets into Claudio Maccone’s interesting notion of using gravitational foci at both Sun and destination star, with a craft at each focus along the line joining them and the two stars. In such a scenario, power requirements are at an absolute minimum, but the trick is the engineering, which assumes a level of technology at the target star that we would not have in place with our early probes. Further into the future, though, a gravitational lensing approach could indeed be used to establish powerful communications links between distant colonies around other stars.

As the Icarus weblog gains momentum, it will be fascinating to watch background articles like these emerge and to keep up with team members as they report on the progress of the project. I recommend adding the Project Icarus blog to your RSS feed. Pat Galea includes a list of references at the end of his article, and I’ll add the Lesh paper, which is “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996), pp. 7–14.

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{ 5 comments }

Tulse January 11, 2010 at 12:43

In such a scenario, power requirements are at an absolute minimum, but the trick is the engineering, which assumes a level of technology at the target star that we would not have in place with our early probes.

Certainly an interstellar probe is more difficult to create technically than a gravitational focus communication hub — why wouldn’t we send such a hub as an integral part of any interstellar mission? The comm hub could separate from the main probe and park itself at the focus, then act as the communications relay.

Eniac January 11, 2010 at 12:43

Looks like a clear-cut case for laser. Pointing accuracy is a false problem, really. It is a limitation, but it applies to all communication methods equally. It is easy to broaden a beam, but difficult to narrow it. If laser allows you to reach the pointing accuracy limit and radio does not, too bad for radio.

And it is not like there is a hard limit. A lot of ingenious ways can be devised to increase pointing accuracy, and the sun is a wonderfully clear and steady beacon to aim by.

Ron S January 11, 2010 at 15:16

Eniac is quite correct, and we already have spacecraft that do a modestly good job of orienting themselves by starlight. There is one advantage of radio, in that the beam isn’t so narrow that a slight misdirection doesn’t result in signal loss. This happens occasionally now with satellites that go into a tumble, and it can take some serious power to regain control, which is not an option for long-haul spacecraft.

However, the problem isn’t beam-width but rather communications reliability and bandwidth. (If the craft can tap into the local stellar energy, power is less of a problem — we don’t have much in the way of light-weight, self-contained power generators that will survive the lengthy voyage time.) What is needed is a combination of gain, power and bandwidth that will allow for reliable and useful communications. Power and directi0nal gain at this end is far less of a problem and, like DSN today, can make up for some (not all) of the constraints at the other end of the link.

yeti January 11, 2010 at 16:26

“b. On Earth (or in near-Earth space), set up a huge array of parabolic dish receivers.”

doesnt Seth shostak have one of these at Hatcreek?

LK January 12, 2010 at 12:08

Oh, but communication by quantum entanglement would be nice just about now, wouldn’t it? ;)
(Which I know close to nothing about, I hasten to add!)

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