Interstellar Communications

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy

If our current understanding of physics is correct, then the speed of light in a vacuum sets an upper limit to the speed of any matter or information transmission system. This speed of light (represented by scientists and engineers with the letter c) is about 300 million metres per second. That means a beam of light can go around the Earth’s equator seven times in one second. Yet despite this seemingly phenomenal speed, the time it takes for light to travel between the nearest stars to us is measured not in seconds, days, or even months, but years. That’s why for convenience we measure the distances to stars in units of “light years”. One light year is the distance that light will traverse in one year. The nearest star system to us, Centauri, is just over four light years from Earth.

When we send a craft to another star, we want to get the data from it back to Earth. There are several ways we could do this:

1. Send another small probe back to Earth carrying all the data.

2. Use radio signals.

3. Use optical signals.

4. Use an exotic technique.

Option 1 isn’t as absurd as it might seem. Although the message probe would take a long time to get back to Earth, it could carry a huge amount of data. In network engineering terms, it is “high latency, high bandwidth”. That is, you get a lot of data, but it takes ages to get it. (This is analogous to the often used example of sending a truck full of data DVDs to a recipient. In some circumstances, this may be the optimal way to send the data, if the initial waiting time isn’t a problem.)

However, the additional mass of an extra interstellar probe together with its propulsion and auxiliary systems probably rules it out as a practical solution for most missions.

Option 2 is an extension of the systems that we currently use on our space probes. Radio is just a longer wavelength of electromagnetic wave than light, and travels at the same speed as light. Radio is a great method for generating and receiving signals because it is easy to create using electronics, it can be piped around systems using wires (or for microwaves, tubes), and it can be easily reflected using metal dishes, allowing effective focusing of signals.

The drawback with radio is that the beam spreads significantly over longer distances, with the result that there is less signal power available at the receiving end, unless the receiving antenna is made very wide to catch as much of the signal as possible. The higher the frequency of the radio signal, the less the beam spreads with distance. Over the decades, space communications have indeed used higher and higher frequencies to reduce the beam spreading, with the result that the transmitter on the craft can use lower power while maintaining the same amount of power received at Earth.

For interstellar distances, the beam spreading at radio frequencies is enormous. The Project Daedalus team came up with two ways to make the use of radio signals viable over the terrific distances involved:

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.)

Option 3 is really just an extension of option 2, because light is just a much higher frequency radio wave. However, in practice the way it interacts with matter, and the way we create, manipulate and receive it is different enough from radio that we generally treat it separately.

As stated above, beam spreading decreases for higher frequency radio waves. This is particularly noticeable with laser light. If you’ve ever played with a laser pointer, you’ll know just how tight the beam is; shine the dot onto a distant wall, and the dot appears no bigger than if you’d shone it on your own hand, and the dot is about the same brightness.

Lasers are very likely to become more common on future deep space probes to distant parts of the solar system because they enable a much higher bandwidth of data to be sent back to Earth for the same amount of power as a radio system.

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).

Option 4 covers a range of suggested techniques which either use new methods of transmission altogether, or use existing systems in a new way to increase their effectiveness. 

Claudio Maccone has suggested the use of gravitational lenses to focus radio (and light), drastically increasing the amount of signal power received from the distant craft. The idea is to have two craft, one at the distant star, and one at our Sun. Each craft sits on the far side of its star on the line joining the two craft and the two stars. (So the system is arranged thus: Receiver Craft – Sun – Interstellar Space – Star – Transmitter Craft.)

The two craft have to be beyond a certain distance from their respective stars; this is the focal distance. In the Sun’s case, this distance is about 700AU (i.e. 700 times the distance from the Earth to the Sun). The transmitting craft sends its data, which is gravitationally focused by its star and sent across interstellar space. Years later, this signal arrives at our Sun, which similarly gravitationally focuses it, and is picked up by the receiving craft. This system is effectively using the two stars as lenses in an interstellar telescope.

There is nothing particularly outrageous about this suggestion. It is fully consistent with the laws of physics, but the big challenge here is in engineering. The two craft need to be kept exactly in line with each other and their stars to a very tight tolerance.

Other suggestions for ‘exotic’ techniques include using gravitational waves or neutrinos to send data. The problems here generally come with difficulties in generating and/or receiving the signals. Creating strong gravity waves would, as far as we know at the moment, require a feat of astrophysical engineering (manipulating large blobs of very dense matter). Neutrinos are easier to create, but they are very hard to detect. It is unlikely that any of these exotic techniques will be implemented in a practical system in the near future.

It is heartening to remember that back in 1977 the Project Daedalus team considered the problem of interstellar communications in detail, and concluded

“We do not need technological breakthroughs in this area. Development of the system could begin now.”

In the early 21st century, we can hope to do at least as well as Daedalus.

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References:

The Hitchhiker’s Guide to the Galaxy, Douglas Adams, 1979 [Wikipedia]

Project Cyclops [Wikipedia]

Deep Space Optical Communications, Ed. Hamid Hemmati, Wiley-Interscience, 2006 [Google Books Preview]

Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, C. Maccone, Springer-Praxis, 2009 [Google Books Preview]

Updating the Gravitational Focus Mission, P. Gilster [Centauri Dreams]

The Gravitational Lens and Communications, P. Gilster [Centauri Dreams]

Project Daedalus: The Vehicle Communications System, A.T. Lawton and P.P. Wright, JBIS, pp. S163-171, 1978. [CD-ROM]

Tags: interstellar communications, signals