▼ Menu

Interplanetary Communication

Wireless World, October & November, 1942.
Extras ▼


The question was: Is it theoretically possible to hold wireless communication with other planets?.

The mere fact that we are able to observe the light which is reflected from other planets shows that there is nothing to prevent an electro-magnetic wave. traversing the space intervening between the earth and those planets - or rather between the earth and the planetary atmospheres. For it must be noticed that this light - which originally comes from the sun - is not necessarily reflected by the surface of the planet itself, but may come from its outer atmosphere. Some planets have very dense atmospheres, others atmospheres of great rarity, while, in the case of Mercury, there is hardly any atmosphere at all. The spectra of some planets contain strongly marked absorption bands, indicating that the light has penetrated the planetary atmosphere, the gases of which have caused absorption of certain frequencies. The light waves in these cases have probably reached the surface of the planets themselves. In other cases the planetary spectra are very similar to that of the solar spectrum, which would indicate either that the planet had no atmosphere, or that the light had been reflected from the outer part of the atmosphere itself.

In some cases, therefore, though not in others, an electromagnetic wave - even one of such a high frequency as that of light - can penetrate the planetary atmosphere and reach the surface of the planet itself. And if a wave of light frequency can do this, why cannot also one of radio frequency? Where there is an atmosphere which is penetrable by the sun's rays there is probably also an ionosphere, brought into being by the action of the rays upon the gas molecules of the planetary atmosphere. And since the nature and distribution of the gases of planetary atmospheres differ from those of our own it is reasonable so suppose that the ionospheres of the planets - if they exist - would exhibit different characteristics from those of the terrestrial ionosphere. There is also the question of the intensity of the sun's rays at the planets to be considered in this connection. It is probable, therefore, that there may be planetary ionospheres which are impervious to different ranges of radio frequencies than those to which our own ionosphere is impervious.

It would appear to be possible, however, for a wave of radio-frequency to penetrate to the surface of a planet in some cases. The frequency used would have to be of such at value that the wave would easily penetrate both our own ionosphere and that of the planet in question, and would not be greatly attenuated by absorption in either of these regions. So far as the terrestrial ionosphere is concerned these conditions are suited by a radio wave in the 'ultra high' part of the spectrum - of a frequency of, say, 50 MHz or higher.

The answer to the first part of the question would therefore appear to be 'Yes - in the case of some planets'. In order to hold wireless communication, however, habitation of the planet by intelligent beings is implied, in order that the communication may be two-way. This would rule out a number of the planets, for it does not seem reasonable to think that intelligent beings could exist on those planets whose density is very low - in some cases it is less than that of water. In other cases there are other reasons for thinking that habitation of the planet is improbable. But in a few cases - such as that of Venus and of Mars - the existence of intelligent life is not so highly improbable.

Attenuation and Absorption

The practicability of holding wireless communication with an inhabited planet is quite another matter, and does not at present appear to exist. When a radio wave travels outward from a transmitter - even when it is sent out in the narrowest possible 'beam' it gradually 'spreads' out in directions at right angles to its direction of travel, so that it covers a greater and greater area the farther it advances. But the energy present in the wave front at a great distance from the transmitter is the same as it was when the wave-front was near the transmitter, and, since the wave front covers a greater and greater area as it advances, the energy present at any one point in it becomes less and less the farther it travels. This weakening of the wave with distance travelled is called 'spatial attenuation' and will occur even when no absorption at all is taking place. Considering the relatively great distances involved between the earth and other planets - 40 to 50 million miles is about the shortest distance - it is evident that spatial attenuation would be very great, and that colossal power would have to be used at the transmitter in order to overcome it and provide a workable signal - according to our standard - at the receiving end. A rough estimate indicates that a transmitter power of the order of 6,000,000 kW would be necessary in order to provide a radio field intensity of 5 μvolts per metre at the nearest planet in the absence of any absorption. True the power necessary could be considerably reduced if a highly directional transmitting aerial array were used, but even so it would still be far in excess of that radiated by any existing transmitting station. So we may rule out the possibility of getting through to the planets at present. As to whether there are any inexplicable radiations reaching us from outer space, so far as the Wireless World Brains Trust are aware, no ionisation which is detectable by present-day apparatus occurs at the earth's surface which cannot be attributed either to cosmic rays, gamma ray radiation from the earth itself or to radioactive emanations in the atmosphere. The cosmic rays themselves are thought to be due to radiations occurring during the creation (or possibly during the disintegration) of atoms in interstellar space, and therefore, not to be associated with any agency on one of the planets). There may, however, be radiations reaching us which are of an entirely different character to those capable of being detected by existing apparatus. The answer to the second part of the question would therefore appear to be 'Not known'. TWB.

November Issue

'Cathode Ray' writes: in my capacity of Member of the Wireless World Brains Trust, and referring to the question of whether there are any inexplicable radiations reaching us from outer space, I recall that in 1933 Karl Jansky reported in Proc IRE, as a result of at least a year's experiments, that he obtained continuous reception of radio waves from a certain stellar region. So far as I know, this matter has never been cleared up.

Arthur C Clarke, Hon. Treasurer of the British Interplanetary Society, adds a note:-

The fantastic figure of six million kW, quoted as necessary to produce a 5 μVolt/metre field on the nearest planet, presumably relates to spherical radiation, which no one for a moment considers. The use of beam technique would reduce power requirements to a minute fraction of this. Moreover, there seems very good evidence that radio waves from comparatively low-powered transmitters have travelled distances which are almost interplanetary. The existence of echoes of several seconds duration (equivalent to the distance of the moon)? is well established, and delays of up to ten minutes or so have been reported - corresponding to distances of several times those of Mars or Venus at perigee.

Secondly, the evidence that radiation reaches the earth from space is quite strong, and I am surprised that your contributor did not mention it. I refer to Janskys reports on the subject (Proc IRE , Oct., 1935). Jansky reports this 'star-static' as lying between 9-21 MHz and being 10-30 dB. above the level of thermal agitation. (See also Jansky, Proc IRE, Dec., 1937, and Friis and Feldman, Proc IRE, July, 1937, for a further discussion of this matter).

Finally, if radio is incapable of really long-range communication (which I doubt) the solution to the problem lies in the modulated light beam. Light can be focussed with extreme accuracy and the sensitivity of a photo-cell collecting light at the focus of a giant reflector, and backed by an electron multiplier and the usual amplifying stages, is so enormous as to be almost. meaningless. It is certainly capable of maintaining communication between all the planets in so small as space as the Solar System! As to the objection that most planets have opaque atmospheres, I would answer that all except Venus have airless satellites very close to them to which they could be linked by UHF.

TWB the Member who wrote the original reply as published last month, now sums up in the light of these comments:-

To the first part of the question, 'Is it theoretically possible to hold communication with other planets?' the answer given was 'Yes - in the case of some planets'. Surely this did not imply that radio is incapable of really long-range communication - on the contrary, the whole of the first part of the answer was intended to show that it is. But, in discussing the present day practicability of holding communication with a planet, the use of an existing transmitting station was assumed. The rough estimate of 6,000,000 kW was for spherical radiation, but was given merely as a starting point, and in order to give some idea of the effect of spatial attenuation at such a distance with ordinary technique. Using a highly directive but perfectly practicable type of short-wave transmitting array, and assuming the required field intensity at the planet to be only 1 μVolt per metre, it would appear that the radiated power might be reduced to somewhat less than 1,000 kW, which still is greater than that used by most existing stations. However, if the frequency used were such as to permit the employment of some type of parabolic reflector, and special equipment were built for the job, then there would not appear to be anything highly-impracticable about it, since the spatial attenuation would be reduced to negligible proportions. It is important, in connection with the choice of frequency, to remember the possible effect of a planetary ionosphere, which might impose severe limitations.

As Mr Clarke says, echoes of several seconds delay have been well established, but the time delays in these cases are too short for planetary distances to be involved, though they do fit in with the distance between earth and moon. As to the echoes of long delay - running into several minutes - these were discovered about 1929 by the Norwegian, Jorgen Hals, and were afterwards confirmed by Professor Stormer, of Norway, and by Dr van der Pol, of Holland. But I think it is true to say - and in doing so I would mention that I have the greatest respect for the opinions of these very eminent scientists - that they have never been repeated, and there is now some considerable doubt as to whether they were, in fact, echoes from space at all. A long period of mass listening about 1937 - in which hundreds of listeners took part - failed to yield any conclusive results as to these long delay echoes, and it is very difficult to account for the strength of the echoes it they had indeed been returned from points in space so far distant as the time delays would imply.

Coming to the matter of the Jansky 'star-static' mentioned both by Mr Clarke and by 'Cathode Ray', this ought perhaps to have been mentioned, but it was omitted from the Brains Trust answer for two reasons. Firstly, Jansky proved early in his investigations that its source was far outside the solar system and so it could not have its origin in a planet - and planets appeared to be the main concern of the questioner - and, secondly, recent work by Reber (see Proc IRE, August, 1942) has shown that these radiations are not entirely inexplicable. They were first noticed by Jansky in 1932, who then thought that their source was associated with the sun. Later he proved that it was outside the solar system altogether, and subsequently found the radiations to come from the direction of the Galaxy. He found that this interstellar noise set the lower limit for the noise-level. in his receiver, i.e., that it was always above that due to the thermal agitation in the receiver. In 1940 Reber reported that he had made some experiments with a view to locating the source of the noise and of explaining its origin, and it is of interest to us here that his repeated attempts to measure radiation from Mars - the only planet with which he seems to have concerned himself - yielded no results, Recently and it must be mentioned that the paper has only just arrived in this country and so may not have been seen by these two correspondents - Reber has done further work with a highly directive system and now suggests that the Jansky noise is really thermal agitation noise caused by electrons which have been erupted from stars and float in interstellar space, all space being the conductor in which the thermal agitation is set up.

Use browser back button to return.