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 electromagnetic 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 frequcncy? 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 ionospheres 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 a 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 Mc / s 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 reasanable 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-4o to o 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 6,00a,000 kW would be necessary in order to provide a radio field intensity of 5 microvolts 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 ` . 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 radioactiye emanations in the atmosphere. The cosmic raya 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 rzdi tions 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." T. W. B.
More Views on Interplanetary Communication
"CATHODE RAY," in a postscript to his letter which is printed on page 271, 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. I.R.E., 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 microvolt/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 Jansky's reports on the subject (Proc. I.R.E., Oct., 1935). Jansky reports this " star-static ''as lying between 9-21 Mc/s and being 10-30 db. above the level of thermal agitation. (See also Jansky, Proc. I.R.E.. Dec., 1937, and Friis and Feldman, Proc. I.R.E., 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 a 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.