National Academies Press: OpenBook

Visual Problems of Space Travel (1962)

Chapter: The Visual Environment of Space

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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
×
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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Suggested Citation:"The Visual Environment of Space." National Research Council. 1962. Visual Problems of Space Travel. Washington, DC: The National Academies Press. doi: 10.17226/18422.
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The Visual Environment of Space The environment of space is not something that becomes apparent at any one specific altitude. As Simons (1958) puts it ". . .there is no one altitude at which space begins for man. Rather, the situation be- comes space-equivalent in more ways as one goes higher and higher until there is no difference." This gradual change is true visually as well as physically. As one ascends from the surface of the earth, the overhead sky takes on a deeper and deeper blue as a result of less optical scattering in the thinner atmosphere. Concomitantly, the lower part of the visual field becomes increasingly brighter and may become the source of severe glare. At about 70,000 feet the colors on the ground begin to fade so that beyond 45 degrees there is only a gray haze. Straight down, greens and reds are still distinguishable but are quite faded and pre- sent a pastel impression. There is a bluish cast as though the earth is being seen through a blue-tinted filter. The overhead sky at 70,000 to 100,000 feet is best described as a dark purple-blue. In the vicinity of 100,000 feet the earth appears very strange. There are several bands of color as one looks from the horizon to the zenith. The first band above the horizon is white. Above this is a narrow band of blue which extends to darker shades into the purple. When examined closely the sky gives the impression of spectral violet. In view of this darkening of the sky at the zenith, it has been suggested by some that daytime celestial navigation may be possible. 3

Laboratory investigations conducted by Taylor (1960a, 1960b) however, have demonstrated that celestial navigation during daylight hours at altitudes as high as 50,000 feet is not feasible either with the unaided eye or with various optical aids. The altitude at which it first becomes possible has not as yet been determined. It is expected that at about 100,000 feet an individual who is dark-adapted will aee flashes of light caused by direct stimulation of the retinal elements by heavy cosmic particles. It is not known at the present time whether or not such stimulation will be of serious physiological consequence. As man goes still higher the features mentioned are even less distinguishable, while the curvature of the earth becomes more ap- parent. At 500,000 feet (about 95 miles) the overhead sky ie com- pletely black except for the stars. Some of the more specific aspects of orbital flight are now considered. "During an orbital flight it will be of importance to maintain a check on the position of the vehicle over the earth's surface, the time at which various check points are passed, and the attitude of the vehicle itself with respect to its orbital path. In the present Mercury sys- tem, external visual cues may play a primary role if the man is involved in these tasks. The normal attitude of the vehicle will be such that he will have a direct view of the horizon to provide a roll reference. In addition, by means of a periscope he will be able to see the ground beneath him out to the horizon in all directions. With the horizon visible at a distance of approximately 900 miles for an altitude of 100 miles, the visible surface of the earth will be represented by a circular area having a diameter of approximately 165 degrees of angle sub- tended at the vehicle. The periscope field will be so

positioned with respect to the vehicle that the vehicle attitude in pitch and roll will be correct when the earth's visible surface is centered in the periscope field. The vehicle, traveling at approximately 18,000 miles per hour at an altitude of 100 miles, will be traversing ap- proximately five miles on the earth's surface every second. In the periscopic field this will be readily discriminable in terms of the motion of the pattern within the field, pro- vided there is a discriminable pattern. The orientation of the vehicle in yaw may then be observed in terms of the direction of relative motion of the earth's surface as seen in the field of the periscope. If there is no discriminable pattern within the periscopic field, control. . . [of] yaw may be achieved by visual reference to star patterns. It may be assumed that in the case of failure of an automatic sys- tem, outside visual reference may be. . .important for control of the vehicle, and that in any case it will provide positive cross-checks of the automatic system and of the proper functioning of instruments. In an orbital vehicle, outside observations will be complicated to some extent by a daylight and darkness cycle of approximately 90 minutes. The portion of the earth's surface traversed in daylight may change on successive cycles. The problems of ex- ternal vision have been considered in detail in connection with the Mercury Project (Jones, 1960)." [Jones has described some of these problems as follows:] "CLOUD COVER - The most conspicuous visual effect at orbital altitude, other than day-night cycle, is the wide variation in cloud cover. Clouds have two significant visual effects: they block the view of the earth and create shadows, and they reflect sunlight to increase illuminance. These have implications for both navigation and protection of the astronaut from high intensity light. The mean cloudiness over the earth has been estimated as 54 per cent for land and 58 per cent for water. However, in the latitudes for the Mercury mission, a 6/10 or more cloud cover is estimated to occur only about 30 per cent of the time. This value will be lower in summer and somewhat higher in winter. Some water or land should be visible almost all the time through the periscope because of its wide field of coverage. However, a NASA Tiros I picture shows a cyclonic cloud 2000 miles in diameter in the central Pacific. The albedo, or proportion of sunlight reflected back in space, has a mean of around 0.35 for the earth, and about 0.50 for clouds. By comparison, the moon's albedo is 0.07. This has significance for the occupant of the vehicle in terms of comfort and adaptation.

At night translucent clouds may serve as a diffusing medium for the light from major population centers, possibly producing a distinguishable landmark." Jones points out further that ". . .The earth-sky discontinuity is an important exterior visual reference for back-up control of the capsule's attitude in pitch and roll." Additional consideration, how- ever, should be given to the fact that "during daylight, the ground horizon is often obscured by haze and ie not sharp." Consequently, the discontinuity just mentioned will not always be available. "A different situation exists on the dark side of the earth since the dis- continuity [here] results from the earth masking the heavens. . .[thus creating the appearance of] a black hole against a star background." To quote from Jones again: "The stars will no longer scintillate, they should be brighter by about 30 per cent, and some differences in color might be apparent. Stars will be visible and constellation patterning recognizable at night when the observer is dark-adapted and capsule lighting conditions are appropriate. The moon, airglow, starlight, galactic light, and zodiacal light in decreasing order of in- tensity, will furnish a very faint light when the vehicle is on the dark side of the earth. Air glow will be below the vehicle." As would be expected, the visual field "will change from dark to light and back to dark every 90 minutes during each orbit." "If for any reason the vehicle should go into a tumbling mode, external cues may afford a reference for regaining stability. Rate indicators will provide a better reference than external cues, but after stabilization has been achieved, locking of gyroscopes during tumbling may require that gyroscopes be caged and that visual cues be used for assum- ing the desired vehicle attitude prior to uncaging gyros .

"The location over the earth of an orbital vehicle at altitudes of 100 to 500 miles may be determined by the visual identification of characteristic features of the earth's surface if cloud-cover does not obstruct visibility. [Such features may be useful as, large lakes, rivers, island patterns, mountain ranges, sizeable cities, etc. Inasmuch as], . .vision at orbital altitudes is characterized by the perception of shape and pattern rather than the resolution of small objects, . . .[any object would have to be of substantial size and possess unique character- istics in order to be of navigational value.]" Outside the earth's atmosphere the light from the sun will be less attenuated and thus the sun will appear as a brilliant disk against the relative blackness of space. Although the sun's corona scatters some of the light emitted from the photosphere, this scatter will not be seen against the brilliance of the solar disk itself. As has been discussed by Strughold and Ritter (1960), an astronaut may well ex- perience functional disturbances in the form of glare. Due to the fact that the sun is surrounded by relative blackness, the problem of glare may be much more serious in space than during flight within the earth's atmosphere, primarily because the pupils often will be dilated during brief exposures. If proper precautions are not taken, the eye may suffer structural damage in the form of retinal burns caused by infra- red and near-infrared rays focused on the retina. Unfortunately, in- frared rays are known to be transmitted by the cornea in sufficient amounts to be harmful both to the lens and the cornea if the source is intense enough. Although on earth it requires about one minute for the develop- ment of eclipse blindness (Cordes, 1948), in space it is estimated that

it may take leas than 10 seconds to produce a retinal burn in the normal eye (Byrnes, Brown, Rose, k Cibis, 1955). Strughold (1960) points out that at least within our own solar system the irradiance of the image on the retina is independent of the distance. Thus, even though the size of the retinal burn will decrease with distance, the critical time of exposure will remain nearly the same. In addition to the danger of retinal burns, consideration must be given to the possibility of ocular damage resulting from ultraviolet radiation. An extensive series of experiments on rabbits by Verhoeff and Bell (1916) showed that abiotic effects could be obtained when the eyes were exposed to wave lengths shorter than 3050A. Inasmuch as the cornea and lens absorb these wave lengths very strongly, 19 times the minimal exposure needed for corneal damage was required to pro- duce an effect on the lens. It was concluded further that due to the absorption of the lena and cornea it is extremely unlikely that an indi- vidual will suffer retinal damage from ultraviolet radiation. This latter conclusion is also supported by Wald (1952). Although the retina itself is rarely damaged by ultraviolet light, a marked fluorescence of the lens and cornea is produced. This can cause a severe blurring of the retinal image which at the least is annoying, and in some cases may produce eyestrain. There is the additional danger of damaging the anterior portion of the eye. This is usually in the form of con- junctivitis (snowblindness), which, if sufficiently severe, can swell the lids to the point that the eyes cannot be opened. Fortunately, the 8

proper design of porthole filters and glasses can all but eliminate the detrimental effects of ultraviolet radiation. Although the astronaut can be fairly well protected from ultra- violet radiation, the picture is not so pleasant with regard to the bio- logical effects of high-energy alpha and proton irradiation. Data ob- tained from earth satellites and deep space probes indicate that these radiations exist at extremely high energy levels. The source of these radiations is the Van Allen Belt, aural displays, coamic rays, and in special cases the intensity is markedly increased by solar flares (Winkler, 1960). It is well established that the lens of the eye is extremely sensitive to radiation. It is protected on the outside by only about 3.5 millimeters of other tissues (aqueous, cornea, tear fluid) and can suffer irreversible damage from a comparatively small dose. The implications of this problem have been stated clearly by Schaefer. "The intricacy of the pertinent relationship is best demonstrated in a concrete example. If we visualize an astronaut in a capsule protected by a vehicle wall of 3/4-inch of aluminum flying through the lower fringes of the Van Allen Belt, the lenses of his eyea are pro- tected by an additional 3. 5 millimeters of tissue, as mentioned above. This additional filtration reduces the dose to the lens to about 94. 5 percent of the skin doae inaide the ship. By closing his eyea, the man could further reduce the dose to the lens to 93 percent, and by squeezing the eyes to about 90 percent. In an auroral proton field under the same conditions the corresponding figures are 67 percent, 59 percent, and 47 percent, respectively. It is obvious that in the first case, i.e., in the Van Allen Belt, closing the eyea and squeezing la not of much advantage, whereas in the auroral radiation field a substantial reduction of the radiation load on the lens would be accomplished. Still more drastic figures

obtain if we visualize the man as freely floating outside the ship in a full pressure suit with his eyes protected merely by 3 millimeters of plastic visor of the helmet. Exact data for this particular case cannot be established yet, since, as has been pointed out before, the range spectra for these very small thicknesses are not known (1960)." A recent laboratory study by Culver and Newton (1961), and Zellmer and Allen (1961), utilizing the synchrocyclotron at the Uni- versity of California at Berkeley, with rhesus monkeys as subjects, has revealed that both immediate and long-term ocular damage may result from irradiation. In order to protect future astronauts from the damaging effects of radiation, extreme care will have to be taken to minimize the time spent in the primary radiation belts and to cease flights during periods of unusual solar activity. In addition, shielding should be provided where possible. In view of the fact that the eyes are the most sensitive to irradiation, the wearing of a specially de- signed eye protector would have the effect of increasing over-all tolerance during passage through high energy fields. A further source of radiation for the crews of space vehicles is that originating in nuclear propulsion systems. A detailed dis- cussion of this problem has been published by Konecci and Trapp (1959). It was concluded that although a nuclear system definitely adds to the radiation hazard, the combined use of structural and chemical shielding should permit such flights to be made, at least from the standpoint of crew protection. In addition to radiation hazards from nuclear systems, the recent advances in rocket propulsion systems makes it imperative 10

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A variety of sensory and perceptual problems will arise in connection with space flight, both for the occupants of space vehicles and in certain instances for support personnel. The solutions to these problems are interrelated and tremendously complex, thus requiring cooperative efforts among many scientific disciplines.

Visual Problems of Space Travel discusses the problems of space flight as they relate to the visual mechanism. This book updates the National Research Council report Sensory and Perceptual Problems Related to Space Flight, and presents additional information regarding specific critical visual problems, as well as a recently compiled, extensive bibliography of research in this file.

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