[327] In the following report ale presented the principal scientific observations made by the Mercury astronauts, arranged according to the sequence: daylight, twilight, and night. The first section is principally concerned with the examination of the earth as seen from these heights, although a section is also presented on the sky. In the second section, the appearance of the sun at sunset is discussed, shell the twilight atmosphere, and the astronomical phenomena peculiar to the early twilight. In the third section are discussed the new data about the earth as seen at night and the difficulty associated with viewing the moon at the horizon.

From the beginning of time, man has looked out upon this world with all active curiosity, cataloging what he saw and eventually developing explanations for wily the earth and sky appear as they do. The results of this type of naturalistic activity as they relate to the earth's atmosphere have been summarized by Minnaert (ref. 1), whose work summarizes at least the main lines of all of the knowledge which man had been able to gain from an earthbound position, by use of the unaided eye. With the advent of manned space flight, it is possible for the first time to observe the earth from outside the atmosphere, and so to extend the naked-eye observations which are summarized in Minnaert's work.

This section compiles and summarizes the observations of the Mercury astronauts and the findings from the principal photographic studies conducted during the Mercury flights. These observation and photographic data were limited by a number of operational constraints discussed in paper 12. The position, transmission polarization structure, and field-of-view of the spacecraft window are described in figure 19-1. As can be seen, this window contains two panes of plate glass and two panes of Vycor, the latter set at oblique angles, which increases the problem of light scattering and window reflections from internal lighting during night time observations. The window transmission cuts off sharply at the lower end of the visual spectrum, precluding photography in the ultraviolet region. Transmission in the infrared range permitted photography in this area for the Weather Bureau. Transmission in the visual range is reduced approximately to the same extent that light is attenuated by the atmosphere. The polarization produced by the window was probably of no significance to any of the observations described in this section. The field-of-view was a limiting factor since control fuel conservation restricted the freedom of the pilot to orient his vehicle for making observations. In addition to the viewing limitations indicated in figure 19-1, during the normal launch, the window frequently becomes covered with a film from the exhaust of the escape tower when it is jettisoned, which reduces slightly the light transmission and increases the problem of scattering. .

Throughout this portion, an attempt is made to present all integrated picture of the appearance of the earth and sky as viewed from space, together with a physical explanation of the phenomenon observed where sufficient information is available to make hypothesis. In general, most of what has been reported by the astronauts confirms data from other sources, such as recent aircraft, balloon, and sounding rocket studies. If much of the information is not novel, it has helped to fill in the basic outlines

of our knowledge about many features of the upper atmosphere.

The program of astronaut observations and their interpretations has been greatly aided by consultation with investigators in a number of fields. The individuals who consulted with Manned Spacecraft Center personnel on the science program are listed at the end of this section.

During the daylight phase of the orbit, the general impression of the earth as seen from a [329] distance of 100 to 150 miles has been characterized by the astronauts as similar to the view from a high-flying jet aircraft. The earth's surface, particularly when viewed obliquely, appears to have a somewhat bluish cast, as would be expected from the longer visual path through the atmosphere. Greens are less readily visible, except when directly below the spacecraft. However, major color variations call be distinguished The coastlines and rivers are easily visible (fig. 19-2) as are mountain ranges (fig.19-3).

In the daytime, the clouds are extremely bright and easily visible. The astronauts have reported that, generally, they can determine relative cloud levels, perhaps by not noting shadows or the apparent motion of cloud tops relative to the surface. Different types of cloud formations are relatively easily discernible. These may be quite spectacular as when the spiral shape of a hurricane a thousand miles in diameter is clearly seen from above (fig.19-4).

The day horizon has been described as a light-blue band, shading off into the blackness of the space above the earth. Photographs taken by the astronauts provide some indication of banding in this horizon layer. Such banding has been reported by Astronauts Shepard, Grissom, and Glenn in references 2 to 4, respectively. The banding appears to be related to the layers in the atmosphere. The width of the daytime horizon appears to correspond to the width of the troposphere and to be approximately 1/2° as viewed from the spacecraft. This is demonstrated in figure 19-5, which shows the moon just above the daylit horizon. The diameter of

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Figure 19-2. Ganges River Basin. MA-9 photograph.

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the moon, which is 1/2°, is approximately equal to the thickness of the daytime horizon as pictured in the photograph.

The visibility of small features on the surface of the earth from space is a complex but important problem since ground landmarks offer a potentially very useful navigational reference. To obtain some information on the operational problems of viewing objects on the surface of the earth in addition to that provided by the ground light study reported in the section on experiments, the pilots were asked to report carefully what could be seen from orbit. These observations have been described in the pilot's report made after each manned flight.

One of the major features of interest to Glenn during the MA-6 flight (ref. 4) was the extent of the he cloud cover over the earth. The only area that has been consistently clear throughout all the orbital flights is the western African desert shown in figure 19-3 and the southwestern United States. Efforts to observe ground signal lights from the spacecraft were frustrated on three of the four flights by overcast conditions (See paper 12). Astronaut Cooper enjoyed the best visibility conditions of any of the astronauts and yet he estimated the cloud coverage to average 50 percent during his flight.

Even where no cloud coverage is present, visibility may be markedly deteriorated by haze produced by smoke, dust particles, or other aerosols. Thus, for example, Astronaut Cooper noted that, while he could see roads and fields and an airport in the El Centro area, he could not see either Los Angeles or San Diego, though he flew right over them. Figure 19-2, which shows I view of the Ganges River Basin [331] photographed on the MA-9 flight, demonstrates this problem since the city of Calcutta with 2 1/2 million population is almost completely invisible and was not seen by Cooper during the flight. Landmarks can be most clearly seen when viewed directly below the spacecraft. The blue haziness, which is seen in photographs of

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the daylight horizon (fig. 19-5), illustrates the reduction in visibility produced by the longer path through the atmosphere. The visibility of features farther from the spacecraft is also reduced by the reduction in size because of viewing distance and foreshortening because of the angle- of-view and the earth's curvature.

Moreover, cloud cover may not simply obscure targets. It may also produce cues which lead to misinterpretation of terrain features. An example of this is shown in the photograph in figure 19-3, taken on the MA-4 flight, which shows a low lying cloud over the Atlantic coast of North Africa. The position of the cloud produces an apparent change in the coastline, which could be confusing if such geographical features were to be used for navigation.

Thus, the extent of cloud cover and atmospheric haze in the latitudes in which the Mercury flights have been made reduces the usefulness of landmarks or ground lights for navigation. On the other hand, in areas where the weather is good, relatively small objects may be sighted. However, this is not a result of magnification produced by the difference in refractive index between the atmosphere and the vacuum of space as had been proposed in reference 5. The effect proposed is the same as the magnification of a penny at the bottom of a cup of water-the penny appears to be a little higher than it really is (fig. 19-6). Because of the relatively small difference between the refractive index of the atmosphere and the vacuum,

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the effect is much smaller than in water. If the index of refraction is computed and summed for each kilometer of atmosphere up to the altitude of 45 kilometers (where it becomes unity) based on the U.S. Standard Atmosphere, 1962, the maximum magnification possible is on the order of only 1.00002, or a rise of 8.5 feet.

The problem of visibility from aircraft has received considerable attention in recent years (ref. 6). This work is too extensive to be reviewed here. However, it is well known that where the dust, smoke, and aerosol content of the atmosphere is low, small objects may be seen for considerable distances if the illumination and the contrast between the object and its background are high. The relationships among illumination, contrast between the object and its background, and the size of the object or angle subtended at the eye is indicated in figure 19-7. These data, taken from the well known work of Blackwell (ref. 7), illustrate that the smallest object that can just be detected 50 percent of the time is dependent on size contrast and illumination. One minute of arc is often taken as a "rule of thumb" for the practical limit of human visual acuity. However, as can be seen from this figure, this is all oversimplification. Under many combinations of illumination and contrast, the smallest object that can just be seen is 10 times that large, while at other combinations of these factors, objects approximately 1/2' of arc can be seen. Where contrast or ;illumination are very high, even smaller

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objects may be seen. Thus, Zoethout (ref. 8) gives a value of 10" of arc for the minimum visible white square on a black background. This corresponds to 30 feet at 100 nautical miles.

These general relationships are complicated by several factors. Thus, for point light sources, such as stars, visibility is independent of size and dependent only on the intensity of the stimulus. For line or ribbon objects, the extended length reduces the necessary diameter for detection, thus the width of a line, which can just be detected, may be one-sixth or less than the minimum diameter of a circular object which can just be seen. This is illustrated in figure 19-8 which shows an infrared photograph taken by a Viking rocket over the southwestern United States (ref. 9). This photograph was taken at a height of 150 miles. From the type of film, the characteristics of the camera lens, the exposure length, and the extent of enlargement, the resolution call be calculated to be 500 feet (ref. 10). Yet roads running across the desert, whose width must be on the order of 50 feet or less, can be clearly seen.

There have been several reports by the astronauts of sightings of small objects on the daylight side of the orbit. These observations have primarily been confined to the area of the southwestern desert of the United States between El Centro, California, and El Paso, Texas. In this region, cities, cultivated fields, roads, airports and railroads have been reported by all four of the pilots who flew orbital flights. These observations have all been made at close to perigee altitude (86, to 90 nautical miles) between 8:00 and 12:00 a.m., local time, under excellent visibility conditions. Astronaut Cooper, who enjoyed unusually good weather conditions, also reported identifying the cities of Dallas and Houston, in Texas, and from the pattern of lakes and wooded areas, the region around Clear Lake where the new Manned Spacecraft Center is being built. In addition, the astronaut made a number of observations in the mountainous and plateau regions of India and Tibet. There he reported what appeared to be individual buildings in Tibetan villages. Some of these observations were apparently aided by trails of smoke from the chimneys of the buildings. In addition, he reported he was able to see {gads on one of which he saw a trail of dust. At the intersection of the dust trail and the road, he saw a spot which he felt might be a vehicle (See paper 20). These observations over Tibet were made from all altitude of 88 nautical miles at approximately 7:30 a.m., local time. The weather conditions were clear with good visibility. Atmospheric attenuation was further reduced by the altitude of the Tibetan plateau which at this point is approximately 16,000 feet.

It should be recognized that all these observations were greatly facilitated by the context in which the observation was made. To be reported, objects must be perceived. Previous training and experience have a marked effect on what an individual will report in any situation. Experience generally increases the likelihood that a small object near visual threshold will be detected, although it may work in the opposite direction as when an unusual angle of lighting or shadow changes the appearance of all object to the point that it goes unrecognized. Such experience and training can also lead to the accurate identification of objects that would otherwise not be recognized. This procedure is much like that of interpretation of a photograph where a set of vehicle treadmarks, running into a forest area, indicate the possible presence of a vehicle among the trees.

Astronaut Glenn described a situation in which he saw a road crossing a river. Each of these could be recognized because they were extended, ribbon-type objects. At the point

where they crossed, he said that he felt he could almost see the bridge, though he recognized that it was too small to be seen.

Since the actual objects which were being viewed at these points cannot be verified, it is not possible to determine the accuracy of these observations. However, from knowledge of the factors which affect visibility under these conditions, there appears to be no reason to suspect that these identifications were not generally accurate. All the astronauts have normal, or better, distance visual acuity. Astronaut Cooper in particular has an acuity, as measured during a recent annual physical, of 20/12, which is significantly better than 20/20 which is the normal standard of good acuity. All the observations were made under high levels of illumination, excellent visibility conditions, and with the aid of many contextual cues. Thus, there appears

to be no need for postulation of either improved visibility resulting from weightless conditions or unexpected atmospheric magnification effects to account for the observations made to date. Despite the impressive nature of these observations, the important feature to be kept in mind is that they are scattered and involve viewing under essentially optimal conditions. As pointed out earlier, the large amounts of haze and cloud cover make ground observations difficult and somewhat unreliable.

As with the problem of direct viewing, it is not possible to put a lower limit on the physiographic and geologic detail which can be delineated on space photographs without an extensive study. However, a rough idea of the useful resolution can be gained by examination of some of the Mercury pictures listed in table

19-I. This table summarizes the general purpose photographs taken on manned and unmanned Mercury flights.

The MA-4 photographs of North Africa are of considerable interest because they are among the best color pictures showing unobscured terrain. The Anti-Atlas Mountains are especially striking (fig 19-3) in the amount of geologic detail which can be seen. The folded structure of the mountains is obvious, and many individual plunging folds can be traced. A linear feature suggestive of the Zemmour fault (ref.11) can be seen intersecting the coast south of Agadir but not identified with any certainty.

Many of the MA-9 photographs show abundant topographic and geologic detail. Figure 19-9, taken over the Tibetan plateau, is particularly useful because of the favorable camera angle. A geologic sketch prepared from this photograph is shown in fig. 19-10. A number of structures of possible economic interest are indicated in the sketch. For example, the domes and anticlines represent potential oil-bearing areas, and intersections of some of the lineaments might be the loci of mineral deposits.

It is interesting to note that manmade features (excepting large areas of cultivation) are generally very difficult to identify on the color photographs. As already noted, figure 19-2 shows the area of Calcutta but the city itself cannot be recognized.

The scientific value of the Mercury terrain photographs depends on several characteristics in which they differ from conventional aerial

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photography. The most obvious of these is the tremendous aerial coverage provided by each picture taken from orbital altitude. This is illustrated by comparison of the 1:800,000 scale of figure 19-9, taken on the MA-9 flight, with the 1:20,000 or 1:40,000 scales of conventional air photos. The area covered increases with approximately the inverse square of the scale, and is so much greater in pictures taken from space as to be virtually a qualitative difference. This great coverage permits continuity of observation which may lead to discovery of large geologic features unnoticed on conventional photographs, such as the very long lineaments illustrated in figure 19-10. It should also be mentioned that the synoptic nature of space photography is valuable in meteorological and oceanographic applications.

Another characteristic of space photographs is the fact that they show the earth, subject to limitations of visibility and resolution, as it is. Stereoscopic vision is possible with even roughly oriented photographs if there is overlap. In addition, subtle tonal differences covering large areas can be detected. Both of these properties are essential for geological interpretation and

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[338] cannot, in general, he provided by mosaics of conventional aerial photographs. This strongly suggests the unique scientific value of terrain photographs from orbital altitudes, not only for unexplored areas such as Tibet, but also for areas previously covered by conventional photography.

In summary, photographs of the earth from orbiting spacecraft are potentially valuable for (1) geologic reconnaissance, (2) topographic mapping, (3) forest mapping, (4) icepack and iceberg monitoring, (5) supplemental weather observations, and (6) mapping of near-surface ocean currents. In addition, experience in interpreting such photographs will prove useful in interpreting similar photographs of the planets when they become available.

Each astronaut has devoted part of his spaceflight program am to visual and photographic observations of value to meteorology (ref. 12). Since high photographic contrast is needed in pictures from weather satellites to aid in distinguishing coastlines and patterns of thin clouds, two photographic studies were initiated to study cloud, land, and water contrast as a function of wavelength. These studies have been described in paper 12.

Astronaut Schirra took a series of 13 black and white photographs of the earth through six color filters in the visible spectral region from 3700 Å to 7200 Å to record some of the spectral reflectance characteristic of clouds, land, and water areas when viewed from outside the atmosphere. In general , the results from this study showed that, as would be expected, photographic contrast increases with increasing wavelength in the visible spectrum.

It might be concluded that the optimum wavelengths for viewing the earth would he in the near infrared spectrum where scattering from atmospheric particles is relatively low. That this is not quite true was demonstrated in a second study conducted by Astronaut Cooper. In this study, three areas of the infrared spectrum were isolated by use of filters and infrared film.

Water has a very low reflectance in the near infrared, while clouds and land have a high reflectance. Therefore, in this second study, coastlines and cloud patterns over water were easily discernible. Unfortunately, however, clouds were more difficult to see over land in the near infrared because of the high reflectivity of both clouds and areas covered with green vegetation containing chlorophyll.

These two studies show, then, that the spectral sensitivity of television camera systems for weather satellites should probably be restricted to the region from about 5000 to 7500 Å as a compromise between the adverse effects of scattering by molecules and aerosols at shorter visible wavelengths and the low contrast effects of clouds over land areas at near infrared wavelengths.

Many of the black and white pictures taken by Astronaut Schirra show bright hand on the earth's horizon. The bright band is approximately 16 kilometers thick, which agrees with the expected thickness of the tropical troposphere. Large light scatterers in the troposphere, such as dust and water droplets, produce this bright band at the earth's limb. The thermal stability of the stratosphere severely limits the convective transport of aerosols to higher levels, so that there is very little scattered light coming from the stratosphere. The apparent brightness of the tropospheric layer varies from picture to picture, suggesting that there are changes in the size or concentration of scatterers over different geographic areas. Changes in brightness in the same picture from one filter to another demonstrate the wave length dependence of the scattering of sunlight; more light is scattered at the shorter wave lengths. However, within an individual picture, both geographical and wave length effects may appear.

The pictures obtained with photographic film contains more meteorological information than do the low resolution pictures from present weather satellite television pictures. Because of the greater resolution and lower altitude the cloud types and patterns can be seen in greater detail in the Mercury photographs. If a meteorologist can see the smaller cloud forms and their orientation, then he may have important clues to the direction of the wind, the wind sheer and possibly a rough estimate of the wind speed in the lower levels of the atmosphere. Photographs from Mercury flights have been useful in cloud studies to help interpret the meteorological information in Tiros pictures.

[339] Several Mercury astronauts have seen lightning in thunderstorms at night, appearing, as Astronaut Glenn described it, "like balls of cotton illuminated from within." Astronaut Cooper observed that each lightning flash was accompanied by static on his high frequency and ultrahigh frequency radio receivers. This observation confirms the findings of a recent research study conducted for the Weather Bureau, which concluded that high frequency energy radiated from a lightning stroke can propagate and be detected and located on a worldwide basis by means of a lightning (or sferics) detector carried on a satellite. Efforts are underway now to develop such an instrument and our confidence that it will work is much higher because of Astronaut Cooper's alert observation.

Cloud systems were visible at night with partial moonlight or none, indicating that low-light level television cameras on weather satellites may photograph cloud cover at night successfully. Photographs of clouds over snow are being studied to seek ways of discriminating one from the other in television pictures. Cooper reported he could detect the difference between snow and clouds. He also reported that smoke trails gave an indication of surface wind direction.

To date, none of the astronauts h as reported seeing stars on the daylight side when the sun or the illuminated earth's surface was within the field of view. Nor was the flashing light released from the MA-9 spacecraft seen by Astronaut Cooper during the daytime, though the possibility that he was looking in the wrong direction cannot be ruled out (See paper 12). However, some of the astronauts hive reported observations of a few bright stars or planets at twilight, but their level of dark adaptation and the degree of cabin lighting are uncertain factors to he considered. There is, of course, no difficulty in seeing the moon (fig. 19-5) since it is even visible from the surface of the earth in daylight.

When the sun and the illuminated earth's surface is not within the field of view, it is possible to look into space and maintain dark adaptation. Under these conditions, Astronaut Cooper reported that the dayside sky appeared less dark than the night sky, and the threshold of star visibility correspondingly raised by as much as two magnitudes. Two hypotheses suggest themselves to account for this observation. The more probable one is that this results from a high altitude dayglow possibly that of the atomic emission at 6300 Å.

A second less likely hypothesis is that the sky appears less dark during the daytime as a result of scattering due to smell solid particles. The argument against this proposal is as follows. If the glow were due to small solid particles, they would have to be at a level low enough so that the sun could not reach them during the night; otherwise, this glow would be apparent from the ground all night long. Since astronomical twilight is defined by saying that at the end of astronomical twilight the zenith has reached full night-time darkness, it is clear that at this time, the dust particles, if any, must be out of the sunlight. It is known that astronomical twilight occurs when the sun is 18° below the horizon; and it is a matter of simple trigonometry to show that at this time an object more than about 350 kilometers high would still be in the sunlight. Hence, if there is a layer of dust particles, they must be below 350 kilometers.

Now, Astronaut Cooper reports that the dayglow as he saw it drowned the light of stars fainter than about the fourth magnitude. This is about the same thing that happens on a night of full moon; the fifth and sixth magnitude stars become very difficult or impossible to see. Hence, the brightness of the sky as Astronaut Cooper saw it was more or less like the brightness of the sky on a night of full moon. We know that from the g round the sky causes a loss of about 30 percent in the light reaching the earth; and, thus, we may think of it as if there were small particles covering about one third of the sky. Above the spacecraft, the sky is so much reduced in scattering power that it scatters only as much light from the surf as the whole atmosphere scatters from the moon. Since the full moon is about 100,000 times fainter than the sun, it follows that the amount of scattering material must be such as to cover about 0.3 of 1/400,000 of the sky, or roughly, one millionth. By the usual laws of optics, this means that in a column one square centimeter in cross- sectional area and 350 kilometers in [340] length, there must be enough matter to cover one millionth of a square centimeter.

It will be shown that this is too much matter. The most efficient size of particle for producing scattered light is about 1 micron in diameter; smaller particles perform the electrical equivalent of bobbing up and down on the light waves without disturbing them, and larger ones simply block the light. A 1-micron particle blocks about 10-S square centimeters; hence about 100 such particles are needed in the above- mentioned column Since the volume of the column is 35 cubic meters, the density is about 3 particles per cubic meter.

A spacecraft moving at 8,000 meters per second will then encounter 24,000 such particles per second per square meter. Actually, however, micrometeorite counters, which are adequately sensitive for these very small particles, show between 1/100 and 1 particle per second per square meter outside of showers. Rates of thousands of particles per square meter per second are never observed (ref. 15). Hence the layer cannot consist of micron-size particles. Neither can it consist of particles of other sizes, because the counts are even lower for these. There is approximately the same amount of mass in each logarithmic increase in size; and the other sizes are less efficient. The hypothesis of a dust layer thus fails by a factor which can be conservatively estimated as 10,000.

The spacecraft window attentuates the average light intensity in the visible range to about the same extent as the atmosphere. It does not, however, produce the same color change. To the astronauts, the sun appears white; they describe it as having the color of an arc light, rather than the yellowish color seen from the earth. As the sun approaches the horizon, a band of orange light spreads from below the sun around the horizon. Above this orange band can be seen the hazy blue layer similar to that of the daytime sky. As the sun comes c loser to the horizon, a v trite layer appears above the orange band. The orange, white, and blue layers are quite distinct, particularly the border between the white and blue layers. Some astronauts have been able to report on layers which do not appear in photographs. The orange, white, and blue layers, however, show up very clearly in the photographs of the setting sun and of the orbital twilight which follows.

As the sun approaches the horizon, the terminator passes below the spacecraft and moves off toward the horizon so that, at sunset, the earth directly below the spacecraft is dark. All that can be seen is the band of light in the west, stretching perhaps as much as 180° around the horizon.

The sun, of course, sets much more rapidly for the astronaut than for the observer on the ground. Since the sun moves for the ground observers at approximately 15° an hour, neglecting the effects of atmospheric refraction, it takes the sun, which is 1/2 ° in width 2 minutes to set from the time it first touches the horizon to the time when it completely disappears. In contrast, for the orbital vehicle, the sun moves at 4° per minute so that, once again neglecting the effects of refraction, it sets in 7 1/2 seconds. Once the sun has set, the glow along the western horizon gradually fades but remains visible for apparently about the astronomical twilight period or until the sun is 18° below the horizon, which is approximately 4 1/2 minutes at orbital velocity.

Just prior to sunset, calculations show that the effects of terrestrial refraction should be to give the sun a football-shaped appearance. The phenomenon lasts such a brief time and is so extremely difficult to observe because of the problem of glare that only the visual report from Astronaut Carpenter (ref. 16) conclusively confirms it. It is, however, plainly visible on photographs, obtained by both Carpenter and Glenn (fig. 19-11), and matches the theoretical shape (ref. 17). The significant point here is not that the path of the ray through the atmosphere is different from the path as seen from the ground. Actually, the distance between the observer and the refractive layer causes the entire atmospheric effect to be compressed in such a way that it results in a completely different phenomenon

During twilight, three atmospheric layers at least are distinguishable (fig. 19-11). As illustrated in figure 19-12 at the top of the atmosphere

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, the light of the sun is scattered in the ordinary way (Rayleigh scattering) by atoms and molecules of the upper atmosphere. This layer is blue for the well-known reason that Rayleigh scattering varies as the minus fourth power of the wavelength and, therefore, effects the shorter blue wavelengths much more than the longer wavelengths. Lower in the atmosphere the scattering approaches saturation in all wavelengths, and so we have a white layer because there is enough atmosphere to scatter even the red light. Close to the horizon the brightness of more distant atmospheric layers exceeds that of the layers at which we are looking. As a consequence, we see, not the light which has been scattered by the atmosphere but that which has come through it either from the sun itself or from bright layers. As a result, this layer appears red, since the beam which reaches us has lost blue light.

Volz and Goody have studied the colors of twilight as seen from the ground (ref.18). They find that in the rare cases when there are no storms between the observer and the sun, the twilight colors change slowly and continuously. Discontinuous changes occur when the contribution to the sky which should be made by some distant region is blocked by a storm. In the same way, if there were no storms along the line of sight as seen from the spacecraft, it would be reasonable to anticipate that the colors of the twilight horizon band would melt uniformly into each other. In general, however, just as the storms interrupt the orderly time sequence of colors as seen from the ground, so also they may interrupt the orderly spatial display of colors as seen from space in the twilight horizon band. In addition, the variation between troposphere and stratosphere may play a part in producing these lines.

In addition to these bands, which can be seen on photographs taken at twilight by Astronauts Glenn and Carpenter, Astronaut Schirra noted further detail in the area in and below the Rayleigh scattering level. He observed the planet Mercury setting through this region and reported a dark-blue band, a light-blue band, and then a dark-blue band near the earth's surface. These observations are still being analyzed; however, there is some indication that the Chappus absorption bands of ozone may play a role in producing the central blue band (ref. 19). Copper confirmed these observations of Schirra by describing the appearance of the blue banding and by examining a sketch prepared from Schirra's report.

On numerous occasions, when the sun was above the horizon, small luminous particles drifting generally backward along the spacecraft line of motion at relative velocities of a few meters per second were observed by the astronauts. Carpenter demonstrated by rapping on the hatch that such particles could be produced from the spacecraft itself. Given the very close coincidence in orbit velocity, which is implied by the small relative velocity, it is considered highly probable that all such particles originate from the spacecraft. From the remark of Glenn that the particles seemed to be about as luminous as fireflies, it is possible to estimate that the sizes of those seen by him are of the order of one millimeter (refs. 20 and 21). Some of them may have been bits of debris. The majority, however, appear to be ice crystals probably formed from the steam which is released by the life-support system.

Astronaut Cooper (paper 20) reported seeing particles emerging from the attitude jet nozzles. He was observing them under especially favorable circumstances, namely at a time when the sun was up but the window faced away both from the sun and from the earth, so that he had a black sky against which to see them. Furthermore, he was dark-adapted. Under these circumstances he could see objects as faint as the fourth magnitude, as compared with an estimated -9 magnitude for the objects seen by Glenn (refs. 20 and 21). They must thus have been as much as 100,000 times fainter, corresponding to the difference of 13 magnitudes. Thus, their diameters may have been as small as 25 microns. For such small particles, it is extremely difficult to be sure of the origin. Given the high temperature of the jet exhaust (approximately 1,300° F.), ice crystals would not be expected. Furthermore, most of the material leaving the nozzles should be moving at supersonic velocities if the jets are to be effective in moving mass of the spacecraft. However, Glenn reported seeing a small "V" of steam each time he activated the pitch down thruster (ref. 4). Such steam, under more favorable viewing conditions might appear as individual particles. It appears possible that some of the material in the periphery of the jet exhaust may be moving relatively slowly and cooling rapidly upon leaving the nozzle producing minute droplets or crystals which can be viewed under very favorable conditions. It is possible that these particles are tiny fragments of the catalyst eroded by the hydrogen peroxide blast. In any case, particles coming from the jets were not seen by Glenn, Carpenter, or Schirra, probably because the latter were observing them under less favorable circumstances. Cooper had the enormous advantage that his cabin lights could completely extinguished and his window covered for extended periods of time to assist him in becoming fully dark-adapted.

At the time of the beginning of the orbital flight program, it was realized that the most promising field for nighttime observations was the study of extended dim objects, especially immediately after sundown or before sunrise. At all times, the astronaut is above a major portion of the airglow layer; and this means a major reduction in the background illumination. Near the time of twilight' the astronaut has the further advantage over the ground observer that his sky is without twilight except for the band along the horizon. Since the majority of comets are found by ground observers in twilight, the astronauts were urged to keep an eye out for them at this time. It should be noted that a new comet was discovered at the eclipse of July 20, 1963 ( ref. 22 ) . It was hoped that the astronaut would observe the no-man's land between the zodiacal light, which can be observed from the ground only at distances of 30° or more from the sun, and the outer corona, which is invisible at distances from the sun more than about 3° (ref. 23). This gap has been partially bridged by airplane flights, but more data are still needed.

Astronaut Cooper reported that at about 20 seconds after sunset, he saw a whitish arch extending some 15° or so out from the sun. Approximately l minute after sunset, Cooper successfully observed the zodiacal light as a faint band concentrated along the ecliptic. The failure of previous astronauts to see it was presumably because of lights in the cabin which could not be extinguished. As part of an experiment developed by Ney and his associates [343] a series of photographs were taken of the zodiacal light, but these were unsuccessful because of the problems described in paper 12.

Once the orbital twilight has faded, the visibility of the earth depends upon the phase of the moon. Even with no moon, the earth's horizon is visible to the dark-adapted eye.

According to Cooper, the earth's surface is somewhat darker than the space above it, which is filled not only with the visible stars, hut also has a diffuse light produced by the countless stars, which cannot be individually resolved by the eye and by dim light phenomena, such as airglow and zodiacal light. With the aid of starlight, zodiacal light, and airglow. clouds and coastlines are just visible to the dark-adapted eye. With moonlight reflected on the earth, the horizon is still clearly defined, but in this case, the earth is brighter than the background of space. With moonlight the clouds can be seen rather clearly and their motion is distinct enough to provide a cue to the direction of motion of the spacecraft. Lights from cities can be distinguished, even through thin clouds. Thus the lights of Shanghai shining through the clouds were used by Cooper to help aline his vehicle in yaw on the last night pass prior to retrofire.

The night sky appears quite black with the stars as well defined points of light which do not twinkle. Lights upon the earth do twinkle w hen viewed from above, according to Cooper.

Comparison of visual estimates of angles near the horizon with the corresponding measurements shows that the so-called "moon illusion" continues to exist in space; that is. objects near the horizon seem to be larger than their true angular dimensions (ref. 21). The fact is interesting' since it shows that this illusion is not related to any sensation of gravity, but is a consequence in some way of the visual perception of the location of the horizon.

Around the horizon, all the astronauts report that they saw a band of light, which appeared to them to be centered at a height of some 6° to 10° above the visible horizon. Astronaut Glenn describes it as "tan to buff"; similar descriptions were given by the others. The nature of the band was made clear by Astronaut Carpenter who employed a filter which passed only the 5577 Å line of the neutral oxygen atom (refs. 21 and 24). Through the filter, the band continued to be visible although all other details of the horizon had vanished. It was thus clear that the band resulted from the phenomenon of nightglow; that is, the emission of light by gases of the high atmosphere. In this emission, the line 5577 plays an important part; it constitutes about 1/6 of the total, according to Tousey and his associates. Carpenter reported that the light seen through the filter seemed to be about the same as that without; this remark should, however, be understood as an indication of order of magnitude rather than as a precise measurement, for which neither time nor instruments were available.

Carpenter also provided a rough estimate of the brightness, indicating that it was comparable with that of a bank of clouds near the horizon illuminated by the quarter moon, or about 30 kilorayleighs, according to later computations. This figure happens to agree closely with rocket measurements (ref. 25).

The height of the nightglow layer was also measured on the MA-7 flight. Carpenter observed the passage of the second magnitude star Gamma Ursae Majoris through the nightglow layer. He timed its entrance into the layer. its passage through the level of maximum brightness, and its emergence. From this information, it has been possible to calculate the height of the nightglow layer by using the standard formulas for the dip of the horizon. A value of 91 kilometers was found; the close agreement with rocket measurements is probably to be expected, since the method is capable of considerable precision.

On the MA-9 flight, a camera with a f/0.8 lens of 3.8 cm focal length using Ansco H 529 color film was carried to photograph the nightglow (see paper 12). A total of 15 usable exposures were made. Some of these were degraded by roll of the spacecraft during the exposure, but a number of them show the nightglow layer as a thin line a few degrees above the horizon as can be seen in figure 19-13(a). The results of this study are summarized in table 19-II.

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The color of the nightglow band, as determined from the photographs, is greenish with respect to the bluish-white illumination of the earth. It is not, however, the same green as a pure 5577 Å line since, as noted above, the light of the 5577 Å line is diluted with other radiations.

On some of the photographs, the atmospheric clouds and haze near the horizon can be seen, illuminated by the moon, then at last quarter (fig. 19- 13(b)). As remarked by Carpenter (ref.24), the brightness of the nightglow layer is comparable with that of the clouds illuminated by the quarter moon; this conclusion is supported by the densitometry of the photographs taken by Astronaut Cooper.

Table 19-II shows the altitudes of the spacecraft as n function of time and the measured angles that the airglow layer has with respect to the observable earth's limb. It also shows the inferred heights of the airglow layer, and these heights vary from somewhat in excess of 100 kilometers down to something just under 80 kilometers. The average height as determined from all the pictures is 88 kilometers, and the thickness of the layer is 24 kilometers. There is an indication (figs. 19-13 (a), (b), and (c) ) that the earlier photographs of the airglow layer show it higher above the horizon as determined by lightning flashes on the horizon

than the later pictures, in which the earth's limb is illuminated by the quarter moon. This could be true latitude effect, and, if it were, would indicate that the airglow layer has a higher altitude at high latitudes-the highest latitude in this case being about 27° S. where the layer is about 108 kilometers as measured from the lightning horizon references. The lowest altitude of the airglow layer is near 17° and is about 78 kilometers.

The width of the nightglow band at the half-intensity points was measured from the films as between 0.66° and 0.92°. By comparison, the distance from the center of the nightglow layer to the bottom was measured by Carpenter and his coworkers (ref. 24) as 0.34°; he did [345] not measure the entry of the star into the layer. Carpenter's half width is in good agreement with the photographed total width; both indicate that the nightglow layer is considerably narrower than the space between itself and the horizon. Table 19-III summarizes and compares the data from the MA-7 and MA-9 flights

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Astronaut Schirra observed on one occasion on the night side, while over the eastern portion of the Indian Ocean and probably while looking in a northerly or northeasterly direction, n large luminous patch which he described as a brownish smog-appearing patch. He saw stars above and below this patch which he felt was higher and thicker (wider) shall the "normal ' nightglow . On the average, this higher patch or layer did not seem to be as bright as the "normal" nightglow layer. Some stars could be seen near the feathered edges of the layer, but he was not certain he could see any stars in the central denser portion (nor is it likely that, at the short period of observation, there was a rich and bright star field in the background). It is tempting to conclude that this phenomenon may have been a view of a

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tropical 6300 Å atomic oxygen emission, first reported by Barbier and his associates (ref. 14). It is believed that the arc observed by Schirra is similar to that observed at Tamanrasset, Algeria, and Maui, Hawaii. On one occasion, Cooper noticed and immediately reported a patch, similar to that described by Schirra, above the "ordinary" nightglow layer while over South America. It had been predicted that there might be visual concomitants of the South Atlantic magnitude anomaly; However neither of these observations were in the correct geographical location to be related to this phenomenon.

Acknowledgements. In addition to the individuals specifically referred to in the text of this section, the following scientists assisted in the development of the Mercury inflight research program as consultants, or members of the "Ad Hoc Committee on Scientific Experiments," or the ''Panel on Inflight Scientific Experiments" of the NASA Office of Space Sciences: Jocelyn R. Gill, Ph. D., NASA headquarters; Gordon C. Augason, NASA Ames Research Center; Maurice Dubin, NASA Goddard Space Flight Center; Frederick R. Gracely , NASA Headquarters; John F. Naugle,

Ph. D., NASA Headquarters; Freeman H. Quimby Ph. D., NASA Headquarters; George P. Tennyson, NASA Headquarters; Ernest J. Ott, NASA Headquarters; Albert Boggess, III, Ph. D., NASA Goddard Space Flight Center; George Swenson, Ph. D., U. of Illinois; Franklin Roach, National Bureau of Standards; Edward P. Ney, Ph. D., U. of Minnesota; Leslie Meredith, Ph. D., NASA Goddard Space Flight Center; and Dale W. Jenkins, Ph. D., NASA Headquarters.

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