The visibility of space objects

The apparent brightness and probability of detection of objects in space depend upon the following important factors, each of which must be specified in order to estimate the visibility accurately:

In normal experience, we are compelled to view extraterrestrial objects through the Earth's atmosphere, which scatters light, is often cloudy, and is in constant motion. The movement of the atmosphere, even in the clearest weather, imposes a severe limit on the resolving power of large telescopes (their ability to produce finely detailed images or photographs). Thus, a large telescope employed above the atmosphere-say, on the surface of the Moon-would have a greatly enhanced ability to resolve details on the surfaces of the planets and the Sun and would tremendously improve man's ability to explore the entire visible universe.

The factors influencing the visibility of objects in space, listed above, are quite obvious and well recognized in ordinary everyday experience. However, to take them all into account simultaneously in calculating the visibility of distant objects usually becomes quite involved. Since simple mathematical expressions which would be useful under a wide variety of conditions cannot be formulated, a number of examples are given below to illustrate some size-distance relationships in the detection of just barely visible objects.1

1 Dole, S. H., Visual Detection of Light Sources on or Near the Moon, The RAND Corp Research Memorandum RM-1900, May 24, 1957.



Specific cases

TABLE 1.-Minimum diameter of objects for detection at the distance
of the Moon as a point of light from Earth under good viewing conditions

 Location of object and optical aid employed ¹Diffuse white disc
(visible by reflected sunlight)
¹Circular plane mirror
(visible by reflected sunlight)
2,000 miles from full Moon:
Naked eye
2,500 feet
14 feet
7 x 50 binoculars
380 feet
2.5 feet
10-inch telescope
47 feet
3.5 inches
100-inch telescope
 4.7 feet
0.35 inch
 Location of object and optical aid employed ¹Diffuse white disc
(visible by reflected sunlight)
¹Circular plane mirror
(visible by reflected sunlight)
On surface of full Moon
Naked eye
25 miles
720 feet
7 x 50 binoculars
3.5 miles
105 feet
10-inch telescope
800 feet
50 inches
100-inch telescope
80 feet
5 inches
¹Approximate diameters

This table illustrates the fact that plane mirrors of even small size, reflecting the Sun's image, may appear to be as bright as quite large expanses of diffuse white material. The mirror reflects the Sun's image in a narrow beam, however, so the orientation of the mirror with respect to the plane of the Sun, the mirror, and the observer is very critical; whereas, with a diffuse white object, the angular relationships are much less critical.

As examples of light emitters:

To a human observer in space, at a distance from any planetary body, the most conspicuous object would, of course, be the Sun, which would appear somewhat brighter than it does when viewed through Earth's atmosphere. In directions away from the immediate vicinity of the Sun and its corona, the sky would appear black but more brilliantly star-studded than the night sky as seen from Earth under the most favorable conditions; the "twinkling" would be absent. All the familiar constellations would be visible at once, from Ursa Minor to Octans (the constellations above the North and South Poles of the Earth). Against this rich starry background, artificial objects would be difficult to detect unless made highly conspicuous by reason of color or brightness. Sophisticated search techniques would be needed to locate faint objects.

To an observer on the surface of the Moon facing the Earth, a similar sky would be presented, except that only one hemisphere could be observed at any one time, and in it the Earth would hang always in almost the same position above the horizon The Earth would display phases (as the Moon does from Earth): about half its surface would be seen to be covered with clouds at all times; where not obscured by cloud cover, the oceans would appear dark, the continents lighter in color; the specular reflection of the Sun's image on the


bodies of water would appear as a bright spot of light. At its brightest, the Earth would provide about a hundred times as much illumination as the full Moon as seen from Earth.

Detection of Earth satellites

The problem of visually detecting satellites of the Earth from the surface of the Earth is especially complicated by the fact that small bodies visible only by reflected light must be in sunlight to be seen at all, and yet to the observer they must appear against a dark background. Thus it is only during the hours shortly after sunset and before sunrise that small satellites may be observed optically. Also, if they are close to the Earth, they may be seen only along a relatively narrow band on the Earth's surface beneath the track of the satellite. Larger satellites, of course, may be seen against more brightly illuminated sky backgrounds. As an extreme example, in order to be visible to the naked eye during daylight hours with clear skies, a satellite at an altitude of 1,000 miles would have to be about 600 to 700 feet in diameter (white or specular sphere) .


Infrared detection systems are playing an increasingly important role in scientific and military applications.2-4 Two fundamental physical phenomena are responsible for this:

Transmission or reflection of infrared radiation can serve as a probe of the structure and composition of chemical and biological matter. Not merely limited to laboratory analysis, infrared probes have permitted man to begin to understand the composition of the atmospheres of the planets and other celestial objects. The principal hindrance to such extraterrestrial observations has been the confusion offered by the intervening Earth's atmosphere. Observations from immediately outside the atmosphere will permit examination of planetary atmospheres with sufficient precision to study the environment that future planetary explorers will have to cope with.

Similarly, infrared observation of the Earth's atmosphere from an orbiting vehicle will permit measurements of cloud cover, water-vapor, and carbon-dioxide concentrations, and the like, which will be valuable for meteorological observations. In addition, infrared sensors in similar vehicles may have some utility for military reconnaissance and surveillance of the Earth. In particular, especially large sources of infrared energy such as afterburning engines of supersonic vehicles and exhaust flames of ballistic missiles may be detectable from orbital platforms.

Infrared detection range is severely limited within our atmosphere by scattering and absorption, so that most military applications, such

2 Tozer, E., Uncle Sam's New Wonder Weapon This Week magazine, November 16, 1958, p. 10.

3 Estey R. S. Infrared: New Uses for an Old Technique Missiles and Rockets, June 1958.

4 Powell, B. W., and W. M. Kauffman, Infrared Application to Guidance and Control, Aero-Space Engineering, May 1958.


as guidance for air-to-air missiles of the Sidewinder type, are of a short-range (i. e., less than 10 miles) nature. However, the space environment will permit full exploitation of the ultimate sensitivity of infrared detectors, permitting extension of detection range by several orders of magnitude. Of course, celestial sources will be detectable at even greater ranges.

Infrared sensors will permit the surveillance of space vehicles in a manner distinct from either optical detection, based on reflected sunlight, or radar, based on reflection of radio waves. The utility of this technique will be dependent in part upon the amount of heat expended in future vehicle power supplies and propulsion systems. One advantage of infrared for ground-based observation of space objects is the ability of infrared to produce a clear image even through haze and scattered light.

Since sunlight has an appreciable portion of its spectrum in the infrared region, it is also possible to observe space vehicles with systems that combine both optical and infrared sensors. A detector such as lead sulfide is sensitive to both these classes of radiation, and may therefore be better in some applications than a narrow sensor limited to a single region of the spectrum.

Infrared can also be useful for detection of celestial objects in space navigation. For such an application, infrared possesses many of the advantages of optical techniques, such as angular accuracy and small equipment size and weight; however, it has one important and unique advantage due to the temperature-magnitude relationship inherent in celestial objects. The tremendous number of optically detectable objects may tend to cause confusion if an optical system is used for navigation or surveillance, unless some method for logical discrimination the basis of space relationships is built in or is available by virtue of the human element. A properly filtered infrared detection system, however, will be limited to detection of a much smaller number of celestial objects which, fortunately, include the most interesting near objects such as the planets, thus offering a simplified background problem. When combined with a measure of spectral analysis, infrared sensors would permit clear identification of a planet, for example, which may be of considerable value in navigation and terminal guidance of space vehicles.


Purpose and problem of tracking

The general purpose of tracking is to establish the position-time history of a vehicle, for guidance, navigation, observation, or attack. The techniques employed are essentially the same for these various purposes. In current satellite and Moon rocket projects the relatively heavy components of guidance equipment are jettisoned after powered flight, and the vehicle is tracked in free flight by other radio or optical means. Navigation in space will undoubtedly require tracking of sources on the planets and in other vehicles as well as tracking of the vehicle itself from Earth and other bases.

Types of tracking systems The principal types of tracking systems are:


Radar and radio systems

These systems employ radio frequencies that are generally in the range from 100 kilocycles to 30,000 megacycles. Below this frequency range, antennas with adequate directivity become impractically large, and ionospheric propagation difficulties become severe. Above this frequency range, there are, at present, practical limitations on the power that can be generated. There are also regions near the upper end of this frequency range (at least for Earth-based stations) that must be avoided because of water-vapor absorption and attenuation due to scattering by rain. In tracking against a background of cosmic noise certain frequencies and frequency regions must also be avoided.

Radar and radio systems may be further classified into active and passive systems, the first requiring transmitting equipment in the vehicle, generally referred to as a beacon or transponder. Passive systems depend upon the reflective properties of the vehicle in bouncing off the incident radio waves. These properties may be enhanced by the use of special reflectors or may be degraded by special surface treatment. Active systems are generally superior to passive systems with respect to range capability and tracking accuracy, but they require special equipment aboard the vehicle. Therefore, in general, active systems can only be used in connection with friendly vehicles in working order.

Radio tracking systems are also categorized as "continuous wave" and "pulse" systems depending upon the scheme used for measurement of range. Angle measurements are sometimes accomplished by a scanning technique in which the antenna pattern is moved either by mechanical or electronic means about the direction of maximum signal return. This method is employed by the more conventional types of radars and by some of the radio telescopes used in radio astronomy. Another method uses the principle of the interferometer to compare the phases of signals received in separate antennas on well-established baselines. This method is employed by Minitrack5 used for Vanguard and Microlock6 7 for Explorer. The frequency of the return signal from a vehicle being tracked depends not only upon the transmitted frequency but also on the relative motion of the vehicle and the tracker. This doppler effect makes it necessary to design the tracker to automatically follow the changing frequency. By the same token it is also possible to use this frequency change to measure the relative velocity of the vehicle and the tracker.

Optical tracking systems Optical systems make use of the visible-light portion of the electromagnetic spectrum. They all consist essentially of a telescope mounted on gimbals to permit rotation about two axes. One type, the cinetheodolite produces a photographic record of the position of the target image with respect to cross hairs in the telescope, along with azimuth and elevation dial readings and a timing indication. With two or more such instruments on accurately surveyed baselines, the position of a

5 Mengel, John T., Tracking the Earth Satellite and Data Transmission by Radio, Proceedings of the Institute of Radio Engineers, vol. 44, No. 6, June 1956, p. 755.

6 Sampson, Willlam F., Henry L. Richter, Stevens, Robertson, Microlock: A Minimum Weight Radio Instrumentation System for a Satellite, Progress Rept. No. 20-308 ORDCIT project contract No. DA-04-495-ORD 18, Department of the Army, Ordnance Corps, Jet Propulsion Laboratory, California Institute of Technology, November 14, 1956.

7 Sampson, William F., Microlock: Capabilities and Limitations, Technical Note No. HTR 58-009, Hallamore Electronics Co., November 4,1958.


target in space is obtained by triangulation. Tracking is usually manual or partially manual.

Another type of optical tracking instrument is the ballistic camera which determines angular position by photographing the vehicle against a star background. This instrument is capable of a very high order of accuracy, but the data require special processing by skilled personnel and the time delay involved is sometimes a disadvantage. Schemes for making some, or all, of the procedures automatic are being considered. An instrument of the ballistic camera type designed especially for optical tracking of Earth satellites is the Baker-Nunn satellite-tracking camera.8

Angle tracking with optical equipment can be accomplished with much greater precision than with radio equipment, but, for Earth-based trackers, darkness, clouds, and haze limit the usefulness of optical equipment. Another limitation of optical trackers is the fact that data reduction sometimes delays the output beyond the period of usefulness. The eventual solution is to make these procedures automatic.

Infrared tracking

For tracking certain objects some advantage is gained by using the infrared portion of the spectrum. Infrared radiation from the object being tracked may provide a better contrast with the background radiation, and certain fog and haze conditions are more readily penetrated. In general, however, infrared radiation is also absorbed by the lower atmosphere, and the range and general utility of infrared tracking are enormously increased by carrying the equipment in high-altitude vehicles essentially above the Earth's atmosphere. The use of photoelectric detection and scanning techniques permits automatic readout of angular position information.

A rather new development in optical and infrared tracking is the use of television techniques to improve sensitivity, selectivity, and rapid readout characteristics of the tracker. Work in this field is being done both in this country 9 and abroad.10 Essentially, a tracking telescope is fitted with the front end of an image orthicon followed by an image intensifier for amplification of photoelectrons. The results provide selectivity to "chop of" the sky background and permit tracking in daylight as well as tracking of fainter objects at night.


Vehicle tracking data are raw material that must be processed mathematically to provide orbit information. Depending upon the application, this orbit information may be needed to accurately establish a vehicle's past behavior and its current position, in order to predict its position in the more-or-less distant future.

Preliminary orbital information is available to the party launching a space vehicle from prelaunch adjustments and from measurements made during the launching operation; and the orbit parameters can

8 Henize, Earl G., The Baker-Nunn Satellite-Tracking Camera, Sky and Telescope, vol. XVI No 3, January 1957.

9 Gebel, Radames K. H., Daytime Detection of Celestial Bodies Using the Intensifier Image Orthicon. WADC Technical Note 58-324, October 1958.

10 Report of the International Astronomical Union Meeting, August 1958, Moscow (to be published).


be corrected and improved as further measurements are made. However, a party not privy to launching operations must have the ability to detect the vehicle and to determine its orbit ab initio from observational data.

At present there are two major orbit computational centers in the United States: the Smithsonian Astrophysical Observatory in Cambridge, Mass., and the Vanguard Computing Center in Washington, D. C. These centers receive data from the various tracking stations around the world; optical data go to the Smithsonian, and radio data to the Vanguard Computing Center. The technique followed is first to derive a preliminary estimate of the orbit configuration, then to refine and correct as further data become available. Actually, two tasks exist for the centers responsible for orbit computation. One is to learn enough about the orbit quickly to be able to predict the future positions for useful periods into the future, and the other is to derive in a more leisurely manner a precision "definitive orbit" giving the satellite's past history.

Prediction of satellite positions for days or weeks into the future is limited primarily by lack of knowledge of air-drag forces. For satellites with orbits which stay sufficiently high-e. g., the Vanguard satellite, 1958 Beta-predictions can be made several weeks in advance with errors of the order of a mile; while for a lower satellite—-e. g., Explorer III-predictions of such accuracy can be made for only a day or so in the future. This situation should improve as information is gathered about the upper atmosphere, but there will always be uncertainty in predicting the drag force which will act on low satellites, particularly nonspherical satellites. If the position of a satellite is to be known to a fraction of a mile, then it will be necessary to continually revise the orbit elements to take care of unpredictable changes in the upper atmosphere.

If it is ever to be possible to predict satellite positions to within one-tenth of a mile or better, it will be necessary to improve the present knowledge of the distribution of mass in the Earth, and the density of air in the upper atmosphere. Satellites represent the best way of studying both; but provision must be made for numerous precise observations around the orbits and use must be made of satellites with known geometry, preferably spherical.

Satellites stay on orbit for long periods-virtually forever if high enough. Thus, an accumulation of hundreds of satellites may be on orbit by the early 1960's as a result of various launchings for scientific, military, and perhaps even commercial purposes. Present satellite data-handling and computing methods will be unable to cope with the problems presented in identifying, cataloging, and keeping track of large numbers of satellites, and it is important that new methods be devised and implemented without delay.


A number of factors-vehicle motion, Earth rotation, and the need for an unrestricted line of sight between the tracker and the vehicle-combine to dictate the number and location of Earth-based tracking stations for a particular application. Refraction of radio and optical rays makes it possible to "see" objects below the horizon, but uncertainty in the refraction correction makes it necessary to restrict


useful tracking data to elevation angles greater than zero. Refraction in the lower atmosphere is due to the presence of air molecules, water vapor, and other constituents, and the refraction of radio waves in the ionosphere is due to the induced motion of the electrons. The later effect is approximately inversely proportional to frequency of the radiation. Since water-vapor content, air densities, and electron densities are variable with time and place, refraction corrections can be made only approximately.11 The refraction correction, as well as the uncertainty in the correction, is greater for low elevation angles. Therefore, useful tracking data can often be obtained only by avoiding low-elevation angles.

Another important factor affecting the selection of the number and location of Earth-based tracking sites is the effect of Earth rotation. For most applications, locations outside the United States are required to provide adequate coverage in longitude.12 The choice of tracker locations also depends upon the particular application at hand, since this determinates the precision required. For a radio tracker, zenith passage yields the most accurate vehicle position and velocity information as well as the longest observation time. This is also true for optical trackers, although for existing optical tracking equipment the relative position of the Sun is a modifying factor.

For some vehicles, tracking is required merely to keep a record of their position, so that a vehicle may be acquired by a tracker when necessary for identification purposes. At the other extreme lie vehicles used for navigation purposes or for determining geophysical or astronomical constants. Here, the number of trackers, their accuracy, and their location are all of critical importance, and the tracking requirement for each application must be analyzed separately.

11 Crain, C. M., Survey of Airborne Refractometer Measurements, Proceedings of the Institute of Radio Engineers vol. 43, No. 10, October 10,1955.

12 Gabler, R. T., and H. R. O'Mara, Tracking and Communication for a Moon Rocket Vistas in Astronautics, Pergamon Press, Inc., 1958.