The Artificial Satellite as a Research Instrument (Nov, 1956)
I love the idea of crowdsourcing the task of actually finding the satellite once its in orbit to an army of amateur astronomers.
The Artificial Satellite as a Research Instrument
Its pay load of 10 pounds will telemeter information about conditions at the edge of space. When its batteries have run down, we can still learn much by observing its flight
by James A. Van Allen
Most persons interested in space travel will be willing to wait until the second or third spaceship has made it to the moon and back before booking their reservations. The artificial earth satellites are another story. If all goes well, the first of them will be on orbit by early 1958, during the International Geophysical Year. Already there is a long waiting list of research projects for these first satellites. Unhappily they will have little space for research apparatus. Only about half of their 20-pound weight can be devoted to instruments for recording and reporting physical conditions at the edge of outer space.
The National Academy of Sciences and the Defense Department have announced that they plan to make enough launching attempts to establish at least one satellite in a durable orbit: there may be 12 such attempts during the I.G.Y.
Each successful flight should vastly enrich our knowledge of the earth and its environment in space. Much has been learned during the past 10 years by means of high-altitude research rockets, and some 200 such rockets will be fired during the I.G.Y. But a rocket flight lasts only a few short minutes. By comparison, a satellite traveling around the earth for days or months will be a semipermanent observatory. From it we can undertake direct and more or less continuous monitoring of the intensities of arriving radiations which are absorbed and obscured by the protecting blanket of the atmosphere. We can get a count and a spectrum of the sizes of the micrometeorites that the earth sweeps up on its orbit. The round-the-world travels of the satellite will make possible surveys of the outer reaches of the geomagnetic field and the cloud cover over vast areas of the earth below. These and other satellite observations can be correlated with observations from the ground to establish more clearly the connection between events inside and outside our atmosphere.
Even without instruments a satellite can be a useful research tool. When conditions are right, against a twilight sky, it will be visible to the naked eye as a very faint and fast-moving “star”—about as dim as the faintest star an acute human eye can see. The direction and speed of its flight can be plotted by sky cameras and by observers equipped with low-power telescopes and binoculars. The variation of its velocity and the perturbations of its orbit will yield precise information about the density of the upper reaches of atmosphere and about the true shape of the earth and the distribution of its mass within. Fixes taken on its position from observatories around the globe will locate reference points on different continents with great precision and reduce present errors in the world map.
The laws of physics set certain inexorable limits on the design and behavior of a satellite. In the first place, to hold an orbit around the earth the satellite will have to have a velocity of at least five miles per second. Man has not yet succeeded in hurling any sizeable object at this velocity. At the present stage in the art of rocketry, the velocity requirement sharply restricts the mass of the satellite. To get a 20-pound object up to orbital velocity at sufficient altitude above the earth to free it from the drag of the atmosphere will require a launching rocket weighing 22,000 pounds. It might seem that with a propulsion system of this size a few extra tens of pounds of payload would make little difference. But to deliver a 40-pound satellite on the same orbit would require a propulsion system weighing 44,000 pounds.
The choice of orbit is likewise restricted. It is not possible, for example, to have a satellite describe a halo over the globe around, say, the Arctic Circle.
The orbit must lie in a plane through the center of the earth. For some purposes a perfectly circular orbit would be ideal, but since perfect directional control of the satellite is impossible, the actual orbits will be mildly elliptical in shape, with the center of the earth at one of the two mathematical foci. Here the question of altitude becomes important. The lifetime of a satellite is determined by the atmospheric resistance it encounters. At 300 miles, the projected launching altitude, it will find the atmosphere about as thin as that in a laboratory vacuum. On an elliptical orbit, however, it will travel through lower altitudes during part of its flight. Air resistance there will slow the satellite so that it will spiral inward to the denser regions where friction will finally burn it up. Because knowledge of atmospheric density at high altitudes is so uncertain, we cannot make firm predictions about the life expectancy of satellites. The objective for the first satellite is an orbit which will take it no closer than 200 miles from the earth’s surface at perigee and no farther than 1,500 miles at apogee. Estimates of its lifetime in such an orbit range from a few weeks to a year.
For convenience of observation, among other reasons, the first satellites will be set on orbit at a 40-degree angle to the Equator. This orbit will keep them circulating overhead in a zone between the 40th latitudes north and south. At the orbital velocity of 18,000 miles per hour, a satellite will circle the earth in about 100 minutes, or 14 to 16 times per day. The eastward rotation of the earth will cause its path to describe a sinusoidal curve around the Equator. The equatorial bulge of the earth, and detailed mass irregularities such as mountain ranges, will produce perturbations of the satellite’s orbit [see diagram at bottom of page 45].
Over a sufficiently long time the satellite will come at least once within sighting distance of everyone within the orbital zone, covering some 125 million square miles of the earth’s surface. It will be sighted most frequently near northern and southern boundaries of the zone. To a casual observer the arrivals of the satellite overhead may appear quite capricious. It will cross the sky at different speeds, at different altitudes and in different directions.
One of the reasons for choosing a lateral orbit around the earth is to take advantage of the earth’s rotation to help launch the satellite. The plan is to launch the objects from Cape Canaveral, Fla., toward the east over the Atlantic Ocean. The earth’s eastward rotation will add, by a kind of slingshot effect, to the velocity given the satellite by the rockets. Every bit of velocity is precious. A velocity of 18,000 miles per hour and an altitude of 300 miles represent an enormous advance over the present record of 6,000 miles per hour and 250 miles established in 1949 by a two-stage rocket. The vehicle that is to accomplish this is a three-stage 72-foot finless rocket [see lower diagram on the preceding two pages].
No less remarkable than this achievement in rocketry will be the feat of control engineering that will carry out the flight plan. The vehicle will be self-guided by an intricate control system housed in the second stage. This system will take command on the launching platform. It will time the ignition and the separation of the spent rockets, and it will direct the jets of the gimbal-mounted motors of the first- and second-stage rockets to swing the vehicle smoothly from its initial vertical trajectory onto a course parallel with the earth’s surface [see upper diagram on preceding pages]. Just before it ignites the third stage, it will fire a pinwheel array of jets which will set the vehicle rotating on its long axis. The third stage, spinning at several revolutions per second, will then streak away in stable flight on the orbit. The shell of this rocket might itself serve as a satellite, without instruments. If it carries an instrument-loaded “bird,” the final propulsion shell and the bird will separate at a pretimed moment. In that case we shall have two companion satellites, for the third-stage shell as well as the bird will continue on orbit. The instrument-carrying satellite may be a sphere, a cylinder or some other shape; there is even a possibility that it may be made inflatable, to improve its visibility.
The launching, if all goes well, will set the stage for the nerve-wracking task of the first sighting of the satellite. Down-range observation of the departing rocket will predict the arrival of the satellite over a given observation point with an error of no less than six minutes and several hundred miles. There may well be doubt that the object is actually in a durable orbit. The number of fully equipped optical observatories will be limited. Their coverage will have to be extended by mobilizing amateur observers all over the world and assigning them systematically plotted areas of sky. If the satellite is not sighted on its first time around, the game of sighting it will assume constantly greater uncertainty. It is needless to enlarge on the haunting fear that a satellite might be on orbit and yet escape detection.
Such a possibility dictates that the first satellites be equipped with a low-power radio transmitter even if they carry no other instruments. A radio beacon and storage batteries to give it several weeks of life can be installed at a cost of about one pound in weight. Its signal can be used to report observations as well as for location. To pick up the signals, an array of tracking stations will be located along the 75th meridian from Washington, D.C., to Santiago, Chile. The satellite will have to pass through this picket fence every time around. Additional stations will be located elsewhere around the globe. Some of them may be in the U.S.S.R., which recently agreed to tune its satellites to the same radio frequencies as ours, and in China. Coverage of the sky by these stations will be extended by enlisting radio hams to monitor the satellites’ frequency, especially during the critical first trip around the earth and during the dying phase.
Once the satellite has been sighted and its first few orbits plotted, prediction of its future orbits can be made with increasing precision. The National Academy of Sciences is establishing special computation laboratories in Cambridge, Mass., and Washington. Their bulletins will alert the observers and the public at large to the satellite’s appearances in the sky at ideal seeing times over observatories and centers of population.
The primary optical observations will be conducted from 12 specially equipped stations. Each will have a 20-inch Schmidt sky camera, capable of registering the image of a 15-inch sphere at 1,000 miles or a three-foot sphere at the distance of the moon. They will take a series of exposures of each passage on strip film. On these pictures the satellite can be located within a minute or two of arc in the sky and within milliseconds in m time. Such precision will make it possible to locate observing stations relative to one another and to the center of the earth to an accuracy of 30 or 50 feet. A dozen such fixes will allow geographers to connect the maps of the continents with new accuracy and will help to establish the shape of the earth. The perturbations of the orbit, observed with the same precision, will give important information about the shape and structure of the earth. The rate of spiraling caused by atmospheric drag will provide an extremely sensitive measurement of atmospheric density as a function both of latitude and altitude.
In the last few revolutions before the satellite disintegrates, orbital changes are likely to be so rapid as to evade prediction and hence observation by the widely scattered “official” stations. The picture of what happens then will have to come from the stop watches, radio receivers and binoculars of amateurs.
All the information recorded by on-board instruments of the early satellites will have to be transmitted by radio, for we cannot expect to recover the instruments after the flight. As a practical matter, to avoid the need for a vast number of receiving stations around the globe, messages will be taken from the low-power satellite transmitters as they pass over a picket fence of receivers after circling the earth. This will require storage of the instrumental observations by some memory device in the satellite. A simple type of memory would be a circuit storing the minimum and maximum readings of a given instrument during a trip. Readings from a number of instruments can be stored in detail with a more elaborate device, such as magnetic tape, but at greater cost in weight. The readout will be triggered by radio command. The command frequency will be kept secret in order to protect the readings and the power supply from dissipation by kibitzers.
Power supply is a knotty problem. Chemical storage batteries appear to be the simplest and most reliable solution for short-life satellites. The best commercial batteries yield about 45 watt-hours per pound. For operation over periods longer than a few weeks we shall have to look to new devices such as solar batteries or radioactive cells. A system which uses several solar batteries to trickle-charge a small storage battery is now being developed by the U. S. Army Signal Corps. This system will provide indefinitely about one fourth of a watt per pound of total weight; during its exposure to the sun it will store a small surplus of energy to supply power for the half-hour or so on each trip when the satellite is on the dark side of the earth.
In the successive passages from the sunlit to the shady side of the earth the outer skin of the satellite will go through marked variations in temperature—from about 100 degrees Fahrenheit to about 70 degrees below zero. It will be necessary to protect the instruments inside from these extremes. By sagacious insulation it should be possible to hold the cycle within the reasonable limits of 40 and 70 degrees.
What instruments shall we put in the satellite observatory? There are a number of good possibilities for the few flights we shall have available. With a simple photocell installed in the satellite we could, for example, make a detailed survey of the cloud cover over large areas of the earth. As the spinning satellite circled the globe, the photocell would alternately look out into space and down at the earth, making a detailed survey of reflected light from points below it. The reflected radiation would be a reliable index of the cloud cover. A small microphone could record the number and momentum of the micrometeorites that beat on the metal skin of the satellite. To measure the density in space of microscopic dust particles, we might paint on the surface a simple stripe of radioactive material, whose erosion would record the rain of particles. Among the interesting questions these observations might settle would be whether micrometeorites play any part in generating the air glow in our upper atmosphere and in creating the noctilucent clouds.
High on the list of things to be clone is a survey of the outer reaches of the ionosphere, the electrified region which is so important to all long-range radio communication on the earth. A satellite should also give us information about the density of electrons in space in the near vicinity of our planet. Another important topic for investigation is the earth’s magnetic field. This might be surveyed with a sensitive, miniaturized magnetometer especially designed for installation in a satellite.
But at the very top of the list of subjects that scientists want to study are the sun’s short-wave radiations and cosmic rays. During 1957-58 there will be a sun-spot maximum bringing heavy fluctuations in both types of radiation. This will provide ideal opportunities for observation of their interactions with the earth’s upper atmosphere. Measurement of ultraviolet radiation and soft X-rays from the sun would illuminate their role in the formation and behavior of the electrically charged layers of the ionosphere. An ionization chamber and photon counters in a satellite could record the varying intensity of this radiation and help determine its relation to flares on the sun. A Geiger counter hooked up to a magnetic tape memory could make corresponding measurements for cosmic rays. During quiet periods the same instrumentation could survey the rays’ geographical distribution above our atmosphere. Such a survey would test the traditional theory that the earth’s magnetic field controls the arrival of cosmic rays against the new notion that their trajectories are shaped by magnetic fields elsewhere in the interplanetary region. An apparatus for cosmic-ray observations in satellites is being developed by George Ludwig and the writer at the University of Iowa [see lower diagram on the opposite page].
It is clear that there is more work to be done than the first satellites can handle. It is equally clear that the Geophysical Year will be only the beginning of this adventure. After the first satellites have proved their usefulness, we can confidently predict that others will be abundantly available to science in the years to follow.