SP-480 Far Travelers: The Exploring Machines


Creating an Exploring Machine


[18] A great deal of engineering is based on previous work. Improvements resulting in lower costs and greater durability are made, weaknesses are corrected, and limitations are reduced. But what if prior experience is absolutely zero? How do we design, build, and test something that has never been built before? That was the daunting challenge that faced the designers of the first Mariners.

To begin with a blank sheet of paper and attempt to create an interplanetary robot is to confront unexpected problems. Simple solutions may be found for problems that at first appear insoluble, but sometimes deceptively simple obstacles are almost insurmountable. Nature has provided countless living creatures with effective solutions to problems of stabilization and mobility, but endowing a robot with these attributes is not easy. Of course, we do not yet know fully how nature's creatures perform many of their functions. Who can say how a migratory bird navigates for thousands of miles or how a hawk stabilizes its head while turning its body? Perhaps the technical approaches engineers use will seem less complex when nature's methods are finally unscrambled. Considering the fact that it was designed to travel 180 million miles to another planet and make observations, the first Mariner was an extremely simple exploring machine, rudimentary and primitive compared with a human being and even with the spacecraft we would launch only a decade later.

Simply put, Mariner was a machine used by man to extend his powers of observation beyond the immediate vicinity and out into space. Lacking the experience and resources to launch an astronaut deep into the solar system, we sent exploring machines as our proxies. Like the specialized robots in a nuclear facility, interplanetary probes were developed to do a job that human beings were unable to do. In the case of Mariner, the assignment was to perform preliminary exploration of a neighboring planet and to learn as much as possible during the journey.

[19] It may not be surprising, therefore, that exploring machines came to resemble living creatures in a number of ways. Designed de novo using engineering principles and technology, Mariner had remarkably human attributes in performance and in its ability to cope with the environment. While dependent on a rocket-propelled launch vehicle for basic transportation, Mariner had to be capable of acting for itself on the way to its destination. Incoming solar energy had to be assimilated to sustain it. Attitude orientation was required to obtain power, to maintain communications, for pointing sensors, and for thermal control. A method of knowing where it was and where it was headed was required; thrusters were needed to serve as "muscles" for attitude and course corrections. It had to have some memory and a time sense, plus an ability to interpret and act on commands and to communicate its state of health and its findings.

A spacecraft, of course, must be held together by a rigid structure, just as a human body is defined by a skeleton. Refined in design yet simple in appearance, the basic structure of Mariner was hexagonal; it was made of magnesium with an aluminum superstructure. Weight is always a primary constraint in vehicles destined to be launched into space; Mariner's frame was as light as possible, since inert parts competed for the same precious weight that might be allocated to sensors, data processors, and the like. The craft was six sided because of its Ranger ancestry; the Ranger was six sided in part to allow efficient structural attachments to the Vega upper stage and in part to allow convenient mounting of solar panels, electronics boxes, and the midcourse propulsion system. On the superstructure were placed antennas, scientific instruments, and other components needing a location with a vantage.

The electronics compartments and the subsystem compartments around the base were modularized so that they could be separated more or less by function, allowing the development of power, guidance and control, instrument signal conditioning, and communications systems in individual laboratories before these bays were brought together and integrated to become a spacecraft. The six boxes were rectangular so that electronic components could be easily packaged in them, yet they all interconnected around the structure, becoming what is called the spacecraft bus, sharing common power systems, thermal control systems, and other basics essential to component integration.

The structure attached to the top of the bus served many functions, but perhaps the most important was that it carried a low-gain antenna at the up [20] per end. The low-gain, or omnidirectional, antenna provided the primary source of command and backup transmission capability, so that signals could be received or engineering data transmitted regardless of the attitude of the spacecraft. The antenna had little or no amplification of signal in any direction, but produced a radiation pattern similar in all directions. Actually this antenna could not be perfectly omnidirectional; some shadowing by the spacecraft bus in some directions was expected. But, located away from the spacecraft, it did have a generally good "view." In addition, since omnidirectional antennas do not have to be very large or very heavy, they fit nicely in the point of the aerodynamic shroud without unduly affecting the center of gravity.

Other components mounted on the superstructure because of the view advantage included scan platforms that could "see" a planet as the spacecraft went by, or, in the case of the Ranger craft to the Moon, cameras that looked out and down as the spacecraft approached the surface. Components that fitted into the superstructure were like the bus compartments, essentially modularized so that they could be assembled and tested in the laboratory before integration with the spacecraft.

Like its human counterpart, a spacecraft needs a regular supply of energy. The stored power of batteries is one alternative, but for missions lasting weeks or months, some means of replenishing battery power is necessary. Mariner's prime energy source was the Sun, which supplied about 150 watts of electricity through solar cells that could charge an internal battery having a storage capacity of 1000 watt-hours. As long as the solar cells were facing the Sun, Mariner had power to lead its own life in its own way. Even if the panels were shaded, automatic switching systems allowed the spacecraft to operate on battery reserves for a time. Lest Mariner grow too independent, however, there were also circuits that could be commanded from Earth at the discretion of the spacecraft's terrestrial masters.

Early solar cells were fairly inefficient at converting solar energy to electrical energy; only 7 to 10 percent of every unit of energy the Sun beamed onto the cells was converted to useful electrical energy. Nevertheless, the Sun offers a clean, dependable source of energy in space. An attractive feature of solar cells is that they are passive devices with no moving parts that wear out. They do have shortcomings, however. In addition to requiring orientation so that they receive full and direct sunlight, they are temperature and radiation sensitive. Solar panels tend to overheat, so a good deal of engineering work is required to develop the proper thermal environment. [21] Fortunately, the backs of panels facing the black sky can be used to allow heat to escape; by judicious engineering, panel temperatures similar to normal room temperatures on Earth can be maintained.

Other worries by designers of solar panels included harmful effects of radiation and micrometeorite impacts. After some bad experiences with early satellites, solar cells were made less radiation sensitive through the use of better materials and protective covers. Micrometeorite protection was limited to wiring cells so that only localized losses of cells would result in case of hits. Some failures are thought to have been due to micrometeorite hits, but the evidence is inconclusive.

An attractive aspect of solar energy, its constancy, also became something of a challenge to spacecraft designers because the usage rate or demands of the spacecraft varied considerably: there were periods when the requirements might exceed the incoming supply and times when excess power would have to be dissipated. This called for an innate capability to adjust the dissipation of energy when the spacecraft requirements were exceeded by the supply.

Thus, power management involved circuitry connecting the solar panels to the batteries in a semiautomatic manner, for it was not logical to try to monitor and control power usage from Earth. However, in emergencies, commands from Earth to adjust power usage were needed, so both data readouts and command functions had to be integrated into automated power system designs.

Finally, the solar panels had to be folded inside the heat shield or nose cone that protected the spacecraft from aerodynamic forces and from the heating that occurred during launch through the atmosphere. This mechanical consideration, involving latching mechanisms, deployment commands, and dynamics of actuation, produced additional headaches for engineers. In a disproportionate way, the success of the sophisticated solar energy collection and conversion system was totally dependent on simple pyrotechnic and mechanical latching systems, for if the panels did not open and were not exposed to sunlight, the consequences would be disastrous.

A human without attitude control would be sadly handicapped, unable to swing a bat, throw a ball, propel himself, or even turn his eyes away from the glaring Sun. Mariner needed attitude control for precisely the same reasons. It must be remembered that inertial space is a most peculiar place, at least by terrestrial standards. There is no up and no down, no day and no night, no air and no true wind. The Sun and other stars are visible at the [22] same time, and surfaces facing the Sun grow very hot while those in shadow grow very cold. Most remarkably, objects that are moving or rotating continue to move or rotate indefinitely until they are stopped by countervailing forces. A spacecraft injected on an interplanetary trajectory in this odd environment would, lacking a stabilizing system, tumble at random, preserving the last impulse imparted to it, plus the resultants of additional impulses that might be derived from particle impacts or from reaction to onboard movements. Naturally, this kind of random movement will not do if an antenna must be pointed precisely, if scientific instruments are to scan a planetary swath, or if an onboard rocket must be aimed carefully to correct an imperfect trajectory.

A simple way to hold a spacecraft fixed in inertial space is to spin it like a top. The whole vehicle then becomes the rotor in a gyro, holding its polar axis in relation to the orbital plane it traverses. This principle has been used with great success for some Earth satellites and the Pioneer class of interplanetary craft, for which simplicity and long life are important considerations. However, the disadvantages are considerable: a spinning scientific platform, an antenna that must be aligned with the polar axis, and the need to mount solar cells in a drum-like configuration so that spinning won't materially affect power generation. The gyro principle can be applied to a reference platform for sensing attitude and maintaining control, but gyros alone would not do the job reliably over the months-long periods needed for even short interplanetary trips; the best of them would be susceptible to drift arising from the accumulation of infinitesimal errors.

An alternative to gyros is an automatic system to hold the entire spacecraft in an established attitude by sighting on distant celestial objects. A principal in the development of guidance and control systems for unmanned spacecraft was John R. Scull, who continued to be involved through all the lunar and planetary missions of the 1960s and 1970s. He and his associates worked out the application of optical sensors and gyroscopes that became standard for spacecraft guidance and control. This type of three-axis stabilization worked fairly well for Mariner and was improved for later missions. The principles are simple: sighting on distant celestial objects, ingenious sensors keep an instrumental eye on distant "spacemarks." If any substantial straying of attitude is detected, the sensors send signals to paired attitude jets. Each jet is a tiny minirocket that releases a spurt of compressed gas to nudge the spacecraft back onto an even keel. Only a modest pulse is required. Too vigorous a push would send the spacecraft bouncing back and [23] forth like a ping-pony ball on concrete, wasting the finite stock of compressed gas. An exploring machine with a well-designed stabilization system will sail placidly through space with only infrequent and gentle pulses from its gas jets.

The bright and ever-present Sun is a logical spacemark for journeys in the solar system, and, although staring fixedly at the Sun doesn't sound comfortable even for an instrument, nothing prevents a simple, reliable little sensor from gazing at the shadow of a small "umbrella," which serves just as well. A Sun sensor attitude control reference system is delightfully simple in principle, sending a "restoring" signal as a shadow moves. Such a system can be made by mounting a small square shade to partially cover two pairs of identical solar cells oriented at right angles, and connected with standard bridge circuitry so that small differences in voltage outputs from matched pairs of cells produce error signals. When two matched cells are exposed to the same amounts of sunlight and shadow, they produce the same voltage output. If the shadow moves so that one receives more sunlight and the other less, the voltage difference can be used as a restoring signal to the attitude control jets. When all four sensors produce the same output, they are oriented at right angles to the Sun, thus providing two of the three axes required for stable reference.

The concept has two minor constitutional weaknesses: (1) it is necessary to preorient the spacecraft roughly in the correct direction in order for the sensors to find the Sun and become effective, and (2) if at the end of a long life the solar cells should chance to age unequally, the spacecraft could develop a list. The initial positioning of the spacecraft is made possible by sensors that determine whether the Sun is shining on the top or bottom of the spacecraft. Careful selection and quality-control processes minimize the risk of varying solar cell lifetimes.

Earth itself seemed to be a good choice for a second spacemark and was used by the Rangers and the first Mariners. It was attractive because the directional antenna needed to be aimed at Earth and the two could be aligned together. Earth proved to be less than ideal, however, for the angle it subtended varied with distance and its apparent brightness diminished greatly as the spacecraft traveled away from home, requiring a sensor of greater sensitivity. As distances grew, the sensor had trouble discriminating between Earth and the Moon, and between Earth and other planets. Earth also moved, introducing still another variable into the calculations. After Mariner 2, for which the Earth served as a workable but somewhat [24] undependable reference, spacecraft designers switched to Canopus, a bright star in the southern hemisphere, for the second spacemark. Shining brightly in an otherwise undistinguished neighborhood, so distant it appears motionless, Canopus has been a guide star ever since for most of the interplanetary exploring machines.

This combination of sensors and systems connected to the on-off valves of the attitude control gas jets allowed stable platform orientation of the spacecraft, maintaining alignment with the Sun and Earth in inertial space so that power, thermal control, and communications needs could be satisfied. The "muscles" providing attitude control of Rangers and Mariners were cold gas nitrogen systems weighing about 4 pounds. They used a small bottle of high-pressure nitrogen and tiny jets mounted on the ends of the solar panels and the superstructure. Because of the finite quantities of gas, duty cycles had to be carefully and accurately controlled to minimize usage.

Reference to remote spacemarks must be temporarily abandoned during midcourse trajectory corrections, and it is desirable to have a temporary set of references if the spacecraft loses its lock on its distant star guides. For this purpose a three axis set of gyros is used. As already mentioned, gyros are not reliable over long intervals, being vulnerable to the accumulation of small errors caused by friction, but they are trustworthy for limited times. (A new design, the laser ring gyro using light beams is now being integrated into aircraft systems, and holds high promise of extreme accuracy for extended intervals. )

Mariner 2 was stabilized with its longitudinal axis pointed at the Sun, holding the spacecraft in both pitch and yaw directions. Roll stability was achieved with an Earth sensor mounted on the directional antenna. Pointing the long axis at the Sun provided the maximum amount of solar energy transfer to the solar panels and aided thermal control of the spacecraft by maintaining a constant Sun impingement angle, allowing the aft end of the spacecraft to point at the dark sky to radiate away excess heat. Initial Sun and Earth acquisitions were performed by internal logic circuits that derived their input from sensors and gyros.

The thermal control of the spacecraft was intended to be as passive or automatic as possible. The greatest part of the heat load came from the Sun and a lesser amount from the onboard electronics equipment, the latter also being among the most heat-sensitive components. For passive control, materials with different absorption and emission properties were used to radiatively balance the heat within the spacecraft. In addition, one of the six [25] boxes around the hexagonal structure was fitted with louvers activated by a temperature-sensitive bimetallic element. If temperatures within the box rose too high, the louvers opened to radiate the heat to black space; when temperatures were too low, the louvers closed to keep in the heat generated by electronic components.

If Mariner's design process had stopped at this point, the craft might have been likened to a beast of burden just able to carry a small load while being led by its master. However, it would not have been able to execute a mission to Venus without some ability to plan and sequence its activities-without a kind of humanoid intelligence. The subsystem that provided these traits was the central computer and sequencer, called the CC&S. Because this was the brain center of the spacecraft, it will be mentioned often. On more advanced spacecraft, units performing similar functions have different names, but understanding the concept of the CC&S will probably enable you to communicate with spacecraft engineers.

The CC&S on Mariner supplied onboard timing, sequencing, and some computational services. Its memory contained a handful of prestored commands, and it was able to respond to a dozen specific commands sent from Earth. Since communication problems might prevent detailed orders from reaching the spacecraft, some preprogrammed intelligence was provided; after receiving the proper initiation commands, the spacecraft could then act on stored information. For example, it could acquire the Sun and Earth, going through a series of actions, after being told to. Parts of the midcourse maneuver sequence were integrated into the spacecraft memory because this was efficient and precise. Onboard sensors could determine how much the velocity had changed and could cut off the rocket after a specified increment; they could do this several minutes before engineers on Earth would have received the information from one action necessary to determine the next. Even though the journey to Venus would take more than 100 days, preprogrammed instructions for actions at encounter were also stored, so that if our command capability had been lost, the CC&S might have ordered the proper spacecraft functions. At the time, we thought that the CC&S was a marvel, little knowing how distinctly limited a brain it would seem when compared with its successors.

Basic to the CC&S was a clock that provided an accurate reference base. The clock was started during the countdown to launch, and it supplied and counted timing signals, much like today's digital watches (in fact, digital watches are an outgrowth of this space technology). Being able to count [26] pulses, it would issue commands at set points of time throughout the mission. Packaged in a box about 6 by 6 by 10 inches, the CC&S included the highly important oscillator that provided the timing base, and it watched particularly over three critical interludes: launch (from liftoff to cruise), the midcourse correction, and encounter.

The CC&S could be given 12 different commands from Earth, although only 11 were used during the mission. It also had the capability of storing three commands that could be actuated at a precise time (not unlike sealed orders given to a ship's captain). The real time commands were specific instructions to be carried out on receipt of a coded signal, such as whether to use the directional or the omnidirectional antenna and whether to turn on or turn off the scientific instruments. To gain maximum use of the limited number of command channels, some would be used more than once by pairing one-time functions when it was possible for the repeated commands to unambiguously relate to different functions. The command for Sun acquisition, for example, was coupled with the command for unlatching the solar panels, because once the pyrotechnic squibs that did the unlatching had been fired, another command to that function would have no effect. All three stored-aboard commands dealt with the midcourse maneuver to improve the spacecraft trajectory. One told the spacecraft to roll a specified amount, one told it to pitch a specified amount, and one told it to achieve a specified velocity change.

The idea of accidental or purely random operation of the command system was horrifying, of course, and much thought went into protecting it from malice or mischance. Engineers had learned this lesson the hard way while working on an Earth satellite program in the early 1960s.

In this case, a sudden rash of mysterious and erratic behavior of the orbiting spacecraft was painstakingly traced to spurious radio signals from a Midwestern taxicab. Though clearly a freak accident, the possibility of sabotage or, more likely, the inadvertent transmission of an improper or mistimed command was ever present and frightening. A complicated tamper-proof system for sending commands was devised that allowed only the correct orders from the correct people to be transmitted and acted upon. Though the system necessitated an often tedious process of reading, writing, and verifying all commands, it very likely prevented potentially ruinous mistakes.

A spacecraft on a one-way trip is useless if there is no way of sending it orders from Earth or retrieving the scientific data it collects. Like the audible or visual contacts required with a roaming hunting dog, the communications [27] system is the only link between a distant robot explorer and its terrestrial masters. In addition to carrying orders and data between man and machine, the communications signals can be used for tracking the spacecraft, yielding startlingly precise computations of where the unseen voyager is and how fast it is going. Amazingly, Mariner's transmitter power of about 4.5 watts-less than that of a good walky-talky rig on Earth-was able to provide a communications link over a distance ranging to nearly 40 million miles.

Communications with spacecraft have usually made use of radio frequencies in the electromagnetic spectrum, although successful experiments have been conducted using lasers. For all the early lunar and planetary missions, radio frequencies in the L-band and S-band were used (about 1000 mhz and 2000 mhz, respectively). Several things affect radio transmissions: one is the distance relationship known as the inverse square law, meaning here that the strength of a given transmission signal is inversely proportional to the square of the distance from the transmitter. Another problem with transmissions to and from the surface of Earth is a serious attenuation of the signal by the ionosphere and its electrical fields. The ionosphere is a boon to shortwave transmissions on Earth because it reflects or bounces signals back toward the surface; these reflective properties tend to bounce short wavelength signals back into space if transmissions are attempted from space to Earth. To overcome this problem, we must transmit at higher frequencies that are able to penetrate this region.

A simple analog to this effect might be useful. Suppose I'm talking to you and someone places a blanket between us. This diminishes the level of the sounds reaching your ears. Depending on the type material used, this filtering might affect different frequencies more or less. This is the case for ionospheric effects it radio frequencies are high enough, they are not attenuated so much that radio communication is inhibited. Fortunately, frequencies of about 1000 megahertz or greater are suitable for communications to and from Earth and deep space.

Another aspect of limited bit rate communications is the need to send all information in a coded shorthand language. The limitations of early spacecraft made this coding very important. Our language has 26 letters, but, compared with shorthand, it is wasteful of bits. Scientific information can be reduced for transmission and then recreated or expanded as necessary A picture contains many bits of information and may truly be worth "a thousand words," but it is possible to compress the bits in a picture by planning Suppose, for example, that we know the spacecraft will be taking [28] a view showing the horizon of a planet with sky above it. If we are only interested in features of the planet and not the sky, we can program the system to send only the portion of the image containing the edge of the planet, simply discarding bits showing the discreetly differing sky. Such a technique presupposes some knowledge of the answers being sought, but it is nevertheless a useful concept.

The sophistication of the communication language depends on the type of data to be sent. One form of compressing data involves indexing transmissions against time to give different meanings to the same signals. There are many schemes that can be applied; what they all have in common is that a coded system for transmission must have a decoding system on the other end to complete the communications process.

From early in a cruise period, tracking a spacecraft leads to two calculated numbers of special interest. One predicts how far from the target planet the spacecraft will come at the moment of closest approach. Having an acceptable miss distance is vital. If the path is too close, scientific instruments can manage only a brief, blurred scan of a huge planetary disk. If the pass is too far away, as is much more likely, the instruments cannot capture all the data they were intended to collect. A desirable flyby range is assumed when the instruments are designed and sighted, and a major departure from it will impair mission results.

The second number that is examined during cruise predicts the time at which closest approach will occur. In effect, this defines the period during which the scientific instruments can reap the richest harvest. It also establishes which of the three Deep Space Network stations, located at three longitudes around the globe, will be in position during those hours to receive the explorer's signals. There may be reasons to change the time of closest approach: for example, if closest approach will occur when the spacecraft is disappearing over the horizon of one station and just rising at the next. The variable quality of equipment or of terrestrial communication links can make it desirable for a particular station to be the one to receive the spacecraft's reports during a critical time. Still another reason for adjusting the time is the angle of the Sun on the hemisphere of the planet being flown past. Pictures taken at local midnight are not very informative, and images under highnoon lighting are not ideal for showing surface relief. Any of the above factors, or a combination of them, can make it desirable to adjust the time of closest approach.

[29] As a practical matter, neither number (time or distance of closest approach) will be perfect. Imprecision in velocity at injection, inexact assumption about the gravitational pull exerted by the Sun and other planets, even the delicate pressure exerted by the particle streams known as the solar wind, extending over the immense length of an interplanetary trajectory, can mean that if the course remains unchanged, the spacecraft will fail to fly through the target hoop of its destination. Distance is the more important factor: there is little sense in fussing over flyby time if flyby geometry is poor. In any event, the numbers are coupled; if one is changed, the other changes. Confidence in the accuracy of the predictions grows steadily during the cruise phase. Early estimates are not altogether trustworthy, but as tracking continues and the data are integrated, it usually becomes evident that for a fully successful mission the trajectory of the spacecraft must be altered.

Assuming that the tracking accuracy provides the necessary knowledge of position and rate, a thrust vector addition can be determined. Once more a rocket becomes the means of producing a vector change in velocity. Integrating a rocket system containing combustibles under high pressure onboard a spacecraft that is carrying delicate sensors calls for careful engineering. The ideal place to put propulsion systems is at the center of gravity of the spacecraft, because as the propellant is used, the balance of the spacecraft will not be affected. Also, the thrust of the motor must be aligned such that it acts through the center of gravity, otherwise the spacecraft might spin up in space like a Chinese pinwheel on the fourth of July.

But how can this knowledge of position, velocity, and attitude be combined with the rocket thrust capability to correct the trajectory? The spacecraft is millions of miles away, moving at high speed, with only the most tenuous radio links connecting it to Earth. The solution is to execute remotely a complex pas de deux called the midcourse correction maneuver. The spacecraft is ordered to abandon temporarily the locks on two spacemarks that have held it stabilized in three axes, to turn according to gyro references until it is pointed in a calculated direction, and then to fire an onboard rocket of known thrust for a precise length of time. A timed rocket burn obviously depends on an Earth-based calibration of thrust under simulated conditions We can also use inertial accelerometers to terminate thrust after the desired change in velocity has occurred. In either case, this adds a vector change to spacecraft velocity and introduces speed and angular deviations in its trajectory The spacecraft then returns itself to cruise [30] orientation, searching for and reacquiring lock on the two spacemarks, and continues on its corrected trajectory.

It is at the very least anxiety provoking for ground controllers to decide upon a course correction. Leaving cruise orientation is in itself a chilling step, for solar power must be abandoned, leaving nothing but finite batteries and their switching circuitry. Also abandoned for the moment is the high-gain antenna pointed at Earth, leaving only a small omnidirectional antenna certain to transmit weaker signals and capable of receiving only strong ones. Also given up for the duration of the process is the laboriously achieved thermal balance that has kept sunlit surfaces from frying and shadowed areas from freezing. The job of orienting the spacecraft with high accuracy in two planes is turned over to gyros that, sophisticated though they may be, are nevertheless intricate electromechanical devices that are heir to all the natural indispositions of the species. Then, central to the entire gamble, the rocket must work as expected, starting smoothly, developing correct thrust, and cutting off cleanly without a burp. Finally, the spacecraft must be brought back to cruise and relocked on its two spacemarks, solar power and the high-gain antenna must be brought back on line, and deviating temperatures must be eased back to normal.

To its anxious masters on Earth, the spacecraft reports by telemetry the approximate execution of all these tasks. However, telemetry cannot communicate immediately how well the tasks have been done. It takes hours, even days of tracking to enable engineers to predict with confidence the new course of the far traveler. With good fortune, skill, and patience, it will be closer to where it should be, carrying its precious cargo to the vicinity of the target planet.

The scientific instruments, sometimes thought of as the payload or passengers onboard the spacecraft, actually become integral parts of the spacecraft system. They depend on the bus for more than just transportation-they need power, thermal control, and telecommunications. As soon as they are chosen for a mission, they are integrated into the spacecraft as if they are basic components.

Although Venus is Earth's closest planetary neighbor, we knew little about it when Mariner was being planned. Men had viewed it for centuries the brightest object (next to the Sun) in the heavens, even supposing it to two objects because of its presence in both morning and evening. T astronomers' telescopes it was a brilliant object without much detail. Excel for its crescent shape (owing to its position between Earth and the Sun during....



Mariner 1 spacecraft.

Mariner 1 spacecraft.


[32] ....most of its orbit), the only variations in its features were occasional changes in light and dark markings that appeared on its dense clouds (impenetrable in the small region of the electromagnetic spectrum visible to the eye), which hid the surface.

Scientific questions about the atmosphere, clouds, and temperatures of Venus were logically chosen for emphasis on the first Mariner mission The instruments devised to address the questions were microwave and infrared radiometers: one to scan the surface at two radiation wavelengths and one to scan the clouds and give us a better idea of the cloud-top temperatures. From an engineering point of view, mechanizing the scan platform that swung the radiometers back and forth as the spacecraft flew by Venus was an entirely new development.

Four other instruments were chosen to provide information about the space environment on the way to and in the vicinity of Venus. These "double duty" instruments were a magnetometer, ion chamber-charged particle flux detectors, a cosmic dust detector, and a solar plasma spectrometer. Although the radiometers were developed specifically for the Mariner mission, the other instruments were adapted from interplanetary counterparts already in use in scientific satellites.

In abbreviated form, the elements and workings of a real spacecraft, designed to be the first official envoy from the United States, Planet Earth, to our neighbor Venus, have been described. Had it been possible to send a human, Mariner might not have been created. It was a machine that had no real consciousness; Mariner did not "know" it had two high purposes: to collect information about interplanetary space and to make scientific measurements of Venus from close by. At the time of its design and development, few of us thought about the similarities of the spacecraft to ourselves or to other living creatures.

But to those who worked on Mariner 2, conscious of the precariousness of the enterprise and the unpredictable behavior of that historic spacecraft, it was not so much a rudimentary automaton as it was a beloved partner, feverish and slightly confused at times, not entirely obedient, but always endearing.