September 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
"Mariner" was the project
name given to NASA's first fleet of interplanetary spacecraft, headed for both Venus (1, 2,
5, and 10) and Mars (3, 4, and 6 - 9). When Mariner 4 launched for Mars in 1964, it marked
the first time mankind had successfully sent a probe to "the red planet." It radioed back
Mars surface images at a resolution of 2 miles across. Fifteen missions later, we now have
vehicles roving the Martian landscape drilling holes for soil samples, crunching rocks, sniffing
the air and determining chemical compositions of the aforementioned, measuring temperature,
wind speed, atmospheric pressure, seismic events, and perhaps most importantly testing for
signs of life. Mariner 4's radio subsystem transmitted data back to Earth at 2300 MHz.
Depending on where Earth and Mars are in their orbits, it can take anywhere between 4 and
21 minutes for signals to span the ether between them. That means if a command signal is sent
from Earth to a Mars craft and a response is immediately signal sent back to Earth, the round-trip
time can be between 4 and 42 seconds - a delay even worse than with two cellphones talking
to each other on Earth. For comparison, it takes about 1-¼ seconds (1-½ round-trip)
to the moon.
Mariner Spacecraft: Explorers of Mars
By Fred W. Holder
Bendix Field Engineering Corp.
Mariners VI and VII, launched early this year, are adding to our knowledge
of the distant red planet to which we may some day travel. Here is what the scientific packages
were designed to accomplish.
This 210-ft antenna at Goldstone, Calif. is used to communicate with the Mariner spacecraft
and other deep-space probes.
On November 28, 1964, an Atlas/Agena rocket roared off the pad at Cape Kennedy, Florida.
The 575-pound Mariner IV spacecraft was on its way. Its mission: To gather scientific information
on the red-orange planet, Mars. Eight months and 325,000,000 miles later Mariner IV was within
6118 miles of the Martian surface. On July 15, 1965, Mariner slewed its TV camera into position
and snapped 21 pictures of Mars. A breakthrough had been achieved. The Mariner photographs
showed surface features as small as two miles across. Even the most powerful telescopes and
the most favorable atmospheric conditions on Earth have never yielded such high-resolution
pictures of the Martian surface; their best resolution shows surface features 100 miles across.
Early this year, two more Mariner spacecraft (Fig. 1), propelled by the powerful Atlas/Centaur
rocket, were hurled into space. Their mission: To fly within 2000 miles of the Martian surface
and to further the exploration begun by Mariner IV. These spacecraft, designated Mariners
VI and VII, were launched in February and March, one month apart. Mariner VI will journey
226,000,000 miles to take close-up TV pictures of the equatorial region of Mars on July 30,
1969. Mariner VII, on the other hand, will travel a somewhat shorter distance and just five
days later, on August 4, 1969, will photograph the polar region of Mars (Fig. 2).
Such lengthy journeys through the bitter cold vacuum of space pose several problems which
the designers must solve if the spacecraft's passenger, the scientific data package, is to
reach its destination. In this article we will investigate some of these problems, see what
the scientific package may accomplish, and what NASA's Jet Propulsion Laboratory (JPL) in
Pasadena, California has planned for the future.
The Mariner IV spacecraft needed almost 200 watts of electrical power to operate its functional
components over the 1,534,000,000 mile journey, which officially ended on December 20, 1967.
This power was furnished by 70 square feet of solar-panel area containing 28,224 solar cells.
When Mariner was in the vicinity of the Earth, these panels generated about 700 watts of electrical
power. As Mariner approached Mars, the output from the solar panels was reduced to 300 watts,
leaving a good margin of power in case of solar-cell damage.
The electrical systems of Mariners VI and VII require a maximum of about 388 watts at the
time of Mars encounter. The power is supplied by 83 square feet of solar-panel area which
contains only 17,472 photovoltaic solar cells. At Earth distances from the Sun, these solar
panels produce about 800 watts of power, but as the spacecraft approaches Mars, the power
capability decreases to 449 watts, leaving an adequate margin in case of degradation caused
by solar flares.
Picture of Martian surface sent to Earth by Mariner IV (1965).
Each spacecraft carries a rechargeable, silver-zinc battery that is a sealed unit containing
18 silver-zinc cells. This battery has a minimum capacity of 1200 watt-hours at launch. This
capability is reduced to about 900 watt-hours at Mars encounter. Power from the solar cells
is directed to the battery to keep it in a state of full charge so that it may be used during
Mars encounter as an emergency backup power source.
Each spacecraft carries two power regulators to provide a redundancy factor. If one regulator
should fail, it is automatically removed from the line and the second regulator is switched
in to assume the full load of the spacecraft. Also, should an out-of-tolerance voltage condition
exist in the main regulator, the standby regulator takes its place in the line.
The primary power distributed to most of the spacecraft systems is a 2400-Hz square wave.
However, the gyro spin motors use a 400-Hz three-phase current. The infrared spectrometer
and the scanning motor are supplied with a 400-Hz single-phase current. The transmitter amplifier
tube, battery charger, and temperature-control heaters use the unregulated d.c. power from
the solar panels or from the battery.
An object in space will be warmed on the side facing the Sun's rays and cooled by its own
radiation into the black sky around it on the side away from the Sun. If the object is not
rotating, the sunny side may be several hundred degrees hotter than the shady side. And an
object at Earth distances from the Sun may be 125° F warmer than it would be at Mars distance.
The Mariner spacecraft and its electronic components couldn't tolerate these adverse conditions.
Therefore, it was necessary for the engineers to devise methods of maintaining certain temperature
limits within which the electronic components could operate properly.
Heating by direct sunlight on the Mariner spacecraft is minimized by the use of a thermal
shield of aluminized Teflon on the sunny side. The side away from the Sun is also covered
with a thermal shield to prevent rapid loss of heat to the cold of dark space.
Fig. 1 - Spacecraft used for Mariner VI and VII missions.
Fig. 2 - Trajectories of Mariner spacecraft past Mars.
The temperature is controlled in the spacecraft's six electronic compartments by the opening
and closing of polished metal louvers that are actuated by coiled bimetal strips. These strips
act as spiral-bound springs that expand and contract as they heat and cool. This mechanical
action is calibrated to provide an operating range from fully closed at 55° F to fully
opened at 90° F. Each pair of louvers operates independently on its own local temperature,
as determined by the internal power dissipation within the compartment.
Other elements of the spacecraft are temperature controlled by means of paint patterns
and polished metal surfaces. For example, the high-gain antenna dish, which is dependent upon
the Sun for its surface heat, is painted green to keep it at or near room temperature during
planet encounter. At Earth distances from the Sun, the antenna dish is at its upper thermal
Broadcasting continuously at 2300 MHz with 10 watts of power, the radio subsystem in Mariner
IV provided a communications link over which scientific and equipment performance data was
returned to Earth at a rate of 8 1/3 bits per second for most of the flight. As Mariner approached
Mars, it took 12 minutes for these radio signals to travel the 134,000,000 miles back to Earth.
The signal reaching the big, 8.5-foot antennas on Earth was 10-19 watt, a bit faint
for conventional receivers. For example, the average home TV set receives a signal of about
All communications between the Mariner spacecraft and the Earth is in digital form. Command
signals transmitted to the spacecraft are decoded, translated from binary form into electrical
impulses, and routed to the proper destination within the spacecraft. Data gathered in the
spacecraft is converted to digital form for transmission back to Earth. In Mariner IV, two
transmission rates were available: 33 1/3 and 8 1/3 bits per second. Early in the Mariner
IV flight, the spacecraft was switched to the 8 1/3 bits-per-second rate. Mariners VI and
VII have five different data rates for transmission: the engineering channel has 8 1/3 and
33 1/3 bits per second at any time, the science channel has 66 1/3 bits per second during
encounter and 270 bits per second during data storage playback, and a high-rate science channel
at 16,200 bits per second. This latter science channel can be used only when the 210-foot
antenna at Goldstone, California is available to receive data.
Unless Mariners VI and VII are receiving uplink signals, they transmit a frequency of 2195
MHz, which originates within the spacecraft transmitter. The transmitter consists of two redundant
exciters and two redundant radio-frequency power amplifiers in any combination. However, only
one exciter-amplifier combination will operate at any one time. Both amplifiers on each spacecraft
employ traveling-wave tubes. They are capable of operating at 10 watts or 20 watts output
and can transmit through either the high-gain or low-gain antenna.
The S-band receiver used in Mariners VI and VII operates continuously during the mission
at a frequency of about 2115 MHz. The receivers in the two Mariners operate at slightly different
frequencies. These receivers are used with the low-gain omnidirectional antenna only and receive
the uplink command and ranging signals from the ground stations of the Deep Space Network.
Fig. 3 - Details on the infrared spectrometer that is used to identify
gases located in the lower atmosphere of Mars.
A brief review of the TV subsystem in the Mariner IV spacecraft provides a basis for comparing
the systems used in Mariners VI and VII. In Mariner IV, a 12-inch focal length telescope with
a 1-degree field of view brought the image to a vidicon tube having a 0.22-square-inch faceplate.
The image was scanned in 200 lines of 200 dots or picture elements each. The TV camera converted
the scanning image to a digital signal of 240,000 bits per picture. The digital picture data
was recorded on a two-track, 1/4-inch magnetic tape loop 300 feet long which was capable of
recording a little more than 21 pictures. Each of the 40,000 elements in a picture was converted
to a six-bit binary number (64 possible numbers) representing picture shading from pure white
(binary 000000) to jet black (binary 111111). It took more than eight hours to transmit each
picture back to Earth at a rate of 8 1/3 bits per second. The bits were transmitted as pulses
which were present (1) or absent (0).
The binary numbers representing picture elements were fed into the computers at NASA's
Space Flight Operations Facility at Jet Propulsion Laboratory. The computers processed the
data so it could be fed into a digital photographic processor. In one method used, this processor
converted each number to an appropriately shaded dot. The dots were then projected in sequence
(200 dots per line for a total of 200 lines per picture) onto a cathode-ray tube. Each completed
picture was photographed by a 35-mm camera.
Mariners VI and VII each carry two cameras. Camera A is similar to the camera on Mariner
IV, except that it has been equipped with a wide-angle lens and covers an area 12 to 15 times
larger than the Mariner IV camera. It has approximately the same resolution - two miles. The
resolution of camera A is one-tenth that of camera B, and its photographs cover an area 100
times larger on the surface of Mars. The best resolution for camera B is expected to be about
900 feet, compared with two miles for the Mariner IV camera.
The vidicon tubes in the TV cameras of Mariners VI and VII operate in a manner similar
to that used in Mariner IV, except that the electronic beam scans 665,280 points on the picture
image. As in Mariner IV, each point is converted to a shading number and recorded on a digital
tape recorder. Each picture will be represented by 3.9 million bits of information. Transmission
of the picture data back to Earth is in binary form. Once the information is received at the
Space Flight Operations Facility, it is converted to electrical pulses, representative of
the pattern of light and dark elements of the original image on the vidicon tube. These pulses
will intensify a beam of light as it is swept across a 70-mm negative to expose it at 665,280
points to recreate the original image.
The standard approach plan programmed in the on-board computer in each spacecraft provides
for 50 approach pictures. Whether or not this plan can be implemented is dependent upon the
availability of the 210-foot antenna at Goldstone, which will allow transmission of these
pictures to Earth at 16,200 bits per second before the flyby. If the "big dish" is not available,
an alternate approach will be used in which eight approach pictures will be taken for transmission
to Earth after the flyby.
As the spacecraft fly by Mars, they are each scheduled to take a series of 24 close-up
pictures of the surface at a closing range of from approximately 6000 to 2000 miles from the
surface. Red, green, and blue filters will be used on camera A to delineate color differences,
while a yellow filter will be used on camera B to reduce haze.
The approach pictures are expected to give scientists the first detailed pictures of features
previously studied from Earth. These photographs may serve to locate haze, clouds, or dust
storms and allow studies of changes during the time each series is made and during the five-day
interval between the two spacecraft. The television pictures may allow scientists to detect
any moist areas on the Martian surface. The innermost of Mars two moons, Phobos, might appear
in one of the approach series.
Other Scientific Experiments
Fig. 4 - Details on the ultraviolet spectrometer that is used to identify
the gases located in the upper atmosphere of Mars.
Fig. 5 - Infrared radiometer for surface-temperature measurements.
The scientific payload of Mariner IV was made up of six devices besides the television
subsystems: (1) a helium magnetometer to measure planetary and interplanetary magnetic fields;
(2) a solar plasma probe to measure the quality rate and energy of positive ions "solar wind";
(3) an ionization chamber and Geiger-Mueller tube to measure the ionization caused by charged
particles; (4) a trapped-radiation detector to measure the Earth's Van Allan belt and to check
for similar formations around Mars; (5) a cosmic-ray telescope to detect protons and alpha
particles; and (6) a cosmic-dust detector that is employed in order to measure momentum, direction,
and the number of hits from cosmic dust.
In addition to the TV subsystem, Mariners VI and VII carry an infrared spectrometer, an
ultraviolet spectrometer, and an infrared radiometer to be used to explore the surface and
atmosphere of Mars. These devices are designed to yield data on the physical, chemical, and
thermal properties of the planet. They will probe the surface and atmosphere of Mars in the
visible and near-visible portions of the electromagnetic spectrum, from the infrared region
through the visible portion of the ultraviolet region. The S-band occultation experiment,
which requires no special equipment other than the spacecraft's radio, will provide new values
for atmospheric pressure and density, and electron density in the Martian atmosphere.
The infrared spectrometer (Fig. 3) will detect infrared radiation in the 1.9 to 14.3-micron
region. It will allow detection of water, carbon dioxide, methane, ethylene, and acetylene,
if present. The presence of organic molecules would provide evidence of the existence of either
past or present life on Mars. This detection, however, would not be conclusive.
The ultraviolet spectrometer (Fig. 4) identifies different species (molecules, atoms, and
ions) by the wavelengths of light they absorb or emit. Each species absorbs the energy of
light (which is composed of a number of different wavelengths) at one or more wavelengths
and re-radiates the absorbed light at the same or longer wavelengths. An atom, for example,
re-radiates the wavelength it absorbs. The spectrometer can detect certain wavelengths and
thus identify the species. This device uses two photomultiplier tubes as detectors. Each of
these tubes is sensitive to different regions of the spectrum. One tube responds to the 1100
to 2150 angstrom region, the other from 1500 to 4350 angstroms.
The infrared radiometer (Fig. 5) is boresighted with the television cameras to allow correlation
of surface temperatures with terrain features and clouds. This provides a map of the surface,
relating temperature variations to surface features. The unit uses two antimony-bismuth, five-junction
thermopile detectors to provide 30 readings every 63 seconds. Twenty-seven of these readings
will be of planetary temperatures, two of them will be calibration readings, and the remaining
reading will be an engineering measurement on the instrument itself. One of the detectors
covers the 8 to 12-micron range; the other covers the 18 to 25-micron range. Filters are used
to determine the wavelengths actually reaching the detectors. If frozen water exists on Mars,
there is a possibility of detecting localized moist areas on the surface. This would require
a higher surface temperature in the area, which would be detectable. The experiment may also
determine if the bright rim seen on the craters in the Mariner IV photographs of Mars are
remnants of carbon dioxide or water ices.
Two more missions to further explore Mars are in the planning stage. The first of these
missions is scheduled for 1971 when two Mariner spacecraft will orbit the planet for three
months. The first spacecraft will orbit at an inclination of 60 degrees to the planet's equator.
This orbit would permit it to examine about 70 percent of the Martian surface. The second
spacecraft would then be placed in the near-polar orbit, inclined 80 degrees to the planet's
equator. This orbit will permit examination of the Mars polar caps and provide high-resolution
coverage of selected areas. It will also permit oblique views of broad areas of the planet's
surface and possibly an examination of the planet's two moons, Phobos and Deimos.
In 1973, NASA plans to send two spacecraft to Mars. Designated Project Viking, the two
spacecraft will be launched in mid-1973, about ten days apart. Each spacecraft will consist
of a Surveyor-type soft-lander mated with a Mariner 1971 class Mars orbiter. The Mars orbiter
will provide power and communications support to the landers during the cruise period.
Upon arrival at Mars, the orbiter propulsion system will be used to place the orbiters
and landers into a Mars orbit. After reconnaissance of potential landing sites by the orbiters,
the landers will be detached and will soft-land, using the techniques developed for Surveyor
and the Apollo Lunar Module. The orbiters will then provide broad-area surveillance in support
of the landers, in the same way that the Lunar Orbiter and Surveyor spacecraft worked as a
team in exploring the Moon.
The exploration of Mars began 3 1/2 centuries ago when Galileo distinguished the disc of
Mars with the first astronomical telescope. Such exploration has continued through the centuries
with ever-increasing sophistication, with larger and more powerful telescopes and more sophisticated
photographic equipment, to the missions of Mariners IV, VI, and VII. We can only hope that
the data provided by the Mariners of 1971 and 1973, as well as those in the present series,
will give us an even more comprehensive picture of the red planet.
Posted September 11, 2017