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HOW WE GET PICTURES FROM SPACE
Since the first cave dweller ventured out to gaze up at the night
sky, people have sought to know more about the mysterious images and
lights seen there. Being limited by what could be seen with the
unaided eye, that early stargazer relied on intellect and imagination
to depict the universe, etching images in stone by hand, measuring
and charting the paths of the wanderers, and becoming as familiar
with the sky as the limited technology would allow.
Although stargazers frequently took the wrong paths in attempting to
explain what they saw, many of them developed new tools to overcome
their limitations. Galileo crafted a fine telescope for observing the
heavens. His hand-drawn pictures of the satellites of Jupiter, the
"cup handles" of Saturn, and the phases of Venus, when combined with
the possible reasons for those facts, shook the very foundations of
the European society in the Middle Ages. Bigger and more powerful
telescopes, combined with even newer tools, such as spectroscopes and
cameras, have answered most of the the questions of those ancient
stargazers. But in doing so, they have unfolded even newer mysteries.
Beginning in the 1960s, our view of the heavens reached beyond the
obscuring atmosphere of Earth as unmanned spacecraft carried cameras
and other data sensors to probe the satellites and planets of the
Solar System. Images those spacecraft sent back to the Earth provided
startling clarity to details that are only fuzzy markings on the
planets' surfaces when seen from Earth-based telescopes. Only two of
the presently known planets, Neptune and Pluto, remain unexplored by
our cameras. In August 1989, Voyager 2 will snap several thousand
closeup frames of the planet Neptune and its largest satellite,
Triton. By the end of the 20th century, only Pluto will not have been
visited by one of our spacecraft.
The knowledge humans have today of outer space would astound Galileo.
Spacecraft have sent back pictures of a cratered and moon-like
surface of the planet Mercury and revealed circulation patterns in
the atmosphere of Venus. From Mars, they have sent back images of
craters, giant canyons, and volcanoes on the planet's surface.
Jupiter's atmospheric circulation has been revealed, active
volcanoes on the Jovian moon Io have been shown erupting, and
previously unknown moons and a ring circling the planet discovered.
New moons were found orbiting Saturn and the Saturnian rings were
resolved in such detail that over 1,000 concentric ring features
became apparent. At Uranus, Voyager sent back details of a planet
that is covered by a featureless, bluish-green fog. The planet is
encircled by rings darker than charcoal and shaped by shepherding
satellites, accompanied by five large satellites, and immersed in a
magnetic field.
Those discoveries, and thousands of others like them, were made
possible through the technology of telemetry, the technique of
transmitting data by means of radio signals to distant locations.
Thus, the spacecraft not only carries data sensors but must also
carry a telemetry system to convert the data from the various sensors
into radio pulses. These pulses are received by a huge dish antenna
here on Earth. The signals are relayed to data centers where
scientists and engineers can convert the radio pulses back into the
data the sensors originally measured.
A camera system on board the spacecraft measures reflected light from
a planet or satellite as it enters the spacecraft's optical system. A
computer converts the measurements into numerical data, which are
transmitted to a receiver on Earth by radio waves. On Earth,
computers reassemble the numbers into a picture.
Because the measurements are taken point by point, the images from
space are not considered "true" photographs, or what photographers
call a "continuous tone," but rather a facsimile image composed of a
pattern of dots assigned various shades from white to black. The
facsimile image is much like the halftones newspapers use to recreate
photographs. ?If you examine a newspaper photograph with a magnifying
glass, you will see that is is composed of many small, variously
shaded dots.?
Even more closely related to the way images are received from space
is the way a television set works. For a picture to appear on a
television set, a modulated beam of light rapidly illuminates long
rows of tiny dots. filling in one line then the next until a picture
forms. These dots are called picture elements, or pixels for short,
and the screen surface where they are located is called a raster.
Raster scanning refers to the way the beam of light hits the
individual pixels at various intensities to recreate the original
picture. Of course, scanning happens very fast, so it is hardly
perceptible to the human eye. Images from space are drawn in much the
same manner on a television-like screen (a cathode-ray tube).
Although cameras on a spacecraft probing the Solar System have much
in common with those in television studios, they also have their
share of differences. For one, the space-bound cameras take much
longer to form and transmit an image. While this may seem like a
disadvantage, it is not. The images produced by the slow-scanning
cameras are of a much higher quality and contain more than twice the
amount of information present in a television picture.
The most enduring image gatherer in space has been the Voyager 2
spacecraft. Voyager carries a dual television camera system, which
can be commanded to view an object with either a wide-angle or
telephoto lens. The system is mounted on a science platform that can
be tilted in any direction for precise aiming. Reflected light from
the object enters the lenses and falls on the surface of a
selenium-sulfur vidicon television tube, 11 millimeters square. A
shutter in the camera controls the amount of light reaching the tube
and can vary exposure times from 0.005 second for very bright objects
to 15 seconds or longer when searching for faint objects such as
unknown moons.
The vidicon tube temporarily holds the image on its surface until it
can be scanned for brightness levels. The surface of the tube is
divided into 800 parallel lines, each containing 800 pixels, giving a
total of 640,000. As each pixel is scanned for brightness, it is
assigned a number from 0 to 255.
The range (0 to 255) was chosen because it coincides with the most
common counting unit in computer systems, a unit called a byte. In
computers, information is stored in bits and bytes. The bit is the
most fundamental counting or storage unit, while a byte is the most
useful one. A bit contains one of two possible values, and can best
be thought of as a tiny on-off switch on an electrical circuit. A
byte, on the other hand, contains the total value represented by 8
bits. The value can be interpreted in many ways, such as a numerical
value, an alphabet character or symbol, or a pixel shaded between
black and white. In a byte, the position of each bit represents a
counting power of 2. (By convention, bit patterns are read from right
to left.) Thus, the first bit (the righmost bit) of the eight bit
sequence represents 2 to the zero power, the second bit refers to 2
to the 1 power, and so on. For each bit in a byte that has a one in
it, you add the value of that power of two (the sequence value) until
all eight bits are counted. For example, if the byte has the bit
value of 00101101, then it represents the number 45. The binary table
at the end of this document shows how translation of bits and bytes
to numbers is done.
If all the bits in an eight-bit sequence are ones, then it will
correspond to the value 255. That is the maximum value that a byte
can count to. Thus, if a byte is used to represent shades of gray in
an image, then by convention the lowest value, zero, corresponds to
pure black, while the highest value 255, corresponds to pure white.
All other values are intermediate shades of gray.
When the values for all the pixels have been assigned, they are
either sent directly to a receiver on Earth or stored on magnetic
tape to be sent later. Data are typically stored on tape on board the
spacecraft when the signals are going to be temporarily blocked, such
as when Voyager passes behind a planet or a satellite. For each
image, and its total of 640,000 pixels, 5,120,000 bits of data must
be transmitted (640,000 x 8). When Voyager flew close to Jupiter,
data were transmitted back to Earth at a rate of more than 100,000
bits per second. This meant that once data began reaching the
antennas on Earth's surface, information for complete images was
received in about 1 minute for each transmission.
As the distance of the spacecraft from Earth increases, the quality
of the radioed data stream decreases and the rate of transmission of
data has to be slowed correspondingly. Thus, at the distance of
Uranus, the data has to be transmitted some six to eight times slower
than could be done at Jupiter. That means that only one picture can
be transmitted in the time six pictures were taken at Jupiter.
However, for the Uranus encounter, scientists and engineers devised a
scheme to get around that limitation. The scheme was called data
compression.
To do that, they reprogrammed the spacecraft en route. Instead of
having Voyager transmit the full 8 bits for each pixel, its computers
were instructed to send back only the differences between brightness
levels of successive pixels. That reduced the data bits needed for an
image by about 60 percent. Slowing the transmission rate meant that
noise did not interfere with the image reception, and by compressing
the data, a full array of striking images was received. The computers
at NASA's Jet Propulsion Laboratory (JPL) restored the correct
brightness to each pixel, producing both black-and-white and
full-color images.
The radio signals that a spacecraft such as Voyager sends to Earth
are received by a system of large dish antennas called the Deep Space
Network (DSN). The DSN is designed to provide command, control,
tracking and data acquisition for deep space missions. Configured
around the globe at locations approximately 120 degrees apart, DSN
provides 24-hour line-of-sight coverage.
Stations are located at Goldstone, California, and near Madrid,
Spain, and Canberra, Australia. The DSN, managed by NASA's Jet
Propulsion Laboratory in Pasadena, California, consists of three
64-meter (210-ft) diameter dish-shaped antennas, six 34-meter
(111-ft) diameter antennas, and three 26-meter (85-ft) antennas. As
antennas at one station lose contact, due to Earth's rotation,
antennas at the next station rotate into view and take over the job
of receiving spacecraft data. While one station is tracking a deep
space mission, such as Voyager, the other two are busy tracking
spacecraft elsewhere in the sky.
During Voyager's contact with Saturn, the DSN recovered more than 99
percent of th 17,000 images transmitted. That accomplishment required
the use of a technique known as "antenna arraying." Arraying for the
Saturn encounter was accomplished by electronically adding signals
received by two antennas at each site. Because of the great distance
Uranus is from the Earth, the signal received from Voyager 2 was only
one-fourth as strong as the signal received from Saturn. A new
arraying technique, which combined signals from four antennas, was
used during the Uranus encounter to allow up to 21,600 bits of data
to be received each second.
Arraying's biggest payoff came in Australia, whose government
provided its Parkes Radio Astronomy Observatory 64-meter antenna to
be linked with the DSN's three-antenna complex near Canberra. The
most critical events of the encounter, including Voyager's closet
approaches to Uranus and its satellites, were designed to occur when
the spacecraft would be transmitting to the complex in Australia. The
data were successfully relayed to JPL through that array.
The DSN was able to track Voyager's position at Saturn with an
accuracy of nearly 150 kilometers (about 90 miles) during its closest
approach. This accuracy was achieved by using the network's
radiometric system, the spacecraft's cameras, and a technique called
Very Long Baseline Interferometry, or VLBI. VLBI determines the
direction of the spacecraft by precisely measuring the slight
difference between the time of arrival of the signal at two or more
ground antennas. The same technique was used at Uranus to aim the
spacecraft so accurately that the deflection of its trajectory caused
by the planet's gravity would sent it on to Neptune.
When the DSN antennas receive the information from the spacecraft,
computers at the Jet Propulsion Laboratory store it for future use
and reassemble it into images. To recreate a picture from data that
has been sent across the vacuum of space, computers read the data bit
by bit, calculating the values for each pixel and converting the
value into a small square of light. The squares are displayed on a
television screen on the spacecraft. The resulting image is a
black-and-white facsimile of the object being measured.
Color images can be made by taking three black-and-white frames in
succession and blending ("registering") them on one another in the
three color-planes of a television screen. In order for that to work,
however, each of the three frames has to be taken by the camera on
board the spacecraft through different filters. On Voyager, one frame
is taken through a blue filter, one through a green, and one through
an orange.
Filters have varying effects on the amount of light being measured.
For example, light passes through a blue filter will favor the blue
values in the image making them appear brighter or transparent,
whereas red or orange values will appear much darker than normal. On
Earth the three images are given the appropriate colors of the
filters through which they were measured and then blended together to
give a color image.
An important feat the interplanetary spacecraft must accomplish is
focusing on its target while traveling at extremely high speeds.
Voyager sped past Uranus at more then 40,000 miles an hour. To get an
unblurred image, the cameras on board had to steadily track their
target while the camera shutters were open. The technique to do this,
called image-motion compensation, involves rotating the entire
spacecraft under the control of the stabilizing gyroscopes. The
strategy was used successfully both at Saturn's satellite Rhea and at
Uranus. Both times, cameras tracked their targets without
interruption.
Once the image is reconstructed by computers on Earth, it sometimes
happens that objects appear nondescript or that subtle shades in
planetary details such as cloudtops cannot be discerned by visual
examination alone. This can be overcome, however, by adding a final
"contrast enhancement" to the production. The process of contrast
enhancement is like adjusting the contrast and brightness controls on
a television set. Because the shades of the image are broken down
into picture elements, the computer can increase of decrease
brightness values of individual pixels, thereby exaggerating their
difference and sharpening even the tiniest details.
For example, suppose a portion of an image returned from space
reveals an area of subtle gray tones. Data from the computer
indicates the range in brightness values is between 98 and 120, and
all are fairly evenly distributed. To the unaided eye, the portion
appears as a blurred gray patch because the shades are too nearly
similar to be discerned. To eliminate this visual handicap, the
brightness values can be assigned new numbers. The shades can be
spread farther apart, say five shades apart rather than the one
currently being looked at. Because the data are already stored on
computers, it is a fairly easy task to isolate the twenty-three
values and assign them new ones: 98 could be assigned 20, 99 assigned
25, and so on. The resulting image is "enhanced" to the unaided eye,
while the information is the same accurate data transmitted from the
vicinity of the object in space.
The past 25 years of space travel and exploration have generated an
unprecedented quantity of data from planetary systems. Images taken
in space and telemetered back to Earth have greatly aided scientists
in formulating better and more accurate theories about the nature and
origin of our Solar System. Data gathered at close range, and from
above the distorting effects of Earth's atmosphere, produce images
far more detailed than pictures taken by even the largest Earth-bound
telescopes.
In our search to understand the world as well as the universe in
which we live, we have in one generation reached farther than in any
other generation before us. We have overcome the limitations of
looking from the surface of our planet and have traveled to others.
Whatever yearning drew those first stargazers from the security of
their caves to look up at the night sky and wonder still draws men
and women to the stars.
_____________________________________________________________________
BINARY TABLE
Bit of Data 8 7 6 5 4 3 2 1
----------------------------------------------------------------------
Sequence Value 128 64 32 16 8 4 2 1
Binary Value 0 0 1 0 1 1 0 1
Byte Value 0 +0 +32 +0 +8 +4 +0 +1 = 45
Sequence Value 128 64 32 16 8 4 2 1
----------------------------------------------------------------------
Brightness Values Binary Values
----------------------------------------------------------------------
0 (black) 0 0 0 0 0 0 0 0
9 (dark gray) 0 0 0 0 1 0 0 1
62 (gray) 0 0 1 1 1 1 1 0
183 (pale gray) 1 0 1 1 0 1 1 1
255 (white) 1 1 1 1 1 1 1 1
______________________________________________________________________
BRIEF HISTORY OF PICTURES BY UNMANNED SPACECRAFT
NAME: Pioneer 4
YEAR: 1959
MISSION: Moon: measured particles and fields in a flyby, entered
heliocentric orbit.
NAME: Ranger 7
YEAR: 1964
MISSION: Moon: 4,316 high-resolution TV pictures of Sea of Clouds;
impacted.
NAME: Ranger 8
YEAR: 1965
MISSION: Moon: 7,137 pictures of Sea of Tranquility; impacted.
NAME: Ranger 9
YEAR: 1965
MISSION: Moon: 5,814 pictures of Crater Alphonsus; impacted.
NAME: Surveyor 1
YEAR: 1966
MISSION: Moon: 11,237 pictures, soft landing in Ocean of Storms.
NAME: Surveyor 3
YEAR: 1967
MISSION: Moon: 6,315 pictures, first soil scoop; soft landed in Sea
of Clouds.
NAME: Surveyor 5
YEAR: 1967
MISSION: Moon: more than 19,000 pictures; first alpha scatter
analyzed chemical structure; soft landed in Sea of
Tranquility.
NAME: Surveyor 6
YEAR: 1967
MISSION: Moon: 30,065 pictures; first lift off from lunar surface,
moved ship 10 feet, soft landed in Central Bay region.
NAME: Surveyor 7
YEAR: 1968
MISSION: Moon: returned television pictures, performed alpha scatter,
and took surface sample; first soft landing on ejecta
blanket beside Crater Tycho.
NAME: Lunar Orbiter 1
YEAR: 1966
MISSION: Moon: medium and high-resolution pictures of 9 possible
landing sites; first orbit of another planetary body;
impacted.
NAME: Lunar Orbiter 2
YEAR: 1966
MISSION: Moon: 211 frames (422 medium and high-resolution pictures);
impacted.
NAME: Lunar Orbiter 3
YEAR: 1967
MISSION: Moon: 211 frames including picture of Surveyor 1 on lunar
surface; impacted.
NAME: Lunar Orbiter 4
YEAR: 1967
MISSION: Moon: 167 frames; impacted.
NAME: Lunar Orbiter 5
YEAR: 1967
MISSION: Moon: 212 frames, including 5 possible landing sites and
micrometeoroid data; impacted.
NAME: Mariner 4
YEAR: 1964
MISSION: Mars: 21 pictures of cratered moon-like surface, measured
planet's thin, mostly carbon dioxide atmosphere; flyby.
NAME: Mariners 6 and 7
YEAR: 1969
MISSION: Mars: verified atmospheric findings: no nitrogen present,
dry ice near polar caps; both flybys.
NAME: Mariner 9
YEAR: 1971
MISSION: Mars: 7,400 pictures of both satellites and planet's
surface; orbited.
NAME: Mariner 10
YEAR: 1973
MISSION: First multiple planet encounter.
Venus: first full-disc pictures of planet; ultraviolet
images of atmosphere, revealing circulation patterns;
atmosphere rotates more slowly than planetary body; flyby.
Mercury: pictures of moon-like surface with long, narrow
valleys and cliffs; flyby; three Mercury encounters at
6-month intervals.
NAME: Pioneer 10
YEAR: 1972
MISSION: Jupiter: first close-up pictures of Great Red Spot and
planetary atmosphere; carries plaque with intergalactic
greetings from Earth.
NAME: Pioneer 11 (Pioneer Saturn)
YEAR: 1973
MISSION: Jupiter: pictures of planet from 42,760 km (26,725 mi) above
cloudtops; only pictures of polar regions; used Jupiter's
gravity to swing it back across the Solar System to Saturn.
Saturn: pictures of planet as it passed through ring plane
within 21,400 km (13,300 mi) of cloudtops; new discoveries
were made; spacecraft renamed Pioneer Saturn after leaving
Jupiter.
NAME: Pioneer Venus 1
YEAR: 1978
MISSION: Venus: studied cloud cover and planetary topography;
orbited.
NAME: Pioneer Venus 2
YEAR: 1978
MISSION: Venus: multiprobe, measuring atmosphere top to bottom;
probes designed to impact on surface but continued to return
data for 67 minutes.
NAME: Viking 1
YEAR: 1975
MISSION: Mars: first surface pictures of Mars as well as color
pictures; landed July 20, 1976; remained operating until
November 1982.
NAME: Viking 2
YEAR: 1975
MISSION: Mars; showed a red surface of oxidized iron; landed
September 03, 1976.
NAME: Voyager 1
YEAR: 1977
MISSION: Jupiter: launched after Voyager 2 but on a faster
trajectory; took pictures of Jupiter's rapidly changing
cloudtops; discovered ring circling planet, active volcano
on Io, and first moons with color: Io, orange; Europa,
amber; and Ganymede, brown; flyby.
Saturn: pictures showed atmosphere similar to Jupiter's, but
with many more bands and a dense haze that obscured the
surface; found new rings within rings; increased known
satellite count to 17; flyby.
NAME: Voyager 2
YEAR: 1977
MISSION: Jupiter: color and black-and-white pictures to complement
Voyager 1; time-lapse movie of volcanic action on Io; flyby.
Saturn: cameras with more sensitivity resolved ring count to
more than 1,000; time-lapse movies studied ring spokes;
distinctive features seen on several moons; 5 new satellites
were discovered; flyby.
Uranus: first encounter with this distant planet; photo-
graphed surface of satellites, resolved rings into multi-
colored bands showing anticipated shepherding satellites;
discovered 10 new moons, 2 new rings, and a tilted magnetic
field; flyby.
Neptune: encounter scheduled for 1989.
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NASA FACTS, HOW WE GET PICTURES FROM SPACE, Haynes, NF-151/7-87