500 lines
24 KiB
Plaintext
500 lines
24 KiB
Plaintext
|
||
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. <20>If you examine a newspaper photograph with a magnifying
|
||
glass, you will see that is is composed of many small, variously
|
||
shaded dots.<2E>
|
||
|
||
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.
|
||
|
||
---
|
||
NASA FACTS, HOW WE GET PICTURES FROM SPACE, Haynes, NF-151/7-87
|
||
|
||
|