HOW NASA 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. ŽIf 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.\ �
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Sunday, 22 May 2016
HOW NASA GET PICTURES FROM SPACE
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