From early theories on how to break Earth's gravitational pull, humankind has traveled into Earth orbit, to the Moon, and even—via robotic proxies—to the outer solar system. Even as the Hubble Space Telescope has made it possible for us to study distant galaxies and stars, other satellites give us up-to-date weather information, enhanced telecommunications, and navigation systems that allow us to pinpoint where we are. These advancements and more have all been made possible through the practical problem-solving that first launched us into space.
| 1903 |
Paper mathematically demonstrates liftoff with liquid fuels
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| 1915 |
Goddard establishes that it is possible to send a rocket to the Moon Robert Goddard experiments with reaction propulsion in a vacuum and establishes that it is possible to send a rocket to the Moon. Eleven years later, in 1926, Goddard launches the first liquid-fuel rocket. |
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| 1942 |
Successful launch of a V-2 rocket Ten years after his first successful rocket launch, German ballistic missile technical director Wernher von Braun achieves the successful launch of a V-2 rocket. Thousands of V-2s are deployed during World War II, but the guidance system for these missiles is imperfect and many do not reach their targets. The later capture of V-2 rocket components gives American scientists an early opportunity to develop rocket research techniques. In 1949, for example, a V-2 mated to a smaller U.S. Army WAC Corporal second-stage rocket reaches an altitude of 244 miles and is used to obtain data on both high altitudes and the principles of two-stage rockets. |
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| 1957 |
Sputnik I On October 4 the Soviet Union launches Sputnik I using a liquid-fueled rocket built by Sergei Korolev. About the size of a basketball, the first artificial Earth satellite weighs 184 pounds and takes about 98 minutes to complete one orbit. On November 3 the Soviets launch Sputnik II, carrying a much heavier payload that includes a passenger, a dog named Laika. |
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| 1958 |
United States launches its first satellite The United States launches its first satellite, the 30.8-pound Explorer 1. During this mission, Explorer 1 carries an experiment designed by James A.Van Allen, a physicist at the University of Iowa, which documents the existence of radiation zones encircling Earth within the planet’s magnetic field. The Van Allen Radiation Belt, as it comes to be called, partially dictates the electrical charges in the atmosphere and the solar radiation that reaches Earth. Later that year the U.S. Congress authorizes formation of the National Aeronautics and Space Administration (NASA). |
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| 1959 |
Luna 3 probe flies past the Moon The Soviet Union’s Luna 3 probe flies past the Moon and takes the first pictures of its far side. This satellite carries an automated film developing unit and then relays the pictures back to Earth via video camera. |
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| 1960 |
TIROS 1 launched Weather satellite TIROS 1 is launched to test experimental television techniques for a worldwide meteorological satellite information system. Weighing 270 pounds, the aluminum alloy and stainless steel spacecraft is 42 inches in diameter and 19 inches high and is covered by 9,200 solar cells, which serve to charge the onboard batteries. Magnetic tape recorders, one for each of two television cameras, store photographs while the satellite is out of range of the ground station network. Although it is operational for only 78 days, TIROS 1 proves that a satellite can be a useful tool for surveying global weather conditions from space. |
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| 1961 |
Alan B. Shepard, Jr. becomes the second human in space On May 5 astronaut Alan B. Shepard, Jr., in Freedom 7, becomes the second human in space. Launched from Cape Canaveral by a Mercury-Redstone rocket, Freedom 7—the first piloted Mercury spacecraft—reaches an altitude of 115 nautical miles and a speed of 5,100 miles per hour before splashing down in the Atlantic Ocean. During his 15-minute suborbital flight, Shepard demonstrates that individuals can control a vehicle during weightlessness and high G stresses, supplying researchers on the ground with significant biomedical data. |
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| 1961 |
Yuri Gagarin becomes the first human in space On April 12, cosmonaut Yuri Gagarin, in Vostok I, becomes the first human in space. Launching from Baikonur Cosmodrome, he completes one orbit of Earth in a cabin that contains radios, instrumentation, life-support equipment, and an ejection seat. Three small portholes give him a view of space. At the end of his 108-minute ride, during which all flight controls are operated by ground crews, he parachutes to safety in Kazakhstan. |
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| 1962 |
John Glenn is the first American to circle Earth John Glenn becomes the first American to circle Earth, making three orbits in his Friendship 7 Mercury spacecraft. Glenn flies parts of the last two orbits manually because of an autopilot failure and during reentry leaves the normally jettisoned retro-rocket pack attached to his capsule because of a loose heat shield. Nonetheless, the flight is enormously successful. The public, more than celebrating the technological success, embraces Glenn as the personification of heroism and dignity. |
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| 1963 |
Syncom communications satellites launched On February 14 NASA launches the first of a series of Syncom communications satellites into near-geosynchronous orbit, following procedures developed by Harold Rosen of Hughes Aircraft. In July, Syncom 2 is placed over the Atlantic Ocean and Brazil at 55 degrees longitude to demonstrate the feasibility of geosynchronous satellite communications. It successfully transmits voice, teletype, facsimile, and data between a ground station in Lakehurst, New Jersey, and the USNS Kingsport while the ship is off the coast of Africa. It also relays television transmissions from Lakehurst to a ground station in Andover, Maine. Forerunners of the Intelsat series of satellites, the Syncom satellites are cylinders covered with silicon solar cells that provide 29 watts of direct power when the craft is in sunlight (99 percent of the time). Nickel-cadmium rechargeable batteries provide power when the spacecraft is in Earth’s shadow. |
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| 1965 |
Edward H. White, Jr. is the first American to perform a spacewalk The second piloted Gemini mission, Gemini IV, stays aloft for four days, (June 3-7), and astronaut Edward H. White, Jr. performs the first extravehicular activity (EVA)—or spacewalk—by an American. This critical task will have to be mastered before a landing on the Moon. |
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| 1968 |
Apollo 8 flight to the Moon views Earth from lunar orbit. Humans first escape Earth’s gravity on the Apollo 8 flight to the Moon and view Earth from lunar orbit. Apollo 8 takes off from the Kennedy Space Center on December 21 with three astronauts aboard—Frank Borman, James A. Lovell, Jr., and William A. Anders. As their ship travels outward, the crew focuses a portable television camera on Earth and for the first time humanity sees its home from afar, a tiny "blue marble" hanging in the blackness of space. When they arrive at the Moon on Christmas Eve, the crew sends back more images of the planet along with Christmas greetings to humanity. The next day they fire the boosters for a return flight and splash down in the Pacific Ocean on December 27. |
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| 1969 |
Neil Armstrong becomes the first person to walk on the Moon Neil Armstrong becomes the first person to walk on the Moon. The first lunar landing mission, Apollo 11 lifts off on July 16 to begin the 3-day trip. At 4:18 p.m. EST on July 20, the lunar module—with astronauts Neil Armstrong and Edwin E. (Buzz) Aldrin—lands on the Moon’s surface while Michael Collins orbits overhead in the command module. After more than 21 hours on the lunar surface, they return to the command module with 20.87 kilograms of lunar samples, leaving behind scientific instruments, an American flag, and other mementos, including a plaque bearing the inscription: "Here Men From Planet Earth First Set Foot Upon the Moon. July 1969 A.D. We came in Peace For All Mankind." |
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| 1971 |
First space station, Salyut 1 The Soviet Union launches the world’s first space station, Salyut 1, in 1971. Two years later the United States sends its first space station, Skylab, into orbit, where it hosts three crews before being abandoned in 1974. Russia continues to focus on long-duration space missions, launching the first modules of the Mir space station in 1986. |
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| 1972 |
Pioneer 10 sent to the outer solar system Pioneer 10, the first mission to be sent to the outer solar system, is launched on March 2 by an Atlas-Centaur rocket. The spacecraft makes its closest approach to Jupiter on December 3, 1973, after which it is on an escape trajectory from the Solar System. NASA launches Pioneer 11 on April 5, 1973, and in December 1974 the spacecraft gives scientists their closest view of Jupiter, from 26,600 miles above the cloud tops. Five years later Pioneer 11 makes its closest approach to Saturn, sending back images of the planet’s rings, and then heads out of the solar system in the opposite direction from Pioneer 10. The last successful data acquisitions from Pioneer 10 occur on March 3, 2002, the 30th anniversary of its launch date, and on April 27, 2002. Its signal is last detected on January 23, 2003, after an uplink is transmitted to turn off the last operational experiment. |
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| 1975 |
NASA launches two Mars space probes
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| 1977 |
Voyager I and Voyager 2 are launched Voyager I and Voyager 2 are launched on trajectories that take them to Jupiter and Saturn. Over the next decade the Voyagers rack up a long list of achievements. They find 22 new satellites (3 at Jupiter, 3 at Saturn, 10 at Uranus, and 6 at Neptune); discover that Jupiter has rings and that Saturn's rings contain spokes and braided structures; and send back images of active volcanism on Jupiter's moon lo—the only solar body other than Earth with confirmed active volcanoes. |
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| 1981 |
Space Shuttle Columbia is launched The Space Shuttle Columbia, the first reusable winged spaceship, is launched on April 12 from Kennedy Space Center. Astronauts John W. Young and Robert L. Crippin fly Columbia on the first flight of the Space Transportation System, landing the craft at Edwards Air Force Base in Southern California on April 14. Using pressurized auxiliary tanks to improve the total vehicle weight ratio so that the craft can be inserted into its orbit, the mission is the first to use both liquid- and solid-propellant rocket engines for the launch of a spacecraft carrying humans. |
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| 1986 |
Space Shuttle Challenger destroyed during launch On the 25th shuttle flight, the Space Shuttle Challenger is destroyed during its launch from the Kennedy Space Center on January 28, killing astronauts Francis R. (Dick) Scobee, Michael Smith, Judith Resnik, Ronald McNair, Ellison Onizuka, Gregory Jarvis, and Sharon Christa McAuliffe. The explosion occurs 73 seconds into the flight when a leak in one of two solid rocket boosters ignites the main liquid fuel tank. People around the world see the accident on television. The shuttle program does not return to flight until the fall of 1988. |
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| 1990 |
Hubble Space Telescope The Hubble Space Telescope goes into orbit on April 25, deployed by the crew of the Space Shuttle Discovery. A cooperative effort by the European Space Agency and NASA, Hubble is a space-based observatory first dreamt of in the 1940s. Stabilized in all three axes and equipped with special grapple fixtures and 76 handholds, the space telescope is intended to be regularly serviced by shuttle crews over the span of its 15-year design life. |
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| 1998 |
International Space Station The first two modules of the International Space Station are joined together in orbit on December 5 by astronauts from the Space Shuttle Endeavour. In a series of spacewalks, astronauts connect cables between the two modules—from the United States and Zarya from Russia—affix antennae, and open the hatches between the two spacecraft. |
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| 2000 |
Expedition One of the International Space Station On October 31 Expedition One of the International Space Station is launched from Baikonur Cosmodrome in Kazakhstan—the same launch-pad from which Yuri Gagarin became the first human in space. Prior to its return on March 21, 2001, the crew conducts scientific experiments and prepares the station for long-term occupation. |
In the days and weeks that followed, the whole world tracked Sputnik's progress as it orbited the globe time and again. Naked-eye observers could see its pinpoint of reflected sunlight tracing across the night sky, and radios picked up the steady series of beeps from its transmitter. For Americans it was a shocking realization. Here, at the height of the Cold War, was the enemy flying right overhead. For the nascent U.S. space program, it was also a clear indication that the race into space was well and truly on—and that the United States was behind.
That race would ultimately lead to what has been called the most spectacular engineering feat of all time: landing humans on the Moon and bringing them safely back. But much more would come of it as well. Today literally thousands of satellites orbit the planet—improving global communications and weather forecasting; keeping tabs on climate change, deforestation, and the status of the ozone layer; making possible pinpoint navigation practically everywhere on Earth's surface; and, through such satellite-borne observatories as the Hubble Space Telescope, opening new eyes into the deepest reaches of the cosmos. The Space Shuttle takes astronauts, scientists, and engineers into orbit, where they perform experiments on everything from new medicines to superconductors. The Shuttle now also ferries crews to and from the International Space Station, establishing a permanent human presence in space. Venturing farther afield, robotic spacecraft have toured the whole solar system, some landing on planets and others making spectacular flybys, sending back reams of data and stunning close-up images of planets, moons, asteroids, and even comets.
In making all this possible, aerospace engineers have also propelled advances in a wide range of fields, from electronics to materials composition. Indeed, even though some critics contend that spaceflight is no more than a romantic and costly adventure, space technologies have spawned many products and services of practical use to the general public, including everything from freeze-dried foods to desktop computers and Velcro.
Sputnik was only the beginning, but it was also the culmination of efforts to get into space that dated back to the start of the 20th century. Of the several engineering challenges that had to be addressed along the way, the first and foremost was building a rocket engine with enough thrust to overcome the pull of gravity and lift a vehicle into orbit. Rockets themselves had been around for centuries, almost exclusively as weapons of war, driven by the burning of solid fuels such as gunpowder. By the 19th century it was clear to experimenters that, although solid fuel could launch missiles along shallow trajectories, it couldn't create enough thrust to send a rocket straight up for more than a few hundred feet. You just couldn't pack enough gunpowder into a rocket to blast it beyond Earth's gravity.
Three men of the 20th century can justly lay claim to solving the problem and setting the stage for spaceflight. Working independently, Konstantin Tsiolkovsky in Russia, Robert Goddard in the United States, and Hermann Oberth in Germany designed and, in Goddard's and Oberth's cases, built rocket engines propelled by liquid fuel, typically a mixture of kerosene or liquid hydrogen and liquid oxygen. Tsiolkovsky took the first step, publishing a paper in 1903 that mathematically demonstrated how to create the needed thrust with liquid fuels. Among his many insights was the notion of using multistage rockets; as each rocket expended its fuel, it would be jettisoned to reduce the overall weight of the craft and maintain a fuel-to-weight ratio high enough to keep the flight going. He also proposed guidance systems using gyroscopes and movable vanes positioned in the exhaust stream and developed formulas still in use today for adjusting a spacecraft's direction and speed to place it in an orbit of virtually any given height.
Goddard was the first to launch a liquid-fuel rocket, in 1926, and further advanced the technology with tests of guidance and stabilization systems. He also built pumps to feed fuel more efficiently to the engine and developed the mechanics for keeping engines cool by circulating the cold liquid propellants around the engine through a network of pipes. In Germany, Oberth was garnering similar successes in the late 1920s and 1930s, his gaze fixed steadily on the future. One of the first members of the German Society for Space Travel, he postulated that rockets would someday carry people to the Moon and other planets.
One of Oberth's protégés was responsible for rocketry's next advance. Wernher von Braun was a rocket enthusiast from an early age, and when he was barely out of his teens, the German army tapped him to develop a ballistic missile. The 20-year-old von Braun saw the work as an opportunity to further his own interests in spaceflight, but in the short term his efforts led to the V- 2, or Vengeance Weapon 2, used to deadly effect against London in 1944. (His rocket design worked perfectly, von Braun told a friend, "except for landing on the wrong planet.") After the war, von Braun and more than a hundred of his rocket engineers were brought to the United States, where he became the leading figure in the nation's space program from its earliest days in the 1950s to its grand achievements of the 1960s.
With the Soviet Sputnik success, U.S. space engineers were under pressure not just to catch up but to take the lead. Less than 5 months later, von Braun and his team successfully launched America's first spacecraft, the satellite Explorer 1, on January 31, 1958. Several months after that, Congress authorized the formation of an agency devoted to spaceflight. With the birth of the National Aeronautics and Space Administration (NASA), the U.S. space program had the dedicated resources it needed for the next great achievement: getting a human being into space.
Again the Soviet Union beat the Americans to the punch. In April 1961, Yuri Gagarin became the first man in space, followed only a few weeks later by the American Alan Shepard. Gagarin's capsule managed one Earth orbit along a preset course over which he had virtually no control, except the timing of when retro-rockets were fired to begin the descent. Shepard simply went up and came back down on a suborbital flight, although he did experiment with some astronaut-controlled maneuvers during the flight, firing small rockets positioned around the capsule to change its orientation. Both were grand accomplishments, and both successes depended on key engineering advances. For example, Shepard's capsule, Freedom 7, was bell shaped, a design developed by NASA engineer Maxime Faget. The wide end would help slow the capsule during reentry as it deflected the heat of atmospheric friction. Other engineers developed heat-resistant materials to further protect the astronaut's capsule during reentry, and advances in computer technology helped control both flights from start to finish. But the United States was still clearly behind in the space race.
Then, barely 6 weeks after Shepard's flight and months before John Glenn became the first American to orbit Earth, President John F. Kennedy threw down the gauntlet in what was to become a major battle in the Cold War. "I believe," said Kennedy, "that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth."
NASA's effort to meet Kennedy's challenge was divided into three distinct programs, dubbed Mercury, Gemini, and Apollo, each of which had its own but related agenda. The Mercury program focused on the basics of getting the astronaut up and returning him safely. Gemini, named for the twins of Greek mythology, fulfilled its name in two ways. First, each Gemini flight included two astronauts, whose main task was to learn to maneuver their craft in space. Second, the overall goal of the program was to have two spacecraft rendezvous and link together, a capability deemed essential for the final Moon missions. Engineers had at least three different ideas about how to accomplish rendezvous. Gemini astronaut Buzz Aldrin, whose doctoral thesis had been on just this subject, advocated a method founded on the basic principle of orbital mechanics that a craft in a lower orbit travels faster than one in a higher orbit (to offset the greater pull of gravity at a lower altitude). Aldrin argued that a spacecraft in a lower orbit should chase one in a higher orbit and, as it approached, fire thrusters to raise it into the same orbit as its target. The system was adopted, and on March 16, 1966, Gemini 8, with Neil Armstrong and David Scott aboard, achieved the first docking in space, physically linking up with a previously launched, unmanned Agena rocket.
Armstrong and Aldrin would, of course, go on to greater fame with the Apollo program—the series of missions that would finally take humans to the surface of the Moon. Apollo had the most complex set of objectives. Engineers had to design and build three separate spacecraft components that together made up the Apollo spacecraft. The service module contained life-support systems, power sources, and fuel for in-flight maneuvering. The conical command module would be the only part of the craft to return to Earth. The lunar module would ferry two members of the three-man crew to the lunar surface and then return them to dock with the combined service and command modules. Another major task was to develop new tough but lightweight materials for the lunar module and for spacesuits that would protect the astronauts from extremes of heat and cold. And then there was what has often seemed the most impossible challenge of all. Flight engineers had to perfect a guidance system that would not only take the spacecraft across a quarter of a million miles to the Moon but also bring it back to reenter Earth's atmosphere at an extremely precise angle that left very little room for error (roughly six and half degrees, give or take half a degree). If the angle was too steep, the capsule would burn up in the atmosphere, too shallow and it would glance off the atmosphere like a stone skimming the surface of a pond and hurtle into space with no possibility of a second chance.
Launching all that hardware—40 tons of payload—called for a rocket of unprecedented thrust. Von Braun and his rocket team again rose to the challenge, building the massive Saturn V, the largest rocket ever created. More than 360 feet long and weighing some 3,000 tons, it generated 7.5 million pounds of thrust and propelled all the Apollo craft on their way without a hitch. On July 16, 1969, a Saturn V launched Apollo 11 into space. Four days later, on July 20, Neil Armstrong and Buzz Aldrin became the first humans to set foot on the Moon, thus meeting Kennedy's challenge and winning the space race. After the tragic loss of astronauts Virgil I. (Gus) Grissom, Edward H. White, and Roger B. Chaffee during a launchpad test for Apollo 1, the rest of the Apollo program was a spectacular success. Even the aborted Apollo 13 mission proved how resourceful both the astronauts in space and the engineers on the ground could be in dealing with a potentially deadly catastrophe—an explosion aboard the service module. But with the space race won and with increasing cost concerns as well as a desire to develop other space programs, the Moon missions came to an end in 1972.
NASA turned its attention to a series of robotic, relatively low-cost science and discovery missions. These included the Pioneer probes to Jupiter and Saturn; the twin Viking craft that landed on Mars; and Voyagers 1 and 2, which explored the outer planets and continue to this day flying beyond the Solar System into interstellar space. Both the Soviet Union and the United States also built space stations in the 1970s. Then, in 1981 the United States ramped up its human spaceflight program again with the first of what would be, at last count, scores of Space Shuttle missions. An expensive breakthrough design, the Shuttle rises into space like any other spacecraft, powered by both solid- and liquid-fuel rockets. But upon reentering the atmosphere, the craft becomes a glider, complete with wings, rudder, and landing gear—but no power. Shuttle pilots put their craft through a series of S turns to control its rate of descent and they get only one pass at landing. Among the many roles the Shuttle fleet has played, the most significant may be as a convenient though costly spacebased launchpad for new generations of satellites that have turned the world itself into a vast arena of instant communications.
As with all of the greatest engineering achievements, satellites and other spacecraft bring benefits now so commonplace that we take them utterly for granted. We prepare for an impending snowstorm or hurricane and tune in to our favorite news source for updates, but few of us think of the satellites that spied the storm brewing and relayed the warning to Earth. Directly and indirectly, spacecraft and the knowledge they have helped us gain not only contribute in myriad ways to our daily well-being but have also transformed the way we look at our own blue planet and the wider cosmos around us.
William A. Anders
Retired Chairman
General Dynamics Corporation
As a youth I was fascinated by science and engineering. I also loved to explore and find out what was on the other side of the mountain. These traits came together beyond my wildest boyhood dreams when I was selected by NASA to be an Apollo astronaut. The Apollo program, with its preeminent role in the "space race" with the USSR, allowed me to serve my country during a critical period of the Cold War.
As astronauts we spent many hours in training, learning how to operate spacecraft systems in our effort to get to the Moon. Those of us with engineering backgrounds (I was a nuclear engineer) were assigned tasks in design and testing, such as determining methods for measuring radiation dosage and shielding. We also studied geology—Earth's surface up close with rock hammer and magnifying glass and the lunar surface from afar with telescopes—so that we would be prepared to describe the lunar surface features and materials when we got there.
I was lucky enough to be chosen for the Apollo 8 crew. This would be man's first flight on the giant Saturn V rocket, with a mission to blaze a trail to the Moon and orbit it 10 times. The mission was set to occur over Christmas 1968. After a successful launch and orbital check of the spacecraft's systems, we re-ignited our rocket engines and boosted our velocity to some 35,000 feet per second. This was easily a new world speed record and enough for us to be the first humans to escape the gravity of our home planet and venture to another celestial body.
After two and a half days of first "coasting" away from Earth and later of "falling" toward the Moon, we retro-fired our spacecraft's rocket to slow us down enough to be captured by the Moon's gravity. We were in lunar orbit!
As the amateur geologist and photographer of the crew (as well as spacecraft systems engineer and pilot), I was especially eager to view the lunar surface. But after several 2-hour "heads down" orbits observing the Moon and photographing lunar features, I was, frankly, getting a bit bored at the Moon's sameness. The surface was a monotonous gray that looked sandblasted. And it was repetitive—crater upon crater upon crater created by countless meteors, large and small through the eons.
Then all of a sudden we saw Earth rising majestically above the Moon's stark horizon. We might have been engineers and "right stuff" test pilots, but the beauty of this sight took our breath away. I grabbed the color camera and snapped the now famous first "Earthrise" photo.
Earth appeared quite small to us—about the size of my fist at arm's length. It was the only color in the dull black "sky"—a fragile Christmas tree ornament to be handled with utmost care, not just a chunk of rock whose inhabitants treated it carelessly. Big or small, we realized that it was mankind's only home and the place where we evolved, the center of our emotional and spiritual universe.
Apollo was designed and operated to go to the Moon and learn about its properties, but its main contribution may well have been the new human perspective it created about the fragility and finiteness of our home planet. And this was done by a bunch of engineers.
Pinpoint Navigation Around the World
Among the thousands of satellites orbiting Earth are 24 that work together as the key elements of the Global Positioning System (GPS), a navigational tool of unprecedented precision. Circling the globe once every 12 hours at an altitude of more than 12,000 miles, the satellites maintain positions relative to one another that enable handheld or vehicle-mounted units to receive signals from as many as six of them at once, from virtually any point on the planet’s surface at any given time. Using signals from at least four satellites, the receivers can calculate their own location—latitude, longitude, and elevation—to within 100 feet; the more satellites the receiver has within its line of sight, the more precisely location can be determined, in some cases within 20 feet.
Each satellite continuously transmits three vital pieces of information an identification code, data on its own location in space, and a time signal. Four atomic clocks on each satellite keep track of time to within 3 billionths of a second, a level of accuracy that enables a receiver’s microprocessor to precisely measure the distance to each satellite from which it receives signals. The microprocessor then applies a sophisticated form of triangulation to determine its own position.
Known officially as NAVSTAR (Navigation Satellite Timing and Ranging), GPS was envisioned as early as the 1950s by Ivan Getting, who subsequently championed its construction. Brad Parkinson, then at the Department of Defense, directed its development for its original military functions. GPS played an important role in Operation Desert Storm, enabling U.S. forces to maneuver during sandstorms and at night; aircraft, ship, tanks, and individual troops made use of more than 9,000 receivers throughout the Gulf region.
Eventually released for civilian use, GPS began finding an increasing number of applications in the 1990s. For example, engineers building the English Channel tunnel used GPS-provided measurements to ensure that the French and British teams digging from opposite ends would meet precisely in the middle. Transportation companies employ GPS receivers to keep track of their fleets, and police cars, fire engines, and ambulances use them to speed their response to emergencies. As receivers become less expensive, GPS is becoming more and more a part of everyday life, helping us all to know quite literally, our place in the world.
HOW GPS WORKS. To calculate a location on or near the earth, a GPS receiver must be able to receive signals from four satellites. Each satellite sends a signal at the speed of light, approximately 186,000 miles per second; the GPS receiver can determine how far it is from a satellite by measuring the time it takes the signal to travel from the satellite to the receiver. (One billionth of a second clock error corresponds to approximately one foot.) This distance represents the radius of an imaginary sphere centered at the satellite and the receiver is located somewhere on its surface. By measuring the distance to a second satellite, the possible receiver locations are reduced to be somewhere on the intersection of the two spheres. To precisely pinpoint its position, GPS must track four satellites. Four satellites are required because the receiver must determine its position (latitude, longitude, altitude) and correct its clock error. By taking multiple “waypoint” readings a hiker can plot the changing elevation of a climb in order to stay on track. In applications where the user receiver knows its own altitude (e.g., a surface ship) and/or has a clock synchronized to the GPS master clocks, signals from fewer satellites are needed.