Astronomy and Space Science: Space Exploration

views updated

Astronomy and Space Science: Space Exploration


Humans dreamed about exploring the heavens for thousands of years, but until the development of the liquid-propellant rocket in the twentieth century, space exploration remained only fiction. World War II (1939–1945) accelerated research into rocket flight, and competition between the United States and Soviet Union during the 1950s and 1960s provided funds for the launch of the first artificial satellite in 1957 and the first lunar landing in 1969. Humans have maintained a presence in space ever since; at the same time, robotic spacecraft have become tools of weather forecasting, military reconnaissance, communications, and astronomical exploration. By the beginning of the twenty-first century, several nations and private groups were exploring space.

Historical Background and Scientific Foundations

Creation stories in virtually every human culture describe a vast realm beyond the terrestrial world, and the lives of early humans revolved around the sky and the perplexing objects in it. The sun—often worshipped as a deity—provided warmth and life. Earth's moon illuminated the nighttime world and helped humans keep track of time; the twinkle of stars in the evening sky entertained and bewildered. Some of the earliest large structures built by humans—and virtually the only ones that still survive—are stone observatories that marked the progress of heavenly bodies over 5,000 years ago. Observation, first with the naked eye and then with instruments, has remained the single most important activity of space exploration, providing information about the origins and structure of the universe—cosmology—and potential destinations for travel.

Ancient humans' inability to visit any of the objects they observed, though, must have been profoundly frustrating. Not only would such voyages be technically difficult and physically dangerous, they were also fraught with spiritual peril. Many traditions equated the sky with divine power, a realm in which mortals could not survive or were not welcome. Western culture in particular is filled with stories of explorers struggling to attain great heights and suffering for their hubris. Among the most famous are the biblical account of the Tower of Babel and the ancient Greek myth of Icarus, whose homemade wax wings allowed him to fly but melted when he flew too close to the sun.

As these stories demonstrate, interest in the possibility of space exploration preceded, by thousands of years, the science and technology necessary to achieve it. Until the development of chemical explosives, humans had no technology capable of propelling anything more than a few hundred feet into the air. Nor did they understand the principles by which bodies move and interact with each other. The physics of ancient Greek philosopher Aristotle (384–322 BC) posited that any object sent upward would eventually return to its “natural” place, an explanation of flight that troubled Western scholars for centuries.

Rocketry and Theory

Scholars and craftsmen in Asia and Europe tackled the problems of flight in new ways around the year AD 1000. Around that time, Chinese artisans discovered the explosive properties of a mixture of charcoal, sulfur, and potassium nitrate. Packed tightly into a cylinder and lit, “black powder” released hot gases as it burned, becoming the means of propulsion in a device that Europeans later renamed the “rocket.” Chinese craftsmen constructed rockets ranging from small fireworks to military missiles several feet long; within centuries, Middle Eastern, South Asian, and European cultures had also adopted the technology. One unreliable legend holds that around 1500 a Chinese astronomer attempted to take flight by strapping rockets to his chair. He was never seen again. Europeans, though, used black powder (known in the West as gunpowder) in firearms that could hurl stone or metal balls with enough force to break castle walls or penetrate armor.

By the 1600s the most advanced theorizing on the nature of the heavens had shifted to Europe, where the work of mathematicians Nicolaus Copernicus (1473–1543), Galileo Galilei (1564–1642), and Johannes Kepler (1571–1630) demonstrated the movement of Earth and planets around the sun in predictable elliptical orbits. In addition, Galileo's improvement and popularization of the telescope enhanced the prospect of space exploration by providing evidence of other potentially habitable worlds. Kepler's Somnium (Dream), published after his death in 1634, is a fantasy in which the protagonist is magically transported to the moon. The book epitomized the problematic concept of space exploration in Renaissance Europe: without technology to carry him, Kepler relied upon magic to take him there, flirting dangerously with forbidden ideas of the occult.

In 1687 English physicist Sir Isaac Newton (1642–1727), in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), articulated laws of motion and gravity that would prove critical to future space exploration. Newton's Third Law of Motion established why a rocket moved. (The hot gases it expelled pushed against it with a force equal to that with which the rocket expelled them.) Newton's work also demonstrated that objects hurled into the air move in predictable paths described by geometric laws. An object accelerated to great enough speed—tens of thousands of miles per hour—might even coast around Earth, forever falling toward a surface it never quite reaches. This elliptical trajectory—the orbit—explained the apparently circular motions of the moon, Earth, and other planets. Accelerated to lower speeds, the object's suborbital trajectory would return it to Earth. At greater speed—25,000 miles per hour (40,233 km/h)—an object would “escape” Earth's gravitation altogether, never returning to the place from which it was launched. Newtonian physics, advanced by mathematicians like Frenchman Joseph Lagrange (1736–1813), provided the theoretical basis for later flight to other worlds.

Space Exploration in the Industrial Age

Newton's theory involved a rock tossed sideways off a tall mountain peak, but others dreamed of machines that might make space flight possible. In nineteenth-century Europe and America, advances in astronomy, chemistry, physics, and engineering produced a succession of technologies and turned attentions further skyward. Research into rocketry, artillery, balloon aviation, and heavier-than-air flight suggested that humans would not long remain on the ground.

Popular literature of the late 1800s, informed by the latest science, offered readers hypotheses about the adventures that might await intrepid humans voyaging into space. The most famous of these “scientific romances,” Frenchman Jules Verne's (1828–1905) book De la Terre à la Lune (From the Earth to the Moon) in 1865, imagined three Americans launched from Florida to the moon using a giant cannon. Beginning in the 1870s, telescopic observations of Mars by American Percival Lowell (1855–1916) and Italian Giovanni Schiaparelli (1835–1910) convinced many that Mars held an advanced society; British futurist H.G. Wells's (1866–1946) novel War of the Worlds in 1898 posited invasion of Earth by Martians.

The Liquid-Propellant Rocket

The modern age of space exploration began early in the twentieth century with innovations in Russia, Germany, and the United States. The techniques for building solid-fuel rockets had been known for centuries, but in the early decades of the twentieth century, Russia's Konstantin Tsiolkovsky (1857–1935), Germany's Herman Oberth (1894–1989), and America's Robert Goddard (1882–1945) provided some of the first descriptions and examples of rockets burning a combination of flammable liquids supplying both the fuel and oxygen for combustion. Liquid propellants delivered far more energy than could solid alternatives like gunpowder; theorists surmised that the use of liquid propellants and of staging—small rockets placed atop larger ones, fired in series—could lift objects to high altitudes or accelerate them to the speeds necessary to place an object in Earth orbit, or escape Earth's gravitation.

In 1919 Goddard, a physics professor at Clark College (now Clark University) in Massachusetts, hypothesized about travel to the moon in a scientific paper that the New York Times found so laughable that it mocked him in an editorial. (When American astronauts landed on the moon in 1969, the Times printed a retraction.) In 1926, Goddard's 11-foot (3.35-m) liquid-propellant rocket—the world's first—flew 41 feet (12.5 m) into the air on a mixture of gasoline and liquid oxygen. In the 1930s, Goddard, operating on a small grant from the Guggenheim Foundation, launched sophisticated rockets from a test site near Roswell, New Mexico.

Goddard avoided attention for much of his career, but most early experimentation in rocketry was open and international. All over the world, amateur clubs like the British Interplanetary Society theorized, built devices, published scientific papers, and corresponded with similar groups in other countries. In Germany, Oberth, with his young engineering student Wernher von Braun (1912–1977), oversaw the activities of an amateur rocketry organization called the Verein für Raumschiffahrt (VfR), or Society for Spaceship Travel. Oberth's 1923 book Die Rakete zu den Planetenräumen (The Rocket into Planetary Space) inspired a generation of rocket enthusiasts, von Braun among them.

In the Soviet Union, Sergei Korolev (1906–1966) co-chaired the state-sponsored Group for Investigation of Reactive Motion (GIRD). Rocket enthusiasts in the 1930s were a counterculture, on the fringes of professional research, often derided and regarded warily. Within only three decades, though, many of these “rocket scientists” became some of the world's most respected intellectuals.

Government-Sponsored Rocketry in World War II

All of the amateur rocket-builders were science fiction enthusiasts; all believed that the liquid-fuel rocket would make space flight possible. These researchers had something else in common—they all sought support from their nations' militaries. Goddard, the most inspired rocketeer, had the hardest time obtaining funding and was never able to interest the American government in large-scale rocketry. Korolev was somewhat more successful; after the military took charge of Soviet amateur rocketry, he enjoyed brief prestige. After a political purge by Soviet leader Joseph Stalin (1879–1953), however, Korolev was imprisoned in a Siberian gulag. He was released shortly before the end of World War II and later led Soviet military and civilian rocket programs, but Korolev died, like Goddard, never witnessing interplanetary flight by humans.


For much of his life, Wernher von Braun (1912–1977) was a study in contradictions. Born to an aristocratic German family in Wirsitz, Germany (now Poland), he died an American citizen in 1977. A failing student in his youth, von Braun later received a bachelor's degree in mechanical engineering from the Berlin Institute of Technology and a doctorate in physics from the University of Berlin. A scientist committed to peaceful uses of outer space, he developed some of the most formidable weapons of World War II and the Cold War.

Von Braun's complex and controversial life demonstrates how closely intertwined military and civilian space flight research became in the twentieth century. Although a member of the Nazi Party and an officer in the Nazi security service, the SS, von Braun later insisted that he joined only when prompted by superiors. He was aware of the party's brutality, however. The V-2 rocket factory exploited concentration camp inmates as slave labor, killing about 150 workers per day. Von Braun, the program's technical director, knew of their conditions, although he is not known to have ever protested their mistreatment. He was later quoted as saying that he did object to the inhumanity he saw, but was told unmistakably that if he persisted he, too, would be imprisoned and enslaved.

Von Braun was never a Nazi diehard, though, and, like Sergei Korolev, eventually ran afoul of the authorities, allegedly for openly criticizing his inability to pursue space flight research under Hitler. Von Braun was arrested and jailed; eventually, however, his army superiors intervened and returned him to his missile work.

As director of development at the United States Army Ballistic Missile Agency (ABMA) during the 1950s, von Braun, as he had in Germany, attempted to interest the government in the possibility of interplanetary flight. Von Braun also collaborated with fellow German-born rocketry enthusiast Willy Ley (1906–1969), Collier's magazine, and Walt Disney to “sell” the idea of space exploration to the American people through popular entertainment. The success of the Explorer 1 satellite greatly enhanced von Braun's reputation; when Congress created the National Aeronautics and Space Administration (NASA) in 1958, the ABMA was merged into the new civilian agency, and von Braun assumed an even greater role in rocket design. To send people to the moon, von Braun's team built upon decades of German and American research to design an entirely new launch vehicle the size of a skyscraper, the Saturn V.

Von Braun's fiancée joined him in the United States shortly after the end of the war; they married and raised a family. To many in the mid-1960s, von Braun was a visionary patriot, and the “father,” not only of three children, but of America's civilian space program. To others, though, he was an opportunist without any real loyalties, willing to sell his talents to who—ever would build his rockets. (Von Braun sought American support after World War II in part because the United States was able to afford his expensive rocket research.) In the 1960s, satirist Tom Lehrer (1928–) sang “‘Once the rockets are up, who cares where they come down? That's not my department,’ says Wernher von Braun.”

Von Braun hoped that Apollo would presage flights to Mars—his ambition ever since coming to the United States—but America was uninterested. In 1970 NASA promoted him to a headquarters job in Washington, D.C., which took him away from NASA operations. He left NASA in 1972 for work in private industry. Many of his beloved rockets were turned into museum displays, but von Braun continued to work on space flight projects until his death from cancer in 1977.

The most politically successful of the early rocket pioneers was Germany's Wernher von Braun. After Adolf Hitler (1889–1945) rose to power in Germany in 1933, he banned civilian rocket experimentation and replaced the VfR with a military program, of which von Braun was invited to serve as scientific director. Shortly before the start of World War II, at Peenemünde on Germany's northern coast, von Braun's team designed a rocket—the alcohol- and liquid-oxygen-powered A-4—nearly 50 feet (15.25 m) tall and carrying enough explosives to destroy a city block over 200 miles (322 km) away. The first flew successfully in 1942; by 1944, Hitler had ordered the missile—renamed Vengeance Weapon #2—fired against mostly civilian targets in Europe, with deadly results.

Postwar Rocket Research

The skills that made von Braun valuable to Nazi Germany also made him useful to the United States Army, which, at the end of World War II in 1945, invited von Braun and dozens of his colleagues to continue their rocket work in the United States. Spirited out of Europe in Operation Paperclip, von Braun set to work firing leftover A-4s on American test ranges and creating a new generation of missiles. The A-4 was the first vehicle to reach space—the realm beyond 50 miles (80.4 km) in altitude, below which virtually all of the Earth's atmosphere is located. In the meantime, the Soviet Union captured German missile parts and most of von Braun's production team. The first postwar “space race” had begun.

Von Braun spent his first years in the United States building on his wartime work and promoting the idea of the human space exploration. In the United States, however, large rocket weapons were regarded warily, although some attention focused on the possibility of launching a small instrument package into orbit several hundred miles above Earth. By the late 1940s, advances in electronics enabled various theorists around the world, including those of the American “Research and Development” (RAND) Project, to contemplate an “earth-circling spaceship”; in the 1950s, the American armed forces designed their own military satellites. By the mid-1950s attention in the United States turned to the use of large rockets to propel a newer, smaller, more powerful generation of nuclear weapons over intercontinental distances, and work to develop these intercontinental ballistic missiles (ICBMs) began in earnest.


Meanwhile, in the Soviet Union, a rocket program under Korolev was developing a liquid-propellant military rocket larger and more powerful than any then available in the West. On October 4, 1957, Korolev's R.7 rocket orbited a metal sphere the size of a basketball. Many in the West greeted the Soviet Union's orbiting of the first artificial Earth satellite, Sputnik, with shock; the rocket that put it into orbit could also send nuclear warheads to New York or Washington, D.C. Sputnik itself was a harmless radio beacon, but it was compelling evidence of sophisticated technical know-how in a country often dismissed as technologically backward.

Not only were many Americans jealous of the Soviet achievement, they feared that space represented a “high ground” from which future military operations might be launched. Increasingly, both American and Soviet leaders equated space exploration with economic and technological strength, and thought success in space was necessary to cultivate allies among the nonaligned nations of the world. In both the United States and the Soviet Union, space exploration enjoyed widespread popular support as a fulfillment of national destiny.

Prior to Sputnik, American President Dwight Eisenhower (1890–1969) supported a limited civilian satellite program—Project Vanguard—partly to establish overflight rights for future spy satellites. The success of Sputnik, though, focused attention on Vanguard as an American response to the Soviet satellite. After a Vanguard test vehicle exploded on the launch pad in a highly publicized failure, President Eisenhower turned to von Braun, whose team launched the first American satellite—Explorer 1—in 1958. That year, Congress created the National Aeronautics and Space Administration (NASA) to coordinate American civilian space exploration. Named director of NASA's Marshall Space Flight Center, von Braun played a pivotal role in designing future American launch vehicles.

Sputnik triggered a competition for further space “firsts.” In late 1957, Sputnik 2 orbited a dog, Laika, who died in space and was not recovered; in 1959, a Soviet probe transmitted the first photographs of the moon's far side, which had never been seen before. In 1960 the United States launched the first communications and weather satellites. Meanwhile, recognizing the symbolic value of human space exploration, both the Soviet Union and United States developed piloted spacecraft and recruited military aviators to operate them. While the Soviet Union concealed most aspects of its space program and its new space-faring “cosmonauts,” NASA sought publicity for its military test pilot “astronauts,” who became celebrities.


On May 25, 1961, soon after astronaut Alan Shepard's (1923–1998) flight, President John F. Kennedy (1917–1963), addressing Congress, committed the United States to answering Soviet achievements with an audacious moon-landing program, which he described as necessary to national security:

Finally, if we are to win the battle that is now going on around the world between freedom and tyranny, the dramatic achievements in space which occurred in recent weeks should have made clear to us all, as did the Sputnik in 1957, the impact of this adventure on the minds of men everywhere, who are attempting to make a determination of which road they should take. Since early in my term, our efforts in space have been under review…. Now it is time to take longer strides—time for a great new American enterprise—time for this nation to take a clearly leading role in space achievement, which in many ways may hold the key to our future on earth….

Recognizing the head start obtained by the Soviets with their large rocket engines, which gives them many months of lead-time, and recognizing the likelihood that they will exploit this lead for some time to come in still more impressive successes, we nevertheless are required to make new efforts on our own. For while we cannot guarantee that we shall one day be first, we can guarantee that any failure to make this effort will make us last. We take an additional risk by making it in full view of the world, but as shown by the feat of astronaut Shepard, this very risk enhances our stature when we are successful. But this is not merely a race. Space is open to us now; and our eagerness to share its meaning is not governed by the efforts of others. We go into space because whatever mankind must undertake, free men must fully share….

First, I believe 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. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish….

SOURCE: Kennedy, John F. “Special Message to the Congress on Urgent National Needs: May 25, 1961.” In Exploring the Unknown: Selected Documents in the History of the U.S. Civil Space Program . Edited by John M. Logsdon. Washington, D.C.: National Aeronautics and Space Administration, 1995.

The urgency of the space race in the late 1950s required the simplest approach to human space flight, placing a small, habitable compartment atop an existing Intercontinental Ballistic Missile (ICBM). In the Soviet Union, a capsule was under development that could serve as both a reconnaissance satellite and a cosmonaut-carrying spacecraft. Meanwhile, U.S. Air Force pilots were approaching the threshold of space aboard rocket-powered airplanes, but attention shifted to capsule designs. As the Soviet Union readied its Vostok vehicle, the United States prepared its Project Mercury spacecraft. Again, the Soviet Union beat the United States to a major “first,” orbiting and recovering cosmonaut Yuri Gagarin (1934–1968) in 1961. Americans Alan Shepard (1923–1998) and Virgil “Gus” Grissom (1926–1967) soon followed on brief suborbital flights; astronaut John Glenn (1921–) orbited in 1962.

The Moon Race

Both President Eisenhower and his successor, John F. Kennedy (1917–1963), were concerned about the extraordinary cost of space flight. Eisenhower was reluctant to authorize expensive space explorations, but amid growing competition in space in 1961, President Kennedy considered many options to counter Soviet successes. After consulting with von Braun and Vice President Lyndon Johnson (1908–1973), he chose an audacious one: During an address to Congress, President Kennedy challenged the United States to land “a man on the moon” and return him “safely to Earth” by 1970.

Records suggest that President Kennedy chose a lunar landing—the most ambitious project considered—not for its chance of success, but for its likelihood of failure. Neither the Soviet Union nor the United States possessed, in 1961, the technology to launch a voyage to the moon, so there was little chance that the Soviets would quickly beat the Americans there. Project Apollo soon became the centerpiece of American space exploration.

Following President Kennedy's challenge, both the United States and Soviet Union turned to the question, not of whether to go to the moon, but how. The technique ultimately chosen in both countries used a single large, multistage, liquid-propellant rocket to send two small spacecraft to the vicinity of the moon, a crew capsule and a small landing vehicle in which a portion of the crew would make the final descent to the lunar surface. In the Soviet Union, government design bureaus competed fiercely for the honor of producing spacecraft. In the United States, Congress appropriated about $100 billion (in 2006 dollars) to private industry, universities, and government facilities to develop the necessary hardware. While other nations, like France and Britain, had small space programs of their own, Cold War “superpower” competition soon made space exploration more expensive than most countries could afford.

Throughout the mid-1960s the Soviet Union and the United States practiced for a moon landing while competing for high-profile “firsts.” The Soviet Union launched the first woman, Valentina Tereshkova (1937–), into space, the first two- and three-person crews in its Voskhod capsules, and the first person to leave a spacecraft while in orbit. The United States, though, led in computer technology and orbital maneuvering using its two-person Project Gemini vehicles. (Less-publicized was the return to Earth of film from a series of photographic reconnaissance satellites, a program that had been America's original motivation for undertaking space flight research.)

Spaceflight in the early 1960s was difficult and dangerous. In flights lasting from minutes to weeks, crewmembers clad in bulky pressure suits worked, slept, and ate in capsules the size of closets. Humans coasting through space experienced the often-sickening feeling that they were falling (weightlessness); at other times, they were exposed to uncomfortable accelerations. Launches were controlled explosions; to return from orbit, spacefarers slowed their vehicles with retrorockets and careened through Earth's atmosphere so violently that heat shields were required to keep the fragile craft from melting. Descending on parachutes into the sea, American space capsules could sink before the navy could recover them. Soviet cosmonauts landed on the frigid Russian steppes, where they might wait for hours before rescue. In the 1960s, numerous astronauts, cosmonauts, and ground personnel were killed or injured in accidents. One, a fire aboard the Apollo 1 spacecraft in 1967, took the lives of three astronauts who were readying for its maiden flight.

Despite setbacks, on December 24, 1968, after a three-day journey, astronauts aboard Apollo 8 became the first humans to orbit the moon. On July 16, 1969, with the Soviet moon program stalled by technical problems, Apollo 11 astronauts Neil Armstrong (1930–) and Edwin “Buzz” Aldrin (1930–), after circling the moon with their colleague, Michael Collins (1930–), descended to the lunar surface in a landing vehicle. After a day on the moon, the three astronauts rocketed back to Earth. The United States had won the moon race; six more moon landings (and the aborted Apollo 13 mission) followed.

Like the United States, the Soviet Union developed lunar vehicles, and a launch vehicle, the N1, to carry them. Korolev's untimely death, however, undermined the Soviet effort. The Soviet Union never flew a piloted vehicle to the moon, although it did succeed in returning lunar soil samples using a series of robotic vehicles.

Space Flight Since the 1970s

In the United States, funding for civilian space exploration increased until 1967, when budget pressures (including the military conflict in Vietnam) brought the first declines in a decade. While many in America and the U.S.S.R. hoped human exploration of space would continue to Mars, funding for such projects was not forthcoming in either country.

During the 1970s, human space flight continued around with the establishment of habitable scientific and military outposts in orbit. The Soviet Union took the early lead with its Salyut space stations, in which some cosmonauts orbited for hundreds of days. In 1973, the United States launched the Skylab Orbital Workshop, a scientific laboratory assembled from leftover Apollo components. The size of a small house, Skylab provided accommodations for three successive crews and room for a solar telescope and a variety of experiments. In 1975 Apollo and Soyuz vehicles docked in Earth orbit, but the symbolic joint flight did not trigger further collaboration between the two nations.

With the end of Apollo nearing, the United States embarked, in 1972, on a program to build a replacement—the Space Transportation System, or “space shuttle.” In 1933, German rocket propulsion engineer Eugen Sänger (1905–1964) had described a vehicle that would launch like a rocket but land like an airplane. This new, reusable shuttle would be the size of a small jet airliner. It would also have a cargo bay large enough to carry equipment and would be able to accommodate crews of up to seven persons, including relatively untrained personnel. Not to be outdone, the Soviet Union soon undertook a similar program. The first American shuttle orbiter, Columbia, flew in 1981; the Soviet shuttle Buran flew unmanned in 1988, but was not used again.

Shuttle crews in the 1980s and 1990s included the first American female, African-American, and Asian-American astronauts. Shuttle astronauts also deployed and repaired the Hubble Space Telescope. Following the disintegration of the Soviet Union in 1991, American shuttles visited the Russian Mir space station.

The cooperation extended to a new effort to create the International Space Station (ISS), supported by American and Russian launches and assembled from components built by several nations.

While the five American space shuttles orbited over 100 crews and placed several satellites in orbit during their first 25 years, they did not prove as safe or inexpensive to operate as their designers had hoped. In 1986 the Shuttle Orbiter Challenger exploded during its launch, killing its seven-person crew. In 2003, Columbia sustained damage during its launch and disintegrated during reentry, killing all aboard. After both incidents, investigators determined that design and maintenance flaws—as well as unrelenting pressure to increase launches—had caused the disasters.

Modern Cultural Connections

Robot Explorers

During the first 50 years of space exploration, many scientists argued that space voyages would best be undertaken by robotic vehicles. Automated probes equipped with cameras and other remote-sensing equipment were the first spacecraft; by the 1970s, a diverse array of Earth observation, communications, military, and astronomical satellites were in orbit. Advances in propulsion and microelectronics also enabled the flight of vehicles to distant planets. American and Soviet probes, for example, mapped and landed on the moon in advance of human explorers.

American Mariner probes flew by Mercury, Venus, and Mars in the late 1960s and early 1970s. In 1975, Soviet Venera probes returned images of the surface of Venus and American Viking landers photographed Mars in 1976. The American Pioneer and Voyager probes visited virtually all of the outer planets of the solar system during the 1970s and 1980s and by 2006, they were nearing the edge of the solar system. All carry a variety of information for any extraterrestrial intelligence that finds them.

Voyages to other planets in the solar system are multiyear journeys that would expose vehicles to dangerous radiation, huge temperature extremes, and interplanetary debris. A variety of technologies have been proposed to turn and propel these probes, including, most recently, “ion engines” that propel vehicles by ejecting charged particles. Various navigational systems (including a device to locate the position of the sun) help to keep the vehicles on course and their main antennas pointed toward Earth to transmit and receive radio data (telemetry). Probes of the inner planets use solar panels to generate electricity to operate instrumentation and communication gear. Outer planetary explorers carry a small quantity of plutonium, the slow decay of which produces heat to power electrical generators. Orbiting or landing on a planet poses other challenges: Landings may require a combination of retrorockets, parachutes, or shock-absorbing balloons. The latest Mars probes slow themselves by skipping through Mars's upper atmosphere, a tricky technique called aerobraking.

Mars, possessing an arctic climate and, likely, abundant water ice, is a continuing source of fascination. Several rovers have traversed the Martian surface, sampling rocks and soil. In 2003 the Galileo vehicle released a probe that descended through Jupiter's dense atmosphere, transmitting data before the planet's atmosphere crushed it. In 2005, the Cassini vehicle, built jointly by NASA and the European Space Agency, released Huygens, a probe that landed on Saturn's moon, Titan. Asteroids and comets have also received robotic visitors.

Spaceflight in the Twenty-First Century

In 2006 a NASA probe returned to Earth carrying material from the tail of a comet and the launch of NASA's first probe to Pluto. At the same time space shuttle flights were growing more infrequent due to safety concerns. In 2004 American President George W. Bush (1946–) announced NASA's intention to retire the shuttle in 2010 and replace it with a new Crew Exploration Vehicle (CEV) to be used for flights to the moon and Mars. The expense of these new projects and the cost of maintaining the ISS, though, threatens the future of other exploration projects. Russia, meanwhile, continues to maintain the ISS with its Soyuz vehicles, while developing a replacement vehicle, Kliper. In 2003 China became the third nation to launch humans into space, aboard a craft similar to the Soyuz, the Shenzou. Like the United States, China has announced its intentions to send its “taikonauts” to the moon in the coming decades.

The exploration of space has always been controversial; commentators have long criticized human space flight in particular as extremely expensive relative to its scientific return. The idea that difficult-to-reach places must be explored, though, has proven irresistible to generations of humans. At the same time, communications, weather, navigation, and military reconnaissance satellites have become so integrated into daily life that civilization can no longer function well without some kind of space exploration program. And though it is not yet clear when such a visit will occur, humanity remains obsessed with visiting its most hospitable planetary neighbor—Mars.

Primary Source Connection

Human space flight is often opposed by scientists who wish to protect funding for their projects from the high cost of putting people in space. This document is a policy paper on present-day space policy distributed by the American Physical Society, one of the world's largest organizations of physicists.


In the 1960s and 1970s, a variety of nations began to develop space launch capabilities; Britain and France both built large rockets, but eventually joined a consortium of European countries to form the European Space Agency, and developed the successful Ariane series of launch vehicles. Other nations, including China, India, and Japan, have developed similar capabilities. Meanwhile, various private groups and business corporations have begun to develop their own satellite launch vehicles.

By the 1980s, most launch vehicles carrying satellites into orbit were versions of American and Soviet military missiles in use since the 1950s. In 1995 a group of individuals established a $10 million prize for a craft capable of carrying two people into space twice within a two-week period. The Ansari X Prize was intended to encourage private space exploration; a team led by aircraft designer Burt Rutan (1943–) won the competition in 2005 with a suborbital vehicle SpaceShipOne. Individuals and businesses involved in the X Prize hope to build vehicles that will allow paying passengers to take short trips into space. While NASA has been reluctant to support “space tourism,” the Russian Space Agency has allowed several private citizens on its Soyuz vehicles and the ISS.

In other ways, a variety of citizens, groups, and institutions play an important role in space exploration. The search for extraterrestrial intelligence has spawned efforts including the nonprofit SETI Institute, and SETI@home, a university-affiliated project to link private computers through the Internet to analyze radio signals from space that may contain evidence of alien life. The Spacewatch Project at the University of Arizona uses telescopes to scan the skies for asteroids and comets that might one day hit Earth. In addition, many of NASA's robotic space programs are built and operated by universities.


The cost of overcoming technological challenges could far exceed budgetary projections. Many approved science programs could be jeopardized.

Executive Summary

Very important science opportunities could be lost or delayed seriously as a consequence of shifting NASA priorities toward moon-Mars. The scientific planning process based on National Academy consensus studies implemented by NASA roadmaps has led to many of NASA's greatest scientific—and popular—successes. We urge the Federal Government to base priorities for NASA missions on the National Academy recommendations.

APS Executive Board Statement

Reestablishing a human presence on the moon and sending astronauts to Mars represents a major national challenge. However such a program could only achieve its full significance as part of a balanced program of scientific exploration of the universe and studies of the interaction between humankind and its environment. In recent years, NASA has captured the public's imagination through its spectacular scientific successes with the Hubble Space Telescope, the Mars Rovers, and Explorer missions that have revolutionized our understanding of the universe.

The technical hurdles facing the moon-Mars initiative are formidable, and the program's overall costs are still unknown. Further, the rapid pace currently envisioned for this program may require a wide redistribution of the science and technology budgets that could significantly alter the broad scientific priorities carefully defined for NASA and the other federal agencies. Launching such a massive program without broad consultation and a clear idea of its scope and budget may hurt rather than enhance, as intended, the scientific standing of the U.S. and the training of its scientists and engineers….

Impact of Moon-Mars on Science Priorities: In Brief

The exploration of the universe is one of the noblest endeavors of humanity. It tugs simultaneously at our emotions and our intellectual curiosity. It is the reason that NASA's spectacular unmanned scientific successes—the images from the Hubble and Chandra Space Telescopes, the Mars Rovers, and the Explorer missions such as Wilkinson Microwave Anisotropy Probe (WMAP)— have captured the public's imagination at the same time that they have revolutionized our understanding of the universe.

We believe that human exploration also has a role to play in NASA, but it must be within a balanced program in which allocated resources span the full spectrum of space science and take advantage of emerging scientific opportunities and synergies. We further believe that our understanding of the moons and planets of our solar system takes its full significance only within the more global context of a systematic study of nature: from the early universe to the formation of planets around other stars; from the fundamental laws of physics to the emergence of life; from the relations between the sun and the planets to the complex interactions in ecological systems and the impact of humanity on its environment. Returning Americans to the moon and landing on Mars would have a powerful symbolic significance, but it would constitute only a small step in the advancement of knowledge, since much will already be known from exploration with the robotic precursor probes that are necessary to guarantee the safety of any human mission.

The moon-Mars initiative presents policy makers with a major challenge: how best to implement the vision of the Administration and modify the NASA priorities without destroying the agency's balanced scientific program that was carefully crafted with strong scientific community involvement. When external factors impose a significant reorientation, it is imperative that NASA not make decisions with undue haste, without serious evaluation of their impacts, and without broad consultation. A number of mechanisms exist to engage the research community in the process, such as NASA advisory committees and the National Academy of Sciences Committee on Astronomy and Astrophysics, but thus far they have received insufficient attention.

Although the moon-Mars initiative began the needed process of addressing the goals and access vehicles for human space flight and the future of the International Space Station, we are concerned that the scope of the proposed initiative has not been sufficiently well defined, that its long-term cost has not been adequately addressed, and that no budgetary mechanisms have been established to limit the potential deleterious impact of the program on other aspects of NASA's missions. The recent analysis by the Congressional Budget Office suggests that the new initiative may only be possible at the expense of canceling proposed robotic exploration that has a much better scientific justification. We are also concerned that the impact of an ill-defined moon-Mars program, whose longterm cost is known only to be very large, could affect programs in other science agencies (such as the National Science Foundation and the Department of Energy) through the pressure of the overall budget allocation process or by putting in question inter-agency collaborative projects.

For these reasons we recommend that before the United States commit to President Bush's proposal, an exhaustive external review of the plans be carried out by the National Academy of Sciences and their likely budgetary impact estimated by the Government Accountability Office (GAO).

Impact of Moon-Mars on NASA Science Priorities: In Detail

The funding agencies, primarily NASA (through its “roadmap” process) and NSF, have used the results of the NAS Decadal Surveys to great benefit in developing their research and funding plans. In formulating their plans, the agencies have also relied on other science-driven NRC reports, such as Connecting Quarks with the Cosmos (2003) and Plasma Physics of the Local Cosmos (2004), which highlight important scientific problems. Constellation-X, a proposed initiative to study the formation and evolution of black holes through space-based X-ray observations, and LISA, a proposed initiative to detect the gravitational radiation from merging supermassive black holes, were ranked as high priority missions in the Decadal Survey Astronomy and Astrophysics in the New Millennium (2002) and were both strongly favored in Connecting Quarks with the Cosmos. NASA embraced both projects as the two “Einstein Great Observatories” in its Structure and Evolution of the Universe theme.

As a consequence of NASA's readjusted priorities in the wake of the moon-Mars initiative, LISA has been delayed at least a year, and Con-X, which was the second highest priority major space mission of the current Decadal Survey, has been delayed until at least 2016. We believe that it will be very difficult to hold the Con-X team together for ten more years, and as a result the project ultimately may have to be aborted. Other scientific missions have been delayed indefinitely, among them the Einstein Probes, which are moderate sized missions aimed at determining the nature of dark energy, observing regions near black holes, and studying the imprint of cosmic inflation on the cosmic background radiation. Their importance was emphasized in Connecting Quarks with the Cosmos and in the recent Office of Science and Technology Policy (OSTP) report, Physics of the Universe (2004).

The Explorer program is another activity that is being affected by moon-Mars. It is arguably the most successful program at NASA in benefit/cost, having produced outstanding science with small (SMEX) and medium (MIDEX) size principal-investigator-led missions—among them, WMAP, GALEX, RHESSI, IMAGE, TRACE, FAST, SWAS, and SAMPEX—covering all areas of astrophysics and solar and space physics. These missions involve academic institutions more actively than any other NASA flight program does.

Explorer spacecraft have provided extensive training for the next generation of space scientists and engineers. Explorer missions, chosen through intense competition to insure cost effectiveness, have also led to innovative instrument design and have produced new and important scientific results of great importance to the advancement of space science. Until now the Explorer budget has been kept at a constant level of funding and has not been raided for other large programs. In the aftermath of moon-Mars, however, while the funding for Explorer missions already selected and in development is still being maintained, budgets for all new missions are being drastically reduced, by 58% in FY05, 32% in FY06, 50% in FY07 and 14% in FY08. These proposed cuts, at best, will postpone the selection and start of new missions by at least a year. At worst, they will cripple the Explorer program.

The moon-Mars initiative has also caused funding cuts for the sun-Earth Connections (SEC) Mission Operations & Data Analysis that could result in the early termination of seven of the present fleet of fourteen operating SEC spacecraft by FY2006. They include the two Voyager spacecraft that are just reaching the boundary of the heliosphere and the Wind and Ulysses spacecraft that provide our best observations for studying space weather for missions to Mars and the moon.

Solar-Terrestrial Probe (STP) missions would also be affected. Although funding for the two missions already under development would be maintained, funding for future STP missions would be severely cut, by 78% in FY05, 82% in FY06, 75% in FY07, 46% in FY08 and 49% in FY09.

The NRC recently released Solar and Space Physics and Its Role in Space Exploration (2004), a report which reconsidered solar and space physics priorities in light of NASA's new space exploration vision. It found that, although the recommendations in the relevant decadal study, The Sun to the Earth—and Beyond (2003), were formulated in 2002, before the 2004 NASA exploration vision, these recommendations remain valid. Accurate predictive tools for space weather are essential for NASA's exploration goals, but without programs such as the STP mission line, the development of such tools would be placed at serious risk.

NASA's suborbital program that supports rocket and balloon-borne experiments, the prime training ground for experimental astrophysicists and space physicists, would suffer reductions as well. The program has already been reduced substantially, but the moon-Mars initiative would force further reductions, 5% in FY05, 17% in FY06, 23% in FY 07 and 26% in FY08 and FY09.

Finally, there is considerable speculation that the budgetary impact of moon-Mars colored NASA's decision to cancel the Hubble Space Telescope service mission. Although NASA cited astronaut risk considerations as the prime motivator for the cancellation, the timing of the announcement, coming just two days after President Bush's moon-Mars speech, suggests that financial considerations, prompted by moon-Mars reallocations, might also have played a substantial role.

To address concerns over the Hubble decision, Congress asked for an independent assessment. As a result, NASA Administrator Sean O'Keefe asked Admiral Harold W. Gehman Jr., the chair of the Columbia Accident Investigation Board, to review the safety of an astronaut service mission. In response, Gehman said that only a “deep and rich study … can answer the question of whether an extension of the life of the wonderful Hubble telescope is worth the risks involved.” NASA subsequently made a formal request that the National Academy of Sciences (NAS) carry out a study of the risks and benefits of using the Shuttle for the servicing mission. A NRC panel reported its preliminary findings to NASA Director O'Keefe on July 13, 2004. They urge NASA to commit to a servicing mission, note that a proposed robotic mission would be quite complex and require significant development, and state that NASA should not preclude a Shuttle servicing mission at this time. The NRC panel will release its final report in fall 2004.

american physical society, panel on public affairs. “the moon-mars program,” november 2004.


See Also Physics: Newtonian Physics.



Braun, Wernher von, and Frederick I. Ordway, III. Space Travel: A History. New York: Harper & Row, 1985.

Burrows, William E. This New Ocean: The Story of the First Space Age. New York: Random House, 1998.

McDougall, Walter A. The Heavens and the Earth: A Political History of the Space Age. Baltimore: Johns Hopkins University Press, 1997.

Siddiqi, Asif A. Challenge to Apollo: The Soviet Union and the Space Race, 1945–1974. Washington, DC: National Aeronautics and Space Administration, 2000.

Wolfe, Tom. The Right Stuff. New York: Farrar, Straus, and Giroux, 1979.

Web Sites

American Physical Society, Panel on Public Affairs. “The Moon-Mars Program.” November 2004. (accessed April 14, 2008).

National Aeronautics and Space Administration. “NASA History Division.” (accessed April 14, 2008).

National Aeronautics and Space Administration. “National Space Science Data Center.” (accessed April 14, 2008).

Smithsonian National Air and Space Museum. (accessed April 14, 2008).

U.S. Centennial of Flight Commission. “Born of Dreams—Inspired by Freedom.” (accessed April 14, 2008).

Wade, Mark. “Encyclopedia Astronautica.” (accessed April 14, 2008).

Matthew Hersch

About this article

Astronomy and Space Science: Space Exploration

Updated About content Print Article


Astronomy and Space Science: Space Exploration