Until the dawn of spaceflight, astronomers were limited in their ability to observe objects beyond the solar system to those portions of the electromagnetic spectrum that can penetrate Earth’s atmosphere. These portions include the visible region, parts of the ultraviolet region, and most of the radio-frequency region. The ability to place instruments on a spacecraft operating above the atmosphere (see satellite observatory) opened the possibility of observing the universe in all regions of the spectrum. Even operating in the visible region, a space-based observatory could avoid the problems caused by atmospheric turbulence and airglow.

Beginning in the 1960s, a number of countries launched satellites to explore cosmic phenomena in the gamma-ray, X-ray, ultraviolet, visible, and infrared regions. More recently, space-based radio astronomy has been pursued. In the last decades of the 20th century, the United States embarked on the development of a series of long-duration orbital facilities collectively called the Great Observatories. They include the Hubble Space Telescope, launched in 1990 for observations in the visible and ultraviolet regions; the Compton Gamma Ray Observatory, launched in 1991; the Chandra X-Ray Observatory, launched in 1999; and the Spitzer Space Telescope, launched in 2003. Europe and Japan have also been active in space-based astronomy and astrophysics. Europe’s Herschel infrared observatory, launched in 2009, studied the origin and evolution of stars and galaxies. A telescope aboard Japan’s Akari spacecraft, launched in 2006, also observed the universe in the infrared spectrum.

The results of these space investigations have made major contributions to an understanding of the origin, evolution, and likely future of the universe, galaxies, stars, and planetary systems. For example, the U.S. Cosmic Background Explorer (COBE) satellite, launched in 1989, mapped the microwave background radiation left over from the early universe, providing strong support for the theory that the universe was created in a primordial explosion, known as the big bang. Precision measurements of this cosmic microwave background by the American Wilkinson Microwave Anisotropy Probe (WMAP, 2001) and the European Planck spacecraft (2009) enabled astronomers to determine the age, size, and shape of the universe. The U.S. satellite Kepler (2009) discovered thousands of planetary candidates of startling diversity orbiting distant suns. The European satellites Hipparcos (1989) and Gaia (2013) precisely mapped the position of more than a billion stars. The striking images of cosmic objects obtained by the Hubble Space Telescope not only added significantly to scientific knowledge but also shaped the public’s perception of the cosmos, perhaps as significantly as did the astronomer Galileo’s observations of the Moon and Jupiter nearly four centuries earlier. Working as complements to ground-based observatories of increasing sensitivity, space-based observatories helped create a revolution in modern astronomy.

Microgravity research

A spacecraft orbiting Earth is essentially in a continuous state of free fall. All objects associated with the spacecraft, including any crew and other contents, are accelerating—i.e., falling freely—at the same rate in Earth’s gravitational field (see Earth: Basic planetary data). As a result, these objects do not “feel” the presence of Earth’s gravity but instead experience a state of weightlessness, or zero gravity. True zero gravity, however, is experienced only at the centre of mass of a freely falling object. With increasing distance from the centre of mass, the influence of gravity increases in both directions perpendicular to the object’s flight path. These constant but tiny accelerations make necessary the use of the term microgravity to describe the space environment. (It is possible to create a similar absence of gravity’s effects only briefly on Earth or in an aircraft.) Human activity or the operation of equipment in a spacecraft causes vibrations that impart additional accelerations and so raise gravity levels, which can make it difficult to carry out highly sensitive experiments under sufficiently low microgravity conditions. Although spacecraft designers cannot totally eliminate gravitational effects, they hope to reduce them in some parts of the International Space Station to one microgravity—one-millionth of Earth’s gravity—by isolating those areas from vibrations and other disturbances as much as possible.

The opportunity to carry out experiments in the absence of gravity has interested scientists from the beginning of activities in orbit. In addition to concern about the effects of the weightlessness on humans sent into space (see above Biomedical, psychological, and sociological aspects), scientists are interested in its effects on the reproductive and developmental cycles of plants and animals other than humans. The overall goal is to use space-based research to add to the general understanding of a wide range of biological processes.

Life-sciences experiments were carried out on the Skylab, Salyut, and Mir space stations and constitute a significant portion of work aboard the ISS. Such research also was conducted on space shuttle missions, particularly within the Spacelab facility. In addition, the Soviet Union and the United States launched a number of robotic satellites dedicated to life-sciences research. Together these experiments have involved a wide range of nonhuman organisms, from bacteria, plants, and invertebrate animals to fish, birds, frogs, turtles, and mammals such as rats and monkeys. Human crew members also have served as experimental subjects for research on such topics as the functioning of the neurological system and the process of aging. In October 1998, the U.S. senator and former Mercury astronaut John H. Glenn, Jr., at age 77 returned to space on a shuttle mission dedicated to life-sciences research, which included studies of similarities between the aging process and the body’s response to weightlessness. The hope is that the results of biomedical experiments conducted in microgravity can be used to improve human health and well-being on Earth.

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The microgravity environment also offers unique conditions for experiments designed to explore the behaviour of materials. Among the areas of inquiry are biotechnology, combustion science, fluid physics, fundamental physics, and materials science. Experiments in the microgravity environment on various materials, including metals, alloys, electronic and photonic materials, composites, colloids, glasses and ceramics, and polymers, have resulted in a greater understanding of the role of gravity in similar laboratory and manufacturing processes on Earth. The microgravity environment offers the potential for producing biological materials, including highly ordered protein crystals for crystallographic analysis and even materials resembling human tissues, that are difficult or impossible to make on Earth. Although microgravity research is still largely at the basic level, scientists and engineers hope that additional work—another major focus for the ISS—will lead to practical knowledge of great usefulness to manufacturing processes on Earth.

Observing Earth

Satellites, space stations, and space shuttle missions have provided a new perspective for scientists to collect data about Earth itself. In addition to practical applications (see below Space applications), Earth observation from space has made significant contributions to fundamental knowledge. An early and continuing example is the use of satellites to make various geodetic measurements, which has allowed precise determinations of Earth’s shape, internal structure, and rotational motion and the tidal and other periodic motions of the oceans. Fields as diverse as archaeology, seismology, and oceanography likewise have benefited from observations and measurements made from orbit.

Scientists have begun to use observations from space as part of comprehensive efforts in fields such as oceanography and ecology to understand and model the causes, processes, and effects of global climate change, including the influence of human activities. The goal is to obtain comprehensive sets of data over meaningful time spans about key physical, chemical, and biological processes that are shaping the planet’s future. This is a coordinated international effort, in which the United States, Europe, Japan, and other countries are providing satellites to obtain the needed observations.