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Spacecraft employ booster rockets for propulsion and small adjustable retro-rockets for changing the orientation of the craft. Rocket propulsion systems vary from the tiny Aerobee sounding rocket to the giant Saturn V used in the Apollo project. For interplanetary flights, propulsion by nuclear or solar energy may be possible. Also being considered are ion and photon engines, which very efficiently provide low thrust that can build up very high velocity during a long flight. Landing on the earth or any planet with a significant atmosphere raises the problem of atmospheric friction, which can instantly burn up any spacecraft. In the manned space program, shielding that comes apart is used to absorb the frictional energy as the material of the shielding vaporizes. Also, a spacecraft enters the atmosphere at a shallow angle to avoid the friction produced by excessively high velocities.
Without the development of modern electronics based on miniaturized transistor circuitry, space exploration would have been practically impossible. Unmanned space probes and satellites carry on-board computers of varying degrees of sophistication, and even on manned missions, maneuvering the spacecraft requires the rapid calculation and response available only through computerized devices. The instruments carried on spacecraft measure almost every conceivable physical parameter. Devices for measuring micrometeorite density, cosmic rays, magnetic fields, and solar wind were aboard even the early artificial satellites. Television cameras for both visible and infrared light are carried by most space probes. In addition, many spacecraft carry telescopes for different wavelengths of the spectrum, ranging from infrared to X rays and gamma rays. An important technique in space science is called multispectral scanning. Images are formed using only certain selected wavelengths; the data can be used to compile a single, detailed color photograph, or can be studied separately. Certain space probes carry more specialized devices, such as ultraviolet spectrographs for studying stars, and coronographs and spectroheliographs for studying the sun.
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