The special field of medicine that deals with humans in environments encountered beyond the surface of the Earth. Aerospace medicine includes both aviation medicine and space medicine and is concerned with humans, their environment, and the vehicles in which they fly (Fig. 1). Aerospace medicine's objective is to ensure human health, safety, well-being, and effective performance through careful selection and training of flight personnel, protection from the unique physiological and psychological effects of the flight environment, and understanding the flight vehicle and human interaction with the vehicle. See also: Airplane; Astronautics; Aviation; Medicine; Space biology; Space flight

The aerospike engine (Fig. 1a) is an advanced liquid-propellant rocket engine with unique operating characteristics and performance advantages over conventional rocket engines. It combines a contoured axisymmetric plug nozzle (Fig. 2), an annular torus-shaped combustion chamber, conventional turbopumps, a turbine exhaust system that injects the turbine drive gases into the base of the plug nozzle, and a simple combustion tap-off engine cycle. The aerospike is one-quarter the length of a conventional rocket engine, yet it delivers comparable performance (efficiency) at high altitude and superior performance at low altitude. The low-altitude performance advantage is primarily due to the fact that the plug nozzle compensates for altitude whereas the nozzle of a conventional rocket engine does not. While the plug nozzle and its benefits are not new to the field of air-breathing propulsion, the aerospike represents the first application of this type of nozzle to the field of rocket propulsion. Typical propellants are liquid hydrogen (fuel) and liquid oxygen (oxidizer). A variation of the aerospike engine is the linear aerospike engine.

The engineering aspects of flight and navigation in space. Astronautical engineering deals with vehicles, instruments, and other equipment used in space (see illustration), but not with the sociological or economic aspects of space flight, except as they influence the equipment. Flight and navigation in space is also known as astronautics, paralleling the term "aeronautics" as applied to aviation. See also: Astronautics

The application of scientific principles and engineering techniques to flight in space. Astronautics deals with spacecraft in the sense that aeronautics deals with aircraft. A distinguishing feature between astronautics and aeronautics is the extent to which the vehicles are influenced by interaction with Earth's atmosphere. See also: Aeronautical engineering; Astronautical engineering; Atmosphere; Earth

The motion of a body traveling from space through the atmosphere surrounding a planet. Entry bodies can be natural bodies, such as meteors or comets, or vehicles, such as ballistic missiles or the space shuttle orbiter. Entry begins when the body reaches the sensible atmosphere (defined as 400,000 ft or 122 km altitude for Earth).

The first interplanetary orbiter and lander mission dedicated to the study of Saturn and its moons. A collaborative mission by NASA (National Aeronautics and Space Administration), ESA (European Space Agency), and ISA (Italian Space Agency), the international Cassini-Huygens spacecraft launched on October 15, 1997, on a 7-year, 3-billion-kilometer (2-billion-mile) journey to reach Saturn. On July 1, 2004, the spacecraft became the first to enter into permanent orbit around the giant, ringed planet (Fig. 1). During Cassini's third orbit, the spacecraft executed a series of maneuvers setting up the release of the Huygens probe for an impact trajectory with Saturn's largest moon, Titan. Huygens parachuted through the moon's atmosphere and touched down on January 14, 2005—the only landing ever attempted and completed in the outer solar system. Overall, Cassini completed 294 orbits in the Saturnian system before mission's end on September 15, 2017. The orbiter's data have allowed for a detailed understanding of the structure and dynamics of the planet's famous ring system. Among the mission's other important advances were plumes erupting from Saturn's moon Enceladus, liquid bodies on Titan's surface, and the finding of hurricanes at Saturn's poles, including a giant hexagonal cloud formation at the northern pole. Cassini also investigated Saturn's rotation period, solved mysteries pertaining to various Saturnian satellites, and discovered seven new moons. See also: Astrobiology; Completion of the Cassini mission to Saturn; Enceladus; Planet; Satellite (astronomy); Saturn; Solar system; Space probe; Titan

An artificial device that circumnavigates the Earth, receiving and retransmitting radio-frequency signals to multiple points on Earth. The communications hardware includes antennas, receivers, signal processing, and power amplifiers. The performance is predicted by calculating the power received by the satellite and by the receiving ground station. The satellite has solar arrays and batteries to provide electrical power, thrusters to keep the satellite in the desired position and orientation, and radiators to dissipate heat. The equipment is designed to operate unattended for many years. Many communications satellites (Fig. 1) are put in a special circular orbit high above the equator. See also: Antenna (electromagnetism); Battery; Celestial mechanics; Electricity; Equator; Heat; Power amplifier; Radiator; Radio receiver; Satellite (spacecraft); Solar energy; Spacecraft ground instrumentation

A space probe mission that made the first visits to the two most massive bodies in the asteroid belt, Ceres and Vesta. A primary objective of the Dawn mission was to learn more about the origin of the solar system by mapping objects that recorded the events of its earliest times. The targets chosen were the icy asteroid Ceres, a dwarf planet and the most massive object in the main belt between Mars and Jupiter, and Vesta, a rocky asteroid that is the second most massive object in the main belt. These bodies are believed to be survivors of the tumultuous history of collisions experienced in the asteroid belt and the inner solar system as the giant planets formed and migrated circa 4.6 billion years ago, when the solar system took shape. Ceres and Vesta thus offer insight into the icy and rocky building blocks of larger planets. A third science driver of the mission was the investigation of two small worlds with contrasting characteristics. Observations of both asteroids by the Hubble Space Telescope had provided some limited information regarding appearance and properties, but humanity did not truly know what Ceres and Vesta looked like until Dawn arrived (Fig. 1). See also: Asteroid; Ceres; Hubble Space Telescope; Jupiter; Mars; Planet; Solar system; Vesta

Nanosatellites, defined as having mass of under 10 kg (22 lb), are being studied to complete numerous missions by the National Aeronautics and Space Administration (NASA), the U.S. Department of Defense, commercial organizations, and other countries. Many of these missions require numerous small spacecraft in a constellation or “swarm”. These include orbital communications networks and swarms of small satellites to conduct remote sensing or even spy missions, and to provide unique perspectives on comets or asteroids. For instance, 20 or more small spacecraft could be deployed from a mother ship to observe a comet from many angles. Figure 1 demonstrates one configuration where a mother ship carries as many as 100 nanosatellites to their final destination in space for deployment. See also: Asteroid; Comet; Communications satellite; Satellite (spacecraft); Scientific and applications satellites; Space communications

Vehicular propulsion that involves electrical heating to raise the energy level of the propellant. In contrast, chemical rockets use the chemical energy of one or more propellants to heat and accelerate the decomposition products (monopropellants) or combustion products (bipropellants) for thrusting purposes. In both instances, the high-energy propellant gases are exhausted through a nozzle where they are accelerated to a high velocity (U_e), and thrust is produced by reaction. By decoupling the heating or energy addition process from the restraints of propellant chemistry considerations, electrothermal devices can be operated on a wide variety of materials, many of which would not otherwise be considered to be propellants. Water and space station liquid-waste streams are two examples of such propellants being considered for electrothermal propulsion purposes. See also: Rocket propulsion