interplanetary material
41Formation and evolution of the Solar System — Artist s conception of a protoplanetary disk The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud …
42small body — ▪ astronomy any natural solar system object other than the Sun and the major planets and dwarf planets and their satellites (moons). The small bodies populate the solar system in vast numbers and include the rocky asteroids (asteroid), or minor …
43Spacecraft propulsion — A remote camera captures a close up view of a Space Shuttle Main Engine during a test firing at the John C. Stennis Space Center in Hancock County, Mississippi Spacecraft propulsion is any method used to accelerate spacecraft and artificial… …
44Project Orion (nuclear propulsion) — An artist s conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion. Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the… …
45earth — /errth/, n. 1. (often cap.) the planet third in order from the sun, having an equatorial diameter of 7926 mi. (12,755 km) and a polar diameter of 7900 mi. (12,714 km), a mean distance from the sun of 92.9 million mi. (149.6 million km), and a… …
46Milky Way Galaxy — Large spiral galaxy (roughly 150,000 light years in diameter) that contains Earth s solar system. It includes the multitude of stars whose light is seen as the Milky Way, the irregular luminous band that encircles the sky defining the plane of… …
47Colonization of Mars — An artist s conception of the colonization of Mars, with a cutaway showing part of the interior The colonization of Mars by humans is the focus of speculation and serious study because the surface conditions and availability of water on Mars make …
48Saturn — /sat euhrn/, n. 1. an ancient Roman god of agriculture, the consort of Ops, believed to have ruled the earth during an age of happiness and virtue, identified with the Greek god Cronus. 2. Astron. the planet sixth in order from the sun, having an …
49Vacuum — This article is about empty physical space or the absence of matter. For other uses, see Vacuum (disambiguation). Free space redirects here. For other uses, see Free space (disambiguation). Pump to demonstrate vacuum In everyday usage, vacuum is… …
50Ion thruster — An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are characterized by how they accelerate the ions, using either electrostatic or electromagnetic force.… …
