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The basic unit of length in the solar system. The astronomical unit (AU) is also used to a limited extent for interstellar distances through the definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean distance a between the…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. The terms atomic clock and atomic frequency standard are often used interchangeably. A frequency standard generates pulses at regular…

Time that is based on physics, specifically on a unit of duration that corresponds to a defined number of periods of radiation of a specific atomic transition of a chosen isotope. The fundamental unit of atomic time is the International System…

A cool and highly luminous star, prominently located in the right shoulder of the constellation Orion and noticeably red in color. Betelgeuse, or α Orionis, is a supergiant star about 130 parsecs (430 light-years) from the Sun. Its spectral type of…

Systems composed of two almost equal-size asteroids orbiting around their center of mass, or one or more satellites revolving around a larger primary body. The discovery of binary and multiple asteroids will aid the work of observers and theorists…

Two stars held together by their mutual gravitational attraction in a permanent (or long-term) association. The stellar universe is hierarchical. Stars exist singly and in binary pairs, multiple systems, and clusters. On large scales, roughly 105…

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a…

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…

The application of dynamical theory to the motion of celestial bodies, including planets, moons, asteroids, comets, and stars, influenced primarily by the force of gravity. Celestial mechanics has enabled humankind to describe with great precision…

The largest object in the asteroid belt between Mars and Jupiter. As an astronomical body, Ceres contains sufficient mass for gravitation to have pulled it into a round shape, in contrast to lumpy, irregularly shaped asteroids. This characteristic…