proton

—protonic, adj.

/proh"ton/, n. Physics, Chem.

a positively charged elementary particle that is a fundamental constituent of all atomic nuclei. It is the lightest and most stable baryon, having a charge equal in magnitude to that of the electron, a spin of ¹/₂, and a mass of 1.673 × 10^-27 kg. Symbol: P

[1915-20; n. use of Gk prôton, neut. of prôtos FIRST]

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Stable subatomic particle (one of the baryons) with a unit of positive electric charge and a mass 1,836 times that of the electron.

Protons are found in the atomic nucleus along with neutrons. For every nucleus of a given element, the number of protons is always the same; this number is the element's atomic number. A single proton is the nucleus of an atom of ordinary hydrogen; as such, it is identical to the hydrogen ion (H⁺). Protons have antimatter counterparts (antiprotons), with the same mass but a negative charge. Protons are used as projectiles in particle accelerators to produce and study nuclear reactions. They are the chief constituent of primary cosmic rays and are among the products of radioactive decay (see radioactivity) and nuclear reactions.

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▪ Russian launch vehicle

      Russian launch vehicle used for both government and commercial payloads. Since 1965 the Proton launch vehicle has been a workhorse means of access to space, first for the Soviet Union and now Russia. Proton has been used to launch spacecraft to Venus and Mars; elements of the space stations Salyut, Mir, and International Space Station; and satellites into low and geostationary Earth orbits.

      Proton was originally designated the UR-500; it was designed as an intercontinental ballistic missile for the most powerful Soviet thermonuclear weapons by the design bureau headed by Vladimir Chelomey (Chelomey, Vladimir Nikolayevich). Its purpose was changed during development, and since its first launch (of the Proton-1 satellite) in July 1965 it has been used only as a space launch vehicle. The name of the launcher was changed to Proton after its initial launch. The launcher has been produced in two-, three-, and four-stage versions and has undergone continuous improvements since it entered service. Its first three stages are fueled by a combination of nitrous oxide and unsymmetrical dimethylhydrazine (UDMH) liquid fuels. Earlier versions of the fourth stage were fueled by a combination of liquid oxygen and kerosene, but the current fourth stage uses the nitrous tetroxide–UDMH combination.

      Launchpads for the Proton are located at the Baikonur Cosmodrome (Baikonur) in Kazakhstan. Proton reliability has been greater than 90 percent over its many years of service. The prime contractor for the vehicle is now the Khrunichev State Research and Production Space Centre, located near Moscow. Commercial launches on Proton are marketed globally by International Launch Services—a joint venture of Khrunichev and the Russian firm RSC Energia (Energia).

John M. Logsdon

 

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