Gray (unit) Gray (unit) The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter. It is used as a unit of the radiation quantity absorbed dose which measures the energy deposited in a unit mass at a certain position, and of the radiation quantity kerma, which is the amount of energy that is transferred from photons to electrons per unit mass at a certain position. The corresponding cgs unit to the gray is the rad (equivalent to
Orbital replacement unit (HST) of fasteners). HST servicing mission 3A (SM3A) replaced (or added) 15 ORUs, e.g. it replaced the DF-224 computer with the "Advanced Computer". The electrical system of the International Space Station also has such subsystems that provide power generation, power distribution and energy storage. Orbital replacement unit (HST) An orbital replacement unit or orbital replaceable unit is a modular component of spacecraft that can be replaced upon failure either by robot or by extravehicular activity. The Hubble Space Telescope (HST) was designed with 70 such parts, including scientific instruments and limited-life items such as batteries. On HST some parts were designed