A form of mass transfer cooling that involves the removal of a special surface material (called an ablative material) from a body, such as a reentry vehicle, a planetary probe, or a reusable aerospace vehicle, by melting, vaporization, sublimation, chipping, or other erosive process, due to the aerodynamic heating effects of moving through a planetary atmosphere at very high speed. The rate at which ablation occurs is a function of the reentering body’s passage through the aerothermal environment, a high-temperature environment caused by atmospheric friction. Ablation is also a function of other factors including (1) the amount of thermal energy (i.e., heat) needed to raise the temperature of the ablative material to the ablative temperature (including phase change), (2) the head-blocking action created by boundary layer thickening due to the injection of mass, and (3) the thermal energy dissipated into the interior of the body by CONDUCTION at the ablative temperature.
To promote maximum thermal protection, the ablative material should not easily conduct heat into the reentry body. To minimize mass loss during the ablative cooling process, the ablative material also should have a high value for its effective heat of ablation—a thermophysical property that describes the efficiency with which thermal energy (in joules [J]) is removed per unit mass lost or “ablated” (in kilograms [kg]). Contemporary fiberglass resin ablative materials can achieve more than 107 J/kg thermal energy removal efficiencies through sublimation processes during reentry. Ablative cooling generally is considered to be the least mass-intensive approach to reentry vehicle thermal protection. However, these mass savings are achieved at the expense of heat shield (and possibly reentry vehicle) reusability.
The apparent angular displacement of the position of a celestial body in the direction of motion of the observer. This effect is caused by the combination of the velocity of the observer and the velocity of light. An observer on Earth would have Earth’s orbital velocity around the sun (VEarth), which is approximately 30 km/s. As a result of this effect in the course of a year, the light from a fixed star appears to move in a small ellipse around its mean position on the celestial sphere. The British astronomer JAMES BRADLEY (1693–1762) discovered this phenomenon in 1728.
A rich (high-concentration density) cluster of galaxies as characterized by the American astronomer GEORGE O. ABELL (1927–83). In 1958 Abell produced a catalog describing over 2,700 of such high-density galactic clusters using photographic data from the Palomar Observatory. Abell required that each cluster of galaxies satisfy certain criteria before he included them in this catalog. His selection criteria included population (each Abell cluster had to contain 50 or more galaxies) and high-concentration density (richness). Abell also characterized such rich galactic clusters by their appearance—listing them as either regular or irregular.
(1927–1983) American Astronomer George O. Abell is best known for his investigation and classification of galactic clusters. Using photographic plates from the Palomar Observatory Sky Survey (POSS) made with the 1.2-m (48-inch) Schmidt Telescope at the Palomar Observatory in California, he characterized more than 2,700 Abell clusters of galaxies. In 1958 he summarized this work in a book now known as the Abell Catalogue.
