Descent

descent, in anthropology, method of classifying individuals in terms of their various kinship connections. Matrilineal and patrilineal descent refer to the mother's or father's sib (or other group), respectively. Bilateral descent refers to descent derived from both sibs equally. Descent groups are of basic significance in the social structure of most nonindustrial societies. They constitute a series of social groups that dominate the domestic organization and the process of socialization, the use and transfer of property, the settlement of disputes, religious activities such as ancestor worship, and certain political relationships. Some lineage systems extend to the limits of the society itself. The Tiv of E Nigeria, for example, all consider themselves descendants in the male line of an eponymous ancestor, and the genealogy of this progeny defines the complete outline of descent group structure.

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descent

System of acknowledged social parentage whereby a person may claim kinship ties with another. Descent systems vary widely. The practical importance of descent comes from its use as a means for individuals to assert rights, duties, privileges, or status. Descent has special influence when rights to succession, inheritance, or residence follow kinship lines. One method of limiting the recognition of kinship is to emphasize the relationship through one parent only. Such unilineal kinship systems are of two main types—patrilineal systems, in which the relationships through the father are emphasized; and matrilineal systems, in which maternal relationships are stressed. These systems differ radically from cognatic systems, in which everyone has similar obligations to, and expectations from, both paternal and maternal kin. The cognatic system is somewhat vague and tends to characterize the more industrialized countries, in which individual rights and duties are increasingly defined institutionally or legally.

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descent

1. derivation from an ancestor or ancestral group; lineage

2. (in genealogy) a generation in a particular lineage

3. Property law (formerly) the transmission of real property to the heir on an intestacy

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descent [di′sent]

(aerospace engineering)

Motion of a craft in which the path is inclined with respect to the horizontal.

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Descent 

in astronautics, the downward trajectory of a spacecraft or descent vehicle through dense layers of an atmosphere or in a vacuum prior to landing on the surface of a celestial body. During descent onto the surface of a planet that possesses an atmosphere, the velocity of a spacecraft is reduced by any force of resistance to the spacecraft’s motion, for example, by aerodynamic drag. Descent with atmospheric braking is most useful for landing spacecraft on the surface of a planet with a rather dense atmosphere. In this case, only a descent vehicle is usually involved, not the entire spacecraft. Atmospheric braking reduces the velocity of the descent vehicle to 150–250 m/sec. Further deceleration and the landing are usually accomplished by means of parachute and other braking systems.

Descent with atmospheric braking may have either a ballistic or a glide trajectory. During a glide descent, the descent vehicle is subject to lift as well as the force of gravity and any resistance forces. The vehicle is subjected to less load in a glide descent than in a ballistic descent, and maneuvering is also possible. If descent in the atmosphere is not preceded by retrorocket braking—using a rocket engine that generates thrust opposite in direction to the motion of the spacecraft—the spacecraft will enter the atmosphere with a high velocity (of the order of orbital velocity for a spacecraft orbiting relatively near a planet or escape velocity for a spacecraft approaching a planet from a great distance). In this case, deceleration is accompanied by overloads and heating of the spacecraft. The reduction of these and other effects at velocities greater than orbital velocity is possible only in a controlled descent or as a result of gradual braking of the landing craft upon multiple passes of the craft through the atmosphere of the planet. Retrorocket braking is used for the descent of a spacecraft onto a celestial body without an atmosphere.

The first return to earth at orbital velocity after an orbital flight was accomplished by the American satellite Discoverer 13 on Aug. 11,1960. The first earth landing of a satellite after an orbital flight was accomplished by the second Soviet orbital spacecraft on Aug. 20, 1960. The first return to earth of a spacecraft with escape velocity was accomplished by the American spacecraft Apollo 4 on Nov. 9, 1967, and the first return to earth of a spacecraft with escape velocity after orbiting the moon was accomplished by the Soviet spacecraft Zond 5 on Sept. 21,1968 (ballistic descent). The first descent and landing on a celestial body when lacking an atmosphere was accomplished on Feb. 3, 1966, when the Soviet space probe Luna 9 landed on the moon. The first descent from escape velocity in the atmosphere of another planet (Venus) was accomplished by the Soviet space probe Venera 4 on Oct. 18, 1967. The first landing on another planet (Venus) was achieved by the Soviet space probe Venera 7 on Dec. 15, 1970, and the first descent and landing on Mars was accomplished by the Soviet space probe Mars 3 on Dec. 2, 1971.