Satellite (redirected from Artificial satellites)

satellite

Natural object (moon) or spacecraft (artificial satellite) orbiting a larger astronomical body. Most known natural satellites orbit planets; the Earth's Moon is the most obvious example and was the only one known until the discovery of the Galilean satellites of Jupiter in 1610. All the solar system's planets except Mercury and Venus have moons, which vary greatly in size, composition (from rock to mostly ice), and activity (from cold and inert to volcanic). Some asteroids are also known to have their own moons. The first artificial satellite, Sputnik 1, was launched into orbit around Earth in 1957. Since then, thousands have been sent into orbit around Earth as well as the Moon, Venus, Mars, Jupiter, and other bodies. Artificial satellites are used for scientific research and other purposes, such as communication (see communications satellite), weather forecasting, Earth resources management, and military intelligence. See also Landsat.

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satellite

1. a celestial body orbiting around a planet or star

2. a man-made device orbiting around the earth, moon, or another planet transmitting to earth scientific information or used for communication

3. a country or political unit under the domination of a foreign power

4. a subordinate area or community that is dependent upon a larger adjacent town or city

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satellite [′sad·əl‚īt]

(aerospace engineering)

artificial satellite

(astronomy)

A small, solid body moving in an orbit around a planet; the moon is a satellite of earth.

(cell and molecular biology)

A chromosome segment distant from but attached to the rest of the chromosome by an achromatic filament.

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Satellite 

in astronomy, a body of the solar system that revolves about a planet under the action of the planet’s gravitational attraction. The four brightest satellites of Jupiter—lo, Europa, Ganymede, and Callisto—were, aside from the earth’s moon, the first satellites to be observed. They were discovered by Galileo in 1610. By 1975, 33 satellites of the planets were known: one, the moon, of the earth, two of Mars, 13 of Jupiter, ten of Saturn, five of Uranus, and two of Neptune.

Satellites travel within the gravitational fields of the planets in orbits that differ only slightly in shape from ellipses. The deviations of the actual orbits from elliptical orbits are a result of perturbations caused by the sun’s attraction and by the deviation of the planets’ shapes from a spherical shape. The perturbations that satellites introduce into each other’s orbits can be used to determine the masses of the satellites. The orbital motion of most satellites is direct. In other words, the satellite revolves about the planet in the same direction that the planet revolves about the sun—that is, in a counterclockwise direction when viewed from the north pole of the ecliptic. The only satellites to have retrograde orbits are the satellites VIII, IX, XI, and XII of Jupiter, Saturn’s satellite Phoebe, Uranus’ satellites, and Neptune’s satellite Triton. Table 1 gives the principal data on the known satellites of the planets.

The satellites Phobos and Deimos of Mars are remarkable for their proximity to the planet and their rapid motion. The inner satellite, Phobos, revolves about Mars faster than Mars rotates on its axis, so that an observer on the Martian surface would see the satellite rise in the west and set in the east. Phobos rises and sets twice in a Martian day. Deimos moves across the sky more slowly: more than 2½ days pass between the time the satellite rises above the horizon and the time it sets. Both satellites move almost exactly in the equatorial plane of Mars. In 1972 the Mariner 9 space probe took close photographs of Phobos and Deimos. Both satellites proved to be irregular in shape. Phobos’ dimensions are 27 km × 21 km × 19 km, and those of Deimos are 15 km × 12 km × 11 km; the measurement error here is 0.5–3 km. The geometrical albedo of the Martian satellites does not exceed 0.05—that is, in terms of reflectivity they are comparable to the darkest parts of the lunar maria. Phobos and Deimos are covered with many craters. One crater on Phobos has a diameter of approximately 5.3 km. The craters were undoubtedly formed by impacts.

The four largest satellites of Jupiter—that is, the satellites discovered by Galileo—are comparatively bright objects of the fifth and sixth magnitude. Their orbits are almost circular, and the

Table 1. Satellites of the planets (1975 data)
Planet	Satellite	Mean distance from planet (thousands of km)	Sidereal period of revolution (days)	Eccentricity	Inclination of orbit to planet’s equatorial plane (degrees)	Diameter (km)	Mass (mass of moon = 1)	Year of discovery
Earth	Moon	384.4	27.3	0.055	23.4	3,476	1.00	–
Mars	Phobos	9.4	0.3	0.016	1.1	27	–	1877
	Deimos	23.5	1.3	0.001	1.8	15	–	1877
Jupiter	V	181	0.5	0.003	0.4	220	–	1892
	l lo	422	1.8	0.000	0.0	3,640	0.99	1610
	I I Europa	671	3.6	0.000	0.0	3,100	0.64	1610
	III Ganymede	1,070	7.2	0.001	0.0	5,270	2.11	1610
	IV Callisto	1,880	16.7	0.007	0.0	5,000	1.32	1610
	XIII	11,100	239	0.15	27	–	–	1974
	VI	11,500	251	0.16	28	160	–	1904
	VII	11,750	260	0.21	25	60	–	1905
	X	11,750	260	0.13	29	18	–	1938
	XII	21,000	625	0.17	147	16	–	1951
	XI	22,500	700	0.21	164	22	–	1938
	VIII	23,500	740	0.38	145	16	–	1908
	I X	23,700	755	0.28	153	20	–	1914
Saturn	Janus	160	0.7	0.000	0.0	220	–	1966
	Mimas	186	0.0	0.020	1.5	400	0.001	1789
	Enceladus	238	1.4	0.004	0.0	500	0.001	1789
	Tethys	295	1.9	0.000	1.1	1,000	0.009	1684
	Dione	378	2.7	0.002	0.0	1,150	0.014	1684
	Rhea	528	4.5	0.001	0.4	1,600	0.03	1672
	Titan	1,223	15.9	0.029	0.3	5,000	1.92	1655
	Hyperion	1,484	21.3	0.104	0.4	350	–	1848
	lapetus	3,563	79.3	0.028	14.7	1,800	0.019	1671
	Phoebe	12,950	550.4	0.163	150	300	–	1898
Uranus	Miranda	130	1.4	0.017	3.4	400	–	1948
	Ariel	192	2.5	0.003	0.0	1,400	–	1851
	Umbriel	267	4.1	0.004	0.0	1,000	–	1851
	Titania	439	8.7	0.024	0.0	1,800	–	1787
	Oberon	586	13.5	0.001	0.0	1,600	–	1787
Neptune	Triton	354	5.9	0.000	160	4,000	1.8	1846
	Nereid	5,510	365.0	0.750	28	600	–	1949

planes of their orbits approximately coincide with the plane of the planet’s equator. The first determination of the speed of light was made in 1676 on the basis of observations of the eclipses of these satellites. Ganymede and Callisto are larger than the planet Mercury. For each of the four satellites, the period of rotation is the same as the period of revolution about Jupiter. Consequently, each satellite always presents the same side to the planet. A considerable part of the surfaces of Europa and Ganymede is covered with ice. The Pioneer 10 spacecraft discovered in 1973 that Io has a dense atmosphere. Jupiter’s satellite XIII was discovered in October 1974.

Saturn’s satellite Titan is larger than Mercury. Titan has an atmosphere, which, like the atmosphere of Saturn, contains methane and ammonia. The closest satellite to the planet, Janus, was discovered on Dec. 15,1966, when Saturn’s rings were invisible. This satellite is usually concealed in the halo of the brilliant rings.

The orbital planes of the satellites of Uranus are close to the equatorial plane of the planet. The satellites revolve in the same direction as Uranus rotates. Since, however, the equatorial plane of the planet is tilted at an angle of 98° to the plane of the planet’s orbit, Uranus moves with its satellites as if it were lying on its side.

Neptune’s satellite Triton was discovered in 1846, two weeks after the discovery of the planet. Triton is larger in size and greater in mass than the moon. The other satellite, Nereid, has a highly eccentric orbit. As a result, its distance from the planet varies from 1.5 to 9.6 million km.

Most of the names of the satellites were taken from ancient mythology and works of literature. The satellites of Jupiter discovered by Galileo are also designated by the Roman numerals I, II, III, and IV, in order of increasing distance from Jupiter. Jupiter’s remaining satellites, which were discovered later, are designated by Roman numerals in the chronological order of the satellites’ discovery.

G. A. CHEBOTAREV