Halley and His Comet

Newton's friend, the astronomer Edmond Halley, endeavoured to compute the orbits of 24 comets for which he had found accurate enough historical documents. Applying Newton's method, he presupposed a parabola as an approximation for each orbit. Among the 24 parabolas, 3 were identical in size and superimposed in space. The three relevant cometary passages (1531, 1607, and 1682) were separated by two time intervals of 76 and 75 years. Halley concluded that the parabolas were actually the end of an...

The Heliopause

Between the Kuiper Belt and the Oort cloud lies the heliopause, the teardrop-shaped region around the Sun that is filled with solar magnetic fields and the outward-moving solar wind consisting of protons and electrons. Nearer the Sun than the heliopause lies the helio-sheath, a region of transition where the solar wind slows to subsonic speeds, that is, slower than the speed with which disturbances travel through the interstellar medium. The tail of the heliopause is estimated to be between 110...

Types of Meteorites

Meteorites traditionally have been divided into three broad categories stony meteorites (or stones), iron meteorites (irons), and stony iron meteorites (stony irons) based on the proportions of rock-forming minerals and nickel-iron (also called iron-nickel) metal alloy they contain. Stony meteorites make up about 94 percent of all known meteorites, irons about 5 percent, and stony irons about 1 percent. There is considerable diversity within each category, leading to numerous subdivisions...

The Ring System

The rings of Uranus were the first to be found around a planet other than Saturn. The American astronomer James L. Elliot and colleagues discovered the ring system from Earth in 1977, nine years before the Voyager 2 encounter, during a stellar occultation by Uranus i.e., when the planet passed between a star and Earth, temporarily blocking the star's light. Unexpectedly, they observed the star to dim briefly five times at some considerable distance above Uranus's atmosphere both before and...

TyPes Of Orbits

In the absence of planetary perturbations and nongravitational forces, a comet will orbit the Sun on a trajectory that is a conic section with the Sun at one focus. The total energy E of the comet, which is a constant of motion, will determine whether the orbit is an ellipse, a parabola, or a hyperbola. The total energy E is the sum of the kinetic energy of the comet and of its gravitational potential energy in the gravitational field of the Sun. Per unit mass, it is given by E v2 - GMr-1,...

The Magnetic Field and Magnetosphere

Saturn's magnetic field resembles that of a simple dipole, or bar magnet, its north-south axis aligned to within 1 of Saturn's rotation axis with the centre of the magnetic dipole at the centre of the planet. The polarity of the field, like Jupiter's, is opposite that of Earth's present field i.e., the field lines emerge in Saturn's northern hemisphere and reenter the planet in the southern hemisphere, then that magnetic moment is about 600 times Earth's, whereas Jupiter's magnetic moment is...

Plutos Moons

Charon, by far the largest, is fully half the size of Pluto. It revolves around the dwarf planet more accurately, the two bodies revolve around a common centre of mass at a distance of about 19,640 km (12,200 miles), equal to about eight Pluto diameters. (By contrast, Earth's Moon is a little more than one-fourth the size of Earth and is separated from the latter by about 30 Earth diameters.) Charon's period of revolution is exactly equal to the rotation...

Sedna

Sedna is a small body in the Kuiper Belt that may be the first discovered object from the Oort cloud. It was discovered in 2003 by a team of American astronomers at Palomar Observatory on Mount Palomar, Calif. At that time, it was the most distant object in the solar system that had ever been observed, at a distance of 13 billion km (8.1 billion miles) from the Sun. Its discoverers named the new object Sedna, after the Inuit goddess said to live in a cave at the bottom of the Arctic Ocean....

Groups Of Comets And Other Unusual Cometary Objects

Some comets travel in strikingly similar orbits, only the time of perihelion passages being appreciably different. Members of such a group of comets are thought to be fragments from a larger comet that was tidally disrupted earlier by the Sun or in some cases by the differential jet action of nongravitational forces on a fragile nucleus. Many such breakups have been observed historically. Slight differences in the resultant velocities though they occur very gently are sufficient to cause...

Orgueil Meteorite

The Orgueil meteorite fell on the village of Orgueil, near Toulouse, Fr., in May 1864 and is often used to infer the relative proportions of elements in the solar system (cosmic abundances). Like the Allende and Murchison meteorites, it is classified as a carbonaceous chondrite, a type that comprises the most primitive meteorites ones having a chemical composition much like that of the Sun and thus of the solid material that formed at the birth of the solar system. About 20 fragments of the...

Moons

Saturn possesses more than 60 known moons. (A table summarizing data for Saturn's moons appears in Appendix B, Moons of Saturn.) Of the first 18 discovered, all but the much more distant moon Phoebe orbit within about 3.6 million km (2.2 million miles) of Saturn. Nine are more than 100 km (60 miles) in radius and were discovered telescopi-cally before the 20th century the others were found in an analysis of Voyager images in the early 1980s. Several additional inner moons tiny bodies with radii...

Geography Of The Asteroid Belt

Geography in its most literal sense is a description of the features on the surface of Earth or another planet. Three coordinates latitude, longitude, and altitude suffice for locating all such features. Similarly, the location of any object in the solar system can be specified by three parameters heliocentric ecliptic longitude, heliocentric ecliptic latitude, and heliocentric distance. Such positions, however, are valid for only an instant of time since all objects in the solar system are...

Classification Of Asteroids

In the mid-1970s astronomers, using information gathered from studies of colour, spectral reflectance, and albedo, recognized that asteroids could be grouped into three broad taxonomic classes, designated C, S, and M. At that time they estimated that about 75 percent belonged to class C, 15 percent to class S, and 5 percent to class M. The remaining 5 percent were unclassifiable owing to either poor data or genuinely unusual properties. Furthermore, they noted that the S class dominated the...

Enceladus

The second nearest of the major regular moons of Saturn and the brightest of all its moons is Enceladus. It was discovered in 1789 by the English astronomer William Herschel and named for one of the Giants (Gigantes) of Greek mythology. Enceladus measures about 500 km (310 miles) in diameter and orbits Saturn in a prograde, nearly circular path at a mean distance of 238,020 km (147,899 miles). Its average density is only 30 percent greater than that of water, which indicates that it is at least...

Notable Asteroids

Among the thousands and thousands of asteroids are some that are particularly notable. Some, such as Ceres, are significant in the history of astronomy. Others, such as Vesta, are geologically interesting. The dwarf planet Ceres is the largest known asteroid in the asteroid belt and the first asteroid to be discovered. Ceres revolves around the Sun once in 4.61 Earth Series of six images showing the rotation of Ceres, taken by the Hubble Space Telescope. ESA STScI NASA years in a nearly...

Proteus and Nereid

Proteus is the second largest moon of Neptune and is named after the prophetic, shape-changing old man of the sea in Greek mythology. It was one of the moons discovered by Voyager 2 in 1989. With a mean radius of about 208 km (129 miles), Proteus is a little larger than Nereid, with a mean radius of about 170 km (106 miles). Because Proteus is so near to Neptune, however, scientists on Earth had been unable to detect it in the planet's glare. Nereid is the third largest known moon of Neptune....

Tethys

Tethys is remarkable for a fissure that wraps around the greater part of its circumference. It was discovered in 1684 by the Italian-born French astronomer Gian Domenico Cassini and named for a Titan in Greek mythology. Tethys has a diameter of 1,060 km (659 miles), and its density of 1.0 g cm3 (0.58 oz in3) the same as that of water indicates that it is composed essentially of pure water ice. It revolves around Saturn in a prograde, circular orbit at a distance of 294,660 km (183,090 miles),...

Info

10 hr 39 min 24 sec (Voyager era) about 10 hr 46 min (Cassini-Huygens mission) 3 major rings comprising myriad component ringlets several less-dense rings *Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth. **Calculated for the altitude at which 1 bar of atmospheric pressure is exerted. A quarter century later, however, some measurements made by Cassini indicated that the field was rotating with a period 6-7 minutes longer. It is...

The Galilean Satellites

Galileo proposed that the four Jovian moons he discovered in 1610 be named the Medicean stars, in honour of his patron, Cosimo II de' Medici, but they soon came to be known as the Galilean satellites in honour of their discoverer. Galileo regarded their existence as a fundamental argument in favour of the Copernican model of the solar system, in which the planets orbit the Sun. Their orbits around Jupiter were in flagrant violation of the Ptolemaic system, in which all celestial objects must...