The Word on the Subject of Comos and Space
The Solar System consists of our local star, the Sun, along with a large number of objects that orbit around it. Just the part containing the planetary orbits is about 6 billion miles (9 billion km) across. Our planet, Earth, orbits within a relatively small central area of this colossal region of space.
THE SUN’S FAMILY
The Sun is by far the largest body within the Solar System, containing 99.85 percent of its mass. Other constituents include: eight planets and their moons; a handful of dwarf planets; and vast numbers of smaller objects that fall into three main groups —asteroids (composed largely of rock and metal), comets (made of ice, rock, and dust), and Kuiper Belt objects (largely ice bodies). Also present are appreciable quantities of rock fragments and particles of dust and ice left in the wake of comets. The planets fall into two distinct groups. These are the relatively small, rocky inner planets (Mercury, Venus, Earth, and Mars—see pp.52–53) and the much larger outer planets (Jupiter, Saturn, Uranus, and Neptune)
The sun
The hot, incandescent ball of gas that we call the Sun provides Earth with life-sustaining heat and light, and dictates its seasons and climate. But, as awesome as the Sun seems to us, it is a fairly typical star—one of at least 200 billion in the Milky Way alone.
A TYPICAL STAR
The Sun lies in the disc of the Milky Way (see pp.192–93), in a region where the average space between stars is about 8 light-years. Like more than 90 percent of all stars, the Sun is in a stable stage of its life.In its hot, dense core, nuclear-fusion reactions convert hydrogen into helium. This releases energy, which gradually makes its way to the surface, where it escapes into space, mostly as infrared radiation (heat) and light. The Sun’s rate of energy production and its surface temperature have both stayed relatively constant for about 4.6 billion years—and will stay so for as long again, until the hydrogen runs out. The Sun will then swell into a red giant, increasing its energy output as it burns helium and destroying the inner planets. After ejecting its outer layers, it will end its days as a white dwarf.
The Meteor and Comet
Meteor
A meteor is a space rock—or meteoroid—that enters Earth's atmosphere. As the space rock falls toward Earth, the resistance—or drag—of the air on the rock makes it extremely hot. What we see is a "shooting star." That bright streak is not actually the rock, but rather the glowing hot air as the hot rock zips through the atmosphere.
When Earth encounters many meteoroids at once, we call it a meteor shower.Diagram shows Sun in center, Earth orbiting, and lop-sided comet orbit intersecting earth orbit.
Why would Earth encounter many meteoroids at once? Well, comets, like Earth and the other planets, also orbit the sun. Unlike the nearly circular orbits of the planets, the orbits of comets are usually quite lop-sided.
Comet
As a comet gets closer to the sun, some of its icy surface boils off, releasing lots of particles of dust and rock. This comet debris gets strewn out along the comet's path, especially in the inner solar system (where we live) as the sun's heat boils off more and more ice and debris. Then, several times each year as Earth makes its journey around the sun, its orbit crosses the orbit of a comet, which means Earth smacks into a bunch of comet debris.
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS). 27 Manx comets were found from 2013 to 2017.
As of July 2019 there are 6,619 known comets, a number that is steadily increasing as more are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.
The Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continents and islands. The remaining 71% is covered with water, mostly by oceans but also by lakes, rivers and other fresh water, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convecting mantle that drives plate tectonics.
Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds). Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day
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