Science and Technology and Engineering: Space Science (Review)

Astronomy
Astronomy is the scientific study of celestial objects and their positions, movements, and structures. Celestial does not refer to the Earth by itself, but does include its movement through space. Other celestial objects include the Sun, the Moon, planets, satellites, asteroids, meteors, comets, stars, galaxies, the universe, and other space phenomena. The term astronomy has its roots in the Greek words 'astro' and 'nomos,' which means 'laws of the stars.'

Origin
The universe can be said to consist of everything and nothing. The universe contains all of space, matter, energy, and time. There are likely still phenomena that have yet to be discovered. The universe can also be thought of as nothing, since a vast portion of the known universe is empty space. It is believed that the universe is expanding. The Big Bang theory, which is widely accepted among astronomers, was developed to explain the origin of the universe. There are other theories regarding the origin of the universe, such as the Steady-State theory and the Creationist theory. The Big Bang theory states that all the matter in the universe was once in one place. This matter underwent a huge explosion that spread it into space. Galaxies formed from, this material and the universe is still expanding.

Structure
What can be seen of the universe is believed to be at least 93 billion light years across. To put this into perspective, the Milky Way galaxy is about 100,000 light years across. Our view of matter in the universe is that it forms into clumps. Matter is organized into stars, galaxies, clusters of galaxies, superclusters, and the Great Wall of galaxies. Galaxies consist of stars, some with planetary systems. Some estimates state that the universe is about 13 billion years old. It is not considered dense, and is believed to consist of 73 percent dark energy, 23 percent cold dark matter, and 4 percent regular matter. Cosmology is the study of the universe. Interstellar medium (ISM) is the gas and dust in the interstellar space between a galaxy's stars.

Galaxies
Galaxies consist of stars, stellar remnants, and dark matter. Dwarf galaxies contain as few as 10 million stars, while giant galaxies contain as many as 1 trillion stars.
Galaxies are gravitationally bound, meaning that stars, star systems, other gases, and dust orbit the galaxy's center. The Earth exists in the Milky Way galaxy and the nearest galaxy to ours is the Andromeda galaxy.
Galaxies can be classified by their visual shape into elliptical, spiral, irregular, and starburst galaxies. It is estimated that there are more than 100 billion galaxies in the universe ranging from 1,000 to 100,000 parsecs in diameter. Galaxies can be megaparsecs apart. Intergalactic space consists of a gas with an average density of less than one atom per cubic meter. Galaxies are organized into clusters which form superclusters. Dark matter may account for up to 90% of the mass of galaxies. Dark matter is still not well understood.

Planets
In order of their distance from the sun (closest to furthest away), the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (Pluto is now considered to be a dwarf planet). All the planets revolve around the sun, which is an average-sized star in the spiral
Milky Way galaxy. They revolve in the same direction in nearly circular orbits.
If the planets were viewed by looking down from the sun, they would rotate in a counter-clockwise direction. All the planets are in, or near, the same plane, called the ecliptic, and their axis of rotation is nearly perpendicular to the ecliptic. The only exception is Uranus, which is tipped on its side.

Terrestrial Planets, Jovian Planets, and Mass of Planets
The Terrestrial Planets are: Mercury, Venus, Earth, and Mars. These are the four planets closest to the sun. They are called terrestrial because they all have a compact, rocky surface similar to the Earth's.
Venus, Earth, and Mars have significant atmospheres, but Mercury has almost no atmosphere.
The Jovian Planets are: Jupiter (the largest planet), Saturn, Uranus, and Neptune. They are called Jovian (Jupiter-like) because of their huge sizes in relation to that of the Earth, and because they all have a gaseous nature like Jupiter. Although gas giants, some or all of the Jovian
Planets may have small, solid cores.
The sun represents 99.85% of all the matter in our solar system. Combined, the planets make up only 0.135% of the mass of the solar system, with Jupiter having twice the mass of all the other planets combined.
The remaining 0.015% of the mass comes from comets, planetary satellites, asteroids, meteoroids, and interplanetary medium.

Definition of Planet
On August 24, 2006, the International Astronomical Union redefined the criteria a body must meet to be classified as a planet, stating that the following conditions must be met:
'A planet orbits around a star and is neither a star nor a moon.'
'Its shape is spherical due to its gravity.'
'It has ‘cleared' the space of its orbit.'
A dwarf planet such as Pluto does not meet the third condition. Small solar system bodies such as asteroids and comets meet only the first condition.

Solar System
The solar system developed about 4.6 billion years ago out of an enormous cloud of dust and gas circling around the sun. Four rocky planets orbit relatively close to the sun.
Their inside orbits are separated from the outside orbits of the four, larger gaseous planets by an asteroid belt. Pluto, some comets, and several small objects circle in the Kuiper belt outside Neptune's orbit. The Oort cloud, composed of icy space objects, encloses the planetary system like a shell.

Earth's Moon
Earth's moon is the closest celestial body to earth.
Its proximity has allowed it to be studied since the invention of the telescope. As a result, its landforms have been named after astronomers, philosophers, and other scholars. Its surface has many craters created by asteroids since it has no protective atmosphere. These dark lowlands looked like seas to early astronomers, but there is virtually no water on the moon except possibly in its polar regions. These impact craters and depressions actually contain solidified lava flows. The bright highlands were thought to be continents, and were named terrae. The rocks of the moon have been pounded by asteroids so often that there is a layer of rubble and dust called the regolith. Also, because there is no protective atmosphere, temperatures on the moon vary widely, from 265°F to -255°F.

Earth's Sun and Other Stars
A star begins as a cloud of hydrogen and some heavier elements drawn together by their own mass.
This matter then begins to rotate. The core heats up to several million degrees Fahrenheit, which causes the hydrogen atoms to lose their shells and their nuclei to fuse. This releases enormous amounts of energy. The star then becomes stable during a stage called the main sequence. This is the stage our sun is in, and it will remain in this stage until its supply of hydrogen fuel runs out. Stars are not always alone like our sun, and may exist in pairs or groups. The hottest stars shine blue-white; medium-hot stars like our sun glow yellow; and cooler stars appear orange. The earth's sun is an average star in terms of mass, light production, and size. All stars, including our sun, have a core where fusion happens; a photosphere (surface) that produces sunspots (cool, dark areas); a red chromosphere that emits solar (bright) flares and shooting gases; and a corona, the transparent area only seen during an eclipse.

Comets, Asteroids, and Meteoroids
Comets are celestial bodies composed of dust, rock, frozen gases, and ice. Invisible until they near the sun, comets emit volatile components in jets of gas and dust when exposed to the sun's heat. The coma is the comet's fog-like envelope that glows as it reflects sunlight and releases radiation. Solar winds blow a comet away from the sun and give it a tail of dust or electrically charged molecules. Each orbit of a comet causes it to lose matter until it breaks up or vaporizes into the sun.
Asteroids are irregularly-shaped boulders, usually less than 60 miles in diameter, that orbit the sun. Most are made of graphite; about 25% are silicates, or iron and nickel. Collisions or gravitational forces can cause them to fly off and possibly hit a planet.
Meteoroids are fragments of asteroids of various sizes. If they come through earth's atmosphere, they are called meteors or shooting stars. If they land on earth, they are called meteorites, and create craters on impact (the Barringer Crater in Arizona).