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Spacepedia Handbook
Sun= '''The Sun, also referred to as "Sol", is the star at the center of the Solar System. Sun’s mass accounts for some 99.86% of the total mass of the Solar System.'''

Composition
Roughly three-quarters of the Sun’s mass is hydrogen, with the rest mostly helium. Only 1.69% of the Sun (which is still 5 628 times the mass of the Earth) is made up of heavier elements like carbon, iron, neon and oxygen.

Distance
The mean distance of the Sun from the Earth is approximately 149.6 million kilometres (1 AU). On average, it takes light from the Sun about 8 minutes and 19 seconds to reach the Earth.

Galaxy
Once thought to be a relatively insignificant star, the Sun is now considered to be brighter than about 85% of the stars in the Milky Way galaxy. It is located about two thirds of the way out from the centre of the Milky Way.

Photosphere
The photosphere is a thin outermost layer of the Sun. Below photosphere, the Sun becomes opaque. However, the photosphere itself is only slightly less opaque than the air on Earth.

Convective zone
Thermal columns carry hot material to the surface of the Sun. Once the material cools off, it plunges back towards the hot radiative zone.

Radiative zone
The radiative zone carries the intense heat of the core outward as thermal radiation, with material rapidly cooling and the density of solar material decreasing a hundredfold from the top of the core to the top of the radiative zone.

Core
99% of all power generated by the Sun comes from within 24% of the Sun’s radius. '''Mercury is the innermost and smallest planet of the Solar System. Because it has almost no atmosphere to retain heat, Mercury’s surface experiences the greatest temperature variation of all the planets.'''
 * -|Mercury=

Observation
Mercury appears as a morning or evening star from Earth, but due to it’s proximity to Sun, it is very hard to see. Still, Mercury can be observed at least twice a year: in spring at dusk and in autumn before dawn.

Crust
The crust is similar in appearance to the that of the Moon with mare-like plains and heavy cratering. A distinctive feature of the surface crust is the presence of numerous narrow ridges, which might have formed as Mercury’s core and mantle cooled and contracted after the crust had solidified.

Core
Mercury’s extreme high density infers that the planet has a large, iron-rich core, with higher iron content than that of any other major planet in the Solar System. Although Venus has very similar size and interior structure as Earth, its vulcanic surface and extremely hot and dense atmosphere makes it one of the most inhospitable places in the Solar System.
 * -|Venus=

Atmosphere
Thick clouds of sulfuric acid and dust reflect most of the Sun’s light back to space, while carbon dioxide traps the Sun’s heat inside the atmosphere, causing a runaway greenhouse effect. The atmospheric pressure on the surface of Venus is 92 times greater than on Earth.

Observation
Venus is one of the brightest objects in our night sky - second only to the Moon. It appears as either an evening or a morning star.

Structure
Currently, we have little direct information about the internal structure of Venus. However, its similar size and density to Earth suggests that they share a common internal structure. The principal difference between the two planets is the lack of plate tectonics on Venus, likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field. Instead, Venus may lose its internal heat in periodic major resurfacing events. '''Our homeworld is the densest of the eight planets in the Solar System. It is also the largest of the four terrestrial planets.'''
 * -|Earth=

Surface
About 71% of the Earth’s surface is covered by salt water oceans, and the remaining 30% is taken up by the seven continental landmasses. Earth’s outer surface is also divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of millions of years.

Axial tilt
Today, the Earth is tilted on its axis by 23.4º, producing seasonal variations in climate and weather across the surface of the planet during the course of a year.

Magnetic field
The Earth’s magnetic field is generated within the molten outer core region of the planet and extends outwards to form the magnetosphere: a barrier surrounding the Earth, deflecting particles of the solar wind and protecting the Earth from Sun's radiation.

Crust
Earth’s crust, together with the uppermost parts of the mantle forms the lithosphere. The lithosphere is broken into a series of tectonic plates which “float” on a more elastic part of the upper mantle. The motion of these plates gives rise to earthquakes and volcanism.

Mantle
The Earth’s mantle is a rocky shell which accounts for 84% of the planet’s volume. The very uppermost regions of the mantle are the most solid and are relatively rigid.

Outer core
The temperature difference within the outer core drives convection currents, with hot liquid metals rising, cooling and then descending back towards the hot inner core. These convection currents power the Earth’s magnetic field.

Inner core
Although the temperature in the inner core is thought to be about the same as the surface of the Sun, the extreme pressure prevents it from becoming liquid. '''The Moon is Earth’s only natural satellite. Next to the Sun, the Moon exerts the greatest influence on the Earth itself, most notably through its affect on Earth’s tides.'''
 * -|Moon=

Mass
The Moon is the largest natural satellite relative to the size of its primary - having 27% percent the diameter and 60% the density of the Earth. The Moon is larger than Pluto.

Origin
The current prevailing hypothesis for the origin of the Moon is that it formed as a result of Mars-sized body colliding with the newly-formed proto-Earth. The material ejected from this impact accreted to form the Moon 4.53 billion years ago.

Observation
The Moon is the brightest object in the sky after the Sun, although its surface is actually dark, reflecting only 7 percent of the light falling on it. Moon’s current orbital distance causes it to appear almost the same size in the sky as the Sun.

Crust
The uppermost parts of the Lunar crust forms broken surface layers referred to as regolith and megaregolith. Regolith comprises small rocks and dust formed by impact processes and is only a few metres thick. Megaregolith is a layer of highly fractured bedrock several kilometres thick which lies directly under the regolith.

Mantle
Moonquakes have been found to occur deep within the Moon’s mantle. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.

Inner core
Lunar core is small, with a radius of about 20% the size of the Moon. '''Mars is the fourth planet from the Sun and the second smallest planet in the Solar System. The reddish appearance of Mars’ surface is caused by iron oxide (rust).'''
 * -|Mars=

Earth-like
While the Mars is about half as big as the Earth, there are still many similarities: Mars has a rocky surface and polar ice caps, the days on Mars are only 40 minutes longer, the axial tilt gives it similar seasons to ours (although each lasts about twice as long)

Surface
Mars has a dramatic landscape, with towering volcanoes and a great canyon system. Of all the planets in the Solar System, it has the highest mountain, Olympus Mons, and the largest canyon, Valles Marineris.

Water
There is strong evidence that the Martian atmosphere was once far denser than it is now, and that water once ran freely on its surface. There is also mounting evidence that much of this water remains locked away underground.

Moons
Mars has two small natural satellites, Phobos and Deimos, that orbit very close to the planet. These moons may be captured asteroids. It has been predicted that in about 50 million years, Phobos will either crash into Mars’ surface or break up into a ring structure around the planet.

Missions
Mars is the focus of intense study, with two rovers (exploration rover Opportunity and science laboratory Curiosity) operating on the surface, while three international missions orbit the planet: NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and ESA’s Mars Express.

Observation
Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude is surpassed by the Sun, the Moon, Venus and Jupiter - but it can briefly match Jupiter’s brightness at certain points in its orbit.

Crust
The outer crust is rich in silicon, oxygen, iron, magnesium, aluminum, calcium, and potassium – all of which could be used in the development of future human outposts on Mars.

Mantle
Silicate mantle was once very active, giving rise to many of the planet’s distinctive surface features, but which now appears to be dormant.

Core
Like Earth, Mars has undergone differentiation, resulting in a dense core primarily comprising iron and nickel, but with around 17% sulfur, which renders the core partially liquid. '''Phobos is the larger and closer of the two natural satellites of Mars. It is a small, irregularly shaped object, closer to its primary than any other known planetary moon.'''
 * -|Phobos=

Proximity
Phobos is so close that it orbits Mars faster than Mars rotates. As a result, from the surface of Mars it appears to rise and set twice each Martian day.

Collision
Phobos is drawing closer to Mars and in 50 million years it will either collide with the planet or break up into a planetary ring. '''Deimos is the outermore and smaller of the two natural satellites of the planet Mars. So small, that it could easily fit inside a medium-sized crater on Earth’s moon.'''
 * -|Deimos=

Moving away
Deimos is slowly moving farther away from Mars. It is believed that it will eventually leave the orbit of Mars.