Luna

**Luna** (The Moon) is Terra's only natural satellite. It is one of the largest natural satellites in the Sol system, and, among planetary satellites, the largest relative to the size of the planet it orbits. It is named after the Roman goddess of itself.

Like almost all natural satellites, Luna is tidally locked with it parent object. It always shows the same face to Terra, which is a third smooth dark land. It is the second brightest object in Terra's sky, and is visible during day. Despite this, Luna has a very low albedo, about black as asphalt. Luna's orbital distance is about thirty times the diameter of Terra, causing it to have an apparent size in the sky almost the same as that of Sol, with the result that the Luna covers the Sun nearly precisely in total solar eclipse.

## Chronology

### Formation

Luna is relatively old, forming some 30–50 million years after beginning of the Sol system. The prevailing hypothesis today is that the Terra–Luna system formed as a result of a giant impact, where a Mars-sized body collided with the newly formed proto-Terra, blasting material into orbit around it that accreted to form Luna.

## Composition

### Surface

The most visible topographic feature is the giant far-side South Pole basin, some 2,240 km (1,390 mi) in diameter, the largest crater on Luna. At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of Luna.  The dark and relatively featureless lunar plains that can clearly be seen with the naked eye are called maria, because they were believed by ancient astronomers to be filled with water. They are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. Almost all maria are on the near side of Luna, covering 31% of the surface on the near side,compared with a few scattered patches on the far side covering only 2%.The lighter-coloured regions of Luna are called highlands, because they are higher than maria. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Luna's near side alone.

#### Conditions

Liquid water cannot persist on the majority of the lunar surface. When exposed to solar radiation, water quickly decomposes. However, water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which survive in cold, permanently shadowed craters at either pole on Luna.

### Structure

Luna is a differentiated body: it has a geochemically distinct crust, mantle, and core. Luna has a solid iron-rich inner core with a radius of 240 km (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 km (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). Luna is the second densest satellite in the Solar System after Vulcan.
Luna is exceptionally large relative to Terra: a quarter its diameter and 1/81 its mass. It is the largest moon in the Solar System relative to the size of its planet, though Rhea is larger relative to the dwarf planet Saturn, at 1/9 Saturn's mass. Terra and the Luna are nevertheless still considered a planet–satellite system, rather than a double planet, the common centre of mass, is located 1,700 km (1,100 mi) beneath Terra's surface.

## Civilization

### Initial colonies

Luna was among the first colonization targets of humans, since it was exponentially closer than any other celestial object in the vast openness of space.

A major advantage in a Lunar colony was that travel times were at a maximum, days, and in case of an emergency, the travel time could be below a day, or even hours. A severe disadvantage is the gravity on Luna, being 16.5% of Terra. Many colonists have to wear weighted shoes and clothes, and have to sleep in rotating gravity bunks. With a combination of these methods, effects of low gravity on bones and muscles were minimized. As little as 0.5 hectares of land could feed 100 people, allowing for the possibility of underground farms.The round trip communication delay to Terra is less than three seconds, allowing near-normal voice and video conversation, and allowing some kinds of remote control of machines from Terra without the need for extremely advanced, and extremely expensive technology, such as wormholes. On the Lunar near side, the Terra appears large and is always visible as an object 60 times brighter than Luna appears from Terra, unlike more distant locations where the Terra is seen merely as a star-like object, much as the planets appear from Terra. As a result, a Lunar colony felt far less remote to humans living there. Building telescopes on the moon were much more advantageous than on Terra. Espicially due to less pressure, telescopes on Luna are much larger; and inside the craters, temperatures are much lower than on Terra, an advantage for infrared telescopes.

Lunar colonies comprise a mixture of underground colonies and surface colonies. Underground colonies were constructed by finding underground lava tubes, reinforcing, and insulating them. When these ran out, tunnels were dug underground and expanded. These first habitats served as the best protection from micrometeoroids and radiation, along with temperature swings. Surface colonies required improved protection from the mentioned. Lunar bricks created out of the lunar soil were first used to create the surface habitats. This loose dirt were melted and formed a hard, ceramic barrier. The first civilian colonists arrived to a prepared Luna in 1981 CE. Water was taken from the poles in the tonnes, which were eventually brought into an artificial water cycle within the colonies. The colonies themselves were built on a rotating base to provide roughly 0.7 g, so inhabitants of Luna could easily return to Terra with no issue. Colonies were protected from radiation by micro Motojima rings. By 2027, Luna as a whole becomes self-sufficient due to artificial farms.

Several power generation types were used for the initial settlement of Luna. It largely depended on region, but for the most part, fusion power came from wealthy Terra. This overcame the difficulty and issues that came with the 354 hour night, opposed to the 12 hour terran night. Solar power serves as a major, crucial backup in case fusion power fails. There are several major solar power plants on Luna, with at least three always being in sunlight. A significant of fuel cells are placed underground for protection and serve as a secondary power source during the lunar night.

### Economy and human geography

Other than serving as a population dump and provides a great deal of recreation for a low-gravity world, Luna serves as the primary trading hub of the Conglomerate and Sol system. Landing on Terra requires atmospheric reentry, and needs a much greater velocity than Luna to escape. In the early years of the human space age, Luna was mined greatly for it's Helium-3, used widely as fusion fuel. With the first arrivals of Plutonian Helium-3, prices dipped lower than ever and Luna could not keep up. Thus, the economy of Luna shifted from mining to trading and ship construction. In addition to this, it also serves as the largest Conglomerate military base in the Sol system, housing over 700,000 servicemen and their families.

Luna serves as the trading center of the Sol system, since shipments headed for Luna don't have to go through atmospheric reentry and can break free of Luna travelling at 4.7 times less than Terra's escape velocity. The vast majority of shipments heading for Terra stop at Luna first.