# Space Transport and Engineering Methods/Physics3

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## 9.0 - Astronomy

Astronomy is the study of the Universe as a whole, and the objects and phenomena within it. While the Earth is part of the Universe, other branches of science study it in more detail. Astronomy mainly studies its place in the Solar System, and the rest of the Universe beyond our atmosphere. This is of course highly relevant for space systems since beyond the atmosphere is where they operate.
This is only a very short introduction. For more background, see
* Nick Strobel's Astronomy Notes website,
* the Wikibook General Astronomy,
* the many topics and articles linked from Wikipedia's Astronomy Portal, and
* Astronomy 2nd Edition from openstax.
One of the key ideas to emerge from astronomy is the Uniformity of the Universe. As far as we can tell, the natural laws and processes that operate here and now have always operated in the past, are the same everywhere in the Universe, and we expect them to continue to be so in the future. Having learned what these general principles are, we can then apply them to specific examples as needed.

### 9.1 - The Universe

The Universe is the totality of existence. The origin and history of the Universe as a whole is of great interest to many people for its own sake, but only selected features are relevant for space systems work. This includes that Baryonic matter (the ordinary kind of matter that we and the Earth are made of) started out as about 76% hydrogen, 24% helium, and almost nothing else. Gravity caused the nearly featureless early Universe to develop denser areas, with emptier regions in between. The denser parts coalesced into many Galaxies. The Milky Way galaxy is the one the Sun and planets are part of.
Within galaxies, gas clouds condense under gravity to form Stars that are powered by nuclear reactions. The reactions convert lighter elements into heavier ones, increasing the proportion by mass of helium to about 27% and heavier elements to about 2%. Stellar nuclear reactions release a great deal of energy, but this source is finite. So the Sun and other stars will eventually run out of fuel, and the composition of the Universe will reach a stable condition.
Several lines of evidence indicate the current age of the Universe is about 13.8 billion years, and the era of stars will last about 100 trillion years. If the expansion of the Universe continues to accelerate, then most of the Universe will be rendered undetectable long before all the nearby stars die out. Regardless of the eventual destiny of the Universe, relative to how long modern humans have existed it will last a very long time in its current form.

### 9.2 - The Milky Way Galaxy

The oldest stars in the galaxy are about 13 billion years old, nearly as old as the Universe. The normal (baryonic) matter is embedded in about five times more of a type that only reveals itself by its gravity. This material does not appear to produce or interact with light, so it has been given the name Dark Matter. It is poorly understood at present. The main concern for space systems is how it affects the motions of objects such as stars, planets, and molecular clouds.
The composition and internal motions of the Milky Way indicate it formed from gas clouds and smaller galaxies falling in under gravity, which continues to the present. Its shape seems to have evolved starting with the halo and central bulge, followed by growth of the surrounding disk-shaped region. The total mass of the Milky Way is estimated to be 1.15 trillion times the Sun's, and contains roughly 100-400 billion stars. That number is uncertain because gas clouds block a full view, and only about 1% of the stars have been observed individually.
The Sun is in the disk region, about 27,000 light years from the center of the Milky Way, and near the central plane of the disk. It orbits the center at about 220 km/s, taking about 240 million years to complete an orbit. We do not know the exact shape of the Sun's orbit but it is suspected to be elliptical.
The random motions of stars near the Sun are on the order of 50 km/s. Over the Sun's life that speed amounts to 450,000 light years (ly) - much more than the 170,000 ly length of the Sun's galactic orbit. This indicates the current nearby stars are not the ones the Sun was born near. In fact, the stars within 100 light years will be replaced by a mostly different set within a million years.

### 9.3 - Life of the Sun

Astronomers see new stars forming in denser regions of the galaxy known as Molecular Clouds. The Sun and the rest of the Solar System are assumed to have formed in such a cloud. Cloud lifetime is estimated to be 10-50 million years, so that cloud is long since gone. Based on radioactive dating the Sun and Solar System are estimated to be 4.6 billion years old, about 1/3 the estimated age of the Universe. The presence of 1.5% heavier elements in the Sun shows it formed from recycled matter that had previously been enriched by older generation stars. Very old stars lack these elements heavier than helium.
Gravity and internal motion caused part of the original molecular cloud form a distinct object called the Solar Nebula. The core of the nebula continued to contract, and the increased pressure caused by self-gravity heated that core to create a Proto-Sun. Once the core of the proto-Sun reached a temperature of 12 million degrees K, hydrogen fusion could begin, and the Sun proper was born. This collapse until ignition took around 50 million years.
Nuclear reactions in the core of the Sun have converted hydrogen to helium, increasing the concentration there to about 60%. Since Helium is heavier, the core has gotten denser and hotter. This has increased the reaction rate of the remaining hydrogen, and the total energy output of the Sun, by about 40%. The current output is 3.846 x 10$^{26}$ Watts. This will continue to increase by about 1% per hundred million years. In about 5 billion years the Sun will run out of hydrogen in the core and expand to a red giant, vaporizing Mercury, Venus, and possibly Earth.

## 10.0 - Planetary Science

Planetary Science is the study of bodies in the Universe smaller than stars such as planets, moons, asteroids, and comets. Prior to the 1980's this was limited to the Solar System. The field of study is now much wider since the discovery of many Circumstellar Discs and Exoplanets around other stars. Since 1998 a few Rogue Planets and two smaller Interstellar Objects have also been found which are not attached to any stars.
The Solar System is known in much greater detail than what is beyond it. The distances from Earth are much smaller, and spacecraft have been sent to many bodies of interest. Fallen objects like Meteorites are natural samples from the Solar System that we can examine. A few spacecraft have brought back "fresh" samples, which provides added information. These have not been altered by passing through the atmosphere or being exposed to weather until found.

### 10.1 - Solar System Formation

The Solar System has Evolved since it formed 4.6 billion years ago, and left to itself will continue to do so for billions of years more.
Angular Momentum is the amount of rotation an object has. Unless it interacts with something else, the amount stays the same. Once the Solar Nebula separated as a distinct object from the cloud it formed in, very little outside interaction has happened. Whatever internal motions it started with became organized into a single rotation. The proto-Sun forming in the center ended up with most of the mass, but the rotation kept a small amount from falling in. It could, however, fall vertically along the rotation axis. The residual mass became a Protoplanetary Disk around 200 AU in diameter, where 1 AU is the Earth's distance from the Sun.
The contraction of the proto-Sun raised its temperature, but distance and optical thickness produced disk temperatures that decreased with distance from the center. The flat shape allowed heat to escape on both sides of the disk so it could cool. Different materials then condensed and froze according to distance. This was only metals and rock close-in, and those plus ices like water, methane, and ammonia farther out. No part was cold enough for hydrogen and helium to condense directly, so they remained as gas. Solid particles could clump at first by sticking to each other, then later by gravity. The mix of objects which formed this way are called Planetesimals. We have fairly good evidence for this by looking at the disks around young stars.
Gravity is a runaway process. As an object gets larger it can attract material from a larger distance, increasing its growth rate. Larger objects have a potential energy well, so smaller ones will accelerate to impact. The impact energy eventually becomes large enough to melt the object. In addition, there were more radioactive elements in the early Solar System than there are now, and decay of those elements added to collision heating.
Larger objects were able to affect the orbits of smaller ones, even if not directly pulling them in. This caused the smaller ones to either impact something or get scattered out of the way. This tended to clear out an orbital region around each large object. The very largest objects had a deep enough gravity wells that they could collect and hold hydrogen and helium, forming the Gas Giants.
Distances are very large beyond Neptune. So some of the objects which formed or were scattered to this region have survived relatively unchanged. The light molecules which could not condense in the inner hot region tended to get pushed out by solar winds and drag. Where they could condense is near Jupiter's orbit, which may account for the large mass of Jupiter and Saturn compared to the rest of the planets. The remaining inner material was mostly metals and rock, which form the four inner planets.
The early formation of the planets took about 100,000 years. There were originally many more smaller protoplanets. The impacts and scattering have since reduced the number, including some getting ejected entirely from the Solar System. The gravity effects were particularly strong inside of Jupiter's orbit, leaving the relatively small amount of material known as the Asteroid Belt. Orbit changes continue to the present day, but collisions are less frequent since the supply of smaller bodies has decreased. The many craters found throughout the Solar System provide a record of past collisions. The objects sent farther away but not entirely lost now make up the various classes of Distant Minor Planets

### 10.2 - Planetary Evolution

Many planets and and asteroids have smaller Natural Satellites or "moons" orbiting them. In some cases they are not so small compared to the larger body and are considered a Double planet or asteroid. There are three ways objects can end up orbiting a larger one:
* The giant planets formed their own disks like the Sun did, but smaller. Moons condensed from these disks.
* The debris from a large impact can collect into one or more moons by gravity. The Earth's Moon is thought to have formed this way.
* Finally, an originally separate object may be captured into orbit around a larger one. This is hard since an object will typically leave with the same speed it approached with. Capture often involves a third object to change the speed.
Once planets, larger moons, and large asteroids form, they can evolve from their original condition to what we see today. If they are large enough, their internal gravity overcomes the strength of whatever they are made of. They become a sphere or Ellipsoid depending on how fast they are rotating. If there is enough heating from collisions, tidal forces, and radioactive decay, they can melt and separate into layers by density and chemical affinity. This is called Differentiation. Low melting-point materials and Volatiles, can remain liquid or form an atmosphere. Some or all of the atmosphere can be lost over time by escape or stripping.
Iron and compatible metals are the heaviest common materials, so they ended up in central cores. Going outwards, the layers include rocky minerals of different densities (a mantle and crust), then liquid and ice layers, and finally an atmosphere. Not every body has all these layers, it depends on what was available at formation and its later history.
The general composition varies with distance from the Sun. There is relatively more of the heavy metals and high temperature minerals in the inner regions, and more of the lighter ices and volatiles in the outer regions. These trends are not strict rules. Random collisions and orbits changed by gravity have affected the make up and location of objects. Smaller bodies tend to lose their original atmosphere, and if it is warm enough their ices, to space.
After the original formation era of about 100 million years, the larger planets continued to interact with each other chaotically until they settled into a relatively stable arrangement about 3.8 billion years ago, or 0.8 billion years after the Solar Nebula formed. Bodies like the Earth have active processes like erosion and vulcanism that tend to erase craters. Smaller ones lack an atmosphere or crustal motion, and preserve them through the life of the Solar System. The resulting current distribution of matter, and the very large energy output of the Sun, are now the main resources available for space projects.

### 10.2 - The Earth-Moon System

Earth Science is the study of the Earth and its component parts. Since the Earth has a complicated structure and history, this broad field is divided into more specific Earth Sciences. The Moon affects the Earth - for example by creating tides. Such effects are generally studied as part of earth science, while the study of the Moon as a separate body is considered part of planetary science.
The study of the Earth predates detailed study of other planets, and it continues to be the best studied planet. In the context of modern astronomy and planetary science, the Earth is now studied as one planet among many. In the context of human history it still has a special place because we evolved here, and until now none of us have left the Earth-Moon system.
So far, nearly all of the design, materials, equipment, and operations for space projects have happened on or came from Earth. For example, only a few percent of a rocket as it sits on the launch pad ends up in orbit. This will continue to be true for at least the near future. So some understanding of the Earth is still needed to carry out space projects, at least for the purpose of building factories and launch sites to leave it. More background information on earth science can be found in a number of Wikipedia Articles and introductory textbooks such as:
* CK12 Foundation - Earth Science Concepts, and the
* Open Textbook Library Earth Sciences collection
The Earth formed in the same way as the rest of the large bodies in the Solar System, mostly by collisions. Debris from a very large collision late in the process is thought to have formed the Moon, explaining the similarities in their compositions. Impacts and radioactive decay released enough energy to melt the entire planet. The high early temperature likely led to loss of some of the more volatile ices and gases. Continued radioactive decay, supplemented by tidal heating, has kept the Earth's interior hot since then.
The Internal Structure of the Earth includes a metallic Inner Core solidified by pressure, even though it is about the same temperature (5700K) as the Sun's surface (5778 K). Outside of this is a liquid metallic Outer Core then a rocky layer called the Mantle. The mantle makes up 84% of Earth's volume and 67% of its mass. Its temperature varies from 4200 K at the core boundary to around 500 K near the surface. Despite the high temperatures, internal pressure keeps most of it solid. However, the rock is able to flow slowly over time in a type of thermal circulation taking on the order of 100 million years.
The least dense and coolest rocks form a rigid upper layer called the Lithosphere, which accounts for about 3% of the planet's mass. It varies from as little 4 km thick at some oceanic ridges to as much as 280 km under the oldest parts of continents. The thickness depends on what depth the temperature gets high enough that rock deforms rather than cracks. The change with depth or Temperature Gradient is higher relative to the rest of the planet. By composition the lower part of the Lithosphere is part of the Mantle, and the upper part is less dense rock called the Crust. Both are primarily made of metal oxide minerals such as Silicates, which contain silicon and oxygen but often additional elements.
The lithosphere, oceans, and atmosphere are the only parts of the Earth accessible enough to be used, even with advanced technology. Projects that need elements (or their compounds) other than the eight most common ones will usually require mining the lithosphere, then separating the rarer elements. The oceans and atmosphere are also sources of particular elements and compounds. The elemental composition of the lithosphere varies by depth and location but on average is
* Oxygen: 46.60%
* Silicon: 27.72%
* Aluminium: 8.13%
* Iron: 5.00%
* Calcium: 3.63%
* Sodium: 2.83%
* Potassium: 2.59%
* Magnesium: 2.09%
* Other elements: 1.41%
The internal motions of the Mantle and heat traveling outwards can cause local temperature to get higher than the melting point dictated by local pressure. The molten rock is called Magma. Its composition can vary because different minerals have different pressure/temperature melting curves. Movement of magma and bulk Mantle circulation slowly cause pieces of the crust, called Plates to move across the Earth's solid surface, grow, collide, split, and dive into the mantle.
There are about 7 major plates and a number of smaller ones. Their movements, melting, crystallizing, Weathering, and other geologic processes explain the geography and geology we find today. These dynamic processes combine to erode most of the early Earth's history. The current solid surface averages ten percent or less of the age of the planet as a whole.
Water being less dense than rock, it lays on top of the crust. The Ocean and smaller bodies of water fill the lower-lying areas. The ocean is the final destination of most rivers, and is in contact with the ground below it. So Seawater has accumulated about 3.5% dissolved material. This is mostly common salt (NaCl), with small amounts of other elements. Besides bodies of open water like rivers and lakes, about 2.8% of the Earth's water is frozen in glaciers, in the ground and soil, as water vapor in the atmosphere, or in living things. Aside from living things, the non-ocean water has had less time to accumulate contaminants and is usually purer.
The uppermost layer of the Earth is the Atmosphere, which at its lowest point is 800-2000 times less dense than the water or land below it. It has the materials too volatile to remain solid or liquid. Dry air has 78% nitrogen, 21% oxygen, 1% argon, and small amounts of other gases. It also holds 0.01-4.24% water vapor that comes from the water and ground below. This sometimes condenses or freezes and then falls. Gravity and its own weight causes the pressure and density to vary with height. When stationary, at each point the pressure equals the weight of all the gas above it. The Sun's heat and the Earth's rotation cause air to move, so the pressure will vary somewhat at a given location.
For space systems, the atmosphere is mainly an obstacle to be gotten through to reach space, and a source of heating and drag when returning. There is no "top" to the atmosphere, the density keeps falling with height until it reaches the level of interplanetary space. Even at 1000 km altitude, below which many satellites operate, it has measurable effects.

### 10.3 - The Moon

Early in the Earth's history, a Mars-sized body called Theia is presumed to have hit the proto-Earth. Most of it stayed with the newly-formed Earth, but some of the debris from that impact collected by gravity to form the Moon. It formed much closer to the Earth than it is now, but tides act to slow the Earth's rotation and increase the Moon's distance, a process that continues.
The Moon is smaller than the Earth and loses its internal heat faster, becoming mostly solid with a billion years. It also separated into layers by density, with a mostly iron core at around 1600-1700K. Around this are rocky mantle and crust layers, with the crust averaging 50 km thick. It is too small to retain an atmosphere because of its low escape velocity. Because of these the Moon retains evidence of its early history in the form of large impact basins and craters of all sizes.
The surface Geology of the Moon is similar to Earth, consisting mostly of silicate minerals. It has been heavily modified by impacts of all sizes, producing a fine-grained broken surface layer of Regolith, and craters ranging from microscopic to giant impact basins.
Some of the larger basins filled with magma to create relatively flat and darker areas mistakenly named Mare (Latin for "Sea"). Before telescopes, they were thought to have water. The greater tides from the time the Moon was closer to Earth slowed the Moon's rotation so the same side now always faces Earth, with a little wobble. It is not yet clear why most of the mare are on the near side.

## 11.0 - Chemistry

The science of Chemistry has historically been considered a separate subject from physics. In a more general sense it can be considered a subset of low energy physics where arrangements of atoms via atomic bonding is important. We humans happen to require living conditions where atomic bonding is important, so we give that energy regime more attention. In reality, something like 99% of the matter in the Universe is in the plasma state, where electrons are no longer bound to atoms, and inter-atom bonding is rare.
Until now, the most important chemical reactions for space projects were the ones that produced high temperature gases to propel rockets. In the future, chemical reactions for life support systems, and extraction and preparation of raw materials in space, will become much more important. At least a basic understanding of chemical principles is useful for space systems design. More background on chemistry can be found from:
* Wikipedia's Outline of Chemistry,
* CK-12 Foundation's Chemistry textbooks at secondary school level,
* Wikibook's Introductory Chemistry and General Chemistry, and the
* Open Textbook Library's Chemistry Textbook Collection

## 12.0 - Life Sciences

Life science in general is the study of living things. Life is a complex phenomenon, so the Life Sciences are divided into multiple branches that study living things for the sake of knowledge, and other branches that apply that knowledge to useful ends like agriculture and health. The life sciences matter for space projects that include living things, especially people. It also includes looking for life beyond the Earth, and eliminating it from probes and equipment to avoid contamination in either direction.
Background information on the life sciences can be found from:
* the many articles linked from Wikipedia's Outline of Biology,
* a large number of open textbooks on Biology, and
* texts in applied fields such as Health Sciences and Agriculture.
For space systems, the applied fields have specialized areas like Space Medicine and Space Farming.