Section 1.1 - Basic Sciences: Astronomy, Planetary Science, Chemistry (page 3)
Astronomy is the study of all objects and phenomena beyond the Earth's atmosphere, and Planetary Science is specifically the study of condensed objects orbiting stars. Since this is where space systems function, a basic understanding of these fields is highly relevant to working on such projects. We provide only a very short introduction here. For more background than will fit in this book, see Nick Strobel's Astronomy Notes and the Wikibook General_Astronomy. One of the key ideas to emerge from these studies 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.
Origin and Evolution of 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 design. 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 uniform early Universe to develop denser regions with emptier regions in between. The denser regions coalesced into many Galaxies, of which the Milky Way galaxy is the one which our Sun and planets are part of.
Galaxies in turn form denser condensations where nuclear reactions occur, which we call stars. 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 13.6 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, on human time scales it will last a long while relatively unchanged.
The Milky Way Galaxy
The oldest stars in our home galaxy are about 13 billion years old, which indicates the galaxy started forming, at least to the point that stars condensed, shortly after the Universe as a whole formed. The baryonic mass is embedded in a five times more massive amount of material which only reveals itself by its gravity. Since this material does not form stars, it is dark, and so we call it Dark Matter. It is poorly understood at present, and our main concern for space projects is how it affects the motions of baryonic objects such as stars, planets, and molecular clouds. Evidence from the composition and motions of parts of the Milky Way indicate it formed by infall of gas clouds and smaller galaxies, which continues to the present. The shape seems to have evolved starting with the halo and central bulge, followed by growth of the disk shaped region. The baryonic mass of our galaxy is estimated at 200-300 billion times the Sun's.
The Sun is in the disk region, about 27,000 light years from the center of the Milky Way, orbiting at about 220 km/s and thus taking about 225-250 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. Random motions of nearby stars are on the order of 50 km/s. Over the age of the Sun these random motions amount to 450,000 light years, which is much more than the circumference of the Sun's orbit. This indicates the current nearby stars are not the ones the Sun was born near. In fact, the current stars within 100 light years will be replaced by an entirely different set in 1 million years.
Formation of the Sun
We observe new stars forming in denser regions of our galaxy known as Molecular clouds, and we assume our Sun and the rest of the Solar System formed in such a cloud, which has since dispersed. Based on radioactive dating we estimate our Solar System to be 4.6 billion years old, which is about 1/3 the estimated age of the Universe. The presence of 1.5% heavier elements in the Sun confirms that it formed from recycled matter that had previously been enriched by older generation stars. Loss of heat from radiation allowed gravity to collect part of the original molecular cloud into 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 kelvins, hydrogen fusion could begin, and the Sun proper was born. This collapse until ignition took around 30 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, thus increasing 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 1026 Watts. This will continue to increase by about 1% per hundred million years.
Solar System Evolution
Whatever internal motions the Solar Nebula had, there was no way to dispose of angular momentum (net rotation). Therefore a small part of the nebula remained orbiting the proto-Sun rather than falling in, and keeping most of the angular momentum. This region is estimated to have been 50 AU in radius, and disk shaped as the net result of rotation. The increasing temperature of the proto-Sun created a temperature gradient based on distance from the center. The innermost part was too warm for icy material to remain solid, while the outer parts were cold enough for water, ammonia, and other ices. No part was cold enough for Hydrogen and Helium to condense. Particles condensed out of the nebula as the flat shape radiated heat to space and the optical thickness radially kept the outer parts from being heated by the Sun. Small particles could grow first by sticking to each other, then later by gravitational attraction. The mix of objects which formed this way are called Planetesimals. We now have fairly good evidence of this process by observing disks around young stars.
Gravitational attraction is a runaway process. As an object gets larger, it can attract objects from a larger distance, thus increasing its growth rate. Larger objects also have a potential energy well, so approaching objects 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 contributed to the heating of the growing bodies. The largest objects were able to affect the orbits of the smaller ones, causing them to either impact or get scattered away. This tended to clear out a region around each large object. The very largest objects had a sufficiently deep gravity well that they could collect gaseous Hydrogen and Helium, forming the gas giants. Some of the scattered objects, and planetesimals which formed at the outer edges of the nebula, have survived relatively unchanged beyond Neptune. The material from the inner Solar System which could not condense there tended to be blown outwards, and the point where they could solidify is near Jupiter's orbit, which may account for the large mass of Jupiter and Saturn compared to the rest of the planets. The entire accretion and clearing process took about 100 million years, with the final growth of the planets perhaps taking 10 million years.
The bodies which were large enough to melt had their denser compounds sink to the center under their own gravity. Iron and related metals are the heaviest common materials, so they ended up in the cores. Going outwards, the layers include rocky minerals of different densities (a mantle and crust), then ices and an atmosphere if the body formed with these components. This layered structure is what we find today in the planets and larger satellites, along with the composition change with distance from the Sun. These trends are not strict rules because random collisions and gravitational scattering have changed the location and make up of the objects since they formed, and smaller bodies have lost original atmosphere and in some cases ices. 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. The planetary shifts affected the smaller bodies, who continued to be scattered or impact. The evidence of this is still visible in craters and the locations of scattered objects.
Bodies in the Solar System continue to interact gravitationally, and impacts and scattering continue, but at a lower rate. Bodies like the Earth have active processes 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 the main resources to work with for space projects.
The Earth-Moon System
Earth Science is the study of the Earth and its component parts. The study of the Earth predates detailed study of other planets because this is where humans started, 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 has left the Earth-Moon system. Nearly all of the design, materials, equipment, and operations for space projects to date has actually occurred on Earth. This will continue to be true for at least the near future. Therefore some understanding of the Earth is still needed to carry out space projects. For a longer introduction to the field see:
- CK12 Foundation - Earth Science Concepts
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 formed the Moon, explaining the difference in its composition relative to the Earth. Impacts and radioactive decay released enough energy to melt the entire planet, and the high temperature likely led to loss of some of the more volatile ices and gases. Continued radioactive decay, supplemented by other energy releases, has kept the Earth's interior hot. It has an inner metallic Core solidified by pressure, even though it is about the same temperature as the Sun's surface (6000 K). Outside of this is a liquid metallic outer core, and then a rocky layer called the Mantle. The deeper parts of the Mantle have temperatures of 2000K or more. Although pressure keeps them solid at these high temperatures, 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 roughly 120 km thick solid layer called the Lithosphere, which has a relatively high temperature gradient relative to the rest of the planet. By composition the lower part of the Lithosphere is part of the Mantle, and the outer part is less dense rock called the Crust.
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, and its composition can vary because different minerals have different melting curves. Movement of magma and bulk Mantle circulation drive about 20 pieces of the Crust, called Plates to slowly move across the Earth's surface and change shape. These movements and Weathering, the mechanical and chemical changes from the surface environment, explain the geography and geology we find today. These dynamic processes combine to erode most of the early Earth's history. The current surface averages ten percent or less of the age of the planet as a whole.
Early in the Earth's history, a Mars-sized body called Theia is presumed to have hit the proto-Earth. 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 orbit, a process that continues. The Moon is smaller than the Earth, and so lost its internal heat faster, and is now mostly solid. It is too small to retain an atmosphere because of the low escape velocity. So the Moon retains evidence of its early history in the form of large impact basins and craters of all sizes. The larger basins filled with magma to create relatively flat and darker areas mistakenly named Mare (Latin for "Sea") because, 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 us, with a little wobble. It also caused most of the Mare to form on the near side.
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.
The importance of chemical reactions to space projects until now was primarily in providing high temperature gases to propel rockets. In future projects chemical reactions for life support systems, and extraction and preparation of raw materials in space, will become much more important. So at least a basic understanding of chemical principles is useful for space systems design. We refer the reader to the following sources for more detail:
- CK-12 Foundation Chemistry pre-university level textbook mentioned at the start of Part 1
- Wikibooks Introductory Chemistry
- Chem1 Virtual Textbook by Stephen Lower