CHEMISTRY PROJECT WORK

SPACE CHEMISTRYTOPIC

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1. Stars and their properties Temperature and spectrum Mass and movement Brightness, luminosity and radius The life of a star

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CONTENTS

The death of a star2. Chemical composition of space

Hydrogen Helium Interstellar dust Interstellar dust and its importance

3. How space suits work What a space suit does Project Apollo space suit Modern space suit: EMU

4. Chemistry of rocket propellants5. Can we harness energy from outer space?

Space based solar power6. References

Stars and Their PropertiesDefinitionsabsolute magnitude - apparent magnitude of the star if it was located 10 parsecs from Earthapparent magnitude - a star's brightness as observed from Earthluminosity - total amount of energy emitted from a star per secondparsec - distance measurement (3.3 light-years, 19.8 trillion miles, 33 trillion kilometers)light year - distance measurement (6 trillion miles, 10 trillion kilometers)spectrum - light of various wavelengths emitted by a starsolar mass - mass of the sun; 1.99 x 1030 kg (330,000 Earth masses)solar radius - radius of the sun; 418,000 miles (696,000 km)

Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are relatively close (the closest 30 stars are within 40 parsecs) and others are far, far away. Astronomers can measure the distance by using a method called parallax, in which the change in a star's position in the sky is measured at different times during the year. Some stars are alone in the sky, others have companions (binary stars) and some are part of

large clusters containing thousands to millions of stars. Not all stars are the same. Stars come in all sizes, brightnesses, temperatures and colors. Let's take a closer look at the features of stars.

Stars have many features that can be measured by studying the light that they emit:

temperature spectrum or wavelengths of light emitted brightness luminosity size (radius) mass movement (toward or away from us, rate of spin)

Temperature and Spectrum Some stars are extremely hot, while others are cool. You can tell by the color of light that the stars give off. If

you look at the coals in a charcoal grill, you know that the red glowing coals are cooler than the white hot ones. The same is true for stars. A blue or white star is hotter than a yellow star, which is hotter than a red star. So, if you look at the strongest color or wavelength of light emitted by the star, then you can calculate its temperature(temperature in degrees Kelvin = 3 x 106/ wavelength in nanometers). A star's spectrum can also tell you the chemical elements that are in that star because different elements (for example, hydrogen, helium, carbon, calcium) absorb light at different wavelengths.

Mass and MovementIn 1924, the astronomer A. S. Eddington showed that the luminosity and mass of a star were related. The larger a star (i.e., more massive) is, the more luminous it is (luminosity = mass3).

Stars around us are moving with respect to our solar system. Some are moving away from us and some are moving toward us. The movement of stars affects the wavelengths of light that we receive from them, much like the high pitched sound from a fire truck siren gets lower as the truck moves past you. This phenomenon is called the Doppler effect. By measuring the star's spectrum and comparing it to the spectrum of a standard lamp, then the amount of the Doppler shift can be measured. The amount of the Doppler shift tells us how fast the star is moving relative to us. In addition, the direction of the Doppler shift can tell us the direction of the star's movement. If the spectrum of a star is shifted to the blue end, then the star is moving toward us; if the spectrum is shifted to the red end, then the star is moving away from us. Likewise if a star is spinning on its axis, the Doppler shift of its spectrum can be used to measure its rate of rotation.

Brightness, Luminosity and RadiusTwo factors determine the brightness of a star:

luminosity - how much energy it puts out in a given time distance - how far it is from us

A searchlight puts out more light than a penlight. That is, the searchlight is more luminous. If that searchlight is 5 miles away from you, however, it will not be as bright because light intensity decreases with distance squared. A searchlight 5 miles from you may look as bright as a penlight 6 inches away from you. The same is true for stars.

Astronomers (professional or amateur) can measure a star's brightness (the amount of light it puts out) by using a photometer or charge-coupled device (CCD) on the end of a telescope. If they know the star's brightness and the distance to the star, they can calculate the star's luminosity [luminosity = brightness x 12.57 x (distance)2].

Stefan-Boltzmann LawThis is the relationship between luminosity (L), radius (R) and temperature (T):

L = (7.125 x 10-7) R2 T4

Units: L - watts, R - meters, T - degrees Kelvin

Luminosity is also related to a star's size. The larger a star is, the more energy it puts out and the more luminous it is. You can see this on the charcoal grill, too. Three glowing red charcoal briquettes put out more energy than one glowing red charcoal briquette at the same temperature. Likewise, if two stars are the same temperature but different sizes, then the large star will be more luminous than the small one. See the sidebar for a formula to that shows how a star's luminosity is related to its size (radius) and its temperature

The Life of a StarAs we mentioned before, stars are large balls of gases. New stars form from large, cold (10 degrees Kelvin) clouds of dust and gas (mostly hydrogen) that lie between existing stars in a galaxy.

1. Usually, some type of gravity disturbance happens to the cloud such as the passage of a nearby star or the shock wave from an exploding supernova.

2. The disturbance causes clumps to form inside the cloud.3. The clumps collapse inward drawing gas inward by gravity.4. The collapsing clump compresses and heats up.5. The collapsing clump begins to rotate and flatten out into a disc.6. The disc continues to rotate faster, draw more gas and dust inward, and heat up.7. After about a million years or so, a small, hot (1500 degrees Kelvin), dense core forms in the disc's center called

a protostar.8. As gas and dust continue to fall inward in the disc, they give up energy to the protostar, which heats up more9. When the temperature of the protostar reaches about 7 million degrees Kelvin, hydrogen begins to fuse to make

helium and release energy.10. Material continues to fall into the young star for millions of years because the collapse due to gravity is greater than

the outward pressure exerted by nuclear fusion. Therefore, the protostar's internal temperature increases.11. If sufficient mass (0.1 solar mass or greater) collapses into the protostar and the temperature gets hot enough for

sustained fusion, then the protostar has a massive release of gas in the form of a jet called a bipolar flow. If the mass is not sufficient, the star will not form, but instead become a brown dwarf.

12. The bipolar flow clears away gas and dust from the young star. Some of this gas and dust may later collect to form planets.

The young star is now stable in that the outward pressure from hydrogen fusion balances the inward pull of gravity. The star enters the main sequence; where it lies on the main sequence depends upon its mass.

Now that the star is stable, it has the same parts as our Sun:

core - where the nuclear fusion reactions occur radiative zone - where photons carry energy away from the core convective zone - where convection currents carry energy toward the surface

However, the interior may vary with respect to the location of the layers. Stars like the Sun and those less massive than the Sun have the layers in the order described above. Stars that are several times more massive than the Sun have convective layers deep in their cores and radiative outer layers. In contrast, stars that are intermediate between the Sun and the most massive stars may only have a radiative layer.

Life on the Main SequenceStars on the main sequence burn by fusing hydrogen into helium. Large stars tend to have higher core temperatures than smaller stars. Therefore, large stars burn the hydrogen fuel in the core quickly, whereas, small stars burn it more slowly. The length of time that they spend on the main sequence depends upon how quickly the hydrogen gets used up. Therefore, massive stars have shorter lifetimes (the Sun will burn for approximately 10 billion years). What happens once the hydrogen in the core is gone depends upon the mass of the star.

LIFE CYCLE OF STARS

The Death of a StarSeveral billion years after its life starts, a star will die. How the star dies, however, depends on what type of star it is.

Stars Like the Sun When the core runs out of hydrogen fuel, it will contract under the weight of gravity. However, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up. This heats the upper layers, causing them to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant. The radius of the red giant sun will be just beyond the Earth's orbit. At some point after this, the core will become hot enough to cause the helium to fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.

Stars More Massive Than the Sun When the core runs out of hydrogen, these stars fuse helium into carbon just like the Sun. However, after the helium is gone, their mass is enough to fuse carbon into heavier elements such as oxygen, neon, silicon, magnesium, sulfur and iron. Once the core has turned to iron, it can burn no longer. The star collapses by its own gravity and the iron core heats up. The core becomes so tightly packed that protons and electrons merge to form neutrons. In less than a second, the iron core, which is about the size of the Earth, shrinks to a neutron core with a radius of about 6 miles (10

kilometers). The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes (supernova), thereby releasing large amounts of energy and material into space. The shock wave from the supernova can initiate star formation in other interstellar clouds. The remains of the core can form a neutron star or a black hole depending upon the mass of the original star.

Chemical Composition of SpaceOuter space consists of a huge vacuum. Through gravitational pull, planets and other astronomical bodies, like the Earth, suck out most of the gas and particles in outer space. The most dominant gases still found in the vacuum of outer space are hydrogen and helium. There are other elements present, but they tend to be labeled together into one term known as "interstellar dust."

The Interstellar Medium

Outer space -- also known as the "interstellar medium" -- contains hydrogen, helium and interstellar dust in very small amounts. According to the University of New Hampshire Experimental Plasma Group, the average density of these particles in the interstellar medium is only "about one atom per cubic centimeter" as opposed to the air on Earth, which has "a density of approximately 30,000,000,000,000,000,000 molecules per cubic centimeter."

Hydrogen

Hydrogen can be found anywhere in the universe, including the vast reaches of outer space. Hydrogen makes up about 75 percent of all atoms in outer space and holds one of the top spots nearby as it is the third most abundant on Earth. Hydrogen's chemical composition is also the simplest of all elements; most of the time, a hydrogen atom contains only one proton and one electron. This simplicity makes it tasteless, odorless and colorless.

Helium

Helium makes up 24 percent of the gas present in outer space, making it the second most abundant element in outer space. This inert gas cannot exist in compound form; its simplistic structure of helium makes it the second simplest element as well, consisting only of two neutrons, two electrons and two protons. It is also tasteless, odorless and colorless. Helium has the lowest boiling point among all elements at minus 452 degrees Fahrenheit.

Interstellar Dust

Hydrogen and helium make up the majority of gas found in outer space, but there are other trace elements present in outer space known as interstellar dust. This "dust" composition consists of several other elements and makes up less than 1 percent of all elements found in outer space. Interstellar dust differs from the dust on Earth; it is composed of very small irregular particles containing carbon, ice, iron compounds and silicates.

Interstellar Dust and Its Importance

Most of this interstellar gas and dust originates from the death of stars which either exploded (supernova) or blew off their outer layers, returning their material to interstellar space. From this material, new stars are formed. Often, the gas and dust between the stars can be detected only in the infrared. Dust grains absorb visible and ultraviolet light which causes them to heat up and radiate in the infrared. Also, by using infrared detectors, astronomers can penetrate the often invisible interstellar gas and dust clouds and gain much information about their composition and structure. It is for the most part a type of small dust particles which are a few molecules to 0.1 µm in size. A smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left the stars. It is called "stardust".

Interstellar dust is an important constituent of the Galaxy. It obscures all but the relatively nearby regions in visual and ultraviolet wavelengths, and reradiates the absorbed energy in the far-infrared part of the spectrum, thereby providing a major part (~ 30%) of the total luminosity of the Galaxy. The FIR radiation from dust removes the gravitational energy of collapsing clouds, allowing star formation to occur. Dust is crucial for interstellar chemistry by reducing the ultraviolet (UV) radiation which causes molecular dissociations and providing the site of the formation of the most abundant interstellar molecule, H2. Probably grain surfaces are responsible for other chemistry as well. Dust controls the temperature of the interstellar medium (ISM) by accounting for most of the elements which provide cooling, but also providing heating through electrons ejected photoelectrically from grains.

The past decade has seen an increase in interest in interstellar dust because of the discovery of spectroscopic features in both emission and absorption, along with laboratory studies of candidate materials. There have been good observations of the extinction law of dust in many

directions. Probably the most important feature to emerge from these studies is that ``interstellar dust'' refers to a variety of materials of widely varying properties.

Many studies of interstellar dust have involved lines of sight through the diffuse, low-density ISM, including some clouds of densities of up to several hundred H atoms per cubic centimeter. This material is reffered to herein as diffuse dust. In the literature, most references to ``interstellar dust'' apply to diffuse dust. Dust in the outer parts of molecular clouds which can be studied by optical and UV observations is called outer-cloud dust. Finally, there have been many studies of sources embedded so deeply within molecular clouds that only the near-infrared or perhaps optical part of the spectrum can be studied. This type of dust is referred to as inner-cloud dust. There is, of course, a continuous gradation of properties from diffuse dust to inner-cloud dust, but these three designations will allow us to emphasize the rather different properties of interstellar dust in the various regions. This review is confined to diffuse dust and outer-cloud dust; for excellent reviews of inner-cloud dust.

Cosmic dust was once solely an annoyance to astronomers, as it obscures objects they wish to observe. When infrared astronomy began, those previously annoying dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of our Solar System.For example, cosmic dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In our Solar System, dust plays a major role in the zodiacal light, Saturn's B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune, and comets.

The study of dust is a many-faceted research topic that brings together different scientific fields: physics (solid-state, electromagnetic theory, surface physics, statistical physics,thermal physics), fractal mathematics, chemistry (chemical reactions on grain surfaces), meteoritics, as well as every branch of astronomy and astrophysics. These disparate research areas can be linked by the following theme: the cosmic dust particles evolve cyclically; chemically, physically and dynamically. The evolution of dust traces out paths in which the universe recycles material, in processes analogous to the daily recycling steps with which many people are familiar: production, storage, processing, collection, consumption, and discarding. Observations and measurements of cosmic dust in different regions provide an important insight into the universe's recycling processes; in the clouds of the diffuse interstellar medium, in molecular clouds, in the circumstellar dust of young stellar objects, and in planetary systems such as our Solar System, where astronomers consider dust as in its most recycled state. The astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the universe's complicated recycling steps.

The detection of cosmic dust points to another facet of cosmic dust research: dust acting as photons. Once cosmic dust is detected, the scientific problem to be solved is an inverse problem to determine what processes brought that encoded photon-like object (dust) to the detector. Parameters such as the particle's initial motion, material properties, intervening plasma and magnetic field determined the dust particle's arrival at the dust detector. Slightly changing any of these parameters can give significantly different dust dynamical behavior. Therefore one can learn about where that object came from, and what is (in) the intervening medium.

MODEL OF AN INTERSTELAR DUST PARTICLE

How Space Suits Work

What a Space Suit Does

By creating an Earth-like environment within the suit itself, space suits allow humans to walk around in

space in relative safety. Space suits provide:

Pressurized Atmosphere

The space suit provides air pressure to keep the fluids in your body in a liquid state -- in other words, to

prevent your bodily fluids from boiling. Like a tire, a space suit is essentially an inflated balloon that is

restricted by some rubberized fabric, in this case, Neoprene-coated fibers. The restriction placed on the

"balloon" portion of the suit supplies air pressure on the astronaut inside, like blowing up a balloon inside

a cardboard tube.

Most space suits operate at pressures below normal atmospheric pressure (14.7 lb/in2, or 1 atm); the

space shuttle cabin also operates at normal atmospheric pressure. The space suit used by shuttle

astronauts operates at 4.3 lb/in2, or 0.29 atm. Therefore, the cabin pressure of either the shuttle itself or

an airlock must be reduced before an astronaut gets suited up for a spacewalk. A spacewalking astronaut

runs the risk of getting the bends because of the changes in pressure between the space suit and the

shuttle cabin.

Oxygen

Space suits cannot use normal air -- 78 percent nitrogen, 21 percent oxygen and 1 percent other gases --

because the low pressure would cause dangerously low oxygen concentrations in the lungs and blood,

much like climbing Mt. Everest does. So, most space suits provide a pure oxygen atmosphere for

breathing. Space suits get the oxygen either from a spacecraft via an umbilical cord or from a backpack

life support system that the astronaut wears.

Both the shuttle and the International Space Station have normal air mixtures that mimic our atmosphere.

Therefore, to go into a pure oxygen space suit, a spacewalking astronaut must "pre-breathe" pure oxygen

for some period of time before suiting up. This pre-breathing of pure oxygen eliminates the nitrogen from

the astronaut's blood and tissues, thereby minimizing the risk of the bends.

Carbon Dioxide

The astronaut breathes out carbon dioxide. In the confined space of the suit, carbon dioxide

concentrations would build up to deadly levels. Therefore, excess carbon dioxide must be removed from

the space suit's atmosphere. Space suits use lithium hydroxide canisters to remove carbon dioxide.

These canisters are located either in the space suit's life support backpack or in the spacecraft, in which

case they are accessed through an umbilical cord.

Temperature

To cope with the extremes of temperature, most space suits are heavily insulated with layers of fabric

(Neoprene, Gore-Tex, Dacron) and covered with reflective outer layers (Mylar or white fabric) to

reflectsunlight. The astronaut produces heat from his/her body, especially when doing strenuous

activities. If this heat is not removed, the sweat produced by the astronaut will fog up the helmet and

cause the astronaut to become severely dehydrated; astronaut Eugene Cernan lost several pounds

during his spacewalk on Gemini 9. To remove this excess heat, space suits have used either fans/heat

exchangers to blow cool air, as in the Mercury and Gemini programs, or water-cooled garments, which

have been used from the Apollo program to the present.

Micrometeroids

To protect the astronauts from collisions with micrometeroids, space suits have multiple layers of durable

fabrics such as Dacron or Kevlar. These layers also prevent the suit from tearing on exposed surfaces of

the spacecraft or a planet or moon.

Radiation

Space suits offer only limited protection from radiation. Some protection is offered by the reflective

coatings of Mylar that are built into the suits, but a space suit would not offer much protection from a solar

flare. So, spacewalks are planned during periods of low solar activity.

Clear Sight

Space suits have helmets that are made of clear plastic or durable polycarbonate. Most helmets have

coverings to reflect sunlight, and tinted visors to reduce glare, much like sunglasses. Also, prior to a

spacewalk, the inside faceplates of the helmet are sprayed with an anti-fog compound. Finally, modern

space suit helmet coverings have mounted lights so that the astronauts can see into the shadows.

Mobility Within the Space suit

Moving within an inflated space suit is tough. Imagine trying to move your fingers in a rubber glove blown

up with air; it doesn't give very much. To help this problem, space suits are equipped with special joints or

tapers in the fabric to help the astronauts bend their hands, arms, legs, knees and ankles.

Communications

Space suits are equipped with radio transmitters/receivers so that spacewalking astronauts can talk with

ground controllers and/or other astronauts. The astronauts wear headsets with microphones and

earphones. The transmitters/receivers are located in the chestpacks/backpacks worn by the astronauts.

Mobility in the Spacecraft

In weightlessness, it is difficult to move around. If you push on something, you fly off in the opposite

direction (Newton's third law of motion -- for every action there is an equal and opposite reaction). Gemini

spacewalking astronauts reported great problems with just maintaining their positions; when they tried to

turn a wrench, they spun in the opposite direction. Therefore, spacecraft are equipped with footholds and

hand restraints to help astronauts work in microgravity. In addition, before the mission, astronauts

practice spacewalking in big water tanks on Earth. The buoyancy of an inflated space suit in water

simulates microgravity.

NASA has also developed some gas-powered rocket maneuvering devices to allow astronauts to move

freely in space without being tethered to the spacecraft. One such device, which was called the Manned

Maneuvering Unit (MMU), was basically a gas-thruster powered chair with a joystick control. NASA has

also developed a nitrogen-gas propelled unit that fits on the backpack, called the Simplified Aid for

Extravehicular Activity Rescue (SAFER). The SAFER can help an astronaut return to the shuttle or

station in the event that he/she gets separated from the spacecraft. The SAFER holds 3.1 lb (1.4 kg) of

nitrogen propellant and can change an astronaut's velocity by a maximum of about 9 feet/second (3

meters/second).

Project Apollo Space Suit

Because Apollo astronauts had to walk on the moon as well as fly in space, a single space suit was

developed that had add-ons for moonwalking. The basic Apollo space suit, which was worn during liftoff,

was the backup suit needed in case cabin pressure failed.

The Apollo suit consisted of the following:

A water-cooled nylon undergarment

A multi-layered pressure suit: inside layer - lightweight nylon with fabric vents; middle layer - neoprene-

coated nylon to hold pressure; outer layer - nylon to restrain the pressurized layers beneath

Five layers of aluminized Mylar interwoven with four layers of Dacron for heat protection

Two layers of Kapton for additional heat protection

A layer of Teflon-coated cloth (nonflammable) for protection from scrapes

A layer of white Teflon cloth (nonflammable)

The suit had boots, gloves, a communications cap and a clear plastic helmet. During liftoff, the suit's

oxygen and cooling water were supplied by the ship.

For walking on the moon, the space suit was supplemented with a pair of protective overboots, gloves

with rubber fingertips, a set of filters/visors worn over the helmet for protection from sunlight, and a

portable life support backpack that contained oxygen, carbon-dioxide removal equipment and cooling

water. The space suit and backpack weighed 180 lb (82 kg) on Earth, but only 30 lb (14 kg) on the moon.

The basic Apollo space suit was also used for spacewalking during the Skylab missions.

During the early flights of the space shuttle, astronauts wore a brown flight suit. Like earlier missions, this

flight suit was meant to protect the astronauts if the cabin pressure failed. Its design was similar to the

earlier flight suits of Apollo.

As shuttle flights became more routine, the astronauts stopped wearing pressurized suits during liftoff.

Instead, they wore light-blue coveralls with black boots and a white, plastic, impact-resistant,

communications helmet. This practice was continued until the Challenger disaster.

After a review of the Challenger disaster, NASA started requiring all astronauts to wear pressurized suits

during liftoff and re-entry. These orange flight suits are pressurized and equipped with a communications

cap, helmet, boots, gloves, parachute, and inflatable life preserver. Again, these space suits are designed

only for emergency use -- in case the cabin pressure fails or the astronauts have to eject from the

spacecraft at high altitude during liftoff or re-entry.

Modern Space Suit: EMUWhile early space suits were made entirely of soft fabrics, today'sExtravehicular Mobility Unit(EMU)

has a combination of soft and hard components to provide support, mobility and comfort. The suit itself

has 13 layers of material, including an inner cooling garment (two layers), pressure garment (two layers),

thermal micrometeoroid garment (eight layers) and outer cover (one layer).

Project Apollo space suit

Rocket PropellantsPropellant is the chemical mixture burned to produce thrust in rockets and consists of a fuel and an oxidizer. A fuel is a substance that burns when combined with oxygen producing gas for propulsion. An oxidizer is an agent that releases oxygen for combination with a fuel. The ratio of oxidizer to fuel is called the mixture ratio. Propellants are classified according to their state - liquid, solid, or hybrid.

Liquid PropellantsIn a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are fed through a system of pipes, valves, and turbo pumps to a combustion chamber where they are combined and burned to produce thrust. Liquid propellant engines are more complex than their solid propellant counter parts, however, they offer several advantages. By controlling the flow of propellant to the combustion chamber, the engine can be throttled, stopped, or restarted.

Liquid propellants used in rocketry can be classified into three types: petroleum, cryogens, and hypergols.

Petroleum fuels are those refined from crude oil and are a mixture of complex hydrocarbons, i.e. organic compounds containing only carbon and hydrogen. The petroleum used as rocket fuel is a type of highly refined kerosene, called RP-1 in the United States. Petroleum fuels are usually used in combination with liquid oxygen as the oxidizer. Kerosene delivers a specific impulse considerably less than cryogenic fuels, but it is generally better than hypergolic propellants.

Cryogenic propellants are liquefied gases stored at very low temperatures, most frequently liquid hydrogen (LH2) as the fuel and liquid oxygen (LO2 or LOX) as the oxidizer. Hydrogen remains liquid at temperatures of -253 oC (-423 oF) and oxygen remains in a liquid state at temperatures of -183 oC (-297 oF).

Because of the low temperatures of cryogenic propellants, they are difficult to store over long periods of time. For this reason, they are less desirable for use in military rockets that must be kept launch ready for months at a time. Furthermore, liquid hydrogen has a very low density (0.071 g/ml) and, therefore, requires a storage volume many times greater than other fuels. Despite these drawbacks, the high efficiency of liquid oxygen/liquid hydrogen makes these problems worth coping with when reaction time and storability are not too critical. Liquid hydrogen delivers a specific impulse about 30%-40% higher than most other rocket fuels.

Liquid oxygen and liquid hydrogen are used as the propellant in the high efficiency main engines of the Space Shuttle. LOX/LH2 also powered the upper stages of the Saturn V and Saturn 1B rockets, as well as the Centaur upper stage, the United States' first LOX/LH2 rocket (1962).

Another cryogenic fuel with desirable properties for space propulsion systems is liquid methane (-162 oC). When burned with liquid oxygen, methane is higher performing than state-of-the-art storable propellants but without the volume increase common with LOX/LH2 systems, which results in an overall lower vehicle mass as compared to common hypergolic propellants. LOX/methane is also clean burning and non-toxic. Future missions to Mars will likely use methane fuel because it can be manufactured partly from Martian in-situ resources. LOX/methane has no flight history and very limited ground-test history.

Liquid fluorine (-188 oC) burning engines have also been developed and fired successfully. Fluorine is not only extremely toxic; it is a super-oxidizer that reacts, usually violently, with almost everything except nitrogen, the lighter noble gases, and substances that have already been fluorinated. Despite these drawbacks, fluorine produces very impressive engine performance. It can also be mixed with liquid oxygen to improve the performance of LOX-burning engines; the resulting mixture is called FLOX. Because of fluorine's high toxicity, it has been largely abandoned by most space-faring nations.

Some fluorine containing compounds, such as chlorine pentafluoride, have also been considered for use as an 'oxidizer' in deep-space applications.

Hypergolic propellants are fuels and oxidizers that ignite spontaneously on contact with each other and require no ignition source. The easy start and restart capability of hypergols make them ideal for spacecraft maneuvering systems. Also, since hypergols remain liquid at normal temperatures, they do not pose the storage problems of cryogenic propellants. Hypergols are highly toxic and must be handled with extreme care.

Hypergolic fuels commonly include hydrazine, monomethyl hydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH). Hydrazine gives the best performance as a rocket fuel, but it has a high freezing point and is too unstable for use as a coolant. MMH is more stable and gives the best performance when freezing point is an issue, such as spacecraft propulsion applications. UDMH has the lowest freezing point and has enough thermal stability to be used in large regeneratively cooled engines. Consequently, UDMH is often used in launch vehicle applications even though it is the least efficient of the hydrazine derivatives. Also commonly used are blended fuels, such as Aerozine 50 (or "50-50"), which is a mixture of 50% UDMH and 50% hydrazine. Aerozine 50 is almost as stable as UDMH and provides better performance.

The oxidizer is usually nitrogen tetroxide (NTO) or nitric acid.

Hydrazine is also frequently used as a monopropellant in catalytic decomposition engines. In these engines, a liquid fuel decomposes into hot gas in the presence of a catalyst. The decomposition of hydrazine produces temperatures up to about 1,100 oC (2,000 oF) and a specific impulse of about 230 or 240 seconds. Hydrazine decomposes to either hydrogen and nitrogen, or ammonia and nitrogen.

Solid PropellantsSolid propellant motors are the simplest of all rocket designs. They consist of a casing, usually steel, filled with a mixture of solid compounds (fuel and oxidizer) that burn at a rapid rate, expelling hot gases from a nozzle to produce thrust. When ignited, a solid propellant burns from the center out towards the sides of the casing. The shape of the center channel determines the rate and pattern of the burn, thus providing a means to control thrust. Unlike liquid propellant engines, solid propellant motors cannot be shut down. Once ignited, they will burn until all the propellant is exhausted.

There are two families of solids propellants: homogeneous and composite. Both types are dense, stable at ordinary temperatures, and easily storable.

Hybrid PropellantsHybrid propellant engines represent an intermediate group between solid and liquid propellant engines. One of the substances is solid, usually the fuel, while the other, usually the oxidizer, is liquid. The liquid is injected into the solid, whose fuel reservoir also serves as the combustion chamber. The main advantage of such engines is that they have high performance, similar to that of solid propellants, but the combustion can be moderated, stopped, or even restarted. It is difficult to make use of this concept for vary large thrusts, and thus, hybrid propellant engines are rarely built.

Can we harness energy from outer space?

People have been searching for clean alternative energy sources for decades to no avail. As soon as one

source seems to pass the test, someone uncovers its fatal flaw. Nuclear, wind, solar and hydropower

have all been dragged through the mud to some degree. Traditional nuclear fission is too risky, winds

aren't consistent, the sun doesn't always penetrate the clouds and hydropower dams disrupt natural

environments.

It seems like any workable solution is light-years away -- literally. Some researchers think the answer to

our energy needs rests in the stars. From wind turbines on Mars to helium-3 fusion, people are

increasingly looking to extraterrestrial sources for the Earth's energy needs.

One of the sources they're looking at is helium-3 to use in nuclear fusion reactions. As opposed to

nuclear fission, which splits an atom's nucleus in half, nuclear fusion combines nuclei to produce

energy. While nuclear fusion has already been tested with the hydrogen isotopes deuterium and tritium,

those reactions give off the majority of their energy as radioactive neutrons, raising both safety and

production concerns. Helium-3, on the other hand, is perfectly safe. It doesn't give off any pollution or

radioactive waste and poses no danger to surrounding areas.

An isotope of the element helium, helium-3 has two protons but only one neutron. When it's heated to

very high temperatures and combined with deuterium, the reaction releases incredible amounts of

energy. Just 2.2 pounds (one kilogram) of helium-3 combined with 1.5 pounds (0.67 kilograms) of

deuterium produces 19 megawatt-years of energy. Roughly 25 tons of the stuff could power the United

States for an entire year.

The only problem is we don't have 25 tons of helium-3 just lying around. But conveniently,

the moon does. In fact, scientists estimate our lunar rock contains more than 1 million tons of the element.

The energy stored in that much helium is 10 times the amount of energy you'd find in all the fossil fuels on

Earth. If you put a cash value on it, helium-3 would be worth $4 billion a ton in terms of its energy

equivalent in oil.

The only issues that remain are the practicalities of extracting the helium and fine-tuning the fusion

process. Current fusion reactors have yet to achieve the sustained high temperatures needed to produce

electricity, and helium-3 extracted from the lunar surface would require lots of refining since it exists in

such low concentrations in the soil.

Space-based solar power

One Small Step for Man, One Giant Leap for Solar Power

Despite the fact that solar power is at our fingertips, there are benefits to outsourcing it beyond the

stratosphere. Aside from the more obvious reason of avoiding the large land-use footprint presented by

collections of solar panels, there's the fact that the sun actually does shine brighter on the other side of

the fence. In this case, eight times brighter.

Without the obstacles like rain, clouds and nighttime, solar arrays based in space would receive more

concentrated solar rays than they would on Earth. The panels also wouldn't be subject to the seasonal

fluctuations that are unavoidable on Earth.

Space-based solar power (SBSP) is the concept of collecting solar power in space (using an

"SPS", that is, a "solar-power satellite" or a "satellite power system") for use on Earth. It has been

in research since the early 1970s.SBSP would differ from current solar collection methods in that the means used to collect energy would reside on an orbiting satellite instead of on Earth's surface. Some projected benefits of such a system are a higher collection rate and a longer collection period due to the lack of a diffusing atmosphere and nighttime in space.

Part of the solar energy is lost on its way through the atmosphere by the effects of reflection and

absorption. Space-based solar power systems convert sunlight to microwaves outside the atmosphere,

avoiding these losses, and the downtime (and cosine losses, for fixed flat-plate collectors) due to

the Earth's rotation.

Space based solar power, or SBSP, would basically work the same way that regular solar power works.

The only difference is that the solar panels would either be attached to orbiting satellites or stationed on

the moon (in which case it would be called lunar solar power , or LSP). The electricity created would be

converted into microwaves and beamed down to Earth. Rectifying antennas, or rectennas, on the

ground would collect the microwaves and convert them back into electricity.

If the concept seems like a stretch, consider that communications satellites already do something very

similar when they transmit your cell phone conversations. Some people have even suggested that the

solar panels could piggyback on communications satellites. In fact, one of the reasons space-based solar

power has gotten so much attention is that all of the necessary equipment and technology is already

developed and understood. The transmission of microwaves is old hat, and solar cells, or photovoltaics,

are almost three times more efficient than they used to be.

Some initial proposals in the 1970s envisioned gigantic 3-by-6-mile (5-by-10-kilometer) solar panel arrays

transmitting microwaves to rectifying antennas of a similar size. These geostationary satellites, 22,300

miles (36,000 kilometers) high would stay in the same place in relation to the Earth at all times. While just

one of these satellites would produce enormous amounts of energy -- twice the energy output of the

Hoover Dam -- launching such a big project proved to be economically impossible.

Recent proposals to have smaller satellites circle the Earth continuously would be more manageable and

still produce considerable energy output. A satellite less than 1,000 feet (300 meters) across orbiting 300

miles (540 kilometers) above Earth could potentially power 1,000 homes.

Even the Pentagon is on board, having released a study detailing applications in powering military

operations. Japan, Russia, Europe and the island nation of Palau are also looking into it.

The major obstacle right now, as with any new technology, is cost. Launching, setting up and maintaining

a solar farm on the moon would require vast amounts of manpower and money. As it is now, launching an

object into space costs 1,000 times more than transporting that objects across the country on a plane --

even though they use the same amount of energy. Besides the cost of implementing such a system,

SBSP also introduces several new hurdles, primarily the problem of transmitting energy from orbit to

Earth's surface for use. Since wires extending from Earth's surface to an orbiting satellite are neither

practical nor feasible with current technology, SBSP designs generally include the use of some manner

of wireless power transmission. The collecting satellite would convert solar energy into electrical energy

on board, powering a microwave transmitter or laser emitter, and focus its beam toward a collector

(rectenna) on Earth's surface. Radiation and micrometeoroid damage could also become concerns for

SBSP.

If NASA and other space agencies like ISRO succeed in finding a new generation of reusable launch

vehicles, though, costs could go down. Not to mention the fact that a solar satellite could pay back the

energy used to send it into orbit in less than five days.

REFERENC

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science.howstuffworks.com en.wikipedia.org www.braeunig.us www.discovery.com