basics of aerospace & satellites detailed)

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Basics of Aerospace & Satellites

Since the dawn of the Space Age only a few decades ago, we have come to rely more and moreon satellites for a variety of needs. Daily weather forecasts, instantaneous world-widecommunication, and a constant ability to keep an eye on not-so-friendly neighbors are allexamples of space technology that weve come to take for granted. The purpose of this brief

astronautics primer is to provide the reader with a conceptual overview of important topics inorbital mechanics. Understanding these key concepts will enhance your insight into the sciencebehind Satellite Took Kit and better equip you to apply these concepts to practical problems inspace. Well begin with a brief overview of space, space missions and space history. Then wellget into the details of orbital mechanics to see how you can use STK to plot your path to thestars.

Why is space so useful?Getting into space is dangerous and expensive. So why bother? Space offers several compellingadvantages for modern society

A global perspectivethe ultimate high ground

A universal perspectivefrom space we have a clear view of the heavens, unobscuredby the atmosphere

A unique environmentfree-fall and abundant resources make space the true finalfrontier

Global PerspectiveSpace offers a global perspective. As you can see in Figure, the higher you are, the more youcan see. For thousands of years, kings and rulers took advantage of this fact by putting lookoutposts atop the tallest mountains to survey more of their realm and fend off would-be attackers.Throughout history, many battles have been fought to take the high ground. Space takes thisquest for greater perspective to its ultimate end. From the vantage point of space, we can viewlarge parts of the Earths surface. Orbiting satellites can thus serve as eyes in the sky to providea variety of useful services.

Global perspective.From space, satellites can observe large-scale features

on the Earth, track weather patterns, monitor the environment and view widely separatedpoints simultaneously, allowing them to communicate.


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Universal PerspectiveSpace offers a clear view of the heavens. When we look at stars in the night sky, we see theircharacteristic twinkle. This twinkle, caused by the blurring of starlight as it passes through theatmosphere, is known as scintillation. Not only is the light blurred, but some of it is blocked orattenuated altogether. This attenuation is frustrating for astronomers who need access to all theregions of the spectrum to fully explore the universe. By placing observatories in space, we cansit above the atmosphere and gain an unobscured view of the universe, as depicted in Figure.The Hubble Space Telescope and the Gamma Ray Observatory are armed with sensorsoperating far beyond the range of human senses. Already, results from these instruments arerevolutionizing our understanding of the cosmos.

Seeing beyond the clouds.Earth-based astronomy is obscured by the

atmosphere. Astronomers dont like the twinkle of star light. Some wavelengths are completelyblocked. Space-based astronomy opens the door to a whole new perspective on

the universe.

Space ApplicationsLets look at some important application of space that affect all of our lives today.

Communications SatellitesScience/Science Fiction writer Arthur C. Clarke first proposed putting satellites into orbitswith periods of 24 hours, 36,000 km above the equator, exactly matching the rotation rateof the Earth. These geostationary orbits could serve as communication hubs to linktogether remote parts of the planet. With the launch of the first experimental communicationssatellite, Echo I, into Earth orbit in 1960, Clarkes fanciful idea showed promise of becomingreality. Although Echo I was little more than a reflective balloon in low-Earth orbit, radio signalswere bounced off it, demonstrating that space could be used to broaden our horizons ofcommunication. An explosion of technology to exploit this idea quickly followed.


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Satellites are now used for a large percentage of commercial and government communicationsand for most domestic cable television. Through satellite technology, relief workers can nowstay in constant contact with their organizations, enabling them to better distribute aid torefugees hungry for food. In addition, our modern military now relies almost totally on satellitesto communicate with forces deployed world-wide. Without satellites, global communication as weknow it today would not be possible.

Remote Sensing MissionsSatellites operating from the global perspective of space have also made possible the science ofremote sensing. Remote sensing is the act of observing Earth and other objects fromspace. For decades, military spy satellites have kept tabs on the activities of potentialadversaries using remote-sensing technology. This same technology has been adapted forcivilian uses such as

monitoring Earths environment

forecasting the weather

managing resourcesAs Satellites can now spy on crops, ocean currents, and natural resources to aid farmers,resource managers, and planners on Earth. In countries where the failure of a harvest may meanthe difference between bounty and starvation, spacecraft have helped planners manage scarceresources and head off potential disasters before insects or other blights could wipe out an entirecrop. Weather forecasting is a further application of remote-sensing technologyone weve allcome to rely on. Overall, weve come to rely more and more on the ability to monitor and map ourentire planet. As the pressure builds to better manage scarce resources and assessenvironmental damage, well call upon remote-sensing spacecraft to do even more.

Satellite remote sensing.From the vantage point of space, we can plan

urban development and plot the course of dangerous storms.

Space-based NavigationEarly seafarers looked to the stars to guide their way. Modern seafarers look only as far as

satellites in Earth orbit. Systems such as the Global Positioning System (GPS), developed by theU.S. military, tell you where you are, in what direction youre heading and how fast youre going.


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Global Positioning System (GPS).GPS allows Earth-based users armed

with a simple, hand-held receiver to triangulate from a constellation of 24 satellites. Theycan then determine their location to within a few meters and velocity, and a few m/sec

anywhere on Earth.


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Magellan at Venus.The powerful synthetic aperture radar on NASAs

Magellan spacecraft pierced the thick clouds of Venus, giving us the first details of theplanets surface. (Photo courtesy of NASA.)

Describing Space Missions

Space missions seem complex, and they are to a certain extent, but if you look at them logically,youll see many similarities. Lets begin with some key definitions:

Mission Objective - Why were going to space and what were going to do once we getthere.

Users - The people or systems that use data or services provided by the satellite orsatellites.

Operators -The people who manage and run the mission from the ground.

Mission Operations Concept -How users, operators, ground and space elements all worktogether to make a mission successful.

All these come together to form the tangible elements of what is collectively called the SpaceMission Architecture. These elements are depicted in following Figure each one is defined in thesubsections following.


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Space Operations

The term space operations encompasses all activities needed to monitor and control satellitesand the other elements that make up a space mission. Space operations are performed by teamsof people located at tracking sites and control centers around theworld.

Spacecraft Bus and PayloadA spacecraft has two basic parts, a payload (or payloads) and a bus. The payload includesspace-borne people and instruments that perform the primary mission. The spacecraft busprovides for the care and feeding of the payloadpointing, heating and cooling, structure,

transportation and power. A simple analogy of a spacecraft bus and its payload is a good old-fashioned school bus, as shown in Figure. It contains all of the same types of systems needed tosupport a spacecraft.

The spacecraft bus. A spacecraft has all the basic systems found in aregular school bus.


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Orbit types. Different types of orbits include the parking orbit, the transferorbit and the final or mission orbit. A satellite normally begins its life in a temporaryparking orbit. From there, an upper stage rocket is used to boost the satellite onto a

transfer orbit. An additional boost places it into the final mission orbit.

Space TransportationSpace transportation includes all of the systems necessary to deliver our spacecraft to its finalmission orbit. Normally, this consists of a booster, such as the Space Shuttle orAriane, an upper stage, such as the Inertial Upper Stage (IUS), and onboard thrusters for finalmaneuvers and station keeping. The Space Shuttle, shown in Figure, is one type of completespace transportation system.

The Space Shuttle. Space transportation includes the systems that put thespacecraft in orbit, keep it there, and rotate and move it if necessary. Space

transportation systems develop the velocity needed to obtain and stay in orbit. Spaceboosters are divided into stages that provide incremental changes in velocity and are

then discarded.

Communications NetworkA space mission is more than just rockets and satellites. An entire system of ground and on-orbitassets are needed to track, command and control all aspects of the mission. Thiscommunications networkties together various links needed to deliver bus telemetry and payloaddata to operators and users, as shown in Figure.


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Communications network. The communications network is the glue

that holds the mission together. The network ties together space assets, groundcontrollers and users in a complex web of links that transfers data among the various

mission element nodes.

Exploring Space

Long before rockets and interplanetary probes escaped the Earths atmosphere,people explored the heavens with just their eyes and imagination. Later, with the aid oftelescopes and other instruments, humans continued their struggle to bring order to the heavens.With order came some understanding and a concept of our place in the universe. Thousands ofyears ago, priests of ancient Egypt and Babylon carefully observed the heavens to plan religiousfestivals, to control the planting and harvesting of various crops, and to understand at leastpartially the realm in which they believed many of their gods lived. Later, philosophers such as

Aristotle and Ptolemy developed complex theories to explain and predict the motions of the Sun,Moon, planets and stars.The theories of Aristotle and Ptolemy dominated the world of astronomy and our understanding ofthe heavens well into the 1600s. Combining ancient traditions with new observations and insights,natural philosophers such as Copernicus and Kepler offered rival explanations from the 1500sonward. Using their models and Isaac Newtons new tools of physics, astronomers in the 1700sand 1800s made several startling discoveries, including two new planetsUranus and Neptune.Lets briefly explore some of these major contributors to our early understanding of space andorbits.Copernicus With the Renaissance and humanism came a new emphasis on the accessibility ofthe heavens to human thought. Nicolaus Copernicus (14731543), a Renaissance humanist andCatholic clergyman, reordered the universe and enlarged mans horizons. He placed the Sun atthe center of the solar system, as shown in Figure 14, and had the Earth rotate on its axis once a

day while revolving about the Sun once a year.Copernicus further observed that, with respect to a viewer located on the Earth, he planetsoccasionally appear to back up in their orbits as they move against the background of the fixedstars. Ptolemy and others resorted to complex combinations of circles to explain this backwardmotion of the planets, but Copernicus cleverly explained that this motion was simply the effect ofthe Earth overtaking, and being overtaken by, the planets as they all revolved about the Sun.However, Copernicus heliocentric system had its drawbacks. He couldnt prove the Earth moved,and he couldnt explain why the Earth rotated on its axis while revolving about the Sun. He alsoadhered to the Greek tradition that orbits follow uniform circles, so his geometry was complex andsomewhat erroneous. In addition, Copernicus wrestled with the problem of parallaxthe


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apparent shift in the position of bodies when viewed from different distances. If the Earth trulyrevolved about the Sun, critics observed, a viewer stationed on the Earth should see an apparentshift in position of a closer tar with respect to its more distant neighbors. Because no one sawthis shift, Copernicus Sun-centered system was suspect. In response, Copernicus speculatedthat the stars must be at vast distances from the Earth, but such distances were far too great formost people to contemplate at the time, so this idea was also widely rejected.

Copernicus redefines the center. Polish astronomer Copernicus reorderedour view of the universe. He promoted a heliocentric (Sun-centered) universe, a simpler,

more symmetric approach with all of the planets in circular orbits about the Sun.Unfortunately, these ideas were widely rejected because they disputed religious

teachings of his day.

KeplerJohannes Kepler (1571 - 1630) revolutionized our understanding of orbits. In Cosmic Mystery,written before he was age 25, he calculated that the orbit of Mars was not circular but elliptical.From this work, he developed three important laws of orbit motion, described in Figures.

Figure 15: Keplers 1st law. The orbits of the planets are ellipses with the Sun at afocus.


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Keplers 2nd law. The orbits of the planets sweep out equal areas in equaltime.

Keplers 3rd law. The square of the orbit periodthe time it takes to goaround onceis proportional to the cube of the average distance to the Sun.

GalileoIn 1609, an innovative mathematician, Galileo Galilei (15641642), heard of a new optical devicethat could magnify objects so they would appear to be closer and brighter than when seen withthe naked eye. Building a telescope that could magnify an image 20 times, Galileo ushered in anew era of space exploration. He made some startling telescopic observations of the Moon, theplanets, and the stars, thereby attaining stardom in the eyes of his peers and potential patrons.Observing the planets, Galileo noticed that Jupiter had four moons or satellites (a word coined byKepler in 1611) that moved about it. This disproved Aristotles claim that everything revolvedabout the Earth. Galileo also took on Aristotles physics. He rolled a sphere down a grooved rampand used a water clock to measure the time it took to reach the bottom. He repeated theexperiment with heavier and lighter spheres, as well as steeper and shallower ramps, andcleverly extended his results to objects in free-fall. Through these experiments, Galileodiscovered, contrary to Aristotle, that all objects fall at the same rate regardless of their weight, asshown in Figure.Galileo further contradicted Aristotle as to why objects, once in motion, tend to keep going.Aristotle held that objects in violent motion, such as arrows shot from bows, keep going only aslong as something is physically in touch with them, pushing them onward. Once this push diesout, they resume their natural motion and drop straight to Earth. Galileo showed that objects inuniform motion keep going unless disturbed by some outside influence. He wrongly held that thisuniform motion was circular, and he never used the term inertia. Nevertheless, we applaudGalileo today for greatly refining the concept of inertia as we know it today.


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Galileo on gravity. Through application of the scientific method, Galileo putAristotles ideas to the test and proved Aristotle wrongall objects fall at the same rate.

Newton

To complete the astronomical revolution, which Copernicus had almost unwittingly started and

which Kepler and Galileo had advanced, the terrestrial and heavenly realms had to be unitedunder one set of natural laws. Isaac Newton (16421727) answered this challenge. Newton wasa brilliant natural philosopher and mathematician who provided a majestic vision of natures unityand simplicity. 1665 proved to be Newtons miracle year, in which he significantly advanced thestudy of calculus, gravitation, and optics. Extending the groundbreaking work of Galileo indynamics, Newton published his three laws of motion and the law of universal gravitation in thePrincipia in 1687. With these laws, you could explain and predict motion not only on Earth butalso in tides, comets, moons, planetsin other words, motion everywhere. Newtons laws areexplained more thoroughly in the next section.

Introduction to Orbital Motion

Orbits are one of the basic elements of any space mission. Understanding a satellite in motionmay at first seem rather intimidating. After all, to fully describe orbital motion we need some basicphysics along with a healthy dose of calculus and geometry. However, as well see, the complextrajectories of rockets flying into space arent all that different from the paths of baseballs pitchedacross home plate. In fact, in most cases, both can be described in terms of the single forcepinning you to your chair right nowgravity. Armed only with an understanding of this singlepervasive force, we can predict, explain and understand the motion of nearly all objects in space,from baseballs to entire galaxies. Once we know an objects position and velocity, as well as thenature of the local gravitational field, we can predict exactly where the object will be minutes,hours or even years from now.

MassMass is a measure of how much matter or stuff an object possesses. For example, a volleyballand a cannon ball are about the same size, but the cannon ball has far more mass because it ismade of a more dense material.

InertiaInertia describes how hard it is to move an object. It is much easier to push a baby carriage thana bulldozer because the bulldozer, being more massive, has more inertia.

WeightWeight actually describes the force produced by gravity acting on a mass. Your weight in varioussituations is illustrated in.


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MomentumLinear momentum describes the resistance a moving object has to changes in either direction orspeed. The more massive an object, or the faster it is moving, the harder it is to stop or change itsdirection of motion. As a result, linear momentum is the product of the mass and velocity of anobject. Momentum for baby carriages and bulldozers is shown in Figure.

Momentum, bulldozers and baby carriages. Linear momentum is theproduct of mass and velocity. For a baby carriage to have the same linear momentum as

a bulldozer, it would have to be traveling at a much higher velocity.

Angular momentum is a measure of the spinning properties of an object. As Figure illustrates, anon-spinning top immediately falls over. However, a spinning top has angular momentum, whichallows it to resist the force of gravity pulling it over (until it finally slows down due to friction).

Angular momentum. A spinning top has angular momentum which keeps itpointing upright even when pulled by outside forces such as gravity.

Newtons Laws

Newtons First LawA body remains at rest or in constant motion unless acted upon by external forces.

In other words, if you were to pitch a baseball, it should continue on its path, in a straight lineforever, unless disturbed by an outside force such as gravity or air resistance.

Newtons Second LawThe time rate of change of an objects momentum is equal to the applied force.Change momentum / Change time =Force Applied

Recall, momentum is the product of mass and velocity. Thus, as long as massstays constant (which it normally does as long as rockets arent firing) this equation can bereduced to:F=maor:Force applied = mass (m ) times acceleration (a)


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The significance of this relationship can be felt every time you hit the brakes in you car.The more force you apply (the harder you hit the brakes) the faster you stop (the fasteryou decelerate). This principle is illustrated in Figure.

Newtons 2nd law. Force is proportional to acceleration (or deceleration). A25,000 N force is needed to stop a 1 m/s bulldozer in 1 second, while much smaller 6.9

N force would take 1 hour to bring it to a stop.Newtons Third Law

For every action, theres an equal and opposite reaction. This basic principle can be illustrated bytwo roller-skating astronautics, as shown in Figure.

Newtons 3rd law. If two people on roller skates push against each other,they both move backward. Their acceleration is proportional to their mass.

Newtons Law of Universal Gravitation

The force of gravity between two bodies is proportional to the product of the asses and inverselyproportional to the square of the distance between them. This is illustrated in Figure 26.Newtons law can be summarized in equation form as follows:


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In other words, the more mass an object has, the more gravitational force it generates.Furthermore, the farther apart two objects are, the less the force is, in fact, the force decreaseswith the square of the distance as illustrated in Figure.

Gravitational attraction. Two masses in space each exert a force on theother. The magnitude of this force depends on the product of their masses and the

square of the distance between them.

Orbits Made Simple

What is an orbit? In the simplest sense, orbits are a type of racetrack in space that a satellitedrives around.

Orbits as racetracks. The simplest way to think of orbits is as giant, fixedracetracks on which spacecraft drive around the Earth.


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Types of Orbits

Geosynchronous Orbits

A geosychronous orbit (GEO) is a prograde, circular, low inclination orbit about Earth having aperiod of 23 hours 56 minutes 4 seconds. A spacecraft in geosynchronous orbit appears to

remain above Earth at a constant longitude, although it may seem to wander north and south.

Geostationary Orbits

To achieve a geostationary orbit, a geosychronousorbit is chosen with an inclination of either zero, righton the equator, or else low enough that the spacecraftcan use propulsive means to constrain the spacecraft'sapparent position so it hangs motionless above a pointon Earth. (Any such maneuvering on orbit is a processcalled station keeping.) The orbit can then be calledgeostationary. This orbit is ideal for certain kinds ofcommunication satellites or meteorological satellites.

A Little GTO

To attain geosynchronous (and also geostationary) Earth orbits, a spacecraft is first launched intoan elliptical orbit with an apoapsis altitude in the neighborhood of 37,000 km. This is called aGeosynchronous Transfer Orbit (GTO). The spacecraft then circularizes the orbit by turningparallel to the equator at apoapsis and firing its rocket engine. That engine is usually called anapogee motor. It is common to compare various launch vehicles' capabilities according to theamount of mass they can lift to GTO.

Polar Orbits

Polar orbits are 90 degree inclination orbits, useful for spacecraft that carry out mapping orsurveillance operations. Since the orbital plane is nominally fixed in inertial space, the planetrotates below a polar orbit, allowing the spacecraft low-altitude access to virtually every point onthe surface. The Magellan spacecraft used a nearly-polar orbit at Venus. Each periapsis pass, aswath of mapping data was taken, and the planet rotated so that swaths from consecutive orbitswere adjacent to each other. When the planet rotated once, all 360 degrees longitude had beenexposed to Magellan's surveillance.

To achieve a polar orbit at Earth requires more energy, thus more propellant, than does a directorbit of low inclination. To achieve the latter, launch is normally accomplished near the equator,where the rotational speed of the surface contributes a significant part of the final speed requiredfor orbit. A polar orbit will not be able to take advantage of the "free ride" provided by Earth'srotation, and thus the launch vehicle must provide all of the energy for attaining orbital speed.

Walking Orbits

Planets are not perfectly spherical, and they do not have evenly distributed surface mass. Also,they do not exist in a gravity "vacuum." Other bodies such as the sun, or natural satellites,contribute their gravitational influences to a spacecraft in orbit about a planet. It is possible tochoose the parameters of a spacecraft's orbit to take advantage of some or all of thesegravitational influences to induce precession, which causes a useful motion of the orbital plane.

Geostationary Satellite
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The result is called a walking orbit or a precessing orbit, since the orbital plane moves slowly withrespect to fixed inertial space.

Sun Synchronous Orbits

A walking orbit whose parameters are chosen such that the orbital plane precesses with nearly

the same period as the planet's solar orbit period is called a sun synchronous orbit. In such anorbit, the spacecraft crosses periapsis at about the same local time every orbit. This can be usefulif instruments on board depend on a certain angle of solar illumination on the surface. MarsGlobal Surveyor's orbit is a 2-pm Mars Local Time sun-synchronous orbit, chosen to permit well-placed shadows for best viewing.

It may not be possible to rely on use of the gravity field alone to exactly maintain a desiredsynchronous timing, and occasional propulsive maneuvers may be necessary to adjust the orbit.

This remarkable image of a Martian aquifer was obtained by the Mars Global Surveyor spacecraftfrom its sun-synchronous Martian orbit in January 2000. The view is to the north.


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Planets & their Orbits in Solar System


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The words "solar system" refer to the Sun or a star and all of the objects that travel around it.These objects include planets, natural satellites such as the Moon, the asteroid belt, comets, andmeteoroids. Our solar system has an elliptical shape and is part of a galaxy known as the MilkyWay. The Sun is the center of the solar system. It contains 99.8% of all of the mass in our solarsystem. Consequently, it exerts a tremendous gravitational pull on planets, satellites, asteroids,comets, and meteoroids.

The Inner Solar System

The Sun

The diameter of our closest star, the Sun, is 1,392,000 kilometers. The Sun is a medium size starknown as a yellow dwarf. It is a star in the Milky Way galaxy and the temperature in its core isestimated to be over 15,000,000 degrees Celsius. In the Sun's core, hydrogen is being fused toform helium by a nuclear fusion process. The energy created by this process radiates up to thevisible boundary of the Sun and then off into space. It radiates into space in the form of heat andlight. Because the Sun is so massive, it exerts a powerful gravitational pull on everything in oursolar system. It is because of the Sun's gravitational pull that Earth orbits the Sun in the mannerthat it does. More information about the Sun and solar features can be foundhere.

A planet is a large space body which reflects the light of a star around which it revolves. Theplanets in our solar system are classified as inner planets and outer planets. The inner planets,the closest to the Sun, are solid spheres of rock and include Mercury, Venus, Earth, and Mars.You will find craters of varying sizes on the inner planets and their satellites. The outer planets,with the exception of Pluto, are large gaseous spheres with rings and include Jupiter, Saturn,Uranus, and Neptune. Between the inner and outer planets is an asteroid belt. Every planet,except for Earth, was named for an ancient Roman god or goddess. Some of the planets havenaturally occurring satellites, or moons, while others do not. All nine planets orbit the Sun in theirown unique way.
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Mercury

Mercury is only about one-third the size of the Earth. It issmaller than any other planet except Pluto. Mercury is veryclose to the Sun and has no substantial atmosphere. Thesefactors contribute to the fact that the surface of Mercury has

the greatest temperature range of any planet or naturalsatellite in our solar system. The surface temperature on theside of Mercury closest to the Sun reaches 427 degreesCelsius, a temperature hot enough to melt tin. On the sidefacing away from the Sun, or the night side, the temperaturedrops to -183 degrees Celsius. Scientists have detected amagnetic field surrounding Mercury, though it is not as strongas the field around the Earth. Scientists theorize thatMercury's field is due to an iron-bearing core or possibly tothe solar winds. Mercury's atmosphere is very thin and iscomposed of helium and sodium. The surface of Mercury hasbeen shaped by three processes: impact cratering wherelarge objects struck the surface resulting in crater formation,volcanism where lava flooded the surface, and tectonicactivity where the planet's crust moved in order to adjust tothe planetary cooling and contracting. Mercury does not haveany naturally occurring satellites. Because Mercury is closerto the Sun than Earth, we sometimes see rare solar transitswhen the planet passes directly in front of the Sun from ourvantage point.

Venus

Venus and Earth are similar in size, composition, and mass. They differ in that Venus does nothave oceans or human life, and its temperature during the day reaches 484 degrees Celsius. Thedaytime temperature is so hot it could melt lead. The dense atmosphere is composed of carbondioxide and sulfuric acid which acts as a greenhouse and traps the heat. Venus revolves aroundthe Sun in a circular orbit once every 225 Earth days. Venus rotates slowly on its axis in aclockwise direction, which is referred to as a "retrograde" rotation because it is the opposite of theother eight planets. A rotation takes 243 Earth days, so a Venusian day is longer than a Venusianyear. As with the other inner planets, the surface of Venus has been shaped by impact craters,tectonic activity, and volcanoes which scientists believe to be ongoing. The volcanic activity isbelieved to be the source of the sulfur found in the atmosphere. Venus does not have anynaturally occurring satellites. For more information, visit the About Venus page.
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Earth

Earth's amazing gaseous atmosphere is responsible for making life possible on this, the thirdplanet from the Sun. Our atmosphere contains water vapour which helps to moderate our daily

temperatures. Our atmosphere contains 21% oxygen, which is necessary for us to breathe, 78%nitrogen, and .9% argon. The other 0.1% consists of water vapor, carbon dioxide, neon, methane,krypton, helium, xenon, hydrogen, nitrous oxide, carbon monoxide, nitrogen dioxide, sulfurdioxide, and ozone. These latter elements are important to have because they help to absorbharmful solar radiation before it can reach the surface of the Earth. If present in larger amounts,most of these latter elements would be poisonous to humans. The atmosphere protects us frommeteors as well. Due to the friction generated between a meteor and the atmospheric gases,most meteors burn up before hitting Earth's surface as a meteorite.

Earth rotates on an imaginary axis which is tilted at a 23.5 degree angle. The rotation is whatcauses the change from day to night. The tilt is what determines our change in seasons. If theEarth was not tilted, we would have the same season all year long. Earth has a core of molteniron-nickel. The rapid spin of the Earth along with the liquid, hot metallic core causes a magneticfield to surround the Earth. This magnetic field traps the charged particles which are hurled at theEarth by the Sun during solar wind activity. When these charged particles react with the gases inour atmosphere, the gases begin to glow. These aurorae, or glowing gases, are mostly seen in
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the Arctic Circle and the Antarctic Circle but can travel as far south as 50 degrees Latitude. Aswith all inner planets, the Earth's surface has been affected by volcanism, tectonic activity, and toa lesser degree, meteorite impacts. Earth has one naturally occurring satellite, the Moon.

The Moon

The Moon travels around Earth in an oval orbit at 3680 kilometers per hour. The Moon does nothave an atmosphere, so temperatures range from -184 degrees Celsius during its night to 214degrees Celsius during its day except at the poles where the temperature is a constant -96degrees Celsius. The Moon is actually a little lopsided due to the lunar crust being thicker on one

side than the other. When you look at the Moon, you will see dark and light areas. The dark areasare young plains called maria and are composed of basalt. The light areas are the highlands. Thelunar surface is covered by a fine-grained soil called "regolith" which results from the constantbombardment of the lunar rocks by small meteorites. The gravitational pull of the Moon on theEarth affects the ocean tides on Earth. The closer the Moon is to Earth, the greater the effect.The time between high tides is about 12 hours and 25 minutes.


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The phases, or changing appearance, of the Moon depend on its position relative to the positionof the Sun. When the Moon is between the Sun and the Earth, the side of the Moon facing theEarth is dark. This is called a "new moon". As the Moon travels eastward in its orbit, more of itssunlit side becomes visible to Earth and the Moon is said to be "waxing". More specifically, thephase after a new moon is called a "waxing crescent" because we can see no more than aquarter of the Moon at this point. As the Moon continues eastward, the Sun, Moon, and Earthform a 90 degree angle and the Moon appears half dark and half light to us here on Earth. This isa "first quarter" phase. After the first quarter phase, more than a quarter of the Moon is visible tous, so it is now in a "waxing gibbous" phase. As the Moon continues its revolution around Earth,the Sun, Earth, and Moon align with the Earth in the middle. The side of the Moon facing Earth isnow fully lit. This is called a "full moon" phase. As the Moon travels further around in its orbit, thelit portion of the Moon visible to Earth becomes smaller, so the Moon is now said to be "waning"as it enters the next phase. After the "waning gibbous" phase, the Moon enters the "third quarter"phase where once again the Moon appears half dark and half light from Earth. As it completes itsrevolution around Earth, the Moon enters a "waning crescent" phase just prior to starting thecycle again as a new moon.

Mars

The orbit of Mars around the Sun is extremely elliptical. Because the distance between the Sunand Mars varies, temperatures range from -125 degrees Celsius in the Martian winter to 22degrees Celsius in the Martian summer. The Martian atmosphere is composed of over 95%carbon dioxide. Solar winds carry the thin, weak atmosphere away because Mars has a weakgravitational and magnetic field. At the Martian poles are polar ice caps which shrink in sizeduring the Martian spring and summer. From data gathered by the Viking 1 and 2 probes, weknow that the Martian surface is covered by various rocks and a soil which is rich in an iron-ladenclay. The presence of iron explains the planet's reddish-orange appearance. Mars containshighlands which occur in the southern hemisphere and are composed of the most heavilycratered crustal material. Mars also contains lowlands which are found in the northern


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hemisphere. The extremely weak magnetic field of Mars suggests that its iron core is no longerfluid and circulating.

The surface of Mars has not only been affected by meteorite impacts, but also by volcanic andtectonic activity. In fact, Mars has some of the largest volcanoes in the solar system; OlympusMons is over 600 kilometers wide and 26 kilometers high! Tectonic activity is in evidence at the

tremendous Valles Marineris canyon system, which is over 8 kilometers deep and 4500kilometers long. Mars has two small natural satellites, Phobos and Deimos.

Asteriods and the Asteriod Belt

An asteroid is a rocky body in space which may be only a few hundred feet wide or it may beseveral hundred miles wide. Many asteroids orbit the Sun in a region between Mars and Jupiter.This "belt" of asteroids follows a slightly elliptical path as it orbits the Sun in the same direction asthe planets. It takes anywhere from three to six Earth years for a complete revolution around theSun. The largest asteriod found in the asteriod belt is called Ceres and is approximately the sizeof the US State of Texas. The gas gaint planet, Jupiter, protects the inner solar system planets

from constant bombardment by these asteriods by exerting its gravitational force on the asteroidsin the belt. The presence of Jupiter actually protects Mercury, Venus, Earth, and Mars fromrepeated asteroid collisions!

The Outer Solar System


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Jupiter

Jupiter is a large gas planet whose rapid rotation causes the planet to flatten at the poles andbulge at the equator. Jupiter emits twice as much heat as it absorbs from the Sun, whichindicates it has its own internal heat source. Astronomers estimate the core temperature at20,000 degrees Celsius, approximately three times greater than the temperature of the Earth's

core. The planet's powerful magnetic field is thought to be generated by the electric currentsproduced by pressurized hydrogen in the mantle. Jupiter's atmosphere is thought to be composedof hydrogen, helium, sulfur, and nitrogen. Clouds in the atmosphere move in alternating bandsfrom east to west or west to east. Lightning, more powerful than any that has been experiencedon Earth, has been noted in Jupiter's atmosphere. Also in Jupiter's atmosphere are oval featureswhich are thought to be circular winds. The most prominent of these is the Great Red Spot, ahurricane-like storm that has been seen in Jupiter's southern hemisphere since Jupiter was firstdiscovered. Jupiter has at least sixteen natural satellites (and may have over 28!) . One of thesesatellites, Io, is volcanically active. Instruments aboard the space probe Galileo have detectedsurface temperatures on Io higher than any other planetary body in our Solar System. Voyager 2,also a space probe, has confirmed that Jupiter is surrounded by a system of thin rings. Themajority of the rings are made up of very small particles thought to be debris from meteoroidcollisions.

Saturn

Saturn is a large gas planet with an atmosphere composed of hydrogen and helium. Saturn'srapid spin tends to flatten out the poles while causing a bulge at its equator. The winds in Saturn'satmosphere reach speeds up to 1800 kilometers per hour! Astronomers see large white spots (orclouds) on Saturn which they believe are storms. Just like Jupiter, Saturn emits twice as muchheat as it absorbs from the Sun indicating it also has an internal heat source. Saturn has anextensive ring system which is formed by a thousand individual rings. The rings contain water iceand dust. The thickness of the rings ranges from 10 to 100 meters and the rings vary in


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brightness. There are gaps between some rings, while other rings appear to be braided together.The particles in the rings closer to the planet, orbit at a faster speed than the particles in the ringsfarther from the planet. There are satellites within the rings which result in the gaps that arepresent between some rings. As with Jupiter, the pressurized hydrogen in Saturn's mantleproduces electric currents which create a strong magnetic field around the planet. Saturn has atleast 30 naturally occurring satellites.

Uranus

Uranus is unique in our solar system because it is tilted 98 degrees. When viewed from Earth, itappears to rotate on its side! At different times throughout its orbit, we can actually view one of


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the planet's poles head-on. The atmosphere is composed of hydrogen, helium, and methane. Thetemperature in the upper atmosphere is so cold that the methane condenses and forms a thincloud layer which gives the planet its blue-green appearance. The winds on Uranus blow mainlyto the east and can reach speeds up to 600 kilometers per hour. The rapid spin of Uranusinfluences the winds in the atmosphere. Uranus has a very strong magnetic field. This planet hasa system of rings which was not discovered until 1977. The ring system contains eleven darkrings composed of varying sized particles. Satellites embedded in the rings create gaps betweenthe rings. Uranus has 21 known natural satellites (and may have at least 27), both within the ringsand outside of the rings.

Neptune

Voyager 2, a space probe, passed within 4900 kilometers of Neptune in 1989. From the datacollected, we know that Uranus and Neptune are very similar in composition. Neptune has amantle of liquid hydrogen while the atmosphere is a combination of ammonia, helium, andmethane. In the upper atmosphere, methane freezes and forms an ice cloud which casts ashadow on the clouds below. Neptune has bands in its atmosphere where wind speeds mayreach 2000 kilometers per hour! Neptune has large, dark ovals on its surface which astronomersbelieve are hurricane-like storms. Neptune generates more heat than it absorbs from the Sun,indicating it has its own internal heat source. Neptune has a very strong magnetic field. It also hasa ring system consisting of four rings; two thin and two thick. The rings are composed of darkparticles which vary in size. Neptune has at least eight natural satellites, four of which orbit within


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the rings. The largest satellite is Triton. Triton has a retrograde orbit abd is thought to be acombination of rock and ice. Its surface temperature is -245 degrees Celsius, and it has a thinatmosphere of nitrogen and methane.

Trans-Neptunian Objects

TNOs are small bodies orbiting the sun beyond Neptune. There are around 350 known "trans-Neptunians" with diameters larger than 100 km extending outwards from the orbit of Neptune (at30 AU) to 50 AU. Current astronomical trends classify these objects incorrectly as Kuiper Beltobjects though the Kuiper Belt, described by cosmologists as "a hypothetical massive flatteneddisc of billions of icy planetesimals beyond the orbit of Neptune", has never been observed ordetected.

Pluto

Pluto is tilted 122.5 degrees on its axis. It has an extreme elliptical orbit. Because of the shape ofPluto's orbit, it actually slips inside of Neptune's orbit once every 248 Earth years for a period oftwenty years. Pluto has one natural satellite, Charon, which is half the size of Pluto. BecausePluto and Charon are comparable in size, many scientists consider them to be a double planet(but many scientists don't consider Pluto a planet at all!). Studies conducted using a spectroscopehave detected methane frost on Pluto and water frost on Charon. Like Triton, Neptune's satellite,Pluto has an atmosphere of nitrogen and methane. Pluto's atmosphere appears to extend out toinclude Charon, which suggests that they may share an atmosphere. Through the Hubble SpaceTelescope, Charon appears to be more blue in color than Pluto. During the time in its orbit whenPluto is farthest from the Sun, its atmosphere condenses and falls to the surface as frost.

Meteoroids

A meteoroid is a piece of stony or metallic debris which travels in outer space. Meteoroids travelaround the Sun in a variety of orbits and at various speeds. The fastest meteoroids move at about42 kilometers per second. Most meteoroids are about the size of a pebble. When one of thesepieces of debris enters the Earth's atmosphere, friction between the debris and atmosphericgases heats it to the point that it glows and becomes visible to our eyes. This streak of light in thesky is known as a meteor. Most meteors glow for only a few seconds prior to burning up beforehitting the Earth's surface. On most dark nights, meteors can be seen. The chance of seeing a


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meteor with the unaided eye increases after midnight. People often refer to meteors as "falling" or"shooting" stars. The brightest of the meteors are called fireballs. Sonic booms often follow theappearance of a fireball just as thunder often follows lightning. At certain times of the year, moremeteors than normal can be seen. When the Earth passes through an orbiting stream of debrisfrom a comet that has broken up, what's known as a meteor shower occurs. Meteor showers takeplace on about the same dates each year.

If the meteor does not burn up completely, the remaining portion hits the Earth and is then calleda meteorite. Over 100 meteorites hit the Earth each year. Fortunately, most of them are verysmall. There has only been one report of a "HBM" (hit by meteorite), and that occurred in 1954.Ann Hodges of Sylacauga, Alabama was slightly injured when a 19.84 kilogram meteoritecrashed through the roof of her home. The larger meteorites are believed to have originated inthe asteroid belt. Some of the smaller meteorites have been identified as moon rock, while stillothers have been identified as pieces of Mars. Large meteorites that crashed onto the Earth longago made craters like those found on the Moon. The Barringer Meteorite Crater near Winslow,Arizona is believed to have been formed about 49,000 years ago by the impact of a 300,000 tonmeteorite. The Hoba iron meteorite is the largest single meteorite known. Its present weight isestimated at 66 tons. Part of the Hoba meteorite has rusted away, therefore it's original weightmay have been as much as 100 tons! It has never been removed from its landing sight inNamibia. The largest single meteorite found in the United States is the fifteen ton Willamette(Oregon) iron meteorite found in 1902.

Comets

A comet has a distinct center called a nucleus. Most astronomers think the nucleus is made offrozen water and gases mixed with dust and rocky material. Comet nuclei are described as dirtysnowballs. A hazy cloud called a coma surrounds the nucleus. The coma and the nucleustogether form the comet's head.

Comets follow a regular orbit around the Sun. If the comet nucleus is pulled into an orbit whichcarries it close to the Sun, the solar heat will cause the outer layers of the icy nucleus toevaporate. During this process, dust and gases which form the coma around the nucleus arereleased. As the comet gets closer to the Sun, the coma grows. The solar winds push the dustand gas away from the coma causing them to stream off into space to form the comet's tail. The

solar winds cause the comet's tail to point away from the Sun. The tails of comets can reach 150million kilometers in length! Each time the comet passes close to the Sun, it loses some of itsmaterial. Over time, it will break up and disappear completely.

Many comets enter an elliptical orbit and repeatedly return to the inner solar system where theycan be viewed from Earth at specific times. Short period comets, of which Halley's Comet is themost famous, reappear within a 200 year time frame. Halley's makes an appearance once every76 years. The comet was named after Sir Edmond Halley.


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A comet has no light of its own. We are able to see a comet because of the reflection of the Sun'slight off of the comet and because of the gas molecules in the coma releasing energy absorbedfrom the Sun's rays.

Illustrations of Satellites Around Earth


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QuickBird Images of Syria Nuclear Site


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Footprint of satellite
http://en.wikipedia.org/wiki/Image:Satellite_Footprint.png


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New geostationary meteorological satellite- Multi-functional Transport Satellite (MTSAT) series -

Space-based Global Observing System
http://www.jma.go.jp/jma/jma-eng/satellite/parts/Space-based%20Global%20Observing%20Systemb.gifhttp://www.jma.go.jp/jma/jma-eng/satellite/parts/MTSAT&Earth.gif


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Geostationary Meteorological Satellite System of MTSAT-1R

Satellite TV System

Early satellite TV viewers were explorers of sorts. They used their expensive dishes todiscover unique programming that wasn't necessarily intended for mass audiences. The dishand receiving equipment gave viewers the tools to pick up foreign stations, live feedsbetween different broadcast stations,NASAactivities and a lot of other stuff transmittedusing satellites.

Some satellite owners still seek out this sort of programming on their own, but today, mostsatellite TV customers get their programming through a direct broadcast satellite (DBS)provider, such as DirecTV or DISH Network. The provider selects programs and broadcaststhem to subscribers as a set package. Basically, the provider's goal is to bring dozens oreven hundreds of channels to your TV in a form that approximates the competition, cable TV.

Unlike earlier programming, the provider's broadcast is completely digital, which means ithas much better picture and sound quality (see How Digital Television Worksfor details).Early satellite television was broadcast in C-band radio -- radio in the 3.7-gigahertz (GHz) to6.4-GHz frequency range. Digital broadcast satellite transmits programming in the Ku

frequency range (11.7 GHz to 14.5 GHz ).
http://science.howstuffworks.com/nasa.htmhttp://science.howstuffworks.com/nasa.htmhttp://science.howstuffworks.com/nasa.htmhttp://electronics.howstuffworks.com/dtv.htmhttp://electronics.howstuffworks.com/dtv.htmhttp://www.jma.go.jp/jma/jma-eng/satellite/parts/Geostationary_Meteorological_Satellite_System_of_MTSAT-1Rb.gifhttp://science.howstuffworks.com/nasa.htmhttp://electronics.howstuffworks.com/dtv.htm


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basics of aerospace & satellites detailed)

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