The space elevator


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The space elevator

  1. 1. Submitted by- Nahid Anjum Praxis Business School1
  2. 2. Praxis Business School The Space Elevator A report submitted to Prof. Prithwis MukherjeeIn partial fulfilment of the requirements of the course Business Information System On 7th November 2010 By Nahid Anjum 2
  3. 3. IndexNo. Topic Page No. 1 Abstract 4 2 Introduction 4 3 Space Elevator 4 4 Structure 5 5 How the space elevator will work 7 6 Space Elevator ribbon 8 7 Riding a space elevator to the top 9 8 Transport system for the space elevator 10 9 Delivery capabilities 11 Cost of suggested space transport10 12 installation11 Cost of delivery 1312 Space Elevator maintenance 1313 Space Elevator impact 1414 Conclusion 1515 Reference 16 3
  4. 4. Abstract: At present, rockets are used for launches and flights into space and to carry people and payloadsinto space. It is only the source to connect us from space. This method is very expensive, and requires a welldeveloped industry, high technology, expensive fuel and complex devices. Their major drawbacks are veryhigh cost of space launching $20,000 – 50,000/kg, large fuel consumption and fuel storage problems becausethe oxidizer and fuel require cryogenic temperatures, or they are poisonoussubstances. In recent years scientists have investigated a series of newmethods for non-rocket space launch, which promise to revolutionize spacelaunches and flight. Especially in this area new, cheaper and more fuelefficient methods are being investigated. Such new methods include the gastube method, cable accelerators, tether launch systems, space elevators,solar and magnetic sails, circle launcher space keepers and more.Introduction: Non-rocket space launch is an idea to reach outer space specifically from the Earth‘s surfacewithout the use of traditional rockets, which today is the only method in use. In the past years the scientistshave published a series of new methods which promise to revolutionize space launching and flight. Theseinclude the gas tube method, cable accelerator, tether launch systems, space elevators, solar and magneticsails, circle launcher and space keeper, space elevator transport system, etc. Some of these have thepotential to decrease launch costs thousands of times, other allow the speed and direction of space apparatusto be changed without the spending of fuel. The idea is very unique to go to space without rockets and withoutfuel consumption.Space Elevator:The space elevator is a cable-like tool which could connect the earth witha fixed structure in outer space. It is a proposal structure designed totransport material from a celestial body‘s surface into space. It wouldprovide a permanent link between earth and outer space which could beable to send material or person to space. The concept most often refersto a structure that reaches from the surface of the earth on near theEquator to geostationary orbit (GSO) and a counter-mass outside of theatmosphere. A space elevator for earth would consist of a cableanchored to the earth‘s surface, reaching into space. By attaching acounterweight at the end or by further extending the cable for the samepurpose, inertia ensures that the cable remains stretched out, counteringthe gravitational pull on the lower sections, thus allowing the elevator toremain in geostationary orbit. Once beyond the gravitational midpoint, carriages would be accelerated furtherby the planet‘s rotation. The space elevator is a theoretical concept which will provide a permanent linkbetween earth and space. 4
  5. 5. Structure: The centrifugal force of earth‘s rotation is the main principle behind the elevator. As the earthrotates, the centrifugal force tends to align the nanotubes in a stretched manner. There are a variety of tetherdesigns. Almost every design includes a base station, a cable, climbers, and a counterweight. Base station- The base station can be categorized into two categories—mobile and stationary. Mobile stations are large oceangoing vessels. Mobile platforms have the advantage of being able to avoid high winds, storms, and space debris. Whereas stationary platforms would generally be located in high- altitude locations, such as on top of the mountains, or even potentially on high towers. They have access to cheaper and more reliable power sources, and require a shorter cable. Cable- The cable in a space elevator must be strong enough to carry its own weight as well as the weight of the climbers. The required strength of the cable will vary along its length, since at various points it has to carry the weight of the cable below, or provide a centripetal force to retain to retain the cable and counterweight above. The cable in a space elevator could only be constructed from an extremely strong, flexible and light weight material such as carbon nanotubes. There are some properties of carbon nanotubes due to which it can be used in the cable of space elevator. They are 200 times stronger than steel. It is the first synthetic material to have greater strength than spider silk. It is heat resistant as it resists burning like a metal. Its molecular structure is carbons atoms in regular, tabular structure. Its properties are strong, light metal-like. Its properties make it possible to be used in cable of space elevator.The tensile strength of several materials and their comparison with carbon nanotubes—Material Young‘s modulus Tensile strength Density (GPa) (GPa) (g/cm3)Single wall nanotube 1054 150 1.4Multi wall nanotube 1200 150 2.6Diamond 600 130 3.5Kevlar 186 3.6 7.8Steel 208 1.0 7.8Wood 16 0.008 0.6 Climbers- The elevator cable anchored to the ground is counter-balanced by an equal length of cable beyond the geosynchronous point, built up by photocell-pushed ―climbers‖. These climbers would also be used to launch payloads up the elevator. Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, most propose to use pairs of rollers to hold the cable with friction. Other 5
  6. 6. climber designs involve magnetic levitation. Climbers must be placed at optimal timings so as to minimize cable stress and oscillations and to maximize throughput. Lighter climbers can be sent up more often, with several going up at the same time. This increases throughput somewhat, but lowers the mass of each individual payload. Both power and energy are significant issues for climbers – the climbers need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload. All proposals to get that energy to the climber fall into three categories—1. Transfer the energy to the climber through wireless energy transfer while it is climbing2. Transfer the energy to the climber through some material structure while it is climbing3. Store the energy in the climber before it starts—this requires an extremely high specific energy Nuclear energy and solar power has been proposed, but generating enough energy to reach the top of the elevator in any reasonable time without weighing too much is not feasible. The horizontal speed of each part of the cable increases with altitude, proportional to distance from the center of the earth, reaching orbital velocity at geostationary orbit. Therefore as a payload is lifted up a space elevator, it needs to gain not only altitude but angular momentum as well. This angular momentum is taken from the earth‘s own rotation. As the climber ascends it is initially moving slightly more slowly than the cable that it moves onto and thus the climber drags on the cable. 6
  7. 7. Counter weight—several solutions have been proposed to act as a counterweight:1. A heavy, captured asteroid2. A space dock, space station or spaceport positioned past geostationary orbit3. An extension of the cable itself far beyond geostationary orbit. The concept of counterweight is like a small asteroid is diverted from deep space and locked into high orbit above earth. The end of the elevator cable beyond geosynchronous orbit is anchored to it as a counterweight. The mass of the asteroid moving in a higher orbit keeps the cable under tension and the cable straight. This way, the overall length of the cable can be greatly shortened. A shorter cable may be desirable for economic reasons; today, carbon nanotubes cost about $500 per gram of mass, or roughly $500 million dollars per ton. A space elevator cable will, of course, weigh many thousands of tons. If this price does not significantly building an equal length of cable beyond geosynchronous orbit. The asteroid could also have the added advantage of being used as a source of raw materials to build space facilities for the elevator, such as the geosynchronous station, or complete additional cables for more tracks along the elevator. The third idea has gained more support in recent years due to the relative simplicity of the task and the fact that a payload that went to the end of the counterweight- cable would acquire considerable velocity related to the earth, allowing it to be launched into interplanetary space. How the space elevator will work— The basic principle of a space elevator is fairly simple to envision. Tie a string to a baseball and twirl the string above your head. The string will remain taut and straight as long as the twirling motion is in effect. The earth is spinning far faster than your hand could ever manage, about 1000 miles per hour. If you anchored an incredibly strong wire to earth‘s surface at the equator, then attached the other end to a large enough mass to keep it taut, you end up with a perfectly straight railroad track right into space. The space elevator‘s center of mass would be at geosynchronous orbit, approximately 22,300 miles above the equator, helping to keep the entire construct fixed over a stable position on earth. The geosynchronous point is also where the cable would be under the most stress, so it would have to be thickest there and taper down exponentially as one move away from it in either direction. Once the cable is set up, elevators can ride it up and down via magnetic rails, delivering cargo straight into orbit. The earth-end of the elevator cable is usually envisioned as being attached to the top of a 7
  8. 8. mountain or a super-high artificial tower. However, though both of these options could imply setting up theelevator, they are not strictly necessary. One scheme, primarily involving the photocell climber elevator,details anchoring the cable to a specially-built but standard-height off-shore platform. The concept is that a very long cable will be laid around 35 degree to -35 degree longitude i.e.highest lifting efficiency at equator, up into space, with the center of mass at the geosynchronous orbit that willbe a counterweight either in form of space station or asteroids will be in higher latitude. From the groundstation, the climber will climb up the cable, powered by, with current idea of ground based laser that will strikethe photovoltaic cells abroad the climber. Its estimated speed is around 190 km/h, so it would take around aweek to get up. Climbers ascend a ribbon, 100,000 km long, strung between an anchor on earth and space ina way never before possible, the space elevator will enable us to inexpensively and completely expand oursociety into space.Space elevator ribbon— The space elevator ribbon is the carbon nanotubes composite ribbon.The counterweight spins around the earth, keeping the cable straight andallowing the robotic lifters to ride up and down the ribbon. Under the designproposed, the space elevator would be approximately 62,000 miles (100,00 km)high. The centrepiece of the elevator will be the carbon nanotubes compositeribbon that is just a few centimetres wide and nearly as thin as a piece of paper.Carbon nanotubes, discovered in 1991, are what make scientists believe that thespace elevator could be built. Carbon nanotubes have the potential to be 100times stronger than steel and are as flexible as plastic. The strength of carbonnanotubes comes from their unique structure. Once scientists are able to makefibres from carbon nanotubes, it will be possible to creates threads that will formthe ribbon for the space elevator. Previously available materials were either tooweak or inflexible to form the ribbon and would have been easily broken. Theyhave very high elastic modulus and their tensile strength is really high, and that all points to a material thatshould make a space elevator relatively easy to build. A ribbon could be built in two ways:1. Long carbon nanotubes—several meters long or longer—would be braided into a structure resembling a rope. As of 2005, the longest nanotubes are still only a few centimetres long.2. Shorter nanotubes could be placed in a polymer matrix. Current polymers do not bind well to carbon nanotubes, which results in the matrix being pulled away from the nanotubes when placed under tension. Once a long ribbon of nanotubes is created, it would be wound into a spool that would be launched into orbit. When the spacecraft carrying the spool reaches a certain altitude, perhaps Low Orbit it would being unspooling, lowering the ribbon back to earth. At the same time, the spool would continue moving to a higher altitude. When the ribbon is lowered into earth‘s atmosphere, it would be caught and then lowered and anchored to a mobile platform in the ocean. The ribbon would serve as the tracks of a sort of railroad into space. Mechanical lifters would then be used to climb the ribbon to space. 8
  9. 9. Riding a space elevator to the top— While the ribbon is still a conceptual component, all of theother pieces of the space elevator can be constructed using knowntechnology, including the robotic lifter, anchor station and power-beaming system. By the time the ribbon is constructed, the othercomponents will be nearly ready for a launch sometime around 2018.Lifter—The robotic lifter will use the ribbon to guide its ascent intospace. Traction tread rollers on the lifter would clamp on to then ribbonand pull the ribbon through, enabling the lifter to climb up the elevator.Anchor Station—The space elevator will originate from a mobileplatform in the equatorial Pacific, which will anchor the ribbon to earth.Counterweight—At the top of the ribbon, there will be a heavy counterweight. Early plans for the spaceelevator involved capturing an asteroid and using it as a counterweight. However, more recent plans includethe use of a man-made counterweight. In fact, the counterweight might be assembled from equipment used tobuild the ribbon including the spacecraft that is used to launch it.Power Beam—The lifter will be powered by a free-electron laser system located on or near the anchorstation. The laser will beam 2.4 megawatts of energy to photovoltaic cells, perhaps made of Gallium Arsenide(GaAs) attached to the lifter, which will then convert that energy to electricity to be used by conventional,niobium-magnet DC electric motors. Once operational, lifters could be climbing the space elevator nearly every day. The lifters will vary insize from five tons, at first, to 20 tons. The 20-ton lifter will be able to carry as much as 13 tons of payloadsand have 900 cubic meters of space. Lifters would carry cargo ranging from satellites to solar-powered panelsand eventually humans up the ribbon at a speed of about 118 miles per hour. 9
  10. 10. Transport system for the space elevator – This section proposes a new method and transportation system to fly into space, to the Moon, Mars, and other planets. This transportation system uses a mechanical energy transfer and requires only minimal energy so that it provides a ‗Free Trip‘ into space. It uses the rotary and kinetic energy of planets, asteroids, meteorites, comet heads, moons, satellites, and other natural space bodies. The main difference in the offered method is the transport system for the space elevator and the use of the planet rotational energy for a free trip to another planet, for example, Mars. The objective of these innovations is to provide an inexpensive means to travel to outer space and other planets, simplify space transportation technology and eliminate complex hardware. This goal is obtained by new space energy transfer for long distance, by using engines located on a planet, the rotational energy of a planet, or the kinetic and rotational energy of the natural space bodies.Free trip to moon— A proposed centrifugal space launcher with a cable transport system which includes an equalizerlocated in geosynchronous orbit, an engine located on earth, and the cable transport system having threecables—a main central cable of equal stress and two transport cables, which include a set of mobile cablechains, connected sequentially one to another by the rollers. One end of this set is connected to the equalizer,the other end is connected to the planet. Such a separation is necessary to decrease the weight of thetransport cables, since the stress is variable along the cable. This transport system design requires aminimum weight because at every local distance the required amount of cable is only that of the diameter forthe local force. The load containers also connected to the chain. When containers come up to the rollers andcontinue their motion up to the cable. The entire transport system is driven by any conventional motor locatedon the planet. When payloads are not being delivered into space, the system may be used to transfermechanical energy to the equalizer. This mechanical energy may also be converted to any other sort ofenergy. The space satellites released below geosynchronous orbit will have elliptic orbits and may beconnected back to the transport system after some revolutions when the space ship and cable are in the sameposition. If low earth orbit satellites use a brake parachute, they can have their orbit closed to a circle. Thespace probes released higher than geosynchronous orbit will have a hyperbolic orbit, fly to other planets, andthen can connect back to the transport system when the ship returns. Most space payloads, like tourists, mustbe returned to earth. When one container is moved up, then another container is moved down. The work oflifting equals the work of descent, except for a small loss in the upper and lower rollers. The suggestedtransport system lets us fly into space without expending enormous energy. This is the reason why themethod and system are named a ―Free Trip‖.Assume a maximum equalizer lift force of 9 ton at theearth’s surface and divide this force between threecables—one main and two transport cables. The mass ofthe equalizer creates a lift force of 9 ton at the earth‘ssurface, which equals 518 ton for K=4. The equalizer islocated over a geosynchronous orbit at an altitude of 10
  11. 11. 100,000 km. Full centrifugal lift force of the equalizer is 34.6 ton, but 24.6 ton of the equalizer are used insupport of the cables. The transport system has three cables—one main and two in the transport system.Each cable can support a force of 3000 kgf. The main cable has a cross section area of equal stress. Thenthe cable cross section area is A=0.42mm^ at the earth‘s surface, maximum 1.4 mm^ in the middle section,and A=0.82 mm^ at the equalizer. The mass of main cable is 205 ton. The chains of two transport cable loopshave cross-section areas to equal the tensile stress of the main cable at given altitude, and the capabilities arethe same as the main cable. Each of them can carry 3 ton force. The total mass of the cable is about 620 ton.The three cables increase the safety of the passengers. If any one of the cables breaks down, then the othertwo will allow a safe return of the space vehicle to the earth and the repair of the transport system. If the container cable is broken, the pilot uses the main cable for delivering people back to earth. If the main cable is broken, then the load container cable will be used for delivering a new main cable to the equalizer. For lifting non-balance loads for example, satellites or parts of new space stations, transport installations, interplanetary ships, and the energy must be spent in any delivery method. When the transport system is used, the engine is located on the earth and does not have an energy limitation. Moreover, the transport system can transfer a power of up to 90,000 kW to the space station for a cable speed of 3 km/s. At the present time, the International Space Station has only 60 kW of power.Delivery capabilities—For tourist transportation the suggestedsystem works in the following manner. The passenger space vehiclehas the full mass of 3 ton to carry 25 passengers and 2 pilots. Oneship moves up, the other ship, which is returning, moves down; thenthe lift and descent energies are approximately equal. If the averagespeed is 3 km/s, then the first ship reaches the altitude of 21.5-23thousand km in 2 h. At this altitude the ship is separated from thecable to fly in an elliptical orbit with minimum altitude 200 km andperiod approximately 6 h. After one day the ship makes fourrevolutions around the earth while the cable system makes one revolution, and the ship and the cable will bein the same place with the same speed. The ship is connected back to the transport system, moves down thecable and lifts the next ship. The orbit may be also three revolutions or two revolutions. In one day thetransport system can accommodate 12 space ships in both directions. This means more than 100,000 touristsannually into space. The system can launch payloads into space, and if the altitude of disconnection ischanged then the orbit is changed. If a satellite needs a low orbit, then it can use the brake parachute when itflies through the top of the atmosphere and it will achieve a near circular orbit. The annual payload capabilityof the suggested space transport system is about 12,600 ton into a geosynchronous orbit. If instead of the equalizer the system has a space station of the same mass at an altitude of 100,00km and the system can have space stations along cable and above geosynchronous orbit, then, thesestations decrease the mass of the equalizer and may serve as tourist hotels, scientific laboratories, orindustrial factories. If the space station is located at an altitude of 100,000 km, then the time of delivery will be 11
  12. 12. 9.36 h for an average delivery speed of 3 km/s. This means 60 passengers per day or 21,000 people annuallyin space.Let us assume that every person needs 400 kg of food for a 1-year-round trip to Mars, and Mars has the sametransport installation. This means we can send about 2000 people to Mars annually at suitable positions ofEarth relative to Mars.Cost of suggested space transport installation— The current International Space Station has cost many billions of dollars, but the suggested spacetransport system can lost a lot less. Moreover, the suggested transport system allows us to create othertransport systems in a geometric progression. Let us examine an example of the transport system. Initially we create the transport system to lift only 50 kg of load mass to an altitude of 100,000 km.The equalizer mass is 8.5 ton, the cable mass is 10.25 ton, and the total mass is about 19 ton. Let us assumethat the delivery cost of 1 kg mass is $10,000. The construction of the system will then have a cost of $190Million then the system costs $1.25 million. Let us put the research and development (R&D) cost ofinstallation at $29 million. Then the total cost of initial installation will be $220 million. About 90% of this sum isthe cost of initially rocket delivery. After construction, this initial installation begins to deliver the cable andequalizer or parts of the space station into space. The cable and equalizer capability increase in a geom etricprogression. The installation can use part of the time for delivery of payload and self-financing of this project.After 765 working days the total mass of equalizer and cables reaches the amount above 1133 ton and theinstallation can work full time as a tourist launcher or continue to create new installations in only 30 monthswith a total capacity of 10 million tourists/year. The new installations will be separated from the motherinstallations and moved to other positions around the earth. The result of these installations allows the deliveryof passengers and payloads from one continent to another across space with low expenditure of energy. Let us estimate the cost of the initial installation. The installation needs 620 ton of cable. Let us takethe cost of cable as $0.1 million/ton. The cable cost will be $62 million. Assume the space station cost $20million. The construction time is 140 days. The cost of using the mother installation without profit is $5million/year. In this case the new installation will cost $87 million. In reality soon after construction the newinstallation can begin to launch payloads and become self-financing.Cost of delivery— The cost of delivery is the most important parameter in the space industry. Let us estimate it for thefull initial installation above. As we calculated earlier the cost of the initial installation is $220 million. Assumethat installation is used for 20 years, served by 100 officers with an average annual salary of $50,000 andmaintenance is $1 million in year. If we deliver 100,000 tourists annually, the production delivery cost will be$160/person or $1.27/kg of payload. Some 70% of this sum is the cost of installation, but the delivery cost ofthe new installations will be cheaper. If the price of a space trip is $1990, then the profit will be $183 millionannually. If the payload delivery price is $15/kg then the profit will be $189 million annually. The cable speedfor K=4 is 6.32 km/s. If average cable speed equals 6 km/s, then all performance factors are improved by a 12
  13. 13. factor of 2 times. In any case the delivery cost will be hundreds of times less than the current rocket poweredmethod.Cost (in comparison to space shuttle)Per kg: $100/kg vs $10,000-$40,000/kgConstruction: $6 billion vs $19 billionSpace elevator maintenance— At a length of 62,000 miles (100,000 km), the space elevator will be vulnerable to many dangers,including weather, space debris and terrorists. As plans move forward on the design of the space elevator, thedevelopers are considering these risks and ways to overcome them. In fact, to make sure there is always anoperational space elevator, developers plan to build multiple space elevators. Each one will be cheaper thanthe previous one. The first space elevator will serve as a platform from which to build additional spaceelevators. In doing so, developers are ensuring that even if one space elevator encounters problems, theothers can continue lifting payloads into space.Avoiding Space Debris— Like the space station or space shuttle, the space elevator will need the ability to avoid orbitalobjects like debris and satellites. The anchor platform will employ active avoidance to protect the spaceelevator from such objects. Currently, the North American Aerospace Defence Command (NORAD) tracksobjects larger than 10 cm. protecting the space elevator would require an orbital debris tracking system thatcould detect objects approximately 1 cm. This technology is currently in development for other space projects.Repelling Attacks— The isolated location of the space elevator will be the biggest factor in lowering the risk of terroristattack. For instance, the first anchor will be located in the equatorial Pacific, 404 miles (650 km) from any airor shipping lanes. Only a small portion of the space elevator will be within reach of any attack, which is 13
  14. 14. anything 9.3 miles or below. Further, the space elevator will be a valuable global resource and will likely beprotected by the U.S. and other foreign military forces.Space Elevator Impact— The potential globalimpact of the space elevator isdrawing comparisons to anothergreat transportation achievement—the U.S. transcontinental railroad.Completed in 1869at Promontory,Utah, the transcontinental railroadlinked the country‘s east and westcoasts for the first time and spedthe settlement of the Americanwest. Cross-country travel wasreduced from months to days. Italso opened new markets and gaverise to whole new industries. By 1893, the United States had five transcontinental railroads. The idea of a space elevator shares many of the same elements as the transcontinental railroad. Aspace elevator would create a permanent Earth-to-space connection that would never close. While it wouldn‘tmake the trip to space faster, it would make trips to space more frequent and would open up space to a newera of development. Perhaps the biggest factor propelling the idea of a space elevator is that it wouldsignificantly lower the cost of putting cargo into space. Although slower than the chemically propelled spaceshuttle, the lifters reduce launch costs from $10,000 to $20,000 per round, to approximately $400 per round.Current estimates put the cost of building a space elevator at $6 billion with legal and regulatory costs at $4billion. Additionally, each space shuttle flight costs $500 million, which is more than 50 times more thanoriginal estimates. The space elevator could replace the space shuttle as the main space vehicle, and be used forsatellite deployment, defence, tourism and further exploration. To the latter point, a space craft would climbthe ribbon of the elevator and then would launch toward its main target once in space. This type of launch willrequire less fuel than would normally be needed to break out of Earth‘s atmosphere. Some designers alsobelieve that space elevators could be built on other planets, including Mars.Conclusion— The Space Elevator is the most promising Space Transportation system on the drawing boardstoday, combining scalability, low cost, qualify of ride, and safety to deliver truly commercial-grade spaceaccess-practically comparable to a train ride to space. Rocket-based space launch systems are inherentlylimited by the physics of rocket propulsion. More than 90% of the rocket‘s weight is propellant and the rest is 14
  15. 15. split between the weight of the fuel tank and the payload. It is very difficult to make such a vehicle safe or lowcost. A target cost of $1000 per kg is proving to be impossible to reach. In comparison, airlines charge usabout $1 per round, and train transportation is in cents per pound. The Space Elevator is based on a thin vertical tether stretched from the ground to a mass far out in space, and climbers that drive up and down the tether. The rotation of the earth and of the mass around it keeps the tether taut and capable of supporting the climbers. The climbers travel at speeds comparable to a fast train, and carry no fuel on board – they are powered by a combination of sunlight and laser light projected from the ground. While the trip to space takes several days, climbers are launched once per day. The Space Elevator is linearly scalable. The first ―baseline‖ design will use 20 ton climbers, but by making the tether thicker we can grow the Space Elevator to lift 100, or even 1000 tons at a time. In addition to launching payloads into orbit, the Space Elevator can also use its rotational motion to inject them into planetary transfer orbits—thus able to launch payloads to Mars, for example, once per day. Imagine the kind of infrastructure we can set up there, waiting for the first settlers to arrive...Looking back from the year 2100, the construction of the Space Elevator will be considered to mark the truebeginning of the Space age; much like the advent of the airplane or steamboat heralded the true commercialuse of the air and sea. 15
  16. 16. References—1. h&source=bl&ots=S-#v=onepage&q&f=false5. preso?src=related_normal&rel=7813749. 16