Believe it or not:<br />The shortest distance between two points in space is a curved line if you are trying to save on fuel.<br />
What is an orbit? <br />An orbit is the path an object travels around a point.<br />An orbit is also the balance between the pull of gravity on an object compared to the speed and direction of the object.<br />An object in low Earth orbit is moving at 5 miles per second.<br />The pull of gravity is 32 feet per second toward the Earth.<br />The Earth curves 32 feet for every 5 miles you go out.<br />So, if an object is moving at 5 miles per second, and in that time it falls 32 feet, and the Earth curves 32 feet for every 5 miles you go out, the spacecraft is still 32 feet above the Earth after the 5 miles it moved forward in one second.<br />
Low Earth Orbit<br /> 5 miles forward and drop of 32 feet<br /> 5 miles forward and drop of 32 feet<br />NOT TO SCALE<br />
What would happen…<br />In essence, you have a balance between Inertia (the force forward on the object) and the pull of gravity.<br />To achieve a higher or lower orbit, what are you going to have to change?<br />Do you change the inertia of the spacecraft or the gravity of the Earth?<br />If you increase the inertia (push the spacecraft forward) it will achieve a higher orbit.<br />If you decrease the inertia (slow the spacecraft down by firing rockets in the opposite direction of the orbit) it will achieve a lower orbit or fall to the surface.<br />
Everything that goes up…<br />In space a straight line may be the shortest distance between two points but it is also the most costly in fuel.<br />If you want something to go straight up you will need to constantly push the object up.<br />As the object goes up gravity wants to pull it down so you will need to keep pushing on it.<br />You will constantly have to fire engines using up fuel and the fuel is heavy so you need more fuel to produce thrust to counter the weight….<br />
If something is in orbit…<br />One you have an object in orbit it will stay in orbit unless something happens to it.<br />Remember Newton’s First Law of Motion: an object in motion tends to remain in motion unless acted upon by an outside force.<br />So, once an object is in orbit you do not need to constantly push it forward.<br />What happens if an object is in orbit and you add to its inertia?<br />
Adding inertia<br />When you add inertia (an engine burn in the direction of travel) the orbit will become elliptical and one end of the orbit will move farther away.<br />
Putting this together:<br />Instead of sending something to the Moon in a straight line, you can increase its orbit until the new orbit intercepts the Moon. <br />Once it runs into the Moon’s gravity it will fall toward the Moon in a curved path putting it in orbit around the Moon.<br />
What about other planets?<br />The same process would be used to reach another planet.<br />
Hoffman Transfer Orbit<br />The Hoffman Transfer Orbit sends an object on a highly elliptical orbit that, if set up correctly, will intercept another object.<br />This transfer orbit uses less fuel than traveling in a straight line but takes longer to get to your destination.<br />
So…<br />If you want to send a probe, lander or satellite way from the Earth you have to put it in a highly elliptical orbit.<br />If you want it to have a stable higher orbit you will need to fire the engines in the direction of the new orbit when it reaches the proper distance from the Earth.<br />To get to the Moon you will need to go 25,008MPH. To get to Mars you will need to go 94,891 mph.<br />
And So:<br />If you want to land something on Mars you are going to have to go from 94,000 miles per hour to zero in a very short period of time.<br />Not only that but your spacecraft or you will have to survive these changes.<br />
Credits<br />A Crewed Mission to Mars...<br />http://nssdc.gsfc.nasa.gov/planetary/mars/marslaun.html<br />Apollo lunar landing launch window: The controlling factors and constraints<br />http://history.nasa.gov/afj/launchwindow/lw1.html<br />The Apollo Flight Journal: Lunar Orbit Rendezvous<br />http://history.nasa.gov/afj/loressay.htm<br />Flight to Mars: How Long? Along what Path?<br />http://www-istp.gsfc.nasa.gov/stargaze/Smars1.htm<br />
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