THERE ARE SEVERAL key things that every spaceship has to do if it wants to leave orbit and come back to Earth. The most obvious is changing its flight path to bend down toward the atmosphere, where the air drag will capture it and bring it relentlessly down to the surface. Next is withstanding the tremendous temperatures of reentry. Changing your flight path angle in an airplane is a relatively easy thing; you push forward on the stick and the air pressure on the elevator moves the nose of the airplane down and the trees get bigger. Pull back on the stick and the trees get smaller.
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However, in space we have Sir Isaac Newton to thank for a very useful trick that allows astronauts to come home. Orbital mechanics are what determine a spacecraft’s motion once in space, and to change your course to the left or right you need a tremendous amount of delta-v, or change in speed. Because of this, it’s very inefficient to change your inclination, or heading. Most human spacecraft carry only
enough rocket fuel to change their heading by a few tenths of a degree to the left or right. The good news is that we don’t have to move left or right to come back to Earth, we just need to go down. Here’s where the useful trick comes in handy—if you slow down, your orbit will descend. Conversely, speeding up makes your orbit climb. The amount of delta-v required for this trick is much less than for changing your inclination.
