Why Earth Orbit is the Key to Getting Almost Anywhere
Some perceptive people may wonder why I focus so on orbital velocity, rather than escape velocity as the key to spaceflight. After all orbital velocity simply gets one falling round the Earth forever; it is escape velocity that takes one interesting places.
The reason why is simple. Reaching orbit must be done rapidly assuming that you are doing it by rocketry; if you almost make orbit you're going to fall back down to Earth on a sub-orbital ballistic trajectory. But once you're in orbit, you can take your time about achieving escape velocity. You need big thundering high-gee boosters to get into orbit; but from orbit a pokey little solar-powered ion drive can get you to escape velocity, because you can add velocity slowly, curving one's orbit out farther and farther until your speed and the escape velocity at that distance from the center of the Earth intersect.
This of course will no longer be the case once we have space elevators, skyhooks or some other mechanical system of achieving orbit, but it is very much still the case now.
Something similar is true on arrival. With good astrogation, you can take almost any old capture orbit and be ok, adjusting the orbit later at your leisure, but you'd better have damn good engines, air braking devices, and/or heat shielding (depending just which world you're orbiting) when you attempt a landing. Which is why, of course, they build specialized lander vehicles.
The reason why is simple. Reaching orbit must be done rapidly assuming that you are doing it by rocketry; if you almost make orbit you're going to fall back down to Earth on a sub-orbital ballistic trajectory. But once you're in orbit, you can take your time about achieving escape velocity. You need big thundering high-gee boosters to get into orbit; but from orbit a pokey little solar-powered ion drive can get you to escape velocity, because you can add velocity slowly, curving one's orbit out farther and farther until your speed and the escape velocity at that distance from the center of the Earth intersect.
This of course will no longer be the case once we have space elevators, skyhooks or some other mechanical system of achieving orbit, but it is very much still the case now.
Something similar is true on arrival. With good astrogation, you can take almost any old capture orbit and be ok, adjusting the orbit later at your leisure, but you'd better have damn good engines, air braking devices, and/or heat shielding (depending just which world you're orbiting) when you attempt a landing. Which is why, of course, they build specialized lander vehicles.