As most respondents have pointed out, something in orbit has a huge
amount of kinetic energy that has to go *somewhere*. Either you spend
that much energy slowing down with rockets, or you dump it into the
atmosphere as heat.

There is another possibility that hasn't been mentioned, though: you
could transfer that energy to something else in orbit. I'm thinking of
a momentum-exchange tether (http://www.tethers.com/MXTethers.html),
probably of the rotating variety. Here's how it would work:

Space travellers in orbit zip around at, say, 17000 kph. Also in orbit
is a large mass connected to a very long, strong tether, rotating
something like a wheel as it orbits, so that the high end (away from the
Earth) is moving much faster relative to ground than the low end (closer
to the Earth). At the high end, the tip of the tether, is travelling at
17000 kph, but at the low end its ground-relative velocity is only (say)
10000 kph.

So, our space travellers wait for the tether to be in the right position
at the high end, when it's travelling at the same speed they are, and
then hook their craft to it. It swings them down, and they unhook at
the low end. Presto, they're now travelling at only 10000 kph -- which
is less than orbital velocity at that altitude, so they're going down,
but they're doing it much more gently. Even larger or stronger (more
rapidly rotating) versions of the tether could of course have even more
benefit -- even dropping the ship down stationary with respect to the
Earth.

Where does all that kinetic energy go? Into the tether system, of
course. It rises up to a slightly higher orbit. How much its orbit
changes depends on the mass of the tether system compared to the ship;
ideally it would mass a lot more, so its orbit wouldn't change much.

But here's the really cool part: the tether system acts as a "momentum
bank". That energy imparted to it from the ship can be used again to
haul the next ship up to orbit. On launch, the ship only has to attain
the speed of the slow end of the tether, i.e. 10000 kph in my example
above. Then the tether imparts the rest of the energy needed to fling
it up into orbit. Its own orbit is reduced as a result, of course, but
it gets some of that back when it drops the ship back down. What energy
is lost to atmospheric drag, mass ejected from the ship, etc. can be
made up for in more leisurely (and efficient) ways, such as
electrodynamic propulsion.

So, a rotating tether helps with the two biggest problems we have today:
getting to orbit, and getting back down. A spacecraft that by itself is
only capable of suborbital launch and reentry, can nonetheless reach and
leave orbit safely with the help of the tether system, and we're no
longer wasting huge amounts of energy both ways -- much of it is simply
being banked and reused instead.

Best,
- Joe

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