The answer
isn't as simple as it may seem. The US government awards wings to astronauts
who have flown in a powered vehicle to a height of 50 nautical miles (about 80 km) above Earth's surface. The
rest of the world recognizes a spaceflight boundary of 100 kilometers; but even at this
altitude, a spacecraft can circle Earth only a few times
before its orbit decays due to atmospheric drag. To maintain orbit for a day or
longer, a spacecraft must attain an
altitude of at least 80 nautical
miles (about 130 km). This may sound high up, but if
Earth were a peach, then this altitude would be right at the top of its fuzz.
Even at
an altitude of 300 kilometers,
there's more than enough atmosphere to make knowledge of its effects essential.
Beyond the drag it causes on spacecraft, the atmosphere at that height has a
different composition than the air at sea level. Significantly, it contains a much
higher percentage of atomic oxygen. Unlike molecular
oxygen, which consists of two oxygen atoms bonded together, atomic oxygen is
highly reactive and will attack spacecraft components susceptible to oxidation
(rust), weakening and eroding them.
In the
deep space beyond Earth's atmosphere, a near-perfect vacuum exists, but this type of environment can
cause problems as well, such as out-gassing (the emission of gases from within a spacecraft) and off-gassing
(the emission of gases from the surface of a spacecraft). Both of these
conditions can cloud sensors and interfere
with instrument readings. The near-perfect vacuum of space can also cause cold-welding, which is a fusing
together that occurs when parts manufactured from similar metals come into contact
in a vacuum.
Gravity
can be another confusing aspect of
the space environment. The idea that vehicles in Earth orbit operate in zero
gravity is a common misconception. Let's say that you could build a tower as tall
as the typical low Earth orbit (about 300 km). Standing at the top of the tower, you would experience
a gravitational pull equal to about 91 percent of Earth's gravitational force
at sea level. The reason that astronauts in orbit float around is not that
they're weightless; the reason is that they're in free fall. Like all objects
in orbit, astronauts are falling
all the time, but they
don't hit the ground because the ground is always curving away from them. (‘The
Bedside Baccalaureate’, edited by David Rubel)
