To answer these question let's start by limiting possibilities:
Cost Of Building International Space Station
The
International Space Station costed us an
estimated a hundred billion dollars to
build. Therefore, everything about it is
engineered with precision because
anything that is designed differently
that in the long term cost us hundreds
of millions of dollars more.
Purpose Of International Space Station
According to the memorandum between NASA
and the Russian space agency article 2, section 2.3, "the purpose of the
International Space Station is to enable
its users to take advantage of human
ingenuity in connection with its low
gravity environment, the near-perfect
vacuum of space and the vantage point
for observing the earth and the rest of
the universe."
Specifically the
International Space Station is aspired
to be a space laboratory Observatory, a
transportation node and possibly a
staging base for future missions, mong
many other capabilities it needs to
provide.
In other words whatever the
orbit might be for the International
Space Station, it needs to provide a
comfortable living space for astronauts
and scientists , and it needs to perform
the aforementioned functions. Therefore
among many other things the astronauts
need to be safe. This hands eliminate the
possibility of having an International
Space Station above 700 kilometers
altitude because the radiation level
there is too high for human astronauts.
Van Allen Radiation Belts
Surrounding our earth is the Van Allen radiation belts that extend from 700 kilometers
altitude to 60,000 kilometers altitude. Within the belt it's filled with
particles which are dangerous for
astronauts. But, because of that anything
under 700 kilometers altitude is
shielded from our sun's radiation which
make it a safe environment for
astronauts to reside.
Also anything
orbiting the earth below 300 kilometers
altitudes will be subjected to
significant atmospheric drag meaning
the air will be dragging the enormous
International Space Station down at an
altitude of 400 kilometers. Our
International Space Station is already
dropping to the earth at the speed of
around 2 kilometers per year and
reboosts are performed every year to
bring it up. Therefore, we can't afford to
have an International Space Station
below 300 kilometers altitude. Either, this effectively limits its orbit height
to anything between 300 kilometers to
700 kilometers, this still doesn't give
us a reason why 400 kilometers was
chosen to be the final orbit of the
International Space Station.
Cost Of Launching Rocket
The last
piece of the puzzle is cost when the
International Space Station was
initiated in 1998, cost of launching
rocket is extremely high and to reach a
higher orbit of 700 kilometers altitude
means tens of millions of additional
funding required per flight. Counting in
the number of flights every year it is a
lot of money. So in conclusion, the choice
of 400 kilometers altitude for the
International Space Station is really
made. Based on the technical constraints
we have and the economic sustainability
we're after. If we can accomplish all
objectives at 400 kilometers altitude at
a much lower cost we just shouldn't
build the International Space Station as
700 kilometers altitude.