basic question on orbits of space ships/stations

"Maarten" writes:

After browsing the internet for some time, I still haven't found an answer
to the following question.
As a novice in this area, I wonder why there is such a specific preference
for having e.g. ISS, Space Shuttle in an orbit of about 400km. (This I
make up from the testimonials from space ships' crew that all tell about the
sun coming up and going down every 1.5 hours).
So my question is, why this preference for 400 km and why isn't it an orbit
of 250km or 600km or even more ?

Too low an orbit and there is too much air drag on the spacecraft, so that
its orbit decays too rapidly.

Too high, and it gets into the lower fringes of the van Allen radiation belt,
which is not healthy for either the crews or the electronics.

Finally, 90 minutes divides evenly into 24 hrs, and it might be operationally
convienient for there to be roughly an integer number of orbits per workday...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Maarten" wrote in message li.nl...

So my question is, why this preference for 400 km and why isn't it an orbit
of 250km or 600km or even more ?

250km would see a lot of atmospheric drag and require considerably
more rocket boosts to keep the station in orbit.

600km would make it somewhat more costly (in terms of fuel) to lift
components and supplies to the station. The shuttle has been pushed to
near its limit to reach 400km with some of the larger station
components, particularly because of the high inclination of the
station's orbit.

Even higher orbits start getting into the radiation belts.

So, 400km is a happy balance.

Mike Miller, Materials Engineer

"Maarten" wrote in message li.nl...
After browsing the internet for some time, I still haven't found an answer
to the following question.
As a novice in this area, I wonder why there is such a specific preference
for having e.g. ISS, Space Shuttle in an orbit of about 400km. (This I
make up from the testimonials from space ships' crew that all tell about the
sun coming up and going down every 1.5 hours).
So my question is, why this preference for 400 km and why isn't it an orbit
of 250km or 600km or even more ?

Maarten

If the orbit is too low, there would be more atmospheric drag on the
station, causing the orbit to decay faster (and necessitating more
frequent reboosts).

If it's too high, you run into the problem of performance--the
vehicles you're flying to the station (Shuttle and Soyuz and Progress)
can't get that high with a reasonable payload. And past that, you
would start to run into radiation problems (Van Allen belts).

"Maarten" wrote in message li.nl...
After browsing the internet for some time, I still haven't found an answer
to the following question.
As a novice in this area, I wonder why there is such a specific preference
for having e.g. ISS, Space Shuttle in an orbit of about 400km. (This I
make up from the testimonials from space ships' crew that all tell about the
sun coming up and going down every 1.5 hours).
So my question is, why this preference for 400 km and why isn't it an orbit
of 250km or 600km or even more ?

I answered this already but somehow the message got lost.

Anywho, short answer: it's the Russians. The Soyuz and
Progress spacecraft have a maximum altitude of somewhere
around 425 km, so the station can't stay above there.
Even though higher altitudes and fewer reboosts would be
preferable.