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basic question on orbits of space ships/stations



 
 
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  #1  
Old April 23rd 04, 08:14 AM
Maarten
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Default basic question on orbits of space ships/stations

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



  #2  
Old April 23rd 04, 01:47 PM
Jorge R. Frank
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Default basic question on orbits of space ships/stations

"Maarten" no_one@nowhere wrote in
i.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 ?


In the altitude bands you give, the orbit period ranges from 89-96 minutes.
That's close enough that most astronauts just round it to 1.5 hours when
giving interviews. That number should therefore not be taken literally for
computing orbital altitude. There *are* spacecraft that have orbited at 250
or 600 km (the Hubble Space Telescope is near 600 km, as one example), but
most spaceflights in the last three decades have visited a space station -
either the Salyuts, Skylab, Mir, or ISS - and those stations have, for the
most part, been placed in orbits around 400 km. Decisions on space station
altitude are a tradeoff between radiation levels, orbit decay lifetime, and
rendezvous performance by visiting vehicles.

The Earth is surrounded by the Van Allen radiation belts. The inner belt
extends from about 650-6300 km in altitude, peaking between 2000-5000 km.
The lower altitude is not uniform over the Earth's surface, because the
belts are created by the Earth's magnetic field and the field is not
centered quite at the Earth's center. So there is a region called the South
Atlantic Anomaly where the belt dips even closer to the Earth's surface.
Space stations have always orbited within the inner belt so that visiting
crews don't have to fly through the belts to get there, so the maximum
altitude would be 650 km - the lower the better, as far as radiation dose
is concerned.

The Earth's atmosphere doesn't just abruptly end at some altitude above the
surface; its density decays exponentially with altitude. So there is still
a measurable atmosphere in low Earth orbit that causes drag on a spacecraft
and will eventually cause it to fall out of orbit if not reboosted. The
higher the orbit, the lower the drag, and the less reboost required. So the
higher the better.

A visiting vehicle must expend more propellant to reach a space station in
a higher orbit, which reduces the amount of payload it can carry. The
Russian Soyuz/Progress spacecraft, for example, have a "rendezvous ceiling"
of about 410 km. So the lower the better. On the other hand, the spacecraft
generally launches into a lower orbit than the station and uses the shorter
orbit period to "catch up" to the station. The difference in period is
roughly proportional to the difference in height, so a space station in a
higher orbit gives a visiting spacecraft more room to "catch up" to it,
therefore widening the rendezvous launch window.

So, to summarize:

The lower the better: radiation dose, visiting vehicle payload performance
The higher the better: drag/decay lifetime, visiting vehicle launch window

400 km seems to be a pretty good compromise between all these factors, so
that's why most space stations to date have been assembled there.


--
JRF

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check "Organization" (I am not assimilated) and
think one step ahead of IBM.
  #3  
Old April 23rd 04, 02:01 PM
bob haller
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Default basic question on orbits of space ships/stations


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



Its about the highest the shuttle can go with much useful payload, and the crew
is protected from the radiation belts too.

Actually ISS would be easier to maintain in orbit if it were higher, but then
we would have no way to get a crew to the station so it doesnt matter.
:
:
:
My opinion is right
  #4  
Old April 23rd 04, 03:19 PM
jeff findley
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Default basic question on orbits of space ships/stations

"Maarten" no_one@nowhere 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 ?


The higher you go, the more fuel it takes to get there, and the closer
you get to the van Allen radiation belts. The lower you go, the
faster your orbit decays due to the increasing density of air
molecules. For a space station, the primary consideration is fuel to
get to the orbit and orbital decay (which requires more fuel for reboost).

Jeff
--
Remove "no" and "spam" from email address to reply.
If it says "This is not spam!", it's surely a lie.
  #5  
Old April 23rd 04, 05:09 PM
Allen Thomson
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Default basic question on orbits of space ships/stations

"Maarten" no_one@nowhere wrote

So my question is, why this preference for 400 km


For a long-duration space station, 400 km is kind of the
"Goldilocks" altitude between atmospheric drag and radiation
exposure from protons in the inner Van Allen belt.


and why isn't it an orbit of 250km


Too much air.

or 600km or even more ?


Too many protons. Also, present transport vehicles are limited
in their ability to carry masses of the desired size to such
altitudes.

http://www.estec.esa.nl/wmwww/wma/Ba...d/rad_env.html
http://www.windows.ucar.edu/spaceweather/sat_drag.html

http://www.heavens-above.com/ssorbitdecay.asp shows rather
graphically, so to speak, why 250 km is too low.
  #6  
Old April 23rd 04, 06:03 PM
Bob Bernatchez
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Default basic question on orbits of space ships/stations

"Maarten" no_one@nowhere 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


I believe that the highest of those orbits would be in or near the Van
Allen belts. Lower orbits would be difficult to maintain due to
increased atmospheric drag on the spacecraft. Others more
knowledgable here will probably chime in with other possible reasons.

Hope this helps.

Bob Bernatchez
  #7  
Old April 24th 04, 08:09 AM
Maarten
external usenet poster
 
Posts: n/a
Default basic question on orbits of space ships/stations

Dear all,

I am amazed by the response and its content: very informative, very useful.
Thank you all a lot, no restless nights anymore!
Maarten

"Bob Bernatchez" schreef in bericht
om...
"Maarten" no_one@nowhere 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


I believe that the highest of those orbits would be in or near the Van
Allen belts. Lower orbits would be difficult to maintain due to
increased atmospheric drag on the spacecraft. Others more
knowledgable here will probably chime in with other possible reasons.

Hope this helps.

Bob Bernatchez



  #8  
Old April 24th 04, 03:48 PM
Ian Stirling
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Posts: n/a
Default basic question on orbits of space ships/stations

Maarten no_one@nowhere wrote:
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 ?


At 250Km, the drag is much, much more, and ISS would reenter much sooner.
Much above 400Km, and the radiation from the Van Allen belts tends to be
a problem.

 




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