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Posted by Jim Oberg on August 31, 2007, 4:28 pm
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NASA: Space Station "moves away" from shuttle after undocking
The page below has a cool image of the ISS with the boot of Italy and the
west coast of Greece in the background -- classical human historical sites
with 'classical' and easily recognizable shorelines. But instead of
identifying the region, the caption suggested that after undocking with
STS-118, the space station "moved away" from the shuttle. Call me
geocentric, or even target-centric, but in MY day in the trench, it was
usually the shuttle that moved away from passive targets. I guess it's all
relative.
http://www.nasa.gov/mission_pages/station/main/index.html
http://www.nasa.gov/images/content/164461main_s118e09467.jpg
Image above: Backdropped by Earth's horizon and the blackness of space, the
International Space Station appears to be very small as it moves away from
Space Shuttle Endeavour. Earlier the STS-118 and Expedition 15 crews
concluded nearly nine days of cooperative work onboard the shuttle and
station. Image credit: NASA
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Posted by Jorge R. Frank on August 31, 2007, 11:03 pm
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Jim Oberg wrote:
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> NASA: Space Station "moves away" from shuttle after undocking
>
> The page below has a cool image of the ISS with the boot of Italy and the
> west coast of Greece in the background -- classical human historical sites
> with 'classical' and easily recognizable shorelines. But instead of
> identifying the region, the caption suggested that after undocking with
> STS-118, the space station "moved away" from the shuttle. Call me
> geocentric, or even target-centric, but in MY day in the trench, it was
> usually the shuttle that moved away from passive targets. I guess it's all
> relative.
Actually, both vehicles get a nudge to their inertial state vectors at
undocking due to the spring plungers in the docking mechanism. Early in
the ISS assembly sequence, ISS was less massive than the orbiter so it
received most of the momentum change from the pushoff. The tipping point
was, I think, flight 98/5A in 2001 - the orbiter was more massive than
the station at docking, but after US lab installation ISS was more
massive at undocking. Since then the majority of the spring pushoff has
gone into the orbiter state, but the change in the station state is
still detectable to the FDOs/TOPOs. And of course, the orbiter crew
performs RCS firings after undocking that cause much larger changes in
the orbiter state.
But yeah - from the viewpoint of either crew, it's all relative.
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Posted by on September 1, 2007, 1:44 am
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>From Jorge:
<snip>
> ISS was less massive than the orbiter so it
> received most of the momentum change from the pushoff.
Sir Isaac would beg to differ with that statement!
~ CT
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Posted by Danny Deger on September 6, 2007, 11:17 am
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> >From Jorge:
>> ISS was less massive than the orbiter so it
>> received most of the momentum change from the pushoff.
> Sir Isaac would beg to differ with that statement!
You are correct. The momentum change is the same, but the delta velociy is
different.
Danny Deger
show/hide quoted text
> ~ CT
>
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Posted by Stephen Williams on September 6, 2007, 1:50 pm
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Danny Deger wrote:
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>> >From Jorge:
>>> ISS was less massive than the orbiter so it
>>> received most of the momentum change from the pushoff.
>> Sir Isaac would beg to differ with that statement!
>
> You are correct. The momentum change is the same, but the delta velociy
> is different.
Well, to beat this one to death...
The momentum change would be equal *magnitude* and opposite direction.
Momentum is a vector quantity, p=mv, where p and v are vectors. If
the shuttle and ISS spring apart, you would have p_s == -p_iss. Or
m_s * v_s = - m_iss * v_iss
Since m_s and m_iss are positive quantities, v_s and v_iss will
show/hide quoted text
be opposite direction values. Further, if m_iss > m_s, then
|v_iss| < |v_s|
This all assumes a reference frame where p_s + p_iss == 0.
High school physics?
- --
Steve Williams "The woods are lovely, dark and deep.
steve at icarus.com But I have promises to keep,
http://www.icarus.com and lines to code before I sleep,
http://www.picturel.com And lines to code before I sleep."
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>
> The page below has a cool image of the ISS with the boot of Italy and the
> west coast of Greece in the background -- classical human historical sites
> with 'classical' and easily recognizable shorelines. But instead of
> identifying the region, the caption suggested that after undocking with
> STS-118, the space station "moved away" from the shuttle. Call me
> geocentric, or even target-centric, but in MY day in the trench, it was
> usually the shuttle that moved away from passive targets. I guess it's all
> relative.