[FPSPACE] How come ISS keeps 'gaining' altitude?

[Geert Sassen]

Several things can cause the ISS to gain altitude:

A) Effects from attitude-control thrusters: once in a while the attitude
controle thrusters need to be used to 'unwind' the gyro's which are normally
used to control the stations attitude. Apart from changing the stations
attitude these thrusters usually also cause a (very minor) velocity change
and if this happens to be in the 'forward' direction it will cause the
station to increase speed and thus gain some altitude.

B) Tidal effects. Tidal effects cause for instance the moon to gain altitude
with several centimeters per day, slowly 'leaving' the earth orbit. The same
happens to any satellite and effect will be proportional to main altitude
and inclination of the orbit. Normally other effects (atmospheric drag for
instance) cancel out this very small altitude-gain but at certain
moments/days it can cause a slight increase in orbital altitude. See
alsohttp://en.wikipedia.org/wiki/Tidal_acceleration

(As a side-note, effect B also causes a change in the earth rotation-period,
so our days are really changing due to ISS although it will be immeasurably
small..)

With ISS gaining more and more mass it should start to cause very, very,
minor gravity-related changes to other satellites, in other words objects
close to ISS should start to 'feel' the stations mass and 'fall' towards it
(or start 'orbiting' the station), and satellites passing 'close' to it
should experience a very very minor change in their orbits. Don't know
whether these effects have already become noticable (they will still be
extremely small).

Regards,

Geert.

On Fri, Apr 3, 2009 at 9:40 PM, James Oberg <jameseoberg at comcast.net> wrote:

>
>
> After the first 'gain' on Mar 29, I asked JSC PAO and they said
> there had been no maneuvers, nothing to raise the station's
> altitude. I wrote it off to a computation fluke.
>
> Now it's happened twice more, the last two days in a row.
>
> What is going on?
>
>
> http://www.hq.nasa.gov/osf/iss_reports/index.htm
>
>
> *ISS On-Orbit Status 03/28/09*
> Mean altitude loss in the last 24 hours -- 167 m
> *ISS On-Orbit Status 03/29/09*
> Mean altitude gain in the last 24 hours -- 15 m
> *ISS On-Orbit Status 03/30/09*
> Mean altitude loss in the last 24 hours -- 70 m
>
> *ISS On-Orbit Status 03/31/09*
> Mean altitude loss in the last 24 hours -- 96 m
>
> *ISS On-Orbit Status 04/01/09*
> Mean altitude loss in the last 24 hours -- 53 m
>
> *ISS On-Orbit Status 04/02/09*
> Mean altitude gain in the last 24 hours -- 46 m
>
> *ISS On-Orbit Status 04/03/09*
> Mean altitude gain in the last 24 hours -- 62 m
>
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>


-- 
Geert Sassen
weblog: http://geertsassen.web-log.nl
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