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This image from Heavens Above that @PearsonArtPhoto links to in this answer shows the orbital altitude of the International Space Station: enter image description here

In general, the ISS altitude decreases gradually with time due to drag, and increases almost as a step function (in the time-scale of this graph) as the ISS is boosted back up to keep it within normal parameters.

However on or around January 27th 2015, there is a sudden loss of altitude of about 1.5km. Given the shape of the rest of this graph, I assume that this was not due to dynamic atmospheric conditions, but more likely a planned maneuver. If it was a planned maneuver, it appears to be in the wrong direction - it looks like the use of (precious) fuel to achieve a result that could have been achieved just by waiting 10 days or so.

Was this a planned maneuver or was it due to natural atmospheric conditions? If it was planned, what was its reason?

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It was a decision made to lower the orbit of the ISS of the ISS on Jan 28, 2015. The goal was to make it easier for spacecraft to make it to the ISS. Rumor is this had something to do with the loss of the Orbital Cygnus spacecraft, which resulted in the cargo resupply missions needing to carry more supplies.

Of note is that this was performed by the ATV, about 2 weeks before the last one ended it's mission. Likely they had surplus fuel that they wanted to use, and decided to use it to lower the orbit of the ISS.

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  • $\begingroup$ So presumably it was cheaper to burn a bit of fuel than wait 10 days or so (and perhaps scrub launches)? $\endgroup$ – Digital Trauma Nov 9 '15 at 21:45
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    $\begingroup$ Not sure how I feel about citations from the Daily Fail ;-) $\endgroup$ – Digital Trauma Nov 9 '15 at 21:46
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    $\begingroup$ Also, I added another source, from NASA. $\endgroup$ – PearsonArtPhoto Nov 9 '15 at 21:49
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    $\begingroup$ The ATV only ever docked at one place, they would have maneuvered the whole ISS. $\endgroup$ – Organic Marble Nov 9 '15 at 23:50
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    $\begingroup$ They did, rotated 180 degrees $\endgroup$ – PearsonArtPhoto Nov 9 '15 at 23:51

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