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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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  • $\begingroup$ That dip (and rebound) in March is also fairly anomalous. $\endgroup$
    – Magic Octopus Urn
    Commented Aug 8, 2019 at 14:50
  • $\begingroup$ Other sudden descents at intervals, by 1 or 2 km. Image from Heavens Above. $\endgroup$
    – mins
    Commented Jan 18, 2021 at 19:37

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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 Orbital Science's Cygnus spacecraft on 28 October 2014, 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)? $\endgroup$
    – Max Q Lagrange
    Commented Nov 9, 2015 at 21:45
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    $\begingroup$ Not sure how I feel about citations from the Daily Fail ;-) $\endgroup$
    – Max Q Lagrange
    Commented Nov 9, 2015 at 21:46
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    $\begingroup$ Also, I added another source, from NASA. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Nov 9, 2015 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
    Commented Nov 9, 2015 at 23:50
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    $\begingroup$ They did, rotated 180 degrees $\endgroup$
    – PearsonArtPhoto ♦
    Commented Nov 9, 2015 at 23:51
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To reduce the time for craft to reach the station, the ISS needs to be at the correct point in its orbit (or "phase") relative to the launch site at the time of launch. Doing this requires changing the orbit, either higher or lower to increase or decrease the orbital period.

While most burns are done to increase the altitude, in 2015 the decision was that a prograde burn would make the altitude higher than desired. It's possible that for this particular launch it was a decision between a slightly lower orbit and a much higher orbit, but I couldn't find anything that supplied those details.

Besides lower fuel costs for visiting vehicles, an ESA article about the ATV mission specifically calls out reduced debris risk at the lower altitude as a factor in the decision.

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