# How often has (or how common is it for) the ISS's orbit been propulsive lowered intentionally?

The question Which engine worked the hardest to keep the ISS in orbit? laments the tireless efforts to keep the ISS from falling from the sky by regularly executing propulsive orbit-raising maneuvers.

How often has, or how common is it for a propulsive orbit-lowering been executed intentionally? One might think never at first, but these rocket scientists play games with fast flights from the ground to the ISS and that sometimes requires phasing and comments note there are collision avoidance maneuvers as well, so it's not inconceivable that it may have happened.

Of course it means there needs to be an engine pointed in the prograde direction, so there's that.

• I would guess that collision avoidance might be another reason to lower the orbit, assuming the maneuver would take less fuel depending on the circumstances. – JohnHoltz Dec 27 '18 at 17:13
• Pendantically-speaking, wouldn't doing a single boost burn lead to an elliptical orbit with a second burn needed to achieve a (lower aphelion) circular orbit? – Alex Hajnal Dec 28 '18 at 0:34
• @AlexHajnal further pedantry: there's no such thing as a truly circular orbit to begin with. It's a mathematical abstraction not possible in the real world. – uhoh Dec 28 '18 at 0:39
• @Cristiano I'm using the convention that "pointed" means where the exhaust goes. To go to space, I point the engine down in order to go up. To lower the altitude of an orbit, I point the nozzle and its exhaust forward in the prograde direction in order to experience a force in the retrograde direction. Hand a thruster to a normal person and say "point this engine away from you" and they will not be looking into the nozzle. – uhoh Dec 28 '18 at 12:32

TLEs and SGP4 may help.
If we calculate the mean radius vector (or the semi-major axis, but not the osculating semi-major axis) for the TLE epoch, we get the following graph:

The reboosts are clearly shown, but there is no deboost (the spikes you see immediately after a reboost are TLE artefacts).

EDIT

This first part of the edit is to clarify about the @uhoh’s misleading message where he say that in this graph:

there is a sudden radius vector drop, while now I’m saying that there are no sudden drops.
It’s absolutely clear that the sudden drop shown in the second graph is caused by a big increase of the air density and not by a “propulsive orbit-lowering executed intentionally” (aka deboost).
Since he asked: "How often has a propulsive orbit-lowering been executed intentionally?", I’m just saying that for the graphed period there are no “propulsive orbit-lowering executed intentionally”.

All that said, a good resource could be the page https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html where there are the lines:

   IMPULSIVE TIG (GMT)   M50 DVx(FPS)      LVLH DVx(FPS)      DVmag(FPS)
IMPULSIVE TIG (MET)   M50 DVy(FPS)      LVLH DVy(FPS)      Invar Sph HA
DT                    M50 DVz(FPS)      LVLH DVz(FPS)      Invar Sph HP
------------------------------------------------------------------------
361/03:07:48.737          -1.0               2.1              2.1
N/A                       -0.5              -0.2              221.5
000/00:05:37.474           1.8              -0.1              215.8


For a deboost, we probably should expect a negative LVLH DVx component (but I’m not totally sure).
Afaik, only the current version of that page seems to be available, but I’m saving a local copy of that page since the day 120 of the year 2017; here’s an example of the list of the maneuvers:

IMPULSIVE TIG (GMT)    DT            DVx   DVy   DVz       DVmag
137/22:15:09.861  000/00:00:19.723   1.0   0.0   0.0  ->   1.00
…
361/03:07:48.737  000/00:05:37.474   2.1  -0.2  -0.1  ->   2.11


The method TLE+SGP4 seems better to me because also the very old TLEs are available. Moreover, if we plot the orbital speed:

we can obtain a good estimation of the deltaV also for values as small as 50 mm/s or better.

• I didn't get a notification of your edits but an upvote brought me here and I see you've added a lot more information; looks better, thank you!! – uhoh Dec 29 '18 at 13:08
• Interesting page about a deboost: blogs.esa.int/orion/2015/02/02/more-details-on-the-deboost – Cristiano Dec 31 '18 at 16:19
• now that's what I'm talk'in about! Consider adding it here or maybe writing a second answer since it's a different approach than this answer. Great find! – uhoh Dec 31 '18 at 17:19

Not having access to any sort of comprehensive data, I was able to find information on 3 occurrences.

Jan 2015 ISS needed a phasing maneuver to prepare for a "fast" (4-orbit) progress rendezvous. If the phasing were done prograde, the increase in altitude would have reduced the cargo load that the progress could bring to dock. Decreased debris at that altitude may also have been a reason for the direction choice.

Aug 2008 ATV did a retrograde avoidance maneuver. The document mentions that it was the first such performed in eight years. So presumably there was at least one prior in 2000.