# How often has 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 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
• @JohnHoltz noted, thank you! – uhoh Dec 28 '18 at 0:06
• 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