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ESA's Venus Express ran out of propellant in late 2014 and has probably decayed out of orbit since then.

A timeline of events from my research for my answer to Are there currently any spacecraft in orbit around Venus that might (or might not) be at risk of meteoric dust from comet Leonard C/2021 A?:

This dramatic of a periapsis drop relative to the change in apoapsis just doesn't add up in my back of envelope thinking.

The lowering of velocity at periapsis affects apoapsis height and, minimally, periapsis height. This is seen dramatically in the decay of Komsmos-482 (1972-023E), via Jonathan McDowell:

High Earth Orbit decay

The periapsis for Kosmos-482 (1972-023E) drops from 228 km (1972) to 199 km (today, ~50 years later!).

Though solar cycle 24 did peak in 2014, it wasn't that strong and the MSISE-90 Model puts Earth's density @ ~200km at ~10e-9 $kg/m^3$ during high solar activity. Compare this with actual Venus Express measurements of densities around 10e-12 $kg/m^3$ @ only ~190 km altitude.

How did Venus Express' periapsis decay so quickly?

BrendanLuke15
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Venus Express (VEX) used a highly elliptical, polar orbit. A peculiar feature of this orbit around Venus is that the pericentre altitude will drift due to third body perturbations from the Sun's gravity (according to this paper).

enter image description here

According to this paper (and implied in this press release from ESA marking the end of mission for VEX), mission operators had to regularly use propellant to raise the orbit's pericentre which would decay by as much as -5 km/day.

This seemingly could explain the rate of the spacecraft's periapsis decay.

Armadillo
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    Wow! and... wow! Great find, it's hard to imagine how it could be ~ 5 km/day, I think there's another question or two in this of the "How the heck...?" variety. – uhoh Jan 12 '22 at 13:24
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    It seems my focus on atmospheric decay was misguided, I suspected this after reading @Ryan C's answer here, nice sources! – BrendanLuke15 Jan 12 '22 at 13:27
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    @uhoh with such a highly eccentric orbit it would really only take a very small force to change the pericentre altitude considerably. – PearsonArtPhoto Jan 12 '22 at 15:58
  • @PearsonArtPhoto that could certainly be true, but it just feels counterintuitive to me for some reason, though I can't say why. Without resorting to math I'd guess that raising the apocenter would be easier, but perhaps the energy stays constant so the pericenter must drop to keep energy and semimajor axis constant. I'm pre-coffee right now so will have to think about that a little later, too few neurons... – uhoh Jan 12 '22 at 20:32
  • From aerodynamics, it makes more sense that it would lower the pericentre quicker, but that isn't the only force involved, not by a long shot... – PearsonArtPhoto Jan 12 '22 at 21:46
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    @PearsonArtPhoto No, just the opposite! For aerodynamic drag the pericenter stays relatively fixed; the orbit circularizes first by lowering of the apocenter. Velocity lost at peri results in apo lowering. https://space.stackexchange.com/a/21226/12102 – uhoh Jan 12 '22 at 22:41
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    @uhoh Ere, yeah, I knew that... Should get more sleep... – PearsonArtPhoto Jan 12 '22 at 22:43
  • @PearsonArtPhoto ya I knew you knew; I'd offer you some coffee but the $1/r^2$ is too large. – uhoh Jan 12 '22 at 22:45