# How did Venus Express' periapsis decay so quickly?

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:

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?

• Maybe it wasn't decay after all... Dec 21, 2021 at 20:01

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).

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.

• 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 at 13:24
• It seems my focus on atmospheric decay was misguided, I suspected this after reading @Ryan C's answer here, nice sources! Jan 12 at 13:27
• @uhoh with such a highly eccentric orbit it would really only take a very small force to change the pericentre altitude considerably. Jan 12 at 15:58
• @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. space.stackexchange.com/a/21226/12102
– uhoh
Jan 12 at 22:41
• @uhoh Ere, yeah, I knew that... Should get more sleep... Jan 12 at 22:43