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A friend of mine noticed that many satellites on the Stuff in Space website can reach an altitude lower than their declared perigee, as you can see in the following screenshot:

Stuff in Space screenshot

We observed these satellites for a while and we noticed that in the same way they never reach the declared apogee (the maximum altitude they reach is a few kilometers below the declared apogee).

Is this a bug in the web application or does this offset have a physical meaning?

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    $\begingroup$ In a sensible world, the apogee and perigee would have shown actual orbital distance, not height-above-ground-assuming-the-planet-was-a-perfect-sphere. But it doesn't, thus misunderstandings like these abound. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Oct 24, 2021 at 14:33
  • $\begingroup$ Can we use this link? sky.rogue.space It seems to host the same application. $\endgroup$
    – xonya
    Oct 22 at 17:12
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    $\begingroup$ xonya - I edited the link to the new URL. Stuffin.space was a somewhat anonymous website that was becoming fairly popular, apparently produced by three-year old startup company Rogue Space Systems. In 2022 they moved it to sky.rogue.space. It's still called Stuff in Space in the thumbnail, and now includes a link to their company website. For some reason they did not redirect the old link, creating some confusion as some articles now refer to Stuff in Space in the past tense. They recently received some Air Force funding so maybe they can now afford to repurchase the old URL for redirect. $\endgroup$
    – Steve Pemberton
    Oct 23 at 12:33
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    $\begingroup$ @StevePemberton StuffInSpace was a high school student at the time just publishing a thing on github. He now works at SpaceX and has stopped maintaining the site. Rogue Space is completely unrelated and has only just copied the front end of the site and isn't using it properly (there was a script running on the server to pull the latest TLEs that they didn't copy over). The spiritual successor to stuff in space would probably be keeptrack.space, which forked the stuff in space repo and has added a ton of new features over the past several years $\endgroup$
    – Tristan
    Oct 24 at 13:25
  • $\begingroup$ @Tristan - thanks, as I mentioned in my comment on the other Stuff in Space question, something seems odd about Rogue Space Systems in general, just my impression that they sort of seem like the somewhat common vaporware startup that gets funding for a while then disappears. Despite some bold claims on their website about what they produce, so far they have only launched a cubesat. They recently received some Air Force funding for satellite maintenance technology, so maybe they will eventually become a viable space company. $\endgroup$
    – Steve Pemberton
    Oct 24 at 13:41

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Great question!

This kind of display is probably possible depending on how the program calculates those things. Real orbits are not actually perfect Keplerian orbits because Earth's gravity field deviates from spherically symmetric by about 1 part in 1000. Earth's $J_2$ is about $1.08 \times 10^{-3}$.

This is off by maybe 1 km out of 7,000 km (distance to center of Earth). It's hard to tell exactly because the apogee and perigee are rounded to the nearest km.

If the program did something fancy and propagated the most recent and complete orbit and used max and min to find them, then there's something wrong here.

However if the program estimates them based on the mean eccentricity and inferred semimajor axis from the mean motion in the TLE (and I'll bet this is what it does) perhaps using only the monopole $GM$, then the current altitude certainly might wander slightly outside those estimates.

The atmosphere is a drag sometimes

At 238 km atmospheric drag is still fairly low and altitude will not drop this much from one orbit to the next, but once it gets a lot lower, say around 120 km, the orbit will be much more of a fast downward spiral and so orbital elements will become meaningless as soon as they're calculated.

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