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Comment by randyrand | Hacker News Reader

It can be tracked, just not 100% of the time.

But here's the thing. Momentum. If you've tracked it once, you know its location for many months to come.

> If you've tracked it once, you know its location for many months to come

Not in low-earth orbit. Look at this graph of the ISS's altitude over time [1]. Atmospheric drag makes orbital dynamics too complicated for long-term predictions. Add in station-keeping [2] jitter, and one has a necessity for reliable tracking.

[1] http://images.huffingtonpost.com/2014-05-17-ISSaltitude.png

[2] https://en.wikipedia.org/wiki/Orbital_station-keeping

In low-earth orbit space debris will just fall into the atmosphere pretty quickly because they have no maintenance burns.

It often takes decades. There is still debris from the 1960s in LEO, which has a lot. Five satellite collisions have complicated things. I suggest this Wikipedia article:

https://en.m.wikipedia.org/wiki/Space_debris

"Below 2,000 km (1,200 mi) Earth-altitude, debris are denser than meteoroids; most are dust from solid rocket motors, surface erosion debris like paint flakes, and frozen coolant from RORSAT nuclear-powered satellites."

And the journal Orbital Debris Quarterly

https://orbitaldebris.jsc.nasa.gov/quarterly-news/newsletter...

2000km is not low earth orbit.

>low Earth orbit (LEO), generally defined as an orbit below an altitude of approximately 2,000 kilometers (1,200 mi)

https://www.swpc.noaa.gov/impacts/satellite-drag

As the article explains its debris below 2,000 km. Such as "DMSP-F13, launched in 1995—exploded while in a sun-synchronous polar orbit leaving a debris field of at least 43 to 100 large fragments and more than 50,000 pieces smaller than 1 millimeter" which is about 450 nautical miles.

The ISS's changes like that though because it's constantly doing burns to keep it "afloat" an abandoned satellite in LEO is going to eventually just burn up in the atmosphere and have a much more consistent orbit. And if it's not abandoned, then there should be _someone_ keeping tabs on where it's otherwise what's the point of it being up there?

"should"

> The ISS's changes like that though because it's constantly doing burns to keep it "afloat" an abandoned satellite in LEO is going to eventually just burn up in the atmosphere and have a much more consistent orbit.

This is not accurate. Even if you ignore the big vertical jumps at each burn, the downward trend is very obviously irregular. Satellites in LEO experience a slight drag force from the Earth's outer atmosphere, and the magnitude of this force can vary unpredictably by orders of magnitude depending on space weather conditions.

Satellites in orbit are moving very quickly, so a slight change in altitude (i.e. orbital period) results in a very large change in position at any given future time. More importantly, the inaccuracies are compounded with each subsequent orbit. As a rough estimate, an uncertainty of 1m in the height of a satellite in LEO translates to a positional error of more than 100m/day. It is not possible to accurately predict the position of a satellite without ongoing observations.

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