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Comment by exitb | Hacker News Reader

Yes, they're designed not to cause the problem, but wouldn't they feed the problem, if it's ever caused by someone else?

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How? If they're in a low orbit how can they get higher to pose a threat? If they collide with something that is en route to a higher orbit, then the collision takes place in this low orbit, and any debris will also be in that low orbit?

generally when two satellites collide some debris can be shot at a higher orbit which will take forever (or will not) reenter atmosphere.

See the scatterplot of debris for the Chinese collision test of 2000: https://en.wikipedia.org/wiki/Space_debris#/media/File:Gabba...

See this video for evolution of debris and altitude over time: https://www.youtube.com/watch?v=mQT5aMa_7iI

> generally when two satellites collide some debris can be shot at a higher orbit which will take forever (or will not) reenter atmosphere.

The lowest point of the new orbit (perigee) is guaranteed to be no higher than the point of collision. Fragments after the collision have no further propulsion, so their new orbit must initially include the point of collision, and can only decay from there. This can also be seen on your scatterplot.

Moreover, the time to decay is most strongly influenced by the perigee, as the atmosphere is the strongest there. If a satellite on an low orbit (decay in decades) explodes, those fragments that "reach a higher orbit" will still (due to the low perigee) decay back to a near-circular orbit in decades. A collision can increase the lifetime for some of its fragments, but not by multiple orders of magnitude (unlike a circular higher orbit).

Your video link also nicely shows that fragments with a low perigee will decay quickly, no matter how high their apogee is.

Yes, this is true, but for this to not be a problem, it also has to suppose that those higher-flying bits of debris don't subsequently collide with other higher-flying satellites.

The Chinese test also happened around 350km higher then Starlink satellites (850km). The natural deorbit time at that altitude is already over 100 years vs under 5 years at 500km.

Also it was literally the worst possible collision being an intentional head on collision. Satellites accidentally hitting each other on orbit are very unlikely to hit each other head on as it is very uneconomical to put satellites in a reverse orbit that low (there is a reason why we do rocket launches towards east if possible)

Debris orbits like anything else. Regardless of the orbits of the colliding objects, the orbit of any debris is only really constrained to intersect the point of collision. For some debris, the point of collision could be the lowest point in its new orbit, meaning it could take longer to reenter and could collide with other stuff higher up until then. On the other extreme, some debris could essentially fall straight down to earth immediately.

No collision of 2 surfboards is going to loft debris high enough to be in a stable orbit. If the common point of orbit is 500km, all of that debris will de-orbit. The only propellent on those things is krypton, so if they collide it is only going to be mechanical effects on the debris. In my search I have not found a single reputable source claiming starlink could contribute to kessler syndrome, only clickbait articles.

The debris orbits from an initial collision don't have to be stable to allow a chain reaction that produces debris in more stable orbits. Debris from even a single low altitude collision/disintegration could collide/disintegrate later at a higher altitude, producing debris with a higher periapsis than any of the original bodies.

There are many factors in whether this matters practically, so I'm not passing judgement on that. None of this is specific to Starlink.

They're operating around 500 km, which is not the lowest of orbits. There's stuff below, including people in the orbit.

As long as the satellites are under 600km or so they will deorbit on their own in under 5 years. So in the worst case scenario caused by Starlink we would not be able to access space for 5 years which while not great is not the end of the world either.

The time to deorbit is not linear. So this 500km orbit is fine while 700km orbit would get us ~25 year deorbit time. 800km over 80 years and so on.

Nope, the satellites are flying so low that they will reenter the atmosphere and burn in few years no matter what happens .

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