[schiffern]

It's important to note that Starlink satellites will maneuver when they calculate a 1-in-100,000 probability of collision.[0] Most other satellite operators will only maneuver when they predict a 1-in-10,000 chance.

The scaling isn't linear either. By setting the threshold 10x lower, SpaceX performs more than 10x as many maneuvers.

Essentially they're complaining because SpaceX is too cautious.

[0] https://www.space.com/starlink-satellite-conjunction-increas...

[wongarsu]

If you have 4000 satellites a 1-in-10,000 chance doesn't seem that unlikely anymore. The scale of their operation forces them to be more cautious

[prepend]

Not really though as the risk is still per satellite. They have a greater chance of an individual loss but their individual risk per satellite is the same (or maybe higher because the collision could be with another starlink).

So it seems like the 1:10k vs 1:100k threshold would be a function of the cost of a loss from a particular incident rather than the risk of a single incident among the 4k satellites.

Thinking from an insurance standpoint, the cost to insure probably depends on the number of satellites overall and total potential loss, not just insuring for a single loss.

[cptcobalt]

Not just cautious—responsible.

[xhkkffbf]

When you've got that many, you've got to expect to lose more than a few. Right?

[wongarsu]

Yes, but when it fails for whatever reason you want it to stay in one piece and just naturally deorbit within ~5 years due to drag. A collision is likely to create more fragments, that in turn have a chance of hitting your other satellites, which would create even more fragments. You don't want that to happen too frequently.

There's enough atmosphere in those low orbits that chain reactions aren't a huge concern for the rest of human spaceflight, but they are a concern for keeping the Starlink constellation alive.

[nordsieck]

Another thing is that Starlink is low enough that space junk spontaneously de-orbits relatively quickly. I think within ~10 years, all of the junk SpaceX is maneuvering around shouldn't be a concern to them any more.

[Joeri]

As long as other satellites keep being launched into higher but not geosync orbits doesn't that provide a steady supply of junk to navigate around?

[nordsieck]

> As long as other satellites keep being launched into higher but not geosync orbits doesn't that provide a steady supply of junk to navigate around?

Sort of.

1. Modern satellites don't really shed junk - and almost all satellites today have to be launched with a plan to make sure that they deorbit in a timely manner. Doesn't always work out, but most are pretty safe.

2. For space junk, the time to deorbit increases super-linearly with orbital height. Which means that you are correct - stuff in higher orbits will slowly "fall" through the Starlink orbits on their way to Earth. But it's a very slow process which means there's really only a thin trickle of stuff falling from above.

What that means is that, Starlink will always have to avoid objects. But the bulk of the problem, which was caused by this[1] Russian anti-satellite missile strike will go away after a decade or so.

---

1. https://www.bbc.com/news/science-environment-59299101

[pclmulqdq]

Many of those de-orbit plans are things like "deploy all the solar panels in the maximal-drag configuration and wait for the thing to burn." That still poses risk to satellites in very low orbits, although they linger for weeks or months rather than years. I wouldn't expect the situation to get a lot better, since despite the reduced time to de-orbit, a lot more satellites are being launched.

[Roark66]

If there is no collision, yes you're right, but if there is one.... Stuff gets thrown into various orbits. Most of the debris will deorbit quickly, but a small percentage can get a substantial kick and go higher.

[nordsieck]

> If there is no collision, yes you're right, but if there is one.... Stuff gets thrown into various orbits. Most of the debris will deorbit quickly, but a small percentage can get a substantial kick and go higher.

That's a misleading description.

While a collision may increase an object's apogee, it will be in an elliptical orbit: it will always intersect the orbit where the collision happened.

This isn't just a collision thing either: all spacecraft do this[1] as a first step (move to an elliptical orbit), and then typically relight their engines at the apogee in order to circularize their orbit.

---

1. https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

[Buttons840]

Are you sure? My KSP intuition tells me this is wrong.

Orbits are circular and constant (accurate enough for my argument). An impact might cause some debris to go up, but when the debris that went up comes back around to the same spot in its orbit, it will again be going up. Well, extrapolate that backward a bit and you realize the debris that is going up was inside the atmosphere moments before.

In other words, if an instant impulse causes an orbit to have a higher apoapsis, it must also lower its periapsis. Which means the piece will go deeper into the atmosphere, and you don't come back from deep in the atmosphere.

[Karellen]

> Stuff gets thrown into various orbits. Most of the debris will deorbit quickly, but a small percentage can get a substantial kick and go higher.

Only at apoapsis. But, any item thrown into an orbit with a significantly higher apoapsis will very likely have a significantly lower periapsis. So even for high-energy fragments, each orbit should have a period of passing through a higher-drag part of its orbit, and lose energy relatively quickly.

Significantly, the periapsis can never be lower than the altitude that the collision occurred. The worst case, IIRC, is for fragments ejected in the direction of orbital travel at the time of collision. But even then, they'll still spend some time in whatever drag environment they were in beforehand.

That's not to say collisions are fine, but nothing is going to get thrown into random high low-eccentricity orbits where they hang around and pose a threat for decades or more.

[Symmetry]

10 years for the satellites. Any debris coming off them will tend to be smaller, have a lower ballistic coefficient, and deorbit faster.