upvote
There's a lot of space in air, and yet we have multiple midair collisions every year.
reply
In the last ten years we have one (1) accidental satellite collision. In 2021 when a Chinese spy satellite in a 788km orbit collided with an old rocket stage from the 90s. There was also that Indian anti-satellite weapon hitting a microsat in 2019, but that was very much intentional (and much condemned because of all the fragments it caused)

In the decade before that (so 2006-2015) we had four accidental collisions and two anti-satellite tests (China and US)

Those are not frequent events. And if you attribute any statistical significance to them, it seems we are getting much better at avoiding collisions

reply
Surface of a sphere (spheroid) is the square of the diameter. Planes fly at ~10kms, satellites at orders of magnitude higher.
reply
Aircraft have both anticollision detection systems and maneuverability often lacking in satellites.
reply
However, the Earth’s own radius dwarfs the height of LEO, so they’re actually roughly the same.

There are other reasons we don’t currently experience major problems with collisions in space, and why airplanes sometimes do, but it is not this.

reply
Respectfully disagree as you're comparing the surface to the size of the object, so it definitely matters.

Here's some math:

Average Earth diameter: 12742kms + 10km Average airplane surface area = 500m2 12752^2*pi = 510,865,389km2 Surface flight/plane = 1021730 planes

Starlink orbit height = ~500km Surface at orbit = ~551,712,377 so ~8% increase (which is non-negligible) Average Starlink satellite surface area = ~7m2 Surface LEO/satelite = 78816053 satellites (77x compared to airplanes)

Daily flights 50k-100k. Total number of satellites <20k.

And this is only for Starlink LEO. If you go for higher orbits the surface grows substantially. Also satellites have predictable paths, altitudes, airplanes maneuver and turn, gain altitude/lose altitude. They gather around points (airfields) etc...

reply
I would argue that 8% is absolutely negligible; however one thing that isn't is that airspace is a narrow band vertically (12 km? Not sure exactly), while LEO is about 800 km thick (from about 200 km, because the Kármán line isn't good enough, to about 1000 km).

Conversely:

> Daily flights 50k-100k. Total number of satellites <20k.

Those 20k satellites orbit roughly every 95 minutes, so they're doing ~15 orbits per day, and even the longest flights from conventional aircraft are about half that distance, so by distance each satellite in LEO is doing strictly more than the equivalent of 30 flights per day each.

Research I'm doing for a blog post has shown me that the exact position of a satellite is surprisingly variable compared to what you'd reasonably expect from a "Newtonian spherical Earth with a perfect vacuum" approximation of the orbits, enough so that it makes sense to treat 1 km as the "collision avoidance manoeuvres needed" threshold.

reply
Airplanes are much more likely to collide, but they don't dump hyperspeed shrapnel into the airspace that persists for months or years when they do. It makes sense to be extremely paranoid about satellite collisions.
reply
I believe the second half of your comment is exactly what I was getting at.
reply
Yes, I agree! But a 10% increase in surface area of LEO isn't small when compared to the size of the satellite, taking into account how the surface area at 10km compares to airplanes.
reply
10% is an outrageously large change in a normal loan interest rate and an irrelevant change when comparing the weight of a paper plane to a real one. Keeping in mind another factor in your comparison was 7700%, I'm inclined to agree the 10% is negligible in context. It's fun to think about though.
reply
But 10% is 50 million square kilometers. If we project this surface area to a theoretical planet with a diameter of 2000kms this planet alone would support 2000+ Starlink satellites in LEO. This is again not negligible.
reply
There are all sorts of such scenarios beyond the one being discussed where it's a relevant amount. This is true of most any measure or metric and why the context that we're talking about the skies of Earth compared to the LEO of Earth is key. Once you remove that context and start comparing against other things you can any difference insignificant or critically massive by choice.
reply
deleted
reply
We have very few midair collisions every year and they almost all happen near the airports.
reply