This is confusing terminology in the field, but you generally talk about the cross sectional area in the plane of the conjunction (https://www.space-track.org/documents/SFS_Handbook_For_Opera...) to calculate the probability of collision.

It’s a conservative definition in the field. It’s generally defined as the hard body radius: take the smallest sphere centered at the center of mass that would entirely enclose the object, then use the maximum cross section of that sphere to define the potential “area” of the colliding object.

Maybe put more simply, it’s the worst case area size / orientation you could be looking at. So yes. Solar arrays have a narrow cross section from the side but looking at them head-on (which is the angle used for Pc calculations) they’ll be very large.

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HPsquared 2 days ago | parent [-]

Shouldn't they try and take some kind of probabilistic average area, rather than worst-case? I assume this is a statistical analysis.

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deddy 2 days ago | parent [-]

It depends on what you're going for.

Generally people really don't want collisions due to cascading effects, so they take the worst-case probability of collision found with bounding assumptions. Additionally, while often all these vehicles have active attitude (orientation) control, sometimes they go into safe mode and are spinning (often spin stabilized to point at the sun), so it will clear the entire potential radius while rotating.

Also how do you define the probabilistic average area for a space object that you don't know how it's control system works or what it's been commanded to do / point at. Yes we can make some pretty good assumptions for things like Starlink, but even those do take safemodes occasionally.

So It's an engineering judgement call on how to model it. It's hard to get a probabilistic average for attitude that you can confidently test and say is "right", it's a lot easier and conservative to take the worst-case upper-bound. That's at least not-wrong.

	▲	notahacker 2 days ago \| parent \| next [-]
		Worth adding that the actual collision avoidance manouevres Starlink (and other satellites with propulsion) makes are based on more conservative assumptions The papers assumptions lead to the conclusion that with no manouevres, we'd see a catastrophic crash between two or more satellites in LEO within 2.8 days. To be on the safe side, Starlink did over 144000 in the first six months of the year (and based on historical doubling rate, will probably be doing 1000 per day by now)...
	▲	2 days ago \| parent \| prev [-]
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