After the pandemic: 2 visions

Those were two fascinating opinion pieces [“Don’t sideline environmental sustainability” and “The coming digital reality,” June], especially read one after the other. Asteris Apostolidis looks forward to a world not much different from that of a few months ago, and urges planning for that future in a manner that would ally further, different calamities. He believes that the physical personal contact and experience, enabled by air travel, will be actively sought by future generations.

Dennis Bushnell’s vision is quite different. Personal contact and physical experience will be unnecessary, with remote data and simulation taking their place. We’ll just lie back and hook up.

But why bother? If the technology gets to a good enough level, why not avoid all those messy interactions with the physical world and other humans, and just dial it all back to, say, N64 level? All your associates could be avatars, much easier to get along with than real people, even those who are remote. No need to argue, no need to lose, no need to have a theory proved wrong.

No need for an aerospace industry. No need to live, come to think of it. I’m rooting for you, Mr. Apostolidis.

William O. Keksz
AIAA senior member
Edgewater, Maryland

More on UVC light

In two articles in September’s Aerospace America, comments were made about validating the efficacy of ultraviolet C light to disinfect planes [“No-contact cleansing” and “Anti-covid leader”]. The shorter wavelength of UVC makes it particularly good for that but could be bad for classes of plastics. What was not asked is what effect does UVC light have either directly on the materials (plastics and fabrics) in the plane or through the ozone that might be generated. Furthermore, are there any compounds that result from the degradation of materials left on the surface that might be an issue with people? Hopefully, someone is also looking into these potential issues.

Raymond F. Maddalone
AIAA senior member
Fishers, Indiana

Getting practical about avoiding space collisions

I was extremely excited to see Moriba Jah’s article on probability dilution in the September 2020 issue [“Needed: Rules of thumb for avoiding collisions in space”]. I’m writing to supplement what Dr. Jah wrote with a little practical guidance for satellite navigators tasked with doing conjunction analysis.

“Probability” of collision is a deeply and provably unreliable risk metric. I know this because Ryan Martin, Scott Ferson and I proved it and published that proof in the Proceedings of the Royal Society in 2019 [“Satellite Conjunction Analysis and the False Confidence Theorem,” July 17, 2019]. Given the typical uncertainties at play in present-day conjunction analysis, “probability” of collision consistently understates collision risk exposure by several orders of magnitude. If used as a plain risk metric, it offers satellite operators absolutely no viable path to controlling the rate at which collisions involving active satellites happen.

Moreover, this is not due to subjectivity in the prior, shaky assumptions in the likelihood, or any of the other usual statistical culprits. It is due to a straightforward mismatch between the mathematics of probability theory and the epistemic uncertainty to which it is being applied in conjunction analysis. This is a problem where Bayesian notions of “coherence” and engineering notions of reliability are fundamentally and provably incompatible.

There are many details and marginal errors to be run down in satellite conjunction analysis, but probability dilution is not merely one of them. It is the dominant source of error in conjunction analysis today. It is the thing that will completely invalidate your analysis, even if you get everything else right.

Fortunately, nobody is stuck with “probability” of collision as a risk metric. Robust alternatives are readily available. Ellipsoid overlap detection is an old standard, and it’s provably reliable, though usually overconservative. Alternatively, navigators looking for a more flexible metric describing collision risk exposure can use plausibility of collision as computed using the “red fuzzy” approach presented in my 2016 AIAA paper on conjunction analysis [“A Corrector for Probability Dilution in Satellite Conjunction Analysis,” SciTech Forum], or equivalently, the two-dimensional “Gaussian possibility distribution” presented in Dr. Jah’s own 2019 AAS/AIAA paper with Emmanuel Delande and Brandon Jones [“A New Representation of Uncertainty for Collision Assessment,” Advances in the Astronautical Sciences AAS/AIAA Spaceflight Mechanics 2019]. More efficient and general solutions are possible, but these references are offered to give satellite navigators adequate tools for immediate use.

Michael Balch
AIAA lifetime member
Alexandria, Virginia

After the pandemic: 2 visions