[DagAgren]

Not really. Flying faster than the speed of sound without complex supporting industry is not possible. Flying faster than the speed of sound is not sustainable, it is something we have to work very hard to achieve, and the second we stopped trying very hard to do it, it would cease to happen.

Grey goo, on the other hand, has to be self-sufficient and sustainable to exist.

[ben_w]

On the other other hand, something only needs to be able to eat us to be a threat. A slightly better bacteria could count.

I wouldn’t call that “grey goo”, but the threat model is a spectrum not a Boolean.