How the heck would we detect a 3 foot spacecraft that weighs a few grams, passing anywhere through the Solar System at speeds over 10% of light? We might be ge

How the heck would we detect a 3 foot spacecraft that weighs a few grams, passing anywhere through the Solar System at speeds over 10% of light? We might be getting them once a week and we would have no idea. We can't even find or track most of our own asteroids, and they're already in solar orbit.