The spacecraft themselves would be essentially invisible: A gram-size interstellar probe striking the Earth’s upper atmosphere at 20 percent the speed of light

The spacecraft themselves would be essentially invisible: A gram-size interstellar probe striking the Earth’s upper atmosphere at 20 percent the speed of light would release roughly a kiloton of energy, indistinguishable from airbursts produced by meter-scale space rocks that regularly pepper our planet at a rate of about once per year. So they also want to fund an array that would start looking for a robot impact from other civilizations and recognize it as such.