I would say, anything whose spacetime geodesic (orbital/freefall path) intersects the spheroid defined by the surface of the Earth. Though by this definition, a comet on a 100-year collision course is already “on Earth”, so I’m not sure if that’s reasonable.
What defines being on earth? Below the atmosphere? stratosphere? Being in contact with the ground? Being more than 10m up?
I would say, anything whose spacetime geodesic (orbital/freefall path) intersects the spheroid defined by the surface of the Earth. Though by this definition, a comet on a 100-year collision course is already “on Earth”, so I’m not sure if that’s reasonable.