USA 245 is an Earth observation satellite, its apogee is only about 1000 km. 42,000km would be for a geostationary satellite. But yes, 300km is pretty close in

USA 245 is an Earth observation satellite, its apogee is only about 1000 km. 42,000km would be for a geostationary satellite. But yes, 300km is pretty close in space. Since you have no issue with atmospheric distortion, the optics become a lot more simple. This is quite a small satellite - "mini fridge" someone said, which does limit what you can put in it. 300km is a fair distance away and you'd need an enormous focal length to resolve small features on the other spacecraft. It's possible they just want to test how well they can track another satellite. For a Hubble class telescope (~50k mm focal length) you'd get around 20cm resolution for a 4MP sensor. A more general way, since we don't know what sensor is onboard, would be to calculate the diffraction limit for the largest mirror you could put on the satellite (sibling comment suggested 1 m, which is about 1 arcsecond at 540nm). That works out to 20cm at 300km, or about 1cm at 20km. Of course you still need the focal length and sensor to make this useful. I guess you could compare Trump's images of the Iranian launch site which were taken at a similar distance, but with a much larger satellite.