Unless i am missing something obvious, there is no way to tell how far away from a body you have to be (or how fast you can go, for that matter) to actually be able to fire probes. I'd suggest adding a display indicating range/distance of the DSS in relation to the targeted body, either to the HUD, or to the lower left target info display. This could be similar to either the distance/speed indicator or the altitude display on the HUD.
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