Astral is a prototyping tool for authoring mobile and smart object interactive behaviours. It mirrors selected display contents of desktop applications onto mobile devices (smartphones and smartwatches), and streams/remaps mobile sensor data to desktop input events (mouse or keyboard) to manipulate selected desktop contents. This allows designers to use familiar desktop applications (e.g. PowerPoint, AfterEffects) to prototype rich interactive behaviours. Astral combines and integrates display mirroring, sensor streaming and input remapping, where designers can exploit familiar desktop applications to prototype, explore and fine-tune dynamic interactive behaviours. With Astral, designers can visually author rules to test real-time behaviours while interactions take place, as well as after the interaction has occurred. We demonstrate Astral's applicability, workflow and expressiveness within the interaction design process through both new examples and replication of prior approaches that illustrate how various familiar desktop applications are leveraged and repurposed.