  This 2020 Decadal Survey White Paper reviews what is known about lunar and
martian lander Plume Surface Interactions (PSI) during powered descent. This
includes an overview of the phenomenology and a description of the induced
hardware and environmental impacts. Then it provides an overview of mitigation
techniques and a summary of the outstanding questions and strategic knowledge
gaps. It finishes with five recommendations: to include dedicated descent
imagers on every surface mission so that PSI can be directly recorded and
reviewed by ground teams; as far as possible, to make all data related to PSI
effects publicly accessible; to develop methods and instruments for making key
measurements of PSI; to assess and record key flight data; and to invest
funding into studies of long-term infrastructure architectures and mitigation
techniques.
