Dr. Killen is a planetary scientist specializing in studies of the exospheres of Mercury and the Moon. She has observed Mercury, the Moon and the Galilean satellites at the McMath-Pierce solar telescope, the W.M. Keck Telescope, The IRTF and at the Anglo-Australian Observatory. She has modeled the exospheres of Mercury and the Moon, and performed radiative transfer calculations for optically thick atmospheres. She is interested in the physics of space weathering and the interactions of the solar wind plasma with the surfaces of planets and satellites.