  We present a navigation concept for solving the lost in space and time
problem using optical observations of $\delta$ Scuti variable stars. Only a
small number of techniques exist that allow a spacecraft to recover from being
lost in both space and time, which can be caused by a failure of the onboard
clock and navigation systems. Optical observations of $\delta$ Scuti stars,
which can be collected onboard from star trackers or navigation cameras, may
enable faster and/or more accurate position and time solutions compared to
existing techniques without requiring additional equipment. Our results
indicate that one day of observation by the OSIRIS-APEX MapCam may enable
position and time determination accuracy within 0.05 au (3$\sigma$) and 15 s
(3$\sigma$).
