The is the original painting created for the first of Robert A. Heinlein's "Juveniles series" of SF novels, from 1947 - for the paperback edition published by Ballantine, 1977.  Sweet produced several covers for this series, this one in particular well known because it was not only the first in the series, but also because it served as the basis for the 1950 film Destination Moon.    Painted in  acrylic on Upson board with dimensions 24" x 16"  and is signed bottom right.  Note: I believe this is the last remaining Heinlein cover to be offered by the estate.