  Rotational dynamics of D$_2$ molecules inside helium nanodroplets is induced
by a moderately intense femtosecond (fs) pump pulse and measured as a function
of time by recording the yield of HeD$^+$ ions, created through strong-field
dissociative ionization with a delayed fs probe pulse. The yield oscillates
with a period of 185 fs, reflecting field-free rotational wave packet dynamics,
and the oscillation persists for more than 500 periods. Within the experimental
uncertainty, the rotational constant BHe of the in-droplet D$_2$ molecule,
determined by Fourier analysis, is the same as Bgas for an isolated D$_2$
molecule. Our observations show that the D$_2$ molecules inside helium
nanodroplets essentially rotate as free D$_2$ molecules.
