The IMP spacecraft series. Prelaunch, the 10 IMP's were
called IMP's A-J. 8 of them were intended for geocentric orbits
(A-C, F-J) and 2 for selenocentric orbits (D,E). Prelaunch, the
latter two were also called Anchored IMPs (AIMPs) A and B.
After launch, the 8 intended for geocentric were also called
IMPs 1-8 and the 2 intended for lunar orbit were called
AIMP 1 and 2. Only IMP E = AIMP B = AIMP 2 achieved lunar
orbit; IMP D = AIMP A = AIMP 1 went into a geocentric orbit.