What does EMPIRE stand for?

Early Manned Planetary-Interplanetary Round-Trip Expedition (SAUS)

Early Manned Planetary-Interplanetary Round Trip Experiment

Early Manned Planetary Interruptionless Reconnaissance Expedition (SAUO)

Early Manned Planetary Interruptionless Round-Trip Expedition (SAUS)

Electromagnetic Performance Information Research (PDAA)

Electromagnetic Phenomena Interference Repository (PDAA)

Electronic Multipurpose Intelligence Retaliatory Equipment (IAA)