And, of course, it's Gravity Probe B.
The Big Bang Theory Season 1, Episode 13
The plaque they sent on the Pioneer PROBE
had a drawing of a naked man and woman.
The Big Bang Theory Season 8, Episode 21
Is this about the PROBE
he's working on without you?
The Big Bang Theory Season 8, Episode 21
So, when the aliens brought you back,
they just left the PROBE in?
The Big Bang Theory Season 9, Episode 23
He helped launch the
New Horizons space PROBE.
The Big Bang Theory Season 12, Episode 5