"Hail Probe!" is a long-running standing in-joke between users of the freeware space flight simulator "Orbiter" and the associated forums, known simply as the "Orbiter Forum".

It originated in 2002, when Dealer McDope created a series of Soviet-era meshes. Included in these was a basic, green, Luna E3 mesh. The Luna E3 was the first probe to photograph the far side of The Moon, launched atop an R7 booster.

When the CCCP Team, co-ordinating on the Orbiter IRC channel, set about writing C++ code for McDope's meshes, the generic test object of choice became the Luna E3. Sticking to the naming convention McDope adopted of simply calling it "Probe", an in-joke cropped up, by which whenever a developer conducted a flight test, cries of "Hail Probe!" would abound.

This in-joke gained popularity when a post to the old M6 off-topic forums introduced the concept of Hailing Probe to a wider audience with a thread titled The Cult of the Probe, posted May 4th, 2004. So ensued the second longest-running thread in the old
Forum's history. Without a single flame.

The Probe has been hailed a great number of times afterwards; one notable example is during the Winter holidays in the end of 2008 (View thread).

Such was the following of the thread that it spanned two years and even spawned Probe Day (4th October -- the date of both Luna E3's launch, and two years earlier, that of Sputnik 1, the first artificial satellite).

Unfortunately, with the migration of the Orbiter forums to a new system in Februrary 2006, the thread was not ported.