Many objects survive from that voyage that shed light on its progress, its mixed meanings and the various activities of James Cook and his companions.

There are botanical drawings and specimens, astronomical observations, documents and charts.

There are objects gathered up in the quest for knowledge, or removed from places and people for less altruistic reasons.

Other objects contain residues of corporate memory among the Indigenous peoples Cook and Endeavour encountered on their voyage. All have stories to tell.

Many of these stories feature in the National Museum of Australia's new exhibition Endeavour Voyage: The Untold Stories of Cook and the First Australians which recently opened in Canberra.

Today and more than ever before, we may reflect on the fateful character of the encounter between Cook and those coastal communities of first nations people dotted all the way up the eastern coast of Australia where Cook made landfall on only a handful of occasions.

The meanings of those relatively brief encounters and the objects associated with Endeavour continue to evolve with the passage of time.

Each generation sees them with fresh eyes, new perspective. Some have become obscure and may at times require explanation or interpretation. Others remain familiar.

None, however, speaks for itself quite so loudly, clearly and emphatically as this cast-iron ship's cannon, one of Endeavour's six four-pounders that were located and recovered from the Great Barrier Reef in 1969.

On 10 June 1770 at about 11 o'clock at night, Endeavour struck coral about 13 kilometres off Cape Tribulation, a little way south of the mouth of what became known as Endeavour River (at modern Cooktown).

Her hull was seriously breached and she began to take on water. It was an appalling incident, one that placed the ship and everyone aboard in immediate peril.

Cook's first response, upon swiftly assessing the extent of the damage, was to lighten the vessel.

He ordered to be thrown overboard "our guns, iron and stone ballast, casks, hoops, staves, oyle jars, decay'd stores etc".

In 12 hours, the ship's company managed to eject a little less than 40 or 50 tonnes of weight.

Prior to her departure from Plymouth on 26 August 1768, Endeavour took aboard ten cannon, 12 swivel guns and a great deal of ammunition, to cover against the possibility of encountering hostilities in the Pacific, as well as 12 heavily armed Royal Marines.

The Royal Society may have supported the voyage for its scientific mission, but Endeavour was an armed vessel of the Royal Navy.

The four-pounders were among the bluntest in the Royal Navy's repertoire of already blunt instruments. For one thing, the cannon were already quite old.

All six carry the cypher of King George II, and therefore date from before 1760; for metallurgical reasons they may even date well before 1750 but not earlier than 1727.

At least three, including ours, were cast by either the gun-founder Joseph Christopher or John Churchill of Essex, both of whom may be associated with the mark "IC".

Before 1969, when an expedition supported by the American Academy of Natural Sciences in Philadelphia successfully located and recovered all six cannon from Endeavour Reef, there had only been one other serious attempt to find them.

That was in 1886-87, and the search coincided with the rapid development of Cooktown and the decision to raise a monument to Captain Cook. That search drew a blank.

So long as they remained missing, the cannon became a sort of holy grail for naval historians and others seeking to memorialise Endeavour.

The stakes for that memorialisation were never higher than 50 years ago.

In 1969, when the bicentenary of Cook's arrival was rapidly approaching, NASA went so far as to draw what were regarded as obvious parallels between Neil Armstrong and James Cook; Apollo 11 and Endeavour.

When the cannon were recovered from the Great Barrier Reef, using magnetic instruments normally used for underwater exploration for minerals and oil, they needed a great deal of conservation.

Enclosing layers of coral, 200 years' worth, needed to be removed, and corrosive salt patiently and permanently extracted from the outermost skin of the metal by means of electrolysis.

However, it was eventually found that before Endeavour went aground two of the six cannon contained a charge of gunpowder, a loaded cannon ball and hemp wadding used to hold both together against the touch hole at the breech for ignition with a gunlock.

In other words, two of the cannon were loaded and therefore ready to fire at a moment's notice.

That is a sobering thought.