Where Spaceman Meet the Outback

Everybody's heard of Australia’s contribution to the first moonwalk by US astronauts in 1969, with the Parkes (NSW) radio telescope transmitting the images of that historic moment to the rest of the world (and also correctly inverting the picture which the American engineers inadvertently had reproduced upside down), but few have heard of the input which took place a few years earlier in simply training those astronauts for what to expect when they got to the moon.

This training took place at the Henbury Meteorite Craters, in the Northern Territory. About 4200 years ago (give or take 1.9 thousand years, and with some sources quoting an age of approximately 4700 years) a meteor, estimated to have been travelling at 40,000km/h and weighing several tonnes, entered the Earth’s atmosphere above the site and tore its way through the atmosphere. The pressure on the face of the meteorite, along with the heat from the aerial friction as it raced towards the Earth’s surface, caused it to shatter not far above the ground, the largest pieces being about the size of 200L drums, and they ploughed into the desert.

They left 13 craters (some reports cite 12 craters, others 14, the varying number probably a result of the difficulty in interpreting some of the smaller strike sites) ranging in size from 9m to 217m in diameter (the largest being interpreted as the coalescing of two smaller craters) and up to 15m in depth. The rim of the largest crater is raised about 7m above the surrounding landscape. The smaller craters, which were only ever a few centimeters deep, are now largely invisible due to erosion. One crater has been broached by a pre-existing stream, which today fills the crater with water whenever it rains. This has resulted in the deposition of silt and the crater remains much damper than the surrounding area. It has become a home for a dense scrub which is refuge for a number of smaller animals and a hunting ground for local dingoes.

A number of fragments of the original meteorite — in total over 500kg in weight — have been found in the craters and its surrounding debris field, something relatively uncommon in most instances of craters. The decay evident in the specimens indicates a temperature of over 850 degrees was reached in the descent. All of the fragments are made up mostly of iron (90 per cent) and nickel (8 per cent), making them quite heavy. This type of material is known as octahedrite because of the octahedral shape of its crystal structure. Pieces of these can be seen at the Museum of Central Australia in Alice Springs.

This sort of construction is not uncommon in meteors, with heavier minerals being a much more common component of the undifferentiated formation of these extra-terrestrial visitors. The Earth’s rocks have been churned over so much, through the processes of volcanism and tectonics that most of this planet’s heavier materials have been moved towards the core and the rocks which form the surface tend to be lighter silicon-rich materials, sort of like the frothy scum on a boiling curry.

Tales of the meteorite fall, and the crater field exist in the traditional tales of the local Indigenous tribes, and older tribal people would not drink water from the craters nor camp within a couple of kilometres of the site, probably because of the stories of what had once gone on there. In fact, the local traditional name for the site is ‘Chindu chinna waru chingi yaku’, which can be broadly interpreted as ‘Sun goes fire devils rock', implying that the locals had a pretty good idea of what occurred.

One elderly Aboriginal who accompanied an early geologist on an investigation of the site said that an ancestor had seen the fire devil there and he had come from the sun.

The European name was adopted from the neighbouring Henbury cattle station, which was named after the hometown of the property’s founders in England. The craters were discovered by the station manager in 1899 but remained only as a curiosity until the Karoonda meteorite fell in South Australia in 1930, which sparked a renewed interest in such phenomena.

So, what’s all this have to do with astronauts walking on the moon?

The Henbury Craters were studied for NASA by the US Geological Survey in 1963 to assess their use in training the astronauts of the Apollo missions to recognise the basic features of a meteorite crater so that these could be identified in their subsequent ventures to our nearest space neighbour. The relatively recent age of the craters was seen as significant and was judged as being amongst the best possible terrestrial examples of the sort of structures that might be encountered on the moon, where erosional decay of landscape features is essentially non-existent. The Henbury craters are today partly softened by post-formation erosion, and some of the smaller impact sites are only recognisable by the absence of surface gravel and pebbles but the basic structures of the larger craters are still obvious enough to form a good training site.

The geological structures of the Henbury craters, though now weathered, are significant in identifying craters elsewhere, and their relatively small size means that they can be viewed on a scale that allows easy interpretation compared to that of a much larger site.

The site is now a conservation reserve that is located just off the Stuart Highway 132km south of Alice Springs, or 70km north of Erldunda. The site is accessible by 2WD vehicles. A basic small camp site is available adjacent to the reserve, with composting toilet and picnic tables available but water and firewood must be carried in. There is a clearly laid out walking track of about 1.5km in length around the four main craters, with good signposting. To get there take the gravel Ernest Giles Road towards Kings Canyon and Watarrka National Park for 8km and then follow the signs on the side road to the north for 5km.