When, after an illegal border-crossing, he eventually emerged near the east coast in Kalimantan, Indonesia, the confrontation with western civilisation gave him such a culture shock that he turned around for another crossing of the island.
Hansen had spent many years preparing for the journey, through research at the University of California, which also had a complete collection of the Sarawak Museum Journals from 1912 onwards.
His aim was to cross the island, following old trade routes, collecting jungle products and medicinal plants of value, and exchanging these and western goods for what he needed.
The few Malay, Indonesian, English and Dutch surveyors who had managed to reach the centre of the island had never agreed on their findings.
According to Hansen: Dr A. W. Nieuwenhuis, two associates and 110 porters and bodyguards had preceded me to succeed in crossing the island, but that had been in 1897, and it had taken them a year.
The challenge was to do it alone, to make myself completely vulnerable, and to be changed by the environment.However, he could only carry enough food for 3 or 4 weeks, and he figured he needed 3 months for the crossing.
For the first eight weeks Hansen made several attempts, going up one river after another, which drained his budget and for which he took along all the wrong trade-items, such as 10 kg of salt, which he ended up giving to a hotel owner, who sympathised with him because he had already seen many westerners try to go so far upriver and failed.
These Penan still used blowpipes, but also had self-made guns, made out of reinforced water pipe, umbrella springs, bicycle inner tubes, metal from flattened oil drums, nails, nuts and bolts and hand-carved hardwood stocks and grips.
Other good trade items were tobacco and, strangely, beads, the sort of thing one would expect ignorant missionaries to take along.
His diet consisted of, among other things, bee larvae, rice soup, roasted rattan shoots, boa constrictors, lizards, monkeys, bats and the large animals - pigs and deer.
When he went further downriver, for what he thought would be the last day, he was shocked to see the river all muddy from logging and the bare shores, motorised longboats and locals in western T-shirts.
Confident that he had learned enough by the first crossing, he decided to test himself by taking a more difficult and uncertain route, through the central Kenyah highlands.
Dayaks never travel through the forest at night, and most certainly not alone, and on top of that his looks fit the description rather too well, which led to some harrowing moments when he had to explain his unlikely appearance at spearpoint.