Rome, July 9 - Italian astronaut Luca Parmitano carried out the most daring and difficult part of Italy's first space walk Tuesday. Head-down, Parmitano crawled along the outside of the International Space Station, clinging onto a robotic arm operated from the inside of the ISS by his American crewmate Karen Nyberg. Parmitano, keenly watched on a live video link from the Italian Space Agency (ASI) near Rome, then teamed up with fellow spacewalker Chris Cassidy, another US astronaut, to carry out the walk's mission. Working together, the pair began to disassemble one of the ISS's platforms so that they can mount a new set of thermal radiators that will enable the ISS to burn off the excess energy from its array of solar panels. "Go, Luca go," said ASI chief Enrioco Saggese from the agency's data centre at Frascati. Several of the Frascati team cheered as Parmitano linked up with Cassidy for the crucial operation. Before setting out across the ISS, Parmitano performed another key task: photographing the Alpha Magnetic Spectrometer (AMS) anti-matter hunter. The AMS, a $2-billion, seven-tonne monster designed to trawl for anti-matter as well as spotting traces of the dark matter that makes up 25% of the universe, has to be regularly checked. "Photographing the AMS is important to get a precise idea of this essential instrument's state of health," said Claudio Sollazzo, director of the European Space Agency's Columbus mission.