The Spanish research vessel Hesperides is halfway through a seven-month circumnavigation of the globe, having set off from Spain in December, before the Japanese earthquake and tsunami damaged the reactors.

Its voyage is part of the Malaspina expedition, an international project to assess the impact of climate change and pollution on the oceans and explore marine biodiversity.

The leader, Professor Carlos Duarte, said the ship could monitor the spread of radioisotopes, such as cesium-137.

No one was studying the extent of contamination in the central Pacific because it was logistically difficult to get a research vessel to remote waters quickly.

On the journey to Spain, via Honolulu, scientists will collect samples of air and water as well as marine life in the top layer of the ocean, where the eggs and larvae of many sea creatures are found.

The expedition, a $23 million Spanish project, involves more than 400 researchers from 11 countries. It is named after Alejandro Malaspina, a Spanish nobleman-turned-explorer who commanded Spain's first voyage of scientific discovery.

His ship arrived in Port Jackson on March 12, 1793, and stayed a month while collecting plants, visiting Parramatta and making political observations of the British colony.

Two centuries later, on its voyage from Perth to Sydney, Hesperides towed a torpedo-shaped device at a depth of 10 metres to collect plankton, or drifting plants, animals and bacteria.

This was the first time a project known as the Australian continuous plankton recorder survey had been carried out around southern Australia.

A CSIRO scientist, Anthony Richardson, said plankton, despite its small size, is essential for life on Earth. The research would provide ''an invaluable baseline'' on the distribution of plankton to work out how this could change with climate change.

Hesperides will arrive in Spain in mid-July.