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SHARK attacks, the Bermuda Triangle and shoals of fish interfering with their instruments 鈥� these are not problems encountered by most robots. They are, however, among the hazards that have to be dealt with by a growing fleet of self-guided underwater 鈥済liders鈥� patrolling the world鈥檚 oceans.

The gliders are being tested by oceanographers who hope to use them as roving robot researchers to study climate change. Far smaller and cheaper than research ships, the gliders can stay at sea for up to 100 days and cover around 3000 kilometres between battery charges.

Three gliders, named Ammonite, Bellamite and Coprolite, are now on their maiden voyage profiling the top 1000 metres of the Atlantic Ocean between the Canary Islands and the west coast of Africa. Since they were launched from the Canary Islands in September by David Smeed and his team based at the , UK, the gliders have been logging their locations using GPS and transmitting measurements of water temperature, salinity and current via a satellite when they surface three times a day. The satellite link also allows the researchers to issue new instructions to the gliders and resolve technical problems without taking them out of the water.

If the trial proves successful, the Southampton team will make the gliders full-time members of its Rapid-WATCH (Rapid Climate Change 鈥� Will the Atlantic Thermohaline Circulation Halt?) project. Rapid-WATCH monitors 25 sets of underwater instruments moored at intervals across the Atlantic Ocean from Miami to north-west Africa. Their mission is to monitor the Atlantic鈥檚 heat circulation, including the Gulf Stream, the system that determines the climate for much of Europe.

The need for the gliders became apparent when the project ran into trouble near the Canary Islands, as the moored instruments kept getting damaged by trawlers. 鈥淲e have not had any permanent instruments in that region for over a year鈥�, says Harry Bryden, one of the project鈥檚 coordinators.

Only 1.6 metres long, the gliders are much less likely to get caught in nets, and the team can also steer them around danger zones. 鈥淲e are hoping the gliders should be less susceptible to this kind of damage,鈥� says Smeed.

Even if a glider does fall foul of a net, its frequent satellite updates mean only a few hours鈥� worth of data will be lost. Information from the moored instruments is retrieved only a few times a year and so months鈥� worth of measurements can be wasted if they are damaged.

The idea of a fleet of gliders roving the world鈥檚 oceans made its first appearance in the form of science fiction, when in 1989 the influential oceanographer and prolific writer Henry Stommel published a short story, , in the journal . He envisaged gliders performing 鈥渁n intricate cotillion-like dance鈥� or wandering 鈥渞andomly, like inspectors on a subway system鈥� throughout the world鈥檚 oceans, communicating via satellite.

One character in the story, the glider鈥檚 chief engineer, was based on Stommel鈥檚 friend and colleague Doug Webb, who had sketched out designs for an underwater glider in 1986. Webb has since founded in East Falmouth, Massachusetts, which developed the gliders being tested by Smeed鈥檚 team; in homage to the gliders in Stommel鈥檚 story, this design was named Deep Slocum.

Two other gliders have also been developed: the glider developed by the Scripps Institution of Oceanography at the University of California, San Diego, and the , produced by the University of Washington in Seattle.

While each of the three gliders is different, all are based on the same engineering principle. The glider rises or sinks in water by changing its density. To do this, it uses some oil and two bladders: one within the glider鈥檚 body, surrounded by low-pressure gas inside a sealed chamber; the other on the outside, surrounded by seawater. When it鈥檚 time for the glider to surface, an electric pump moves oil from the internal bladder to the external one, making the glider less dense than the seawater around it, so it rises. Descent requires nothing more than the opening of a valve between the external and internal bladders, allowing the pressure of water on the external bladder to force the oil back inside, increasing the glider鈥檚 density again and making it sink.

Because the glider has fins, it doesn鈥檛 just rise and sink vertically, but instead traces out a saw-tooth pattern that moves it across the ocean as it moves up and down. This allows it to make readings across a range of depths and locations. The current fleet carries instruments that measure salinity, temperature and ocean current data, which are enough for the Rapid-WATCH team.

There is also the option of trading some of the glider鈥檚 battery life for a more extensive instrument suite. Karen Heywood at the University of East Anglia in Norwich, UK, is one of those with a more sophisticated payload in mind. She is planning to buy three Seagliders to launch into the waters off Antarctica. Her Seagliders will measure oxygen levels, chlorophyll and the cloudiness of the water to help monitor the abundance of krill. They will also measure the amount of water spilling off the Antarctic continental shelf.

For her, the advantage of gliders is that they can work in winter, when conditions are too dangerous for a research ship. 鈥淭he gliders offer an opportunity to go to places that are really rough and windy, where there is lots of sea ice. With the glider data our observations won鈥檛 be biased towards the summer months.鈥�

鈥淭he gliders can work when conditions are too dangerous for a research ship鈥�

Meanwhile, the next generation of gliders is already under development. Rather than using a battery-powered pump to control buoyancy, Webb鈥檚 new Thermal Slocum exploits the different temperatures of seawater at different depths: a mass of wax that liquefies and expands at surface temperatures squeezes oil into a system of bladders near the surface and sucks it out again further down. Needing little electrical power, the glider can swim further and for longer than the Deep Slocums.

The Thermal Slocum successfully completed one voyage around the islands of the Caribbean early this year but ran into mysterious difficulties on its second trip. 鈥淲e wanted to test the glider鈥檚 endurance with a trip around the Virgin Islands and Bermuda,鈥� says Clayton Jones, who led the development team. 鈥淲e made it up and around the islands but just as we hit the corner of the Bermuda triangle we didn鈥檛 hear from the glider again,鈥� he says. The team suspects a shark attack may be to blame.

Despite such hazards, Smeed believes gliders will soon be invaluable for oceanographers. As for Ammonite, Bellamite and Coprolite, their three-month trial is due to come to an end next month. Smeed and his team will then compare measurements collected by the gliders with those from moored instruments and decide whether it is worth continuing with the gliders. 鈥淎t the moment it looks very promising,鈥� he says.