The number of visitors to the icy continent is likely to fall to 39,000 in the 2008-09 summer season from a record 46,000 a year ago, according to the International Association of Antarctica Tour Operators (IAATO).

"It's been impacted by recession," Steve Wellmeier, executive director of the association, said in a telephone interview. He projected that numbers could rebound to 43,000 next season but not challenge the record until 2011 or 2012.

First seen by sailors only in 1820, Antarctica has been drawing ever more rich tourists with wildlife such as penguins and seals along with jagged mountains, glaciers and icebergs. Fewer than 1,000 a year visited until the early 1980s.

Environmentalists and some nations want tighter controls to protect visitors and wildlife. Worries include shipwrecks, oil spills and an aggravation of stresses on animals and plants that may already be suffering from global warming.

"The growth trajectory for tourism has been too steep," said Jim Barnes, head of the Antarctic and Southern Ocean Coalition (ASOC), which groups 100 environmental organizations. "Sooner rather than later (governments) will need to address the issue of overall visitor numbers," ASOC wrote in a submission to the 47 members of the Antarctic Treaty before an annual meeting in Baltimore

from April 6-17.

"ASOC would prefer to see a conservative overall figure that is in the order of magnitude of present total numbers," it said. Some governments have spoken of caps but have not set numbers. But the tourist association says fears are exaggerated.

"We're talking about a continent that is larger than Australia and we're talking about a number of tourists that would fill a football stadium," Wellmeier said. "Seriously, is this a number for concern?" Wellmeier said there was no evidence that tourism damaged Antarctica. Still, governments are worried even though many agree that tour operators have set themselves strict standards. And almost all tourist visits are to the peninsula, the part of the southern hemisphere that has warmed most in the past 50 years, apparently because of global climate change stoked by human use of fossil fuels.

"The increase in visitor numbers is in the context of global warming. The presence of visitors enhances the risks to the environment," said Yves Frenot, deputy head of the French Polar Institute. Visitors could inadvertently bring in new seeds or spores -- even diseases -- that might gain a foothold in a slightly warmer environment. And accidents including the first sinking off Antarctica of a cruise ship, the MV Explorer, in November 2007 highlighted risks for tourists.

More than 150 passengers and crew were rescued before the vessel sank off King George Island. In another incident, a Norwegian cruise ship ran aground on Deception Island off Antarctica in early 2007. Apart from tourists, several thousand scientists, other staff and some reporters also visit. The Rothera base on the Antarctic peninsula run by the British Antarctic Survey, for instance, has about 100 summer staff.

Among exotic draws, the clear Antarctic air lacks almost all smell -- there is nearly no vegetation or pollution. Colors are largely limited to white snow, blue sky or sea and dark rocks. On the shore the sound is sometimes a fizzing as thousands of bubbles from thawing icebergs pop at the surface. Jose Retamales, head of the Chilean Antarctic Institute, criticized use of large cruise liners off Antarctica. Non-binding guidelines say operators must have their own search and rescue backups -- many tour ships stay within range of each other.

"I'm worried about some excesses," he said. "Just 200 people aboard a vessel is a big risk." The biggest ships, run by Princess Cruises, have more than 3,000 people aboard. Wellmeier said such vessels were modern, stayed clear of icebergs and do not let tourists land on Antarctica. Instead, they view the coast through binoculars.

Smaller cruise ships moor just offshore and run small fuel-sipping inflatable launches to land, enabling tourists to make day trips

ashore and return to their cabins overnight. There are no hotels in Antarctica and IAATO opposes any construction -- in line with a treaty for protecting the Antarctic environment.

The few tourists who visit the interior stay in tents. Scientific bases, such as at Rothera, have hotel-like rooms for scientists and visitors, with two to a room. The United States -- from where a third of all tourists come -- wants measures including a ban on landings by vessels with more than 500 people aboard. Among other proposals preferred by some nations are that all vessels should have ice-hardened hulls, should burn only light diesel fuels, and that lifeboats should have covers to protect against the cold. A few visitors go inland. "We aim to leave only footprints, take back only photos and memories," said Keith Heger of PolarExplorers, who led five people to ski the last 110 kilometres (68 miles) to the South Pole in January.

"Trash and even human waste created is brought out." Bad weather forced the group to abandon a climb of Mount Vinson, Antarctica's highest peak at 4,897 metres (16,070 ft). The fee to take part in a similar trip next season -- $67,000. Summer temperatures at Rothera are around zero degrees Celsius (32 Fahrenheit), with top temperatures of about 7 Celsius (44.6 Fahrenheit). It rains nearly as frequently as it snows. Tourists visit the South Pole in summer, when temperatures are around minus 40 Celsius (-40 F).

Polish businessman and climbing instructor Rafal Szczepanik, aged 32 and on the trip, reckoned it was probably less-environmentally damaging than flying to the tropics and staying in an air-conditioned hotel. "You probably produce more carbon dioxide in a four-star Caribbean hotel than you do walking in Antarctica," he said.