The first element of the station, known as Zarya, was launched last November. Yet as long ago as September 1997, the Khrunichev State Research and Production Space Centre in Moscow warned that noise generated by fans, air filters, dust collectors and pumps would result in a "probable worsening of the health of cosmonauts and in unintelligibility of voice communications".

In parts of Zarya, astronauts could be subjected to a perceived noise level of 72.5 decibels (dBA). That's louder than an air conditioner in a closed room, says Robert Lilkendey of Jaffe Holden Scarbrough Acoustics in Norwalk, Connecticut. "It's not something that's intolerable, but you've got to raise your voice." Even the quietest parts of Zarya exceed the space station's design requirements of 50 to 55 dBA.

Under normal circumstances, these noise levels are too low to cause hearing damage, but experience with long, constant exposures in Russia's Mir station -- which is of similar design to Zarya -- worries NASA engineers. "There's an indication that a significant number of astronauts have lost hearing in Mir," says Jerry Goodman, who works on the acoustics of the space station. Goodman declines to give further details, citing medical confidentiality, but says that the astronauts later recovered a percentage of their hearing loss.

Even if astronauts' hearing is not damaged, noise could have other undesirable effects. "Crew ability to sleep, work, communicate will be impaired," warns one NASA document, dated July 1998. Another document, dated November 1998, notes that the warning tones that signal an emergency would be "minimally audible" in Zarya.

Rich Patrican, a NASA official responsible for safety aboard the space station, says that the launch of Zarya went ahead despite the problem because foam covers and other devices would be installed later to dampen the noise. "It will be under control," he says. But other NASA documents predict that the installation of acoustic mufflers will be "minimally effective", reducing the noise by only a few decibels. And Goodman admits he can't be sure the mufflers will work.

Less than a month before Zarya's launch, the Russians and Americans were still arguing about the solution to the noise problem. In October 1998, a team from Khrunichev suggested that astronauts spend no more than four hours a day in Zarya and no more than two hours a day in the noisiest segments. They also suggested wearing earplugs. However, documents from the Johnson Space Center in Houston brand the time restrictions "unacceptable", and say that earplugs should be prohibited because they would interfere with astronauts' ability to hear warning signals.

With Zarya already in orbit, the focus of the arguments has now shifted to the Russian service module, currently scheduled for launch in September, which will serve as the astronauts' living quarters during much of the station's construction. At a meeting in January this year, Russian officials announced that noise levels in the service module will be as high as 74 dBA. This did not include the added rumbles of refrigerators and other equipment.

Goodman says that NASA is now paying additional attention to the acoustics within its station modules, to provide astronauts with quiet havens. "When you're locked up in a spacecraft, you can't get away from the noise unless you have a good sleep station," he says.

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