In 1997, an un-piloted cargo ship collided with Mir and punctured the Spektr module, which has remained depressurized ever since.

The hatch separating Spektr from the core module is located in a small ball-shaped compartment in the front end of Mir's core module. Known as PKhO (which means "transfer compartment"), it serves as a connecting hub between the core, four add-on modules, including Spektr, and a Soyuz spacecraft.

Following the puncture, the docking port between the Spektr module and the PKhO transfer compartment was replaced with a special hatch. It allowed electrical cables that had to be cut when Spektr was sealed off after the collision to be reconnected to the rest of the station while keeping the damaged module isolated. As a result, three of four solar panels on the battered module could still be used to supply electricity to the rest of the station.

This hatch also features a valve used to mount a pressure gauge and a device for pumping gas inside Spektr in an effort to detect the leak left by the collision.

Engineers believe the valve in the Spektr's hatch was at least one source of the leak.

For the test, which finally revealed the location of the leak, the crew had to disconnect numerous cables leading into the Kvant 2 and Kristall modules, so that their hatches could be closed. As a precaution, the crew took shelter in the Soyuz transport ship rather than in the core module during the test.

Only 3.5 hours into an eight-hour test, the pressure inside the small transfer compartment dropped.

Cosmonauts told ground control that they actually heard a hissing sound of air escaping through the valve into the depressurized Spektr module.

The crew was then told to install a standard plug on the valve and resume the pressure test. The preliminary data showed that the leak from the compartment had stopped. Currently, the pressure inside the station is stable.

The engineers on the ground will spend the next five or six days evaluating the latest pressure measurements made by the crew, and comparing them with the daily pressure readings received by Mission Control since last August.

"If we are lucky, we will be able to confirm that this is the only place of the leak," Grigoriev said.

The engineers hope to avoid the next and more daunting part in the search for the leak, which would require isolating the Kvant 1 module from the core module. The hatches between the two have never been closed since they docked in 1987.

Meanwhile, the crew proceeded with planned pressure checks inside the Kvant 2 and Kristall modules, since the communications with them were already disconnected for the tests in the transfer compartment. The hatch between the core and Kvant 2 modules had to be reopened yesterday, when the initial attempt to isolate the module caused overheating in its compartments.

The search for the leak was one of the main tasks of the expedition, which is the 28th to occupy the Mir. This Friday, cosmonaut Sergei Zaletin will celebrate his 38th birthday aboard Mir.

To support Mir operations, the launch of the Progress M 1-2 cargo ship is scheduled for April 26 from Baikonur Cosmodrome.

The craft will deliver supplies, experimental equipment and fuel to the station. The craft's engines will also be used to adjust the station's orbit, which has been degrading faster then usual due to high solar activity.

The propellant stored on board the Progress M-1-1 spacecraft, currently docked to the station, is almost exhausted. On April 16 and 17 the crew used the engines on their Soyuz spacecraft to slightly adjust the station's orbit. Mir is currently flying in a 202- by 217-mile (326- by 350-kilometer) orbit.

According to a Mission Control representative, the station currently looses 1,312 to 1,640 feet (400 to 500 meters) in altitude daily, as opposed to an average 328 feet (100 meters) a day in previous years.