The deviation may be caused by a small helium leak in the engine used to slow a spaceship when it leaves orbit.
"The problem does exist but it would be wrong to exaggerate its importance," a spokesman for the Mission Control Center, Valery Lyndin, said Vladimir Solovyov, the head of the Russian part of the ISS, said. According to Mr. Lyndin, there is no danger whatsoever for Michael Foale, an American astronaut, Alexander Kaleri, a Russian cosmonaut, or Andre Kuipers, an astronaut from the European Space Agency, during their decent. "This does not pose any serious threat to the crew, we will be working according to the routine program," he said. "Such leaks happened on other Soyuz series spaceships before, but the helium leaks did not exert any influence to speak of on the operation of the spacecraft."
A Russian specialist who helped design Soyuz spacecraft said pressurized helium is used to move fuel components from the tank to the engine and that if the system leaks, deviation from the set trajectory of descent is probable.
A minor helium leak in the engine was discovered on the Soyuz TMA-3 in mid April when it was docked to the station. The leak appeared during the flight to the station in October 2003 but, since there are two Soyuz engines, the crew replaced the malfunctioning system.
The expert said that after the Soyuz undocks from the ISS, the thrusters will take the spacecraft a safe distance away from the station. After a short flight, when the Soyuz is over the equator, the descent system will be activated. The spacecraft will turn its engines toward Earth and the crew will wait for a braking impulse.
"The Soyuz engine must operate for about 200 seconds while the ship slows from its orbital speed and starts descending," the expert said. According to him, the Soyuz will then break into three parts. "The daily compartment, the instrument-assembly module and the descent capsule with the crew will fly to Earth separately.
"The first two elements of the spacecraft will burn up in the dense layers of the atmosphere, while the descent module and the crew will continue descending and land smoothly with a parachute 70 kilometers north of the city of Arkalyk in Kazakhstan at approximately by 4.00 a.m. on April 30."