Throughout the operation Michael Foale, the British-born guest astronaut from Nasa, will sit in the escape capsule to which his crewmates, Anatoly Solovyov and Pavel Vinogradov, would beat a hasty retreat if an emergency meant they had to evacuate Mir. But both Russian and US space officials expressed confidence yesterday that the repairs would succeed and give the 11-year-old station a new lease of life.
Mir, which has suffered a string of technical faults because, on their own admission, the Russians are saving money by only replacing spare parts when they break down instead of when they reach the end of their life expectancy, lost half its power in June when a Progress cargo craft hit and holed Spektr.
The module had to be disconnected from the mother ship and stopped contributing its share of energy from the solar panels which cover the entire exterior of Mir.
The aim of today's repairs is to reconnect the cables between Mir and Spektr. But the task is a good deal tricker than putting plugs back into sockets.
Because Spektr is airless, Solovyov and Vinogradov can only enter it wearing oxygen-giving suits of the kind they would put on to walk in open space. Igor Goncharov, the chief doctor at Mission Control, said the "internal space walk" was easier than leaving the space ship from a psychological point of view because there would be "no hanging in space, no seeing the Earth below your feet". But it will be more difficult in physical terms.
The Russian-made Orlan space suits are bulky, the passage into Spektr is narrow, the module itself has little room for cat-swinging and debris such as broken glass could be floating around inside. David Wolf, the US astronaut training to replace Foale, ripped his glove during a simulation of the Spektr operation in a swimming pool at Star City outside Moscow this week.
If the cosmonauts tear their suits for real today, they will have little chance of surviving.
Commander Solovyov, who has 43 hours of space-walking experience, will bear the main burden of the work, gliding into Spektr feet-first while Vinogradov gives him support from behind. Foale will sit in the Soyuz rocket but not in a space suit.
"If something goes seriously wrong," said Valery Ryumin, head of the Mir-Nasa programme, "all three cosmonauts will have to abandon Mir in the Soyuz."
Mission Control was not anticipating problems. The computer failure which earlier this week forced the crew to save energy by switching off all but life-support systems was corrected. Mir recovered its orientation in relation to the sun and recharged its batteries. The spacemen also recharged theirs by resting yesterday and going to bed early ahead of the repairs which were due to start first thing in the morning.