Hooked to each other since last Thursday, the Russian space station Mir and the American shuttle Atlantis are scheduled to part at 1118 GMT. The shuttle returns to Earth on Friday, while Mir will stay in orbit.
The final preparations were being made for the separation yesterday. After a farewell ceremony, the two teams were to return to their respective vessels. The ships should then be depressurised and closed.
Today's separation will begin with a series of precise moves that will permit the shuttle to push about 2ft away from the space station. By firing its motors, the shuttle will push farther away.
For months the Russians have proposed a daring additional manoeuvre - filming Atlantis's departure live from Soyuz. Nasa accepted, but only on the condition it could refuse at the last minute if it deemed necessary.
Immediately after the departure of the shuttle, the plan calls for Atlantis to position itself 660ft from the space station, and then the three ships are to fly in formation at 17,360 miles per hour.
It will then be Atlantis's turn to film Soyuz, which will reattach itself to the station so that the two occupants can re-enter Mir.