1.   The generating system breaks down periodically, but the Mir is so large that the existing oxygen supply is sufficient to last for several days.

2.   The existing oxygen supply is good for several days, and there are also backup oxygen canisters if needed.

3.   The existing oxygen supply lasts for several days and the crew also has a supply of air canisters.

4.   The existing oxygen supply is good for several days, and the crew also has a two-month supply of oxygen canisters if needed.

5.   The oxygen generating system breaks down periodically, but the Mir is so large that the existing oxygen supply is sufficient to last for several days.

6.   Photosynthesis -- the process by which plants take carbon dioxide and water to produce organic matter and oxygen -- only sustained the amount of existing oxygen, he said.