Mir's natural decay makes controlled reentry tricky
BY ANATOLY ZAK
Posted: February 1, 2001
Although Russian space officials express full confidence in their ability to control the station during its final days in orbit, they also say that the wild fluctuations in the density of Earth's atmosphere make it more difficult to predict the exact date of Mir's reentry.
Currently, the regular peak of solar activity, which takes place approximately every 11 years, causes the Earth atmosphere to "bulge," increasing atmospheric drag. As a result, low-orbiting spacecraft, including Mir, spiral down toward denser atmosphere faster and less evenly than usual.
According to high-ranking officials at RKK Energia, the company which operates Mir, the station's altitude currently drops as little as 200 meters one day and as much as 650 meters on another, which complicates the work of the team monitoring the station's orbit.
On January 30, Mir descended to the altitude of 294 kilometers and it was expected to go as low as 291-292 kilometers above the Earth surface on Friday, February 2. (During routine mission, Mir was maintained at the altitude of around 350 kilometers.)
Currently ground controllers keep the station in the slow spin, as it circles the Earth, to distribute evenly Mir's exposure to the sunlight. However, around February 15, the mission control in Korolev expects to switch Mir back to the active orientation, which will be maintained with the use of small attitude control thrusters.
RKK Energia has ruled out the possibility of activating Mir's gyrodines, the station's electrically powered reaction wheels, which allow highly accurate orientation in space without use of onboard propellant. The decision is dictated by the fact that Mir approaches the altitude of around 260-270 kilometers, where the upper atmosphere is dense enough to disturb gyrodines' super-sensitive performance.
Despite inactive gyrodines, the RKK Energia officials remain optimistic that the amount of fuel onboard the station will be enough to deorbit the station fully under control into designated area of the Pacific Ocean, east of New Zealand.
The Progress M1-5 cargo ship successfully docked with Mir on January 27, delivering 2,677-kilogram of propellants. The fresh Progress craft is expected to perform all active maneuvers, which will led to the station's plunge into the atmosphere at the beginning or middle of March.