However, officials at the Russian Aviation and Space Agency (Rosaviacosmos), and experts at the Moscow-based Center for Analysis of Strategies and Technologies believe the station's lower altitude may impact the MCC's ability to control the engine burns necessary to bring the station back to Earth.
The station is to pass the 137-mile mark sometime on March 19 and will hit a designated area in the Pacific Ocean some 1,200 to 1,500 miles (1,930 and 2,415 kilometers) east of Australia either on Tuesday, March 20 or in the early hours of March 21, Ivanov said. He said these two dates "can shift back or forth by one or two days."
The center booted up Mir's main computer on Monday in order to run a series of tests prior to the planned deorbiting of the outpost. According to Viktor Blagov, deputy flight control chief at Korolyov, the station has been descending by an average of more than a half-mile (0.8 kilometer) a day over the last week.
The recently selected plan provides for the station to be sunk in one day rather than three as initially planned, according to Ivanov and Blagov.
The plan was selected at a March 6 meeting in Moscow of space station specialists chaired by Yuri Koptev, the director general of Rosaviacosmos. It provides for the Progress supply ship, which docked with Mir in January, to be used for three braking impulses, rather than four as previously announced, to slow the aged station so it drops out of orbit, Ivanov said.
Once Mir hits the 137-mile mark, the center will verify that the Progress cargo ship's engines are pointed in the exact opposite direction of the station's flight path, according to Ivanov.
When it reaches this altitude, Mission Control in Korolyov will wait for Mir to pass above the equator at 20 degrees east longitude. They will watch the station over another 14 orbits before firing the cargo ship's engines; the first of the so-called braking impulses. The second impulse will occur during the 16th orbit, Ivanov said.
The center will then wait for the station to descend to an altitude of some 130 miles (210 kilometers) before firing the engines of the cargo ship again somewhere above Africa for a third and final time.
This impulse will last some 20 minutes and end while the station still remains within the MCC's zone of radio visibility. It will then take the station anywhere between 45 and 60 minutes to crash into the Pacific Ocean, according to the new plan.
"We have chosen this scheme because it allows us to use standard schemes without shifting back and fourth between inertial and orbital modes. It is safer that way," Ivanov said, though he would not elaborate on why the new plan is safer.
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According to one of Koptev's staff, the new scheme was chosen because Rosaviacosmos and the MCC doubted whether there would be enough fuel in the Progress tanks for an additional impulse if anything went wrong during implementation of the previous three-day deorbiting plan.
"The reason is that we need more fuel," the Rosaviavcosmos official told SPACE.com.
The official, who asked not to be named, said the deorbit plan has "some risk" since the atmosphere is thicker at an altitude of 137 miles than at 155 miles. There is a possibility that this could make it more difficult for MCC to orient the station for the first impulse.
Experts at the Moscow-based Center for Analysis of Strategies and Technologies also believe that the lower Mir descends the more difficult it will be to control the station.
"It is easier to aim and calculate the descent trajectory at higher altitudes, whereas trying to orient the station at 137 miles may prove too be difficult," the center told SPACE.com in a March 12 written statement.
According to Ivanov, however, the Korolev center can keep the station under control as long as it remains above 124 miles (200 kilometers). "Only lower can such problems begin," he said.
Next page: Lessons from Salyut and Skylab?
However, Ivanov admitted that the center has never tried to issue descend commands to stations, such as Salyut, at 137 miles in the past, opting to begin the deorbiting sequence at higher altitudes. The Korolev control center lost contact with the Salyut 7 station in 1991 as it orbited the Earth unmanned.
As a result, the station's solar-power panels lost their orientation toward the Sun and Salyut 7 froze before the Korolev center could send an emergency crew to the station.
Unable to send a rescue team, Korolev controllers could not keep fragments of the 40-ton spacecraft from crashing in the Argentine Andes near the Chilean border.
According to Ivanov, there is a 2-percent chance that Mir will veer off and crash outside the designated area that is some 3,000 miles (4,830 kilometers) long and 300 miles (480 kilometers) wide.
And if Mir goes into an uncontrolled dive, there would be a 10-percent chance that debris would crash on land, including a 1.7-percent chance that it would hit the continental United States, according to estimates of Koptev televised in Moscow on February 2.
Whatever happens to Mir after it drops below 125 miles (200 kilometers), the Korolyov center will know only after the fact, Ivanov said. The MCC has no means to download telemetry after the third and final impulse, but he hopes the U.S. military and European Space Agency will keep tracking Mir further and feed data to the center.
"Fortunately or unfortunately, we will not see it," Ivanov said.
It will be the U.S. Army station at Kwajalein Atoll in the Marshall Islands that will be the last to track Mir as it begins its fiery reentry into Earth's atmosphere.
Deorbiting of the station will be rather spectacular, according to Nikolai Anfimov, director general of the Central Machine-Building Scientific Research Institute, which incorporates the Korolev control center.
The station will start disintegrating somewhere between 55 to 70 miles (90 and 115 kilometers) in altitude, Anfimov said at a recent press conference in Moscow.
He said the station would rapidly heat up as it enters the planet's atmosphere. The heat and friction will first tear away the station's solar panels and cause its fuel tanks to explode.
Then, said Blagov, Mir's hermetic modules will be splintered, torn away from each other to disintegrate as the station plunges deeper into the atmosphere. Most of the fragments will burn up before hitting the water.
Some parts, however, such as the heat-resistant spherical gas tanks, parts of the station's engines and some of its gyrodines will probably survive the overheated plunge and smash into the ocean, he said.
Up to 1,500 fragments, weighing a total of some 13 to 19 tons, said Ivanov, will survive the planned burning dash through the Earth's atmosphere and hit the surface.
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