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MIR De-orbit



Russian Plan For Shorter, Simpler Deorbit Poses Risks

By Simon Saradzhyan
Special to SPACE.com
posted: 06:00 am ET 
13 March 2001     

MOSCOW -- The Mission Control Center (MCC) in Korolyov chose to shorten and 
simplify the Mir deorbiting plan in an effort to minimize risks, despite 
safety concerns expressed by an independent think tank.

Rather than begin deorbiting the station once it reached an altitude of 155 
miles (250 kilometers), the center opted for a new plan which provides for a 
controlled descent of the outpost once it passes the 137-mile (220-kilometer) 
mark, the center's chief ballistics expert Nikolai Ivanov told SPACE.com. 
Between March 11 and 12, the 15-year-old station dropped by a little more 
than a mile (2 kilometers) to an orbit of 152.6 miles (245.6 kilometers).


(Mir Facts 
STRUCTURE: Mir has a core module and five other components weighing about 143 
tons in all. With a cargo ship and an escape capsule attached, it weighs up 
to 154 tons. The modules are arranged in a T-shaped structure, 86 by 96 by 99 
feet (26 by 29 by 30 meters).
AVERAGE SPEED: 17,885 miles (28,780 kilometers) per hour.
NORMAL ORBIT: about 250 miles (400 kilometers) above Earth. 

COST: $4.2 billion to build and maintain Mir, according to the Russian space 
agency.
CREWS: Since the launch of its core module on Feb. 20 1986, Mir has had 104 
people aboard, including 42 Russian cosmonauts, seven NASA astronauts and 
others from Britain, France, Germany and other countries.
GLITCHES: More than 1,500, including near-fatal collision with cargo ship in 
June 1997 and onboard fire earlier that year. WORLD'S LONGEST SPACE MISSION: 
Cosmonaut Valery Polyakov, 438 days in 1994-1995. 

WORLD'S LONGEST TOTAL TIME IN SPACE:
Cosmonaut Sergei Avdeyev, 747 days on three Mir missions between 1992 and 
1999.
SPACEWALKS: Cosmonauts and astronauts conducted a total of 78 spacewalks, 
lasting 352 hours altogether.
CHAMPION SPACEWALKER: Cosmonaut Anatoly Solovyov, 16 spacewalks totaling 77 
hours. )


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.
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.
 
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