Russian Plan For Shorter, Simpler Deorbit Poses Risks

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).

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.