Ensuring Controlled Descent of the Mir Orbital Station 

N.A. Anfimov
Director of TsNIIMash, RUSSIA

The history of the world cosmonautics is on the eve of witnessing a very remarkable event. Fifteen years ago, on February 20, 1986 the launching of the Core Block marked the beginning of the Mir Orbital Station construction. Currently the Mir Station consists of 6 modules. They are the Core Block, Kvant, Kvant-2, Kristall, Spectr and Priroda as well as the Docking Compartment (for docking with US Space Shuttle orbiters) and two transport vehicles known as Progress and Soyuz.

The Mir Orbital Station is undoubtedly one of the most important engineering achievements of cosmonautics in the XX century. The Mir Station total mass is near 130 tons. Through 15 years of its orbital life the Mir Station was inhabited for 12 years and 7 months. The station housed 104 cosmonauts and astronauts including 62 foreign citizens from 11 countries to stay and work there at that period. US astronauts constantly stayed and worked at the Mir Station from March 24, 1996 till June 08, 1998. The Mir Station accommodated 28 long-term basic expeditions accompanied by 16 visiting expeditions lasting from a week to a month. Of them 15 expeditions were international ones. More over, 9 visiting expeditions were conducted with Space Shuttle orbiters ferrying 37 US astronauts, 1 Canadian astronaut, 1 ESA astronaut, 1 French astronaut and 4 Russian cosmonauts. The Mir Station became essentially the first international space station. 

23 research experiments and studies to Russian and international programs have been conducted for the Mir Station lifetime. Many of those experiments and studies were unique in the world.

The Mir Station has become essentially a flying testbed for actual testing of many technological solutions and processing techniques to be used at the International Space Station.

Currently the Mir Station mission program has been completed and the Russian Federation government agreed with a proposal by the Russian Aviation and Space Agency (Rosaviakosmos) to terminate the Mir Station operation program in February-March 2001 providing its controlled safe deorbiting and sinking in the World Ocean waters. A special Interagency Commission was established to ensure the Mir Station final stage of flight and controlled descent. The Commission is headed by Ju.N. Koptev,General Director of Rosaviakosmos. Preparations for the final operations are under way.

It is worth to mention that the world community expresses a great interest and, in some sense, concern about forthcoming deorbiting of that huge space object. Therefore it seems appropriate to advise this high-level forum of the procedures for conducting the forthcoming unique and previously unparalleled operation to deorbit the Mir Station. Also, it is to the point to discuss some aspects defining available capabilities and limitations for the Mir Station descent realization.

The information on the final stage of the Mir Station flight is constantly updated at the TsUP TsNIIMash web-site (http://www.mcc.rsa.ru) and Rosaviakosmos web-site (http://www.rosaviakosmos.ru). 

Two aspects are the most principle in relation to implementation of the forseen strategy for the Mir Station deorbiting and sinking in a specified zone of the Pacific Ocean. They are:

-         the Mir Station technical status, that ensures implementation feasibility for the defined program and

-         ballistic & navigation support of defined dynamic operations.

The Mir Station technical status is constantly monitored by the Energia RSC as a primary designer as well as by TsNIIMash and associated entities. At the moment the Mir Station current technical status is estimated as satisfactory one without precluding from implementation of the specified operations.

The TsNIIMash Mission Control Center (TsUP) undertakes a ballistic & navigation support for the Mir Station flight control. However surveillance from Russian territory could be conducted only for a half of daily orbits. The other half (so called 'dark orbits') cannot be tracked from the Russian territory. That is why it is impossible in TsUP normally to update the Station motion parameters from those orbits. Since the requirements for constant control of the Mir Station flight is of a primary importance for the whole period of final station deorbiting preparation the Interagency Commission has made up and implemented an extensive set of technical and administrative arrangements to provide the final Mir Station operation stage. Specifically, all Russian satellite orbit monitoring facilities were involved. Moreover agreements were reached with NASA and ESA to attract their appropriate technical facilities for Mir Station orbit tracking.

Discrepancies in ballistic estimates are referred to uncertainty of the Earth upper atmosphere density and significant inaccuracies in its forecasting. To update forecasts of solar activity and geomagnetic perturbations decisively affecting the upper atmosphere density Russian primary associated institutes are attracted to the mission. Also data from ESA and NASA are in use. Cooperation of all participating organizations and services is arranged under TsUP control.

For sinking unburned Mir Station fragments after reentering atmosphere a Southern Pacific Ocean area limited by the following point coordinates: 53њS, 175њW; 23њS, 175њW; 23њS, 132њW; 30њS, 127њW; 30њS, 90њW; 53њS, 90њW is choosen. That area located between Australia and South America is free from shipping routes and is usually used for sinking debris of launch vehicles and spacecraft by Russia and other countries.

The forseen controlled deorbit of the Mir Station would cover three stages. They are:

1.     Passive waiting the date when the orbital altitude decreases to 250-240 km (so called pre-descent orbit). This stage started after docking the Progress M1 cargo vehicle on January 27, 2000.

2.     Construction of the approximately 160 ´ 230 km descent orbit with a perigee located above the sinking area using several burns of the Progress M1 cargo vehicle engines.

3.     Progress M1 executes the final deorbit impulse and Mir Orbital Station passes to the reentry trajectory.

Undestroyed station fragments could impact the outlined Pacific area from three diurnal orbits. According the basic estimated case the final deorbit impulse would be executed at the orbit arc from the Gulf of Guinea to the Caucasus. The coordinates of the aiming point for the Mir Station deorbit are 47њS, 140њW (can be updated depending on a specific deorbit date and selection of a basic orbit for reentry).

During Mir Station descending along the last reentering orbit the station main body and its external elements would sustain progressively increasing stress and thermal loads due to atmosphere density increase. Outer antennas and solar panels will burn first at altitudes of 110-100 km. The main body structural elements destruction and hence the primary fragmentation of the whole object can take place at altitudes 90-80 km. The formed separate fragments continue their descent in the atmosphere independently. They can further split into parts, and the fragmentation process terminates at altitudes of 50-40 km. The majority of fragments melts and completely burns down, but the most heavy and high-melting ones can reach the Earth surface. 

Aerodynamic characteristics of separate fragments can differ essentially and hence their descent downrange is also different, that forms the fragments dispersion area. Moreover, the descent downrange of fragments depends on an altitude where they are formed. The dispersion area width is determined by the lateral force coefficient of each fragment during its motion in the atmosphere. The development of the destruction model for a complex artificial space object is a non-deterministic problem that can not be solved precisely, at least at the present time. The main reasons of this situation are the following:

-       A sequence of the fragments separation and their shapes are determined by an initial object orientation, uncontrolled object motion relative to the center of mass under the action of aerodynamic moments that is impossible to analyze precisely.

-       Precise determination of aerodynamic characteristics of separate fragments along the descent trajectory is not possible due to the uncertainty of their shape and also fragments rotation around their centers of mass.

Therefore it is practically impossible to determine a complete set of fragments formed during the object reentry into the atmosphere, and also longitudinal and lateral dispersion of the fragment impact points basing on precise solution of the space object fragmentation problem and fragment motion in the atmosphere problem. Therefore, the problem of successive disintegration of the Mir Station into fragments and that of their final set was solved in a approximate way.

A total estimated mass of unburned fragments would be 20 - 25 tons with a total expected number of fragments equal to about 1500 pieces. Length of the fragment impact area along the reentry path is estimated to be 6 thousand km with about 200 km in width.

The nominal implementation of the developed Mir deorbit and sinking plan practically entirely excludes any damage infliction. Nevertheless it is not possible to exclude entirely an occurrence of contingencies onboard the station or in the flight control loop. All such possible situations have been analyzed and methods to control them have been developed, including a launch of the Soyuz transport vehicle with the crew to dock the station and to perform repair woks there. Of course there is a small probability that the planned three stages program of the Mir deorbit could not be implemented. But even in this case an option remains to correct the station trajectory using engines or changing an orientation of the station or its separate solar panels in order to avoid hitting the land. Thus a risk of the Mir deorbit in a completely uncontrolled mode and fragments falling on the land are inessential.

Conclusion

The set of implemented preparatory efforts and the developed program of controlled MIR Station deorbit should provide a safe sinking of unburned fragments in pre-determined area of the Pacific Ocean at high enough reliability.