Safety concerns are mounting over the de-orbiting of the Mir space station next June and an international debate has now begun on how to dispose safely of the flagship of Russia's aerospace industry.
While Russia embarks this month on reducing Mir's orbit from its present 450km, critics of the de-orbiting scheme are claiming that the space station is not junk, is still operational and could conceivably have several years of service ahead of it. Others are saying that trying to de-orbit the large space laboratory so hastily could have dangerous consequences on the earth.
The controversy has been sparked by three factors: NASA wants to be rid of Mir; Russia's space programme lacks funds to support the station; and, critics argue, the de-orbit plan has not been thought out fully.
"It is a great tragedy to allow it to crash," says author Arthur C Clarke. "It is a piece of space history. The Russians have done a wonderful job with Mir." Clarke says that, rather than de-orbit the Mir, it should be boosted into a higher orbit. "It could be a major tourist attraction in 40 years time. The USA and its Russian colleagues ought to be working together to save the station," Clarke says.
The de-orbit plan is "-far more important an issue", he says. "To say that it will fall under control and harmlessly into the Atlantic Ocean is nonsense. Pieces will come down uncontrollably along an entire band, going off in different trajectories. It could land anywhere," the author believes.
Mir would be safer in a higher orbit, he says, giving scientists the time to find a safer way of disposing of the station, if it is really necessary.
Raising Mir's orbit presents many difficulties, however. It would require a very large tanker craft to provide the propulsion required and, for safety, the Mir "-would have to be placed into an orbit about 2,000km above the earth, to ensure it does not present danger to the highly populated lower orbits, which are becoming increasingly busy now that mobile communications satellite constellations are being introduced", says Richard Crowther, space debris scientist at the UK's Defence Evalalution and Research Agency in Farnborough. The DERA is involved in the final planning for the de-orbiting project.
French space agency CNES is hosting the Inter Agency Debris Committee meeting in Toulouse this November and the Mir issue will be at the top of the agenda.The de-orbit plan will be studied in detail and, although several alternatives will be discussed, it is likely that the initial plan will be adhered to and only a "damage limitation" exercise will be introduced.
This will try to gain as much understanding as possible into what parts of the Mir are likely to survive re-entry and how the craft can be controlled until its last minutes. US Space Command and other radar stations will work together to ensure close monitoring of the re-entry. An additional difficulty is that June 1999 will be a period of maximum solar activity, which causes the atmosphere to heat. This could result in an earlier natural re-entry than planned, Crowther says.
Most of the 140t-plus station would be vapourised during re-entry, but it is estimated that about 10% may survive, the equivalent of 14t of metal hurtling towards earth.
The 20t Salyut 7 space station was deliberately de-orbited in February 1991 and aimed at the Pacific Ocean. Large parts of the craft landed in South America, in sparsely populated regions. In 1979, the US Skylab space station made a natural re-entry, which was predicted to end with debris raining over the Pacific, but large parts crashed in Western Australia.
The problem with the Mir is that it is not just one large symmetric cylinder. It consists of six modules, five of which are the size of a bus, plus huge arrays of solar panels. "It is asymmetric and it is impossible to predict how it will behave when it hits the upper layers of the earth's atmosphere at about 17,000mph [27,400km/h]. Parts will break off, taking their own different trajectories," says Crowther.
Russia may be aiming for an impact in the Pacific, but the timing of the re-entry of surviving sections of the station will be difficult to predict and the debris could hit the earth anywhere along the flight path up to 1,000km from the predicted impact zone.
"We would not expect any deviations across the flight path, just along it," says Crowther, adding that "-there is very low probability of debris hitting populated areas within the target flight path". The de-orbit plan is the easiest to accept since the re-entry is not controlled and no-one can be held responsible for damage.
Of the 10% of the Mir that may survive in small pieces, "-the most likely will be those made of exotic metals such as titanium, which has been used for module's propellant tanks", Crowther says. "The Mir is very robust because it was over-engineered."
A safer strategy, but one which would be far more expensive, would be to separate each module and de-orbit each progressively. If the modules were able to break up one at a time, "-there would be less probability that they would survive", says Crowther. The modules would have to be equipped with attitude control systems, however, otherwise "gravity gradient and aerodynamic torques" would cause them to rotate out of control.
Another option would be to attach a large propulsion module to the space station and "punch it in" at such a high speed, says Crowther, that re-entry forces break it apart with little chance of pieces surviving. Again, this would require additional resources.
When the former Soviet Union developed its ill-fated Buran space shuttle, it planned to carry Mir modules back and forth from orbit. This would have been the method used to assemble the planned Mir 2 space station in orbit, but the collapse of the Soviet Union resulted in the grounding of Buran and its Energia launcher and the budget difficulties that the Russian space programme finds itself in now.
With no Mir 2 to develop, Russia approached the USA and suggested that it participate in and contribute modules to the International Space Station (ISS).This approach was eagerly accepted by a NASA desperate to save the ISS from being cancelled by Congress.
Russia made promises that its budget difficulties have made hard to deliver, frustrating NASA, which has paid a high price for accepting the offer. Changing the orbital inclination of the Station from 28° to the Russian 51° has cost millions of dollars and limited the weight of components that would otherwise be too heavy for the Space Shuttle's reduced payload capability to the higher inclination orbit.
One attraction of collaboration for NASA was that Russia could offer a wealth of experience in space station operations and this resulted in the Phase 1 Shuttle Mir Missions. Six NASA astronauts experienced long duration flights aboard the Mir from 1995 to 1998 and nine Shuttle missions were flown, some with Russian cosmonauts on board. The USA paid Russia over $500 million for these missions.
Operating a space station is not always easy and problems were to be expected. They had occurred on Mir missions before the Shuttle visitors arrived, but NASA's involvement exposed these difficulties to the media.
One was last year's collision of the Progress M tanker with the Spektr module. That the resultant depressurisation was not instant and fatal to the crew, including NASA's Mike Foale, was thanks to the durability of the Mir, which was able to take a hit and sustain damage no larger than a pinhole on the module itself.
The latest Russian resident crew has been working on board the Mir without major technical problems for many months and, without any US participation, has not been under the hypercritical glare of the media.
The Mir is described unfairly as an old station. Although the core module was launched in 1986, the Kvant 1 in 1987, Kvant 2 in 1989 and the Kristall module in 1990, the Spektr module did not dock until 1995 and the Priroda arrived just two years ago. The Spektr was damaged, but can be repaired and made operational again, which was the plan before premature de-orbiting was forced on the Russians.
The former Soviet Union and then Russia have sent 29 crews to the Mir, including 16 international visiting missions, 10 of which were fully commercial flights generating over $200 million in revenue. The space station has been manned almost continuously for about 5,500 days.
International visitors have accumulated over 270 days' flight experience aboard the Mir, plus 900 days by six NASA astronauts - far exceeding the 171 days' experience NASA gained during Skylab operations in 1973-4. Over 70 spacewalks outside the Mir have been completed, exceeding 300h.
Russia's wealth of experience and the commercial revenues Mir was generating started to rankle NASA, particularly as it felt this was diverting attention - and money - from Russia's contribution to the ISS.
As the first ISS flights loomed, Russia made it clear that the Mir would continue operations until the ISS was operational. The station could be a fallback in case of delays, it said. The thought of the ISS being delayed and then cancelled before it gets up and running, and Russia still having a space station on which to fall back, has proved too much for NASA, which wants the Mir out of the way and quickly as possible.
The de-orbiting could have been delayed until 2000, when the Mir would make a natural re-entry without periodic boosts. Operations could have continued even longer. The de-orbiting has been brought forward to June 1999 because of political pressure, including the threat of withdrawing US financial support for the Russian space programme.
Many Russian space chiefs are angry at the US pressure. "It is the national government which must take responsibility of the state's space programme," not the USA, says Yuri Semenov, general manager of space company Energia. Former cosmonaut Alexander Serebrov says: "To sink such a station is a sin. If it happens, not only Russia but the whole world will be left without space research," for the next six to seven years before the ISS is fully operational.
One day, the ISS too will look like a "used car". It, too, will experience operational problems and become untidy, with equipment and cables everywhere. By then, Mir and its triumphs and troubles will be history.