What makes Mir more difficult than previous space re-entries is the magnitude of the forces of nature already acting on Mir, in comparison to the stabilization and impulse capabilities of the Progress tug sent to do the job.

At 137 tonnes, the Mir space station is about 20 times heavier than the Soyuz and Progress craft which the Russians are accustomed to de-orbiting.

Like a tug maneuvering an ocean liner, the little Progress M1 craft now docked with Mir has to steer and push MIR around. Able to carry about 3.8 metric tons of fuel, much of which is needed for orientation, Progress can produce only a very limited velocity change. This means that the controllers must wait until Mir is teetering on the brink of a natural re-entry.

As many readers are aware the Drop Zone has been carefully chosen, but what exactlly is so attractive about the South Pacific?

Firstly, a craft in an orbital inclination of 52 degrees only overflies areas between 52N and 52S latitude, ruling out the polar regions.

Secondly, having a drop zone near to this latitude (as opposed to near the equator) means their are several chances to deorbit in this area, should the first attempt fail.

Thirdly, and most obviously, the "graveyard" provides an ocean area without islands and consequently people (see Figure 1).

Figure 1, Drop Zone in South Pacific. Credit: Dane Ikin.

Fourthly, the drop zone needs to be about 180 degrees opposite the de-orbit burn, to enable the final burn to be excuted within the coverage area of Soviet tracking stations, as outlined on the big screen in Mission Control in Moscow (se Figure 2).

Figure 2, Mir Mission Control as TsUP, Korolev. Credit: Ian Bryce.

Left to its own devices, Mir will come down at a random location. The upper atmosphere (currently swollen by the solar maximum) is exerting an increasing drag force. On my estimates, at 250 km the density is about 9E-11 kg/m(3) at this time. This would cause a drag on Mir of about 2 Newtons (0.44 lb, a light push from one's little finger).

Producing a deceleration of 1.5 millionths of a G, this force is slowly but inexorably bringing Mir down. Drag forces in the 250 km region thus rob Mir of around 2.8 km in height every day. Every 27 km lower, the density doubles. Mir is "falling exponentially" - an accelerating process.

In a natural decay, the final moment would be around 13 days after the 250 km marker. But with an uncertainty of plus or minus 5 days, the remaining life is impossible to predict accurately. As Mir's path wraps around all the earth between 52�N and 52� S every day, it passes over major population areas.

Planning the re-entry with minimum risk to populated areas is a complicated task for mission planners in Moscow, due to the small impulse available. The first crucial step is picking the day for re-entry. This requires a combination of science, experience and good luck.

Burn a few days too early (when Mir is too high) and it will pass over the drop zone, only to come down further around the earth, possibly over Europe Burn a few days too late, and nature will beat them to it, again at a random location. Or the orientation system may be overwhelmed and Mir will start tumbling, so an attempted burn would have unpredictable consequences.

Russian space engineers first estimated from simulations that the day when Mir passes an imaginary milestone at the altitude of 250 km is the best time (this was later lowered to about 220 km). The actual date of reaching the milestone can be estimated in advance by charting the daily decrease in altitude.

The second crucial step is choosing the particular orbit. The time of day is fixed by the period when the orbital plane passes over the drop zone; this will occur when the orbit matches "Pass 1" in the maps. Should the burn not be achieved on Pass 1, then two or three more passes are available which still pass over the drop zone.

The third step is choosing the moment of the final burn. This is like jumping off a very fast merry-go-round, and trying to land on a handkerchief. Mir is expected to travel about 180 degrees of its last orbit before impacting. However the exact distance cannot be predicted due to air density variations, unknown tumbling of Mir, and the unknown moment of final breakup.

Control of eccentricity is important. Atmospheric drag always tends to circularize an orbit, as reducing the velocity at perigee lowers the apogee. Unfortunately, a craft in a circular final orbit can come down anywhere. A targetted re-entry requires some eccentricity on its final descent, to achieve any predictability.

Figure 3, Progress tug cutaway. Credit: RKK Energia

Progress (see Figure 3) will transfer about 2.7 tons of fuel to Mir, perhaps 1 ton of which will be allocated for orientation control. Additional fuel will also be used in Mir's maneuvering engines to provide some eccentricity to the orbit in the 2 days before re-entry, probably to around 240 x 120 km.

For the final burn, the operating commands for Mir and Progress will be time tagged with the chosen date and time, by Mission Control in Korolev, Moscow. The radio signal will be transmitted from Russian earth stations. Over the Caspian Sea, Progress's main engine will be turned on (Figure 3, Progress main engine). Its 3 kiloNewtons thrust (660 lb) will put the brakes on Mir, decelerating it at 0.003 g. Hopefully, Mir's orientation control will keep working, assisted by Progress's own steering system.

After 10 minutes, Progress's fuel will be exhausted. But Mir will have slowed by another 21 m/s. With the increasing loss of speed will cause Mir to descend even further, towards a perigee of about 50 km over the drop zone. Ath this point Mir will start to tumble but it won't matter any more. As it passes by New Zealand, the thickening air will slow it further, reinforcing the process until re-entry sets on in earnest.

The Challenges Involved
The attached Progress craft has a history of reliable operation by remote control, so the remote operarations are not a big risk. Mir modules were originally designed for a lifetime of 3-5 years, and long ago completed their designed tasks. Moreover, after 15 years in orbit, it's no surprise that problems have developed.

The batteries cannot supply large currents and the solar panels do not always face the sun, so erratic voltages and surges sometimes trip off the electronics. The gyrodynes for orientation need heavy power and cannot be used now, so the gas thruster system is needed to keep the station pointing as needed. But it has limited fuel, and the thickening atmosphere is trying to tumble the station. As such, Mission Control plan to turn the solar panels to minimize aerodynamic moment on the whole station.

If something goes wrong, and the burn is aborted, the backup plan comes into effect. Ninety minutes later, Mir will emerge on "pass 2", providing a second opportunity. Although Mir will be in the same orbital plane, the earth has meanwhile turned through 23 degrees, so the flight path is about 2000 km further west.

Figure 4, Intersecting Orbits on the globe. Credit: Dane Ikin.

Russian has three or four passes available when Mir passes over the graveyard, and thus several chances to correct any problems or amend radioed commands. This is the advantage of making the drop zone near the extreme latitude. (see Figure 4). The third pass corresponds roughly to the "Koptev Corridor", which is used routinely for disposal of space ferries after completing their missions.

The Russians have spared no effort in planning and executing Mir's final mission, to the benefit of world safety, and increased knowledge of de-orbit operations.

A Fiery End Will See Mir Out
If any of the four burn opportunities is successful, and the simulations accurate, then Mir will descend into the drop zone (see Figure 5). But what will the last minutes of Mir be like?

Figure 5, Mir final orbit and Descent. Credit: Dane Ikin.

The top of the atmosphere is very blurry, the density doubling about every 27 km of descent in this region. Attitude control of Mir will be lost when drag forces overcome the thrusters (or earlier if they are not working). Then the complex will tumble end over end, so it is impossible to be specific about which parts are exposed first.

Two effects compete in the cremation of Mir. There is a drag force proportional to velocity squared, and a heating effect dependent on velocity cubed. Both are also proportional to air density, and both act only on forward-facing surfaces. Thin and weak elements will burn away or beak off before thick and strong parts.

The light solar panels and masts will start burning at about 80 km, and then break away. These parts will decelerate quickly, and their charred remains will fall to earth at the first (most westerly) end of the debris footprint. Being light, they are unlikely to do serious damage on the ground.

Next to feel the heat will be equipment from outside the hulls: antennas, sheet metal covers, navigation sensors, the Kurs docking antennas, a small manipulator arm, and orientation thrusters mounted on several masts.

The joints between modules and nodes are naturally a weak point, and it is considered likely that the various modules (in size like a railway carriage) and nodes will break apart at about 50 km. As they tumble, lift forces could drive them apart so that each follows its own trajectory to the ground, probably resulting in clusters of debris.

As the modules reach denser air the heat input will greatly increase, like being attacked by a giant blowtorch.

A space station like Mir requires many tanks and pressure vessels. For propulsion they contain the rocket fuels nitrogen tetroxide and hydrazine (thankfully they will be consumed in re-entry), and pressurising gases helium and nitrogen. All manned spacecraft also carry extensive life support systems for oxygen, water and waste storage. Heavy external equipment like tanks and rocket motors may break off and partly burn, but many of these parts will survive to reach the surface.

The pressure hulls are relatively strong and thick, however aluminium cannot withstand extreme temperatures so they will burn through and be mostly vaporised. Then the internal frames, fittings, instruments, experiments, cameras, and so on, from inside will be exposed and attacked by the fiery blast.

Mir contains about 18 heavy metal flywheels called gyrodynes, turned by electric motors and thus able to control the pointing of Mir in operation. These with the docking adapters and the many 75 kg nickel cadmium batteries are the heaviest items, and will be the last to slow down, thus travelling to the east end of the debris footprint.

When the orbital speed is spent (between 50 and 20 km altitude), the surviving parts will fall under gravity. Cooling down, they will not be glowing, and will be hard to see, even in daylight.

Light, Sound and Lots of Waiting
Can we predict what parts will survive and may be found on the surface? Heavy parts take longer to burn away, but their inertia means they also take longer to slow down, being exposed to the flame for a longer time.

Keys to survival include temperature-resistant materials such as steel and titanium, layered components such as kevlar-wound tanks, and the luck of being shielded by something else.

In total, about 35 tons of the original 137 tons is estimated to reach the surface. Strong candidates are the gyrodynes, batteries, tanks, and docking adapters.

An American group led by Bob Citron of Spacehab is planning a flight from Tahiti to experience the re-entry at first hand. Because of the unknowns and the need to keep their distance, their chances of seeing anything are doubtful. They plan talks by Mir veterans and live video linkups to celebrate the event.

People on the surface or in the air near the re-entry track will also be treated to a light and sound show. Best light effects will be around 80-50 km altitude. Re-entry of the modules and components will cause a series of sonic booms which can be heard around the debris footprint area. Heavy parts are expected to arrive before the sonic boom, light parts after.

All in all, the Mir re-entry is potentially a spectacular fireworks show with light and sound, followed by a treasure hunt for debris and souvenirs.

Ian Bryce works with Russian space companies regularly in his job as a space engineer, and has gained much insight from their ballistics experts into the planning behind this historic event. Ian can be contacted via ibryce@[email protected] - remove @NOSPAM@ and replace with a single @

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