The Final Days of Mir

A recent example of this was the controlled reentry of NASA’s Compton Gamma Ray Observatory (CGRO). This 17-ton spacecraft was deorbited via a series of four burns on June 4, 2000. Its reentry debris footprint was spread over a region of the Pacific Ocean about 2,173 nmi (4,025 km) southeast of Hawaii. For more on CGRO, see this Space.com article.

Nominal Mir Deorbit

The burns required for Mir’s controlled deorbit will be provided by the Progress M1-5 spacecraft, which was launched on January 23, 2001 and docked with Mir’s Kvant 1 module on January 27. The station's debris footprint will be targeted such that its center of mass impacts at 47 degrees south latitude and 140 degrees west longitude, a point in the south Pacific roughly 1620 nmi (3000 km) east of New Zealand's southern tip and situated well away from populated islands and commercial shipping lanes. This map shows the nominal locations of Mir's reentry breakup point, debris footprint, and the associated danger area, designated by New Zealand's Civil Aviation Authority (CAA) in a Notice to Airmen (NOTAM).

The nominal deorbit plan, developed by Russia’s TsNIIMash Mission Control Center (TsUP) under the direction of the Russian Aviation and Space Agency (RASA, Rosaviakosmos), consists of three stages:

1. A waiting period for atmospheric drag to decrease Mir’s orbit to a 119 nmi (220 km) mean altitude pre-descent orbit. This stage began with the docking of Progress M1-5. For Mir’s current altitude, click here.
2. Transfer to a lower 89 nmi perigee x 119 nmi apogee altitude (165 x 220 km) descent orbit via two braking burns (delta Vs), using the Progress' attitude thrusters. The first of these burns (burn 1) will be executed during Mir's 15th daily orbit while the second (burn 2) will be done during the next orbit, orbit 16 (Mir orbits the earth approximately 16 times a day). The first and second burns will decrease Mir’s velocity by 30 ft/s (9.1 m/s) and 33 ft/s (10.1 m/s), respectively.
3. Transfer from the descent orbit to the reentry trajectory. A final delta V, burn 3, of 82 ft/s (25.0 m/s) is applied two orbits after burn 2, during daily orbit 2 of Mir. Both the Progress' attitude thrusters and 882 lbf main engine will be used for burn 3. This burn will transfer the station to an approximately 45 x 115 nmi (83.3 x 213.0 km) orbit, placing its perigee well within the dense region of Earth’s atmosphere.

The exact date of Mir’s reentry will be determined based on how long it takes for aerodynamic drag to lower its orbit, as indicated in Stage 1, the passive phase, of the deorbit plan. The aerodynamic drag depends on the density of earth’s atmosphere at high altitudes. The density in turn depends on the near-earth space weather conditions, the sun’s activity and the earth’s magnetic field. The greater the solar activity and the more intense the geomagnetic field, the greater the density and the corresponding drag on Mir.

Space weather conditions vary with time, roughly following an 11 year long solar cycle. Like terrestrial weather, however, the finer day-to-day variations in space weather are uncertain. Therefore, since the space weather conditions influence the aerodynamic drag experienced by Mir, the exact time of the end of Stage 1, when its orbit has decayed to a mean altitude of 119 nmi, and, consequently, its reentry date, is uncertain. The current reentry date and time, designated by TsUP and supported by simulations of Mir’s orbit decay, is March 23 2001 at about 1:00 am EST.

The active phase of the deorbit plan, Stages 2 and 3, will occur over 4 1/2 orbits of the station. Since each orbit has a period of approximately 90 minutes, the deorbit sequence will take approximately 5 1/2 hours to complete from the beginning of burn 1 to the impact of Mir's debris.

All three of the deorbit burns will be executed when Mir is over Earth’s northern hemisphere. Applying braking burns at the apogee of a satellite’s elliptical orbit opposite to the direction of its velocity will decrease the orbit’s perigee (for a diagram depicting an elliptic orbit, click here). Thus, the first two delta Vs will change Mir’s orbit from circular to slightly elliptical in shape, with apogee occurring over the northern hemisphere, on the arc of the orbit where the burns will be applied, and perigee over the southern hemisphere, where reentry is to occur. The third and final delta V will also decrease the station’s perigee, lowering it into the denser layers of Earth’s atmosphere. As the station approaches perigee approximately 35 minutes after the completion of burn 3, it will reenter earth's sensible atmosphere, slow down, and break up (at an altitude of about 42 nmi) under the influence of reentry loads and heating, spreading its debris in a footprint over three thousand miles long (according to TsUP).

A chart depicting Mir’s changing apogee and perigee altitudes during the active phase of deorbit, Stages 2 and 3, is shown here. The drops in perigee due to the three burns is readily apparent on the chart, as well as the continuous decay of apogee between burns due to drag.

As indicated, all three burns occur around the apogee point of Mir's orbit, which is situated over Earth's northern hemisphere. This figure of the ground track for Mir's final four orbits shows where the burns will nominally occur. In addition to the ground track of the final orbits, the figure also shows the locations of Russian, U.S., and European Space Agency (ESA) radar ground stations that will be tracking Mir. The first two burns begin out of sight of TsUP (at Kolpas in the figure), requiring them to be programmed into Mir's main computer prior to the controlled deorbit sequence.

After determining the effects of the first two burns on Mir's orbit via the data from the ground stations, TsUP will compute and issue the command for the third and final burn necessary to target the station's impact in the designated impact area. Unlike the first two deorbit burns, the beginning of burn 3 will be in site of TsUP, allowing ground controllers to abort the final burn in the event of a serious malfunction. If a malfunction occurs, controllers will have another opportunity to deorbit the station with a final burn approximately 24 hours later, after Mir's ground track completely circles the earth's surface.

The final Russian ground station to track the descending station will be at Ulan Ude. The final station to track Mir will be the U.S. Army station on Kwajalein Atoll in the Marshall Islands (Alcor in the ground track figure).

• World map showing Mir's nominal cm impact point
• World map showing Mir's reentry breakup point, debris footprint, and NOTAM warning area
• Plot of Mir's apogee and perigee altitudes during controlled deorbit
• Ground track of Mir's final four orbits, showing the locations of the deorbit burns, ground stations, and impact point