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The Re-Entry of the Compton Gamma Ray Observatory

On June 4th, 2000, the Compton Gamma Ray Observatory (CGRO) successfully entered the Earth’s atmosphere over the targeted Pacific Ocean. This was the first time NASA conducted a controlled re-entry of an unmanned spacecraft from Low Earth Orbit (LEO). The criticality of this GN&C intensive operation was enhanced due to the large mass of the spacecraft, 14,000 kg (15.5 tons) post final burn, and the loss of two of the four orbit control thrusters soon after launch in 1991. Over the spacecraft’s nine year lifetime, two orbit re-boosts were done to raise the operational orbit of the spacecraft and extend it’s science lifetime. During the beginning of the first re-boost effort, one of the redundant attitude control thrusters had a major drop in performance. This thruster had since recovered to near nominal performance and was considered fully operational during the re-entry. The spacecraft is shown below.

The Compton Gamma Ray Observatory
The Compton Gamma Ray Observatory

In December 1999, it was recognized that a detailed plan needed to be developed and exercised for a controlled re-entry. Initial plans included an “Implementation Phase” for developing and testing ground procedures, using a preliminary mission design, before a critical failure occurred. These critical failures or "Trigger Points" would initiate an "Execution Phase" controlled re-entry process, including a final mission design. Before the detailed plan could be completed, the spacecraft lost the number 3 gyro. This accelerated the process. Procedures were developed and rehearsed using a preliminary mission design, as the final mission design was developed. Approximately one month before re-entry, all procedures were frozen and rehearsed with the final mission profile. The Flight Operations Team (FOT) and Applied Engineering and Technology Directorate (AETD) sub-system engineers repeatedly rehearsed various nominal and failure scenarios. Failure scenarios included thruster performance and failures, gyro and battery loss, through ground operations breakdowns.

The re-entry maneuver scenario consisted of an engineering burn and four 26+ minute burns each centered at apogee that dropped the spacecraft from it’s 510 km circular orbit to a 510 x 50 km terminal orbit. On May 28th 2000, 19:44:00 GMT, the engineering burns were executed. These engineering burns consisted of short firings of the 5lb and 100lb thrusters to verify performance. Re-entry burn #1 was conducted on May 31st 01:51:05 GMT and put the spacecraft in a 510km X 350km orbit. Re-entry burn #2 was conducted on June 1st 02:36:52 GMT and placed the spacecraft in a 510km X 250km orbit. The final two re-entry burns were done on June 4 at 03:56:00 GMT and 05:22:21 GMT respectively. Re-entry burn 3 placed the spacecraft in a 510km X 150km orbit and re-entry burn 4 placed the spacecraft in its 510km X 50km terminal orbit. Within 30 minutes after the end of burn 4, the spacecraft had completed its re-entry into the Pacific. Confirmation of the re-entry from NORAD was received at approximately 10:00:00 GMT.

Between Burns 1, 2 and 3, the spacecraft remained in a power positive "Parking Attitude" under wheel control. Six minutes before the burn, the spacecraft entered Thruster Maneuver Mode (TMM) control and maneuvered to a "Burn minus 2 minute attitude" using the Attitude Control Thrusters (ACTs). At the burn minus 2 minute mark, the spacecraft pitched at 1 revolution per orbit to maintain the thrusters parallel to the velocity vector. A command from ground transitioned the controller to Velocity Control Mode (VCM) and fired the 100 lb Orbit Adjust Thrusters (OATs). During VCM, attitude control followed the velocity vector by on-modulating the ACTs for pitch and yaw and off-modulating the OATs for roll. An on-board timer automatically switched the controller back to TMM upon completing the burn. The spacecraft attitude continued to follow the velocity vector until a new parking attitude was loaded. After the slew to the new parking attitude was complete, FOT commanded the spacecraft to a wheel based Normal Maneuver Mode (NMM). The attitude errors reduced to a level where an automatic switch to Normal Pointing Mode (NPM) occurred.

After Burn 3, the perigee altitude was reduced to approximately 150 km. At this altitude, aerodynamic torques would cause the controller to saturate the wheels. Therefore, the spacecraft remained under thruster control during a final perigee pass. On the next orbit, a 30 minute Burn 4 reduced the perigee to less than 50 km over the South Pacific Ocean. The predicted footprint is shown in the figure below, represented as the three stars connected by a line in the ocean. Since the burns were of nominal performance, the spacecraft objects fell around the green (center) star.

Predicted Debris Footprint
Predicted Debris Footprint

The objects with a low mass to area ratio, i.e. the solar arrays, fell on the upper left side of the center star and the objects with a high mass to area ratio, i.e. the titanium bolts, fell down stream from the center star. All objects fell within the yellow (first) and white (third) stars. Visual contact from an U.S. Air Force plane, contracted by NASA to track re-entry, verified the proper time and location of impact. Further analyses are continuing to verify actual impact location of the debris.

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