• SCIENCE CORNER |

      A video frame shows bright spots in Occator crater on dwarf planet Ceres, generated from data from NASA's Dawn spacecraft. (Photo - jpl.nasa.gov).

      A video frame shows bright spots in Occator crater on dwarf planet Ceres, generated from data from NASA’s Dawn spacecraft. (Photo – jpl.nasa.gov).

      Control of Dawn’s orientation in the weightless conditions of spaceflight is the responsibility of the attitude control system.

      By Dr. Marc D. Rayman

      (To maintain a mystique about their work, engineers use the term “attitude” instead of “orientation.” This system also happens to have a very positive attitude about its work.) Dawn (and all other objects in three-dimensional space) can turn about three mutually perpendicular axes. The axes may be called pitch, roll and yaw; left/right, front/back and up/down; x, y and z; rock, paper and scissors; chocolate, vanilla and strawberry; Peter, Paul and Mary; etc., but whatever their names, attitude control has several different means to turn or to stabilize each axis.

      Earlier in its journey, the spacecraft depended on devices known as reaction wheels. That method is now used only rarely, because two of the four units have failed. The remaining two are being saved for the ultimate orbit at about 230 miles (375 kilometers), which Dawn will attain at the end of this year. Instead of reaction wheels, Dawn has been using its reaction control system, shooting puffs of hydrazine, a conventional rocket propellant, through small jets. (This is entirely different from the ion propulsion system, which expels high velocity xenon ions to change and control Dawn’s path through space. The reaction control system is used only to change and control attitude).

      Whenever Dawn is firing one of its three ion engines, its attitude control system uses still another method. The ship only operates one engine at a time, and attitude control swivels the mechanical gimbal system that holds that engine, thus imparting a small torque to the spacecraft, providing the means to control two axes (pitch and yaw, for example, or chocolate and strawberry). For the third axis (roll or vanilla), it still uses the hydrazine jets of the reaction control system.

      June 30

      Dawn’s spiral descent from its 2nd mapping to its 3rd, with the two mapping orbits shown in green. Dawn’s trajectory color progresses from blue, when it began ion-thrusting in survey orbit, to red, when it arrives in HAMO. The red dashed sections show where Dawn is coasting for telecommunications. (Photo - NASA/JPL-Caltech).

      Dawn’s spiral descent from its 2nd mapping to its 3rd, with the two mapping orbits shown in green. Dawn’s trajectory color progresses from blue, when it began ion-thrusting in survey orbit, to red, when it arrives in HAMO. The red dashed sections show where Dawn is coasting for telecommunications. (Photo – NASA/JPL-Caltech).

      On June 30, engine #3 came to life on schedule at 10:32:19 p.m. PDT to begin nearly five weeks of maneuvers. Attitude control deftly switched from using the reaction control system for all three axes to only one, and controlling the other two axes by tipping and tilting the engine with gimbal #3. But the control was not as effective as it should have been. Software monitoring the attitude recognized the condition but wisely avoided reacting too soon, instead giving attitude control time to try to rectify it. Nevertheless, the situation did not improve. Gradually the attitude deviated more and more from what it should have been, despite attitude control’s efforts. Seventeen minutes after thrusting started, the error had grown to 10 degrees. That’s comparable to how far the hour hand of a clock moves in 20 minutes, so Dawn was rotating only a little faster than an hour hand. But even that was more than the sophisticated probe could allow, so at 10:49:27 p.m., the main computer declared one of the “safe modes,” special configurations designed to protect the ship and the mission in uncertain, unexpected or difficult circumstances.

      The spacecraft smoothly entered safe mode by turning off the ion engine, re-configuring other systems, broadcasting a continuous radio signal through one of its antennas and then patiently awaiting further instructions. The radio transmission was received on a distant planet the next day. (It may yet be received on some other planets in the future, but we shall focus here on the response by Earthlings.) One of NASA’s Deep Space Network stations in Australia picked up the signal on July 1, and the mission control team at JPL began investigating immediately.

      Engineers…chose
      to switch to
      another gimbal

      Engineers assessed the health of the spacecraft and soon started returning it to its normal configuration. By analyzing the myriad diagnostic details reported by the robot over the next few days, they determined that the gimbal mechanism had not operated correctly, so when attitude control tried to change the angle of the ion engine, it did not achieve the desired result.

      Because Dawn had already accomplished more than 96 percent of the planned ion-thrusting for the entire mission (nearly 5.5 years so far), the remaining thrusting could easily be accomplished with only one of the ion engines. (Note that the 96 percent here is different from the 69 percent of the total time since launch mentioned last week, simply because Dawn has been scheduled not to thrust some of the time, including when it takes data at Vesta and Ceres.) Similarly, of the ion propulsion system’s two computer controllers, two power units and two sets of valves and other plumbing for the xenon, the mission could be completed with only one of each.

      Although engineers likely could restore gimbal #3’s performance, they chose to switch to another gimbal (and thus another engine) and move on. After all, Dawn’s goal is to explore a mysterious, fascinating world that used to be known as a planet, not to perform complex (and unnecessary) interplanetary gimbal repairs.

       

      > Dr. Marc D. Rayman is the Dawn Mission Director and Chief Engineer at JPL. Marc greatly enjoys sharing the thrill of interplanetary adventures with the public.


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