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The Forefront of Space Science

Open Up New Routes in Outer Space
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Various swingbys

The U.S.’s Voyager Program is a famous swingby mission. Voyager performed a series of swingbys past the outer planets and finally escaped from the solar system. While it may give the impression that the swingby is “used for accelerationEor “only possible when planets align in very rare cases,Ein fact it is adopted for a variety of purposes and situations. In this article, I would like to introduce three cases of swingby.

Launched in September 2007, KAGUYA was put into orbit around the Moon about three weeks after its launch, as initially planned. Careful preparations were made in advance, however, just in case. One example was the “trajectory ready for main engine malfunction.EIn case the main engine failed for any reason, we intended to use small thrusters instead. Because of large difference in performance, however, we had to devise a large change in trajectory using small thrusters. Fig. 1 shows the trajectory we designed (note that the orbit prior to arriving at the Moon is omitted). The important point in this trajectory is to decrease its approach velocity to the Moon as much as possible to insert KAGUYA into an orbit around the Moon with its small thrusters. To achieve this, KAGUYA would pass by the Moon at its first approach. Then, it would be injected into a large orbit with its apogee about 1 million km using lunar swingby. Next, the velocity for its second lunar approach would be gradually decreased by effective use of perturbation by solar gravity. This technique was also used in the trajectories of the HITEN and NOZOMI missions.

Figure 2	Broadband circular-polarized wave donut-beam antenna

Figure 1. Trajectory of KAGUYA. A backup trajectory is prepared in case of main engine malfunction.

Most bodies in the solar system move on almost the same plane as the earth’s revolution plane (i.e., ecliptic plane). Therefore, most deep-space explorers also flew on the ecliptic plane except for a few cases. The environment outside the ecliptic plane and the solar system viewed from outside the plane are unknown to us. Thus, we planned to insert spacecraft into a trajectory largely inclined to the ecliptic plane. To deploy spacecraft into such a trajectory is very difficult. The required velocity increment (delta V) is more than 20 km/sec. To realize this, we created an orbit to make maximum use of both the launching rocket capability and the ion engine’s delta V. Fig. 2 indicates an orbit where inclination increases gradually (when viewed from side). The point of this trajectory is to exert the maximum velocity-increment capability of ion engines and, from this standpoint, the direction of velocity increment in each orbit is determined. Since the velocity increment itself does not contribute to the inclination increase, however, we convert spacecraft velocity to the appropriate direction by performing earth swingbys every two years. This method that converts efficiently velocity acquired by the ion engines to the desired direction by using swingby was used on HAYABUSA’s trajectory.

Figure 2	Broadband circular-polarized wave donut-beam antenna

Figure 2. Trajectory to put explorers into a trajectory largely inclined to the ecliptic plane.

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