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The Forefront of Space Science

Open Up New Routes in Outer Space
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Draw trajectory and create mission

Deep space explorers such as the lunar orbiter KAGUYA and asteroid explorer HAYABUSA are leading players in space science since they reach celestial bodies far from earth and bring back many new findings. Unlike earth-orbiting satellites that are deployed to their destinations by launchers, deep space explorers must reach their target objects by their own power. Setting the spacecraft’s flight course from departure from earth to arrival at the target body (and, in some cases, its return to earth) is the first step of space exploration planning. The spacecraft’s course is called the “trajectoryEand the work of devising the trajectory is called “trajectory design.ETrajectory design is my job.

Once the trajectory design is completed, main schedule of the exploration mission is determined, which includes the launch date and the estimated time of arrival at the target. This information is indispensable for the decision on the implementation of the exploration mission. With the trajectory design fixed, we can also calculate the energy required for the launcher and the propellant consumption until arrival at the target. These are important design parameters to decide the scale of the spacecraft. The distances between the flying spacecraft and the Sun and the earth are estimated based on the trajectory design. The positional relationship among the Sun, the earth and the target body from the spacecraft’s point of view is also given. These data provide critical parameters for spacecraft design such as electric power, heat, communications, and allocation of instruments. In summary, trajectory design is not just a simple matter of plotting a course in outer space, but an important task in the drafting of the exploration plan taking into account various factors. For this reason, trajectory design is also frequently called “mission design.E

Limitation of trajectory design and swingby

You may think that drawing a spacecraft’s trajectory in open space is an untrammeled task just like painting a picture on a blank canvas. In fact, however, there is little room for freedom in trajectory design.

As Kepler discovered in the 17th century, most bodies in the solar system have an elliptical orbit, one of which focuses is centered around the Sun. This is true for spacecraft. Where artificial spacecraft differ from natural celestial bodies, however, is that a spacecraft can change its trajectory by intentionally changing its velocity. For example, at launch the spacecraft is given high velocity by launcher. With this high velocity, the spacecraft is able to escape the earth’s revolution orbit and enter a trajectory heading for its target object. Using its onboard propulsion system, a spacecraft can modify its trajectory at any time by changing its velocity. This is also true for spacecraft capable of generating continuous thrust, such as the ion engines used in HAYABUSA or the solar sail now under research.

Trajectory change made using this method, however, is extremely restrictive. The earth's revolution velocity, for example, is about 30km/sec. Except in special cases, the amount of velocity change (also referred to as “velocity incrementEor “delta VE attained in spacecraft by the method above is just several km/sec. In other words, it is as though we were rowing a small boat with tiny oars in a strong whirlpool (i.e., strong solar gravity field). This is the real meaning of the sentence above, “there is little room for freedom in trajectory design.E

There is one orbit-altering method not yet mentioned, “swingby.EThis technique alters the trajectory by using velocity change that is caused by the gravity of planets or the Moon as the spacecraft passes by them. Swingby’s greatest advantage is that it enables large velocity change with almost no consumption of spacecraft fuel. Moreover, if we can plot the trajectory to pass by the same planets again, the spacecraft can repeat swingby many times. Swingby greatly expands the freedom of trajectory design. On the other hand, one of its disadvantages is that the time required to arrive at a target object is extended as the spacecraft makes a detour. Another point to keep in mind is the difficulty. Trajectory designers must set swingby timings and conditions according to the mission’s purpose or combine swingbys with other trajectory-altering methods as necessary. In Japan, SAKIGAKE performed the first successful earth swingby in 1987. Since then, HITEN, GEOTAIL, NOZOMI, and HAYABUSA made successful lunar and earth swingbys in exploration missions. Apart from Japan, only the U.S. has as much swingby experience. Even the former Soviet Union and Europe have only a few records of swingby in the past. It can be said that swingby is a Japanese specialty.

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