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# NOZOMI's Swingby and its Basic Principle

On June 19, the Mars explorer NOZOMI came close to Earth at a distance of approx. 11,000 km and implemented the earth swingby. This means that the "2nd swingby" in the NOZOMI's new orbit (see Fig. 1) was accomplished and, accordingly, we succeeded in putting NOZOMI into the orbit that enables to arrive at Mars in mid-December of this year.

The technology called "Swingby" has been often used for solar system explores such as Voyager and Galileo. The purpose of this article is to give an explanation to answer the question, "What is really swingby", that readers must have. I am not an orbital design specialist, so, conversely taking advantage of my non-expertise, I try to explain as plain as possible.

Usually, making change of spacecraft's direction or velocity in space requires much onboard propellant. The swingby is a technology to enable large change of spacecraft's orbit by making use of gravity of celestial bodies such as Earth without consuming propellant. Gravity of a celestial body is in proportion to its mass and in inverse proportion to square of distance from it. Spacecraft cruising interplanetary space in the solar system is under the influence of the gravity of Sun, the biggest and exceptionally huge celestial body in the system. But, when the spacecraft comes very close to a planet (or its satellite), gravity of the planet exceeds that of Sun. The spacecraft, which is naturally to fly straight near by the planet, is pulled by the force of the planet and, accordingly, its orbit is curved. When the spacecraft passes through the field that the gravity of the planet works, its trajectory describes a hyperbolic curve centering the planet (see Fig. 2 (a)). Relative velocity of the spacecraft to the planet is the same at both entering into its gravitational field and escaping from it. However, since the planet goes around the sun, velocity of the spacecraft to the absolute coordinate system is changed. In case that the spacecraft passes by the opposite face of the direction of the planet's revolution, it is accelerated (see Fig. 2 (b)). In case that the spacecraft passes by the face of the direction of the planet's revolution, it is slowed down (see Fig. 2 (c)).

To conduct swingby, precise orbital control in advance is essential. In preparation for the swingby in June, position of NOZOMI was accurately determined by operations prioritizing ranging and VLBI and its orbit was precisely adjusted. Following the orbital determination efforts conducted for one week after the swingby, we could confirm that NOZOMI entered into the planned orbit.

October 1, 2003