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TOP > Report & Column > The Forefront of Space Science > 2014 > Aiming for a Much Higher Sky

The Forefront of Space Science

Physics of Fermi Acceleration Explored by Fermi Gamma-ray Space Telescope
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The design of a moon tour is even more challenging for moon orbiter mission: in this case, the tour terminates with the spacecraft performing a large burn to get captured around a moon. Typically the target orbit is a polar, low-altitude orbit from which the spacecraft can perform high-resolution remote sensing of the entire surface of the moon. More constraint are added to the tour design, for example: the lighting conditions of the moon, as seen form the spacecraft orbit, should be optimal for imaging and infrared measurements; no Earth occultation by the Sun or by the primary (which impede communication with Earth) should occur in the final part of the tour and until the end of the nominal science mission. In addition, the amount of propellant required by the final orbit insertion burn is even larger then the one used to capture the spacecraft around the primary. For these reasons, orbiter missions are much harder to implement and have not flown yet. However, a number of new orbital mechanics techniques have been developed recently which significantly mitigated the propellant requirements. One of such techniques (called Tisserand-leveraging) enabled the JUICE mission, which will be the first moon orbiter to fly. Another new technique, called non-tangent leveraging, is shown to enable a potential mission to Enceladus or to other low-mass moons, which are deep within the gravity well of a giant planet.

These same techniques were recently applied to proposed or flying JAXA missions. Tisserand-leveraging techniques were used to design the fuel-efficient four lunar flybys of DESTINY (figure 3). DESTINY is a proposed low-cost JAXA deep-space mission that would test a number of key technologies for future space missions. Non-tangent leveraging techniques were used to compute the recovery trajectory for Akatsuki: deep-space manoeuvres were carefully planned and executed to reduce the return time to Venus of more than one year, and at the same time, to enable a new orbit insertion possibility in 2016 by lowering the required amount of propellant.

Figure 1
Figure 1. Example Neptune Orbiter tour. Left: Viewed from Neptune’s north pole. Right, top: Close-up of the tour. Right, bottom: close-up of the Triton flybys during Phase I and Phase III.

Figure 2
Figure 2. Pump angle-crank angle graph used for the design of the Neptune Orbiter tour. Every point of the graph represents an orbit around Neptune.

Figure 3
Figure 3. Lunar flyby phase of the proposed DESTINY technology demonstration mission.

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