Mission Design Overview for the Gravity Recovery and Interior Laboratory (GRAIL) Mission

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) Mission -a NASA DiscoveryProgram mission currently in development and scheduled to launch in September 2011 -will make a high resolution determination of the lunar gravity field in an effort to understand the internal structure and thermal evolution of the Moon "from crust to core" and will extend that knowledge to other terrestrial planets within the inner solar system. This will be accomplished by placing two nearly identical orbiters, flying in tandem, in a low-altitude, near-polar orbit around the Moon. Precise measurements of the relative velocity between the two orbiters combined with measurements of the absolute position of the orbiters about the Moon as determined via Earth-based tracking will allow the global lunar gravity field to be mapped to unprecedented accuracy and resolution. The development of the baseline strategy to achieve the objectives of this mission involves the integration of a variety of design elements into a coherent mission plan, including: trans-lunar trajectory and navigation design, in-flight calibration and checkout planning, lunar orbit insertion design, design of the maneuvers for orbit circularization and for the establishment of the formation necessary for gravity science data collection, and gravity science data collection design. These mission design elements are described in chronological order as the mission progresses through its seven mission phases:

The Scientific Measurement System of the Gravity Recovery and Interior Laboratory (GRAIL) Mission

“…Ebb and Flow are in a common near-polar, near-circular orbit with a mean altitude of approximately 55 km during the prime mission. However, as described in Roncoli and Fujii (2010) the periapsis altitude ranges from approximately 16 km to 51 km above a reference lunar sphere. The Ebb-Flow separation distance is designed to slowly vary.…”

“…Mission design, navigation, deep space stations, and the ground data system critically contribute to the quality of science data. Specifics of each factor were described above and additional details on the mission system can be found in Roncoli and Fujii (2010), Hatch et al (2010), Beerer andHavens (2012), andZuber et al (2013, this issue).…”

“…Figure 14 of Roncoli and Fujii (2010) illustrates the time history of the solar beta angle as well as the relationship between beta angle and the duration of solar eclipse during the science phase; eclipse durations are a maximum at the beginning and end and no solar eclipses when the solar beta angle is near 90 degrees near the middle. For the simulations the Sun-angle is represented after with the inclination computed with respect to the lunar pole vector, as:…”

“…The shorter separation distance is optimum for data exploring the local and regional spatial features while the segment around the maximum separation is optimum for the determination of the global studies such as the lunar core parameters, which are the Love number and the periodic signature of degree 2. The separation distances are designed to ensure that there is no degradation of the Ka-band signal due to multipath off the lunar surface and, according to Roncoli and Fujii (2010), a shorter spacecraft separation is required because of the lower spacecraft altitude.…”

“…Klipstein et al (2013, this issue) describes the design and implementation of the GRAIL payload. Hoffman (2009) described GRAIL's flight system and Roncoli and Fujii (2010) described the mission design. This paper illustrates how a team of scientists and engineers prepared to meet GRAIL scientific objectives and data quality requirements through simulations and modeling of the design and configuration of the flight and ground systems.…”