GRAIL (Gravity Recovery and Interior Laboratory)

The Gravity Recovery And Interior Laboratory (GRAIL) will unlock the mysteries of the Moon. By mapping the lunar gravitational field globally – not just on the nearside – to unprecedented accuracy and resolution, GRAIL will, in essence, peer deep inside the Moon (see right) to reveal its internal structure and thermal history.

Knowledge acquired about the Moon from GRAIL will be extended to understand the broader evolutionary histories of the other rocky planets in the inner solar system: Earth, Venus, Mars, and Mercury. Indeed, the Moon is a lynchpin for understanding how the terrestrial planets evolved.

GRAIL is the lunar analog of the very successful Gravity Recovery and Climate Experiment (GRACE) twin-spacecraft terrestrial gravity recovery mission that was launched in 2002 and continues to operate. GRAIL will be implemented with a science payload simplified from GRACE and a spacecraft derived from the Lockheed Martin Experimental Small Satellite-11 (XSS-11) launched in 2005.

GRAIL will place two spacecraft (represented as GRAIL-A and GRAIL-B to the left) in a low-altitude (50 km), near-circular, polar lunar orbit to perform high-precision range-rate measurements between them using a Ka-band payload. Subsequent data analysis of the spacecraft-to-spacecraft range-rate data provides a direct measure of the lunar gravity.

The payload, flight system and mission design ensure that all error sources that perturb the gravity measurements are contained at levels well below those necessary to meet science requirements. We can illustrate performance margin between the science requirements (red and green), the allocated performance (black), and Current Best Estimate (CBE) performance (gold). These margins enable GRAIL's low-risk implementation.

GRAIL's short mission duration (270-days) includes a 90-day gravity mapping Science Phase. Initial science products will be available beginning 30 days after the start of the Science Phase and will be delivered to PDS no later than 3 months after the end of the Science Phase.

Included on the spacecraft is an "E/PO MoonKam" assembly that provides images and video of the lunar surface as part of the E/PO and Student Collaboration segment of GRAIL.

Science Objectives and Investigations -- The Moon is the most accessible and best studied of rocky, or "terrestrial", bodies beyond Earth. Unlike Earth, however, the Moon's surface geology preserves the record of nearly the entirety of 4.5 billion years of solar system history. Orbital observations combined with samples of surface rocks returned to Earth, show that no other body preserves the record of geological history so clearly as the Moon.

The structure and composition of the lunar interior (and by inference the nature and timing of internal melting and heat loss) hold the key to reconstructing this history. Longstanding questions such the origin of the maria, the reason for the nearside-farside asymmetry in crustal thickness, and the explanation for the puzzling magnetization of crustal rocks, all require a greatly improved understanding of the Moon's interior. Deciphering the structure of the interior will bring understanding of the evolution of the Moon itself, and also extend knowledge of the origin and thermal evolution of the Moon to other bodies in the inner solar system. For example, while the Moon was once thought to be unique in developing a "magma ocean" shortly after accretion, and now such a phenomenon has now been credibly proposed for Mars as well.

This need to understand the internal structure in order to reconstruct planetary evolution motivates the GRAIL primary science objectives, which are to determine the structure of the lunar interior from crust to core and to further the understanding of the thermal evolution of the Moon. The GRAIL mission will accomplish these goals by performing global, regional and local high-resolution (30x30 km), high-accuracy (<10 mGal*) gravity field measurements with twin, low-altitude (50 km) polar-orbiting spacecraft using a Ka-band ranging instrument.

The GRAIL Science Team will conduct six lunar science investigations to:
(*) Map the structure of the crust & lithosphere.
(*) Understand the Moon's asymmetric thermal evolution.
(*) Determine the subsurface structure of impact basins and the origin of mascons.
(*) Ascertain the temporal evolution of crustal brecciation and magmatism.
(*) Constrain deep interior structure from tides.
(*) Place limits on the size of the possible inner core.

The GRAIL spacecraft was launched side-by-side on a single Delta II vehicle during a 26-day launch period that opens on September 8, 2011. The mission was designed to avoid the December 10th, 2011 and June 4th 2012 lunar eclipses which interfere with the mission. The Goldstone complex will implement initial acquisition including solar array deployment.