GRAIL will carry out its scientific investigations to map the lunar gravitational field and infer the Moon's internal structure and composition by using the tools and techniques of "gravity science," a branch of "radio science." Scientists apply additional knowledge from geological, chemical, and physical observations to deduce the events and processes that could have led to the observed state. Researchers have used this combination extensively throughout the Solar System, but the usual method for acquiring the gravitational data is not applicable to half the Moon.
The standard technique is to track, from Earth, a radio signal transmitted by a spacecraft flying over a planet or moon. Variations in the gravitational field through which the spacecraft flies make it speed up or slow down a little from one location to another, and those typically small motions reveal themselves as changes in the frequency of the radio signal Earth receives, a phenomenon known as Doppler shifting (the same principle at work in the radar gun which tells a police officer how fast you're driving).
This technique requires a direct line of sight from Earth to the spacecraft. But the Moon keeps one side facing Earth at all times (with a slight wobble from side to side), so we never have a direct view of a spacecraft flying over the far side. GRAIL solves that problem by flying two spacecraft in the same orbit around the Moon, and using each spacecraft to track the other's relative motion via the Doppler-shift technique. As in the more traditional scenario, variations in relative motion correspond to variations in the gravitational field.
While flying over the Moon's near side, the GRAIL team will supplement the inter-spacecraft measurements by using the standard technique to analyze radio signals sent from the two spacecraft to Earth. The team will use Doppler shifting to measure changes in speed, and will also employ a technique called "ranging," in which the travel time of the radio signal, multiplied by the speed of light, reveals the absolute distance of the spacecraft from the receiving station on Earth.
Gravitational reality check
Lunar researchers have an assortment of ideas about the Moon's possible interior structure, composition, and thermal history. With powerful computers, they can run simulations of how the Moon's gravitational field would look as a result of each hypothesized set of conditions. GRAIL will enable them to compare their virtual gravitational fields with the real thing and see which model most closely matches reality.
GRAIL's high-resolution gravitational map will complement other information about the Moon. In particular, it will be used in conjunction with a high-resolution topographic map from the Lunar Reconnaissance Orbiter (LRO). Subtracting the gravitational effects expected from surface topography (for example, a crater or mountain) from the effects actually observed by GRAIL will reveal how much gravity must be coming from subsurface features in any given location. Researchers will also incorporate data from other sources such as the historical lunar seismometers from the Apollo missions, and an ongoing process called "lunar laser ranging," which tracks the Moon's motion from Earth by bouncing laser beams off of special reflectors, also placed by Apollo astronauts.
Love and the heart of the Moon
The Love number (named for mathematician A.E.H. Love) measures the Moon's response to the tidal pull from Earth's gravity, which varies as the Moon follows its elliptical orbit around Earth. This response is sensitive to the size and elastic properties of the lunar core, and so is a valuable tool for investigating the heart of the Moon.
GRAIL will determine the Moon's Love number by measuring the gravitational field of the Moon on a global scale while the two spacecraft are at their highest altitude and greatest separation distance, and observing how it changes in response to the changing distance between Moon and Earth. At the other extreme, the spacecraft will be sensitive to the gravity of local features at or near the surface while at their lowest altitude and closest separation distance.
For more information:
JPL Radio Science Systems Group