Grail lunar probes will use gravity to map the inside of the moon

Precise measurements of the distance between the two probes will allow scientists to infer the moon's interior structure

Nasa will launch a pair of probes on Thursday that will examine the interior structure of the moon and shed light on the formation of the rocky planets of the solar system.

Part of the agency's Discovery Program, the Gravity Recovery and Interior Laboratory (Grail) mission consists of two satellites that will spend almost four months on a journey to the Earth's companion. Once they arrive – a day apart at the moon's south pole – they will take measurements for a further three months before being directed to smash into the lunar surface.

The Grail mission will measure the moon's gravitational field in an attempt to reconstruct the history of how and when different parts of it cooled and solidified. While the shifting geology of the Earth continually erases memories of its origin through the movement of tectonic plates, the moon's surface and interior has kept a near-perfect record of the 4.5bn-year history of our solar system.

Though there have been a number of probes and astronauts sent to the moon, the missing piece of the puzzle has been the study of its interior.

"I like to use the analogy of the moon being Earth's closest family member," said Maria Zuber of the Massachusetts Institute of Technology. "If you think about the people that you know and love it is not what they look like but what they are like on the inside that makes them special. We will not really understand the history of the moon until we know what the moon is like on the inside."

One of the most enduring questions about the moon is why the nearside (the side we see from Earth) is so different from the farside. While the nearside has large impact basins that were flooded by massive lava flows (the dark areas we see from Earth), the farside does not contain these despite the presence of an impact basin far larger and deeper than any on the nearside. "It suggests the inside of the moon is very different on the sides towards and away from Earth and, whatever the reason, played a large role in how the moon got to be what it is like today," said Zuber.

The two spacecraft will launch from Cape Canaveral in Florida atop a Delta II rocket. Grail-A will travel almost 2.6 million miles in its circuitous route to the moon, while its companion, Grail-B, will travel about 2.7 million miles.

As they orbit the moon, the Grail spacecraft will transmit radio signals that will allow precise measurement of the distance between them. As they pass over the surface, the distance between the probes will fluctuate because of the varying gravitational field. Recording these fluctuations will allow scientists to infer and map the moon's gravity field.

"As the first spacecraft goes over an area with higher gravity it speeds up and increases the distance to the other; the second spacecraft catches up as it passes over the higher gravity region," said Zuber. "As the two spacecraft chase each other around the moon we can construct a map of gravity. The two spacecraft perform precision formation flying. We will be able to determine the position of one spacecraft with respect to each other to the size of a human blood cell. Fundamentally the experiment is measuring the distance between two points, but doing it very, very well."

Contributor

Alok Jha, science correspondent

The GuardianTramp

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