Two American ships are about to fly around the moon

The pair of spacecraft launched by the US Aerospace Agency (NASA) in September will fly to the moon's orbit around the weekend.

Grail-A and Grail-B , the names of the two spacecraft, were launched on September 10. Their task is to measure gravity and study the inner structure of the moon. Grail-A will reach the lunar orbit at the last hours of 2011, while Grail-B ships will arrive at the first hours of the new year, Xinhua reported.

Illustrations of two Grail-A and Grail-B spacecraft flying around the moon.

"The purpose of pairing Grail ships is to collect data about the gravitational pull of the moon. With new data, scientists can explore the moon's internal structure, from the top shell to the core. " NASA scientist David Lehman said.

After arriving in orbit, the couple will fly around the moon for two months. The process of sending data to the earth will start from March.

The moon is a celestial body with the most irregular gravitational field in the solar system. So NASA scientists want to know what happens above and below the surface of the moon to understand why the moon's permanent dark half has a rougher surface than the face facing the globe. They also want to know if the ridges on the moon are the result of a collision 4 billion years ago with a smaller Earth satellite.