SPACE/ Journey to the center of the Moon

A few months before the two American astronauts, Neil Armstrong and Buzz Aldrin, touched lunar soil, in that magical July 1969, some scientists from the JPL (Jet Propulsion Laboratory) of NASA, while analyzing the flight data of the Lunar Orbiter probe, saw some striking anomalies in the gravitational field of the satellite. The most intense variations were attributed to the presence of large concentrations of mass buried underneath some lunar seas. These areas immediately entered into astrophysics lingo with the term mascon (mass concentration). Their intense gravitational attraction is such as to create stability problems for low altitude spacecraft flights in their vicinity.

These were the signs of something strange in the distribution of mass on the moon, which was later confirmed, though particularly accurate studies were not conducted to measure the details and verify the causes. Now this gap will be filled with the GRAIL (Gravity Recovery and Interior Laboratory) mission, composed of two twin probes, Grail-A and Grail-B, which were launched a few days ago by NASA with the primary goal of making the first complete map of the gravitational field of the Moon and exploring its internal structure in detail, to the nucleus.

The fact that gravity on the moon is different from gravity on Earth is common knowledge, if only because of the images that we all have in mind of the astronauts bouncing lightly in their moon walks. Those who are preparing for future holidays in space, know that one weighs less on our satellite. A song from the 60s says, "weight on the moon is half of a half". The author, however, was not well informed, because weight, or gravitational attraction, on the Moon is about one-sixth that of Earth, so if a body weighs 60 kg where we are, on the moon it would weigh about 10 kg. But this is a generic number, instead, the values change in different areas, with considerable variations in the vicinity of the aforementioned mascons: an object's weighing 100 kg on the Moon in a "normal" zone, could weigh up to 450 grams more next to a mascon.

Here's how the Grail twins will try to measure these and other irregularities in lunar gravity: in three and a half months, in the days at the turn of the New Year, after traveling more than four million kilometers, they will enter lunar orbit and will start to emit microwave signals to each other to calculate precisely the distance that separates them. They will use a system called the Gravity Lunar Ranging System, which is able to measure variations in distance between two spacecraft while flying over the lunar surface with great sensitivity. In fact, the differences in mass in different zones make the gravitational field irregular, and change the distance between the two probes: by measuring such changes, one can trace the distribution of the mass.