The Apollo Experiments
On July 20, 1969, during NASA’s Apollo 11 mission, astronauts Neil Armstrong and Edwin ‘Buzz’ Aldrin became the first humans to set foot on the Moon. One of the less known facts about them is that they placed a seismometer on the Moon (Fig. 1) along with a few other scientific instruments. The astronauts went home a few earth days later, but the seismometer kept working for 21 days and transmitted the data to Earth. It turned out that the seismometer detected a number of moonquakes.
Figure 1. Apollo 11 astronaut Buzz Aldrin setting up a seismometer.
Starting on Apollo 12 (Nov. 19-20, 1969 on the Moon), the astronauts deployed a more expansive array of geophysical instruments. These instruments were collectively called the Apollo Lunar Surface Experiments Package (ALSEP). Apollo 14, 15, 16, and 17 also deployed ALSEPs (Figs. 2 and 3). Apollo 17 (Dec. 11-14, 1972 on the Moon) was NASA’s last human mission to the Moon. Astronauts Harrison ‘Jack’ Schmitt and Gene Cernan are the last humans to walk on the Moon till this day (Video 1).
Figure 2. A panoramic photograph of the Apollo 15 ALSEP station by astronaut James Irwin.
Figure 3. The Apollo landing sites.
Each of the Apollo 12, 14-17 landing sites had a mix of 5 to 8 ALSEP instruments. Powered by radioisotope thermoelectric generators (RTGs), many of these instruments kept working till September 30, 1977.
Apollo 17 Lunar Module leaving the Moon
Modern planetary scientists owe most of their knowledge about the interior of the Moon to the ALSEP data and the rock samples brought back by the Apollo astronauts. Even though the ALSEP data are nearly 50 years old, they represent the only geophysical data gathered on the surface of the Moon. Thanks mainly to technological advancement in the last half century, researchers now have more diverse and powerful tools for analyzing the ALSEP data. We are still learning new things about the Moon to this day because of that.