Fifty years ago, between July 26 and August 7, 1971, astronauts David Scott, Alfred Worden, and James Irwin journeyed to the Moon and back. On their return, they brought back 170 pounds of lunar rock and soil. A piece of one of those rocks, the sample designated 15016, has been on loan from NASA to the National Air and Space Museum for decades, and was on display for many years in the Apollo to the Moon gallery. Since that gallery closed in 2018 as part of our project to revitalize and transform the entire Museum, this sample has been in storage, but it will be on view again when a fantastic new gallery called Destination Moon opens in 2022.
Every rock can tell us a story, once we know how to read it. We can learn the history of how it formed and when, what it is made of, and how it has weathered the eons. But this particular Moon rock also has another story, a story of how two visitors from another world happened to collect it.
Sample 15016 is the rock type called basalt, and it formed as molten rock cooled about 3.3 billion years ago. It was exposed on the lunar surface for about 300 million years. A basalt is a dark-colored fine-grained igneous rock rich in iron, magnesium, and plagioclase feldspar, which is a common rock-forming mineral on Earth. The rock also contains the mineral olivine, which in gem form is known as peridot—the August birthstone. In the photo above, you’ll notice it is covered with small holes— these are called vesicles and are caused by expansion of gas bubbles as a rock solidifies. So, if we want to give the rock a technical name, it would be a vesicular olivine basalt. But it has also another less formal name.
The Apollo 15 landing site was located on the eastern edge of Mare Imbrium, a vast dark volcanic plain. On their first traverse using the Lunar Roving Vehicle (LRV), astronauts Scott and Irwin drove to Hadley Rille, a long and winding valley that was one of the major features of the site. As they were driving back to the Lunar Module (LM), Scott spotted our sample at the location marked with an arrow on the traverse map above. It grabbed his interest due to its vesicular texture. He wanted to stop and pick it up even though that wasn’t in the planned traverse they were supposed to stick to for time and safety reasons. He stopped anyway, and pretended to Mission Control listening back home that he needed to pause to adjust his troublesome seatbelt. Irwin played along, and Scott was able to quickly collect the sample. Even though it was acquired by not following the rules, it is now a well-known rock and it has contributed to our understanding of lunar geology. As a result of its colorful story, it has gained a popular nickname, and once the new exhibit opens you will be able to come and see the “Seatbelt Basalt”!