In the late 1960s and early 1970s, NASA sent a dozen men to the Moon and returned them safely to Earth. Apollo started as a political stunt, a way for America to thumb its nose at the USSR. But along the way, a remarkable thing happened: science.
We learned a lot about the Moon during those missions. And the engineers and scientists at NASA figured out clever ways of learning more: Seismographs were placed on the surface by several missions, starting with Apollo 11 (which only provided data for a few weeks; later devices worked for years). Waiting for moonquakes was a pain, though, so they decided to make their own: Starting with Apollo 13, the upper stage of the Saturn rocket that brought them to the Moon was purposely steered toward the surface. When it impacted, it created a series of seismic waves that could be measured.
Fast-forward to 2015. The Lunar Reconnaissance Orbiter has been mapping the Moon’s surface in detail for years, and over time has found all the impact sites for those boosters, except one: that of Apollo 16. The S-IVB boosters were equipped with radio devices, so they could be tracked to the surface. The impact locations for the others were well known, and relatively easily found in LRO images. But the Apollo 16 transmitter failed, so the location of the impact site was only poorly known …
Until now. The impact crater of the Apollo 16 S-IVB has finally been located! It was about 30 km off from tracking estimates. That may not seem like much, but the LRO images map the Moon in strips 25 or 50 km wide, and those images have half a billion pixels in them. Worse, the Moon is saturated in craters the size of the booster impacts, making this like looking for a particular grain of sand on a huge beach.
But the good folks working with LRO found it. The impact site is in the western half of the Moon in a region called Mare Insularum, just to the southwest of the bright crater Copernicus.
The photo at the top of this article shows the impact crater. It looks different than those around it; the other craters are softer looking, with less distinct features. That’s a clear sign of age, perhaps billions of years; micrometeorite impacts and the solar wind take their toll over the eons.
The impact site also has obvious rays: Streamers of excavated material shot out as plumes, which then fall as linear features pointing away from the center of the crater. Those same erosive events fade the rays over time, so we know this is a very fresh feature. And the size, about 40 x 30 meters (roughly a quarter the area of an American football field), is just what you’d expect from a high-speed impact from the Saturn V booster.
Very cool. And this is more than just idle curiosity: The exact location of the booster impact will help refine the models of the Moon’s interior made using those seismographs. The devices used timing differences to figure out how the seismic waves traveled through the Moon, so knowing the exact location of the sources of the waves is very helpful. The shape and depth of the crater also help scientists understand the nature of impacts on the Moon, too.
We spend a lot of time, effort, and money on Apollo, and here we are, 43 years later, still learning from it. I expect that will be true for many, many decades to come, too.