Geological Chronicle
Hertzsprung is a massive multi-ring impact basin located on the lunar far side at 1.4° N, 128.7° W, positioned just beyond the western limb. Spanning 536 kilometers across its outer rim, the structure exceeds the diameter of several basaltic maria on the near side. The basin formed during the Nectarian period and lies within the northwestern blast zone of the younger Mare Orientale impact. Altimetry measurements place the basin depth at 5.31 kilometers from the outer rim crest to the interior floor.
The basin has a classic double-ring morphology consisting of an outer topographic rim and a distinct inner mountain ring measuring 272 kilometers across. This inner floor is smoother and less rugged than the terrain between the two rings, although it contains several satellite craters, including Hertzsprung D (48 km) on the eastern side and Hertzsprung S (49 km) to the west. Doppler tracking from the Lunar Prospector spacecraft identified a prominent mass concentration (mascon) at the basin center, caused by dense mantle material that rebounded after the impact.
Subsequent impacts have altered Hertzsprung's rim over billions of years. The prominent crater Michelson overlaps the northeast rim, Vavilov cuts across the western rim, and Lucretius lies to the southeast. A chain of impact craters named Catena Lucretius begins at the southeastern margin and extends west-northwest toward the inner basin perimeter. In March 2022, the area near Hertzsprung D became the site of an artificial impact when a spent Long March 3C rocket upper stage struck the surface, producing an unusual double crater that was later photographed by NASA's Lunar Reconnaissance Orbiter.
The International Astronomical Union named the basin in 1970 for Danish astronomer Ejnar Hertzsprung, who co-developed the Hertzsprung-Russell diagram classifying stars by color and luminosity. Because the lunar equator passes directly through the southern portion of the basin on the far side, the feature remained hidden from Earth until orbital spacecraft mapped the Moon's farside hemisphere in the 1960s.