Finding spacecraft impacts on the Moon
Posted by Phil Stooke
20-02-2017 6:00 CST
Over nearly 60 years of spacecraft exploration of the Moon, lots of spacecraft have crashed on the lunar surface. Some were accidental impacts, like the NASA robotic lander Surveyor 2 which failed during its flight to the Moon, or the Soviet Union’s Luna 15 which was attempting a sample return even while the Apollo 11 astronauts were on the lunar surface. Others were deliberate crashes. Some of those crashed as part of the mission, like Ranger 7 or LCROSS, the first taking pictures all the way down to the point of impact and the second using its crash to excavate a plume of debris and volatile substances for analysis by a companion spacecraft. Others were simply commanded to hit the Moon at the end of a mission, like the gravity-mapping GRAIL spacecraft or the Apollo Lunar Module ascent stages. Some of those still did useful work, like the Apollo ascent stages which generated seismic signals for equipment deployed by the astronauts. A few other spacecraft such as the Soviet orbiter Luna 19 and the Indian Chandrayaan 1 orbiter were abandoned in orbit and will have crashed later as their orbits evolved, at unknown locations.
For many of these crashes tracking or imaging data allowed the impact site to be located fairly precisely, and often an impact crater can been found in the spectacular high resolution images from NASA’s Lunar Reconnaissance Orbiter. The LRO camera team have a great website in which these craters and their ejecta deposits (and lots of successful landers) can be seen under different illumination conditions, sometimes with before and after images for recent impacts.
But some impact sites are missing from this site. In some cases we don’t know the location well enough to look for it—Surveyor 2 and Luna 15 both fall into that category. Others should be easier to find but have not been located. In particular the Apollo LM ascent stage impacts have proven elusive, despite prolonged efforts by several researchers. The Apollo Lunar Module ascent stage was massive enough (2300 kg) to make its mark on the lunar surface but the large hollow structure and the very low angle oblique impact (about 3°) make it hard to estimate what sort of feature might mark the impact site.
Four impact sites which have not been identified previously are described here based on recent work I have been doing in the Centre for Planetary Science and Exploration at Western University in London, Ontario. Two are Apollo Lunar Module ascent stages, from Apollo 12 and Apollo 14. The third site is that of the Chinese orbiter Chang’E 1, whose impact site was previously imaged by Apollo 16 for comparison with LRO images, forming a before-and-after pair which conclusively demonstrates that the correct location has been found. The fourth is SMART-1, the European mission to the Moon launched in 2003 and crashed at the end of its mission in September 2006.
The Apollo 12 impact site was discovered by means of a very unusual (in fact apparently unique) field of small dark markings suggestive of shrapnel strikes, located at 3.90° S, 21.23° W just west of the tracking location of the impact. By following this line back along the orbit path the impact site was discovered. The expected fan-shaped spray of ejecta which LRO has also seen at the GRAIL and LADEE impact sites was visible and seems to originate from a linear gouge oriented along track (C in the figure below). The gouge is roughly 25 m long and 2 to 3 m wide at 3.920° S, 21.172° W. Topographically, the gouge is on the crest of a small rise in topography, and the shrapnel markings commence 800 m to the west and extend for about 1500 m. The gap between those markings and the impact site is a shallow depression in LRO laser altimeter (LOLA) topography. It seems that the cloud of debris from the impact, which was probably generated when a grazing impact induced catastrophic rotation in the spacecraft, flew over the depression before striking the surface downrange.
Ewen Whitaker, the veteran lunar scientist formerly at the Lunar and Planetary Laboratory at the University of Arizona, who sadly passed away late last year, looked for artificial impact sites and other features of interest in Apollo images after each mission. Writing in the Apollo 16 Preliminary Science Report, he described a dark marking near the expected location of the Apollo 14 LM ascent stage impact which he found in Apollo 16 Metric image 2508. LRO images do not reveal anything obvious at that dark spot, but a smaller dark marking closer to the tracking location has now been identified. It also shows a fan of ejecta emanating from a gouge-like feature in the expected direction. A more distant spray of shrapnel-like impacts has not been observed here. The proximity to the tracking location and the similarity of the site to the Apollo 12 feature suggests that this is the LM Ascent Stage impact site. The location is 3.420° S, 19.637° W.
After a successful lunar mapping mission, Chang’E 1, China’s first lunar orbiter, struck the Moon on 1 March 2009, travelling from south to north in Mare Fecunditatis. Professor Jianjun Liu of China’s National Astronomical Observatories and his colleagues described the location based on analysis of images taken during the final orbit of the spacecraft. The expected location was 1.80° S, 52.23° E. Luckily this area was also imaged by Apollo 16 at high resolution (unlike the Apollo LM sites where only low resolution images are available). The Apollo 16 Panoramic Camera images are sufficiently detailed to compare with LROC-NAC images to search for new impacts.
An Apollo-era index of image coverage and the online Apollo image resources at http://apollo.sese.asu.edu/ brought me to the appropriate image, AS16-P 5202. When the Apollo image is exactly registered with a modern LRO image and the two are displayed alternately any new feature should seem to ‘blink’ on and off. This is how Pluto was discovered! I was actually looking specifically at craters with odd ejecta deposits and found that all of them were present in the old image. But in the process of doing this I found what I was looking for. Sure enough, a fan-shaped spray of ejecta can be seen at 1.66° S, 52.27° E in LROC images (Fig. 7) which is clearly not present in the older image (Fig. 8). This must be the Chang’E 1 impact site. There is a crater at the south end of the fan, but it is not clear if this is the impact crater itself or a pre-existing crater struck by the spacecraft. A small dark pit on the crater’s south side is more probably the impact site.
After its science and technology-testing mission, the Swedish-built European Space Agency mission SMART-1 (SMART stands for Small Missions for Advanced Research in Technology) was decommissioned by crashing it on the surface of the Moon. Its impact flash was imaged by the Canada-France Hawaii Telescope, just on the night side of the terminator. Orbit tracking and the impact flash give us a good estimate of its location, and very close to that point is a very unusual small feature shown in Figures 8 and 9. Like the Apollo LM features, this is a linear gouge in the surface, about 4 m wide and 20 m long, cutting across a small pre-existing crater. At its south end a faint fan of ejecta sprays out to the south. The location is 34.262° S, 46.193° W (313.807° E)
All of these spacecraft impacts from orbit, the ones described here and the well-established craters made by the GRAIL and LADEE spacecraft, show that kind of fan of ejecta extending downrange. Earlier searches for these sites often assumed that the ejecta would be very bright and largely spread out sideways, a so-called butterfly ejecta pattern typical of hypervelocity impacts at low angles. But these are not hypervelocity impacts (20 or 30 km/second), they are relatively slow impacts (2 km/second) by lightweight hollow structures. It now seems well established that impacts of orbiting spacecraft will form elongated craters, most of whose rather faint ejecta extends downrange.
What about the other Apollo LM impacts? Apollo 10’s ascent stage was sent into a heliocentric orbit rather than impacted. Its descent stage was abandoned in a low orbit and crashed at an unknown location. Apollo 11’s LM ascent stage was abandoned in orbit and its impact location is unknown. Apollo 16’s crashed at an unknown location after control of it was lost prior to impact. That leaves Apollos 15 and 17. Both of their ascent stages struck the surface. Neither has yet been located, leaving something for future work.