Samuel LawrenceJul 26, 2009

The Power of Lighting Conditions

For over four decades, the lunar science community has absorbed the information from the Apollo missions. Although many important questions were answered, many important new questions are waiting to be tackled -- which is the very essence of science and exploration. To answer these compelling scientific questions, we need to send human explorers to many different places on the Moon, including the lunar poles and the farside.

During the Apollo era, NASA and the lunar science community identified scientifically compelling landing sites from telescopic observations and orbital photography (from the US Lunar Orbiter spacecraft, as well as the Apollo missions themselves). Among many drivers of site selection for the Apollo missions (especially Apollo 11) was landing site safety. Apollo mission planners needed smooth surfaces with shallow topographic slopes for lunar landings.

NASA's Project Constellation will return human explorers to key scientific and resource targets on the Moon. The key first step in this process is the Lunar Reconnaissance Orbiter, which (similarly to the Lunar Orbiter missions that preceded the Apollo voyages) is returning a phoenomenal dataset that enables future human lunar exploration and utilization. One of the primary goals of the Lunar Reconnaissance Orbiter Camera scientific investigation is to collect comprehensive imaging datasets for fifty notional human exploration design reference sites selected by our friends in Project Constellation in order to assist with eventual landing site safety determinations. But what exactly does this mean, and what kinds of measurements are required?

Since LRO is going to be in a low polar orbit, one of the most important things LROC is going to do is collect images of proposed landing sites under a variety of illumination conditions. When LRO first arrived at the Moon, it was in an orbit that carried it over the lunar terminator. From a practical standpoint, that meant that the terrain in most early LROC images was ensconced in shadows. Some of these images had stunning interplays of darkness and light, giving them an almost ethereal beauty (we've put many of these images up in our LROC Image Gallery). One of the first images we released on July 2nd was of the lunar highlands within the Deslandres impact structure southeast of Mare Nubium. The low Sun angle gave this region a harsh, forbidding look. The terrain looked rugged and, to the untrained eye, impassable. However, as LROC Principal Investigator Mark Robinson was careful to point out in the press release that accompanied the Deslandres image, in actuality the region is quite similar to the Descartes region that was very successfully explored by the Apollo 16 astronauts in 1972 on one of humanity's great voyages of exploration. From this one example, we can deduce that lighting conditions are really important, and can easily affect your perception of a potential exploration site. This is especially true on the Moon, where the surface has lots of texture and lighting conditions can be extreme. As the LRO mission continues, LRO will continue to move out of the shadows and collect imagery over a wide variety of solar lighting conditions. If you were an astronaut, would you land here?

Simply looking at the same surface under different solar illumination conditions is a powerful tool. Shadows bring out topographic details (such as small craters or even boulders) that might not be visible near noon. In fact, repeat imaging of a region on the lunar surface under different illumination conditions (both in terms of sun angle and direction) enables scientists to create detailed topographic maps - known as digital elevation models, or DEMs. DEMs allow quantitative scientific studies of a region's landforms, (also called geomorphology). Accurate measurements of local slopes are also critical for landing site evaluations, since shallow slopes are easier landing targets.

Same place, different lighting conditions
Same place, different lighting conditions Four Apollo Metric frames of the same area in Mare Imbrium each taken under different solar illumination conditions. (AS15-M-2170, AS15-M-1145, AS15-M-0596, and AS15-M-0423).Image: NASA

In the image above, we see the same region on the lunar surface under progressively different illumination conditions. Each picture was taken on a different orbit as the relative positions of the Sun, the Moon, and the Endeavour progressed (upper left: solar incidence angle 48 degrees upper right: 73 degrees, lower left: 83 degrees, and lower right 88 degrees). In the upper left, Mare Imbrium appears flat and inviting; the image of the same area in the lower right looks starkly forbidding.

The reality is that there are many safe landing places in Figure 1. This is why it is so important to re-image potential landing sites at various illumination angles to accurately portray the roughness and slope of a particular landing site. Data from both the upcoming Lunar Reconnaissance Orbiter Camera and the Lunar Orbital Laser Altimeter, which are both returning data from the Moon as you read this, will be used by NASA to fully assess the roughness and topography of potential landing sites on the the lunar surface as we prepare for the seventh human lunar landing and beyond.

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