With the cancellation of plans to return Americans to the Moon, at least anytime soon, you might think that NASA's Lunar Reconnaissance Orbiter is a spacecraft without a mission. After all, LRO's two key objectives were to scan the lunar landscape for places where future crews could safely land and to assess how much water might be stashed in the Moon's polar regions. But don't worry: the global observations being returned by this highly capable orbiter will keep lunar specialists busy for decades. Here's a guide to the spacecraft's seven instruments.
Much ado has been made of LRO's views of landing sites from previous missions. Most famous among these is the revered Apollo 11 site, Tranquility Base, snapped a couple of times last year. The stunningly detailed views are made possible by LRO's high-resolution cameras, dubbed LROC. The twin sets of 8-inch (195-mm) f/3.6 optics, viewing from an orbit only 30 miles (50 km) up, resolve the lunar terrain at about 2 feet (0.5 m) per pixel.
In the past few months, the LROC team has chronicled a host of other spots where landers set down. For example, there's now complete coverage of the six Apollo sites. But the image cache also includes the robotic Surveyors 3, 5, and 6, as wel as the Soviet Union's lunar rovers (including Luna 21) and the automated sample-return missions Luna 20 and 24.
Diehard space cadets will have to excuse me, but I can only get excited to a point about these trips down memory lane.
However, a view released last week caught my eye. It's the site where, on April 14, 1970, NASA controllers forced a Saturn IV-B rocket stage (the one used by Apollo 13) to slam into the surface. The spent rocket, 58 feet (18 m) weighing 14 tons, hit at 1½ miles (2½ km) per second. That wallop — equivalent to the explosive power of 10 tons of TNT — created a distinctive crater about 100 feet (35 m) across. The energy of the impact created small tremors picked up by a seismometer left behind by Apollo 12's astronauts.
Located at 27.9°W, 2.6°S (southwest of Copernicus between Oceanus Procellarum and Mare Cognitum), the crater has bright mounds in its interior, and the surrounding veneer of ejecta includes bright rays that extend for more than a mile. LROC took this view with the Sun well up in the sky, which brings out subtle differences in the albedo (reflectivity) of the surface. Look carefully, and you'll spot some dark rays among the bright ones.
You might be wondering how this crater compares with the one excavated last October by the Centaur rocket used to carry LRO (and its kamikaze impactor, LCROSS) to the Moon. Although the impact speeds were comparable, the Centaur is much smaller and weighs only 2 tons. Still, LCROSS scientists estimate the resulting crater at 70 to 100 feet across. That blast, famously, took place in the permanently shadowed floor of Cabeus crater — so we might never have views of it showing this kind of detail.