It's been about 17 months since the launch of the most productive spacecraft ever to orbit the Moon. No, it wasn't a NASA project — in fact, as remarkable as this might seem, since its final Apollo landing 36 years ago the U.S. space agency has launched only one dedicated lunar mission (Lunar Prospector, in 1998).
Instead, the rave reviews are pouring in for a spacecraft built and operated by the Japan Aerospace Exploration Agency (JAXA). Its formal name is the cumbersome and contrived Selenological and Engineering Explorer, or Selene for short. But everyone knows it by its nickname, Kaguya, who was an enchanting Moon princess in Japanese folklore.
I got my first glimpse of Kaguya's results last March, when some of the mission's scientists trekked to the annual Lunar and Planetary Science Conference in Houston, Texas. They razzled and dazzled with incredibly detailed surface images, topographic maps, and (especially intriguing) gravity maps.
Now the Kaguya team has made a scientific splash with a suite of four articles in this week's Science. These emphasize what's been learned about the lunar far side, the mysterious, poorly understood half of the Moon that can't be seen from Earth.
For example, Kaguya's best images, taken with its Terrain Camera, have captured details as small as 35 feet (10 meters) over huge swaths of far-side real estate. Some of these recorded Mare Moscoviense ("Sea of Muscovy"), a lava plain that partially fills one of the far side's most prominent basins — and so named after being discovered in 1959 by the historic lunar flyby made by the Soviet Union's Lunik 3.
The Kaguya images are so crisp that geologists led by JAXA's Junichi Haruyama have found an unexpected paucity of small impact craters on some of the lava surfaces. As they reckon it, these flows must have erupted about 2½ billion years ago — making them fully 500 million years younger than the last known episodes of far-side volcanism.
Another eye-opening result came from Kaguya's laser altimeter, which has been measuring the lunar terrain's highs and lows with 100 times better coverage than previous craft had achieved. The far-side crust, report Hiroshi Araki (National Astronomical Observatory) and others, is rather rigid — that is, less likely to even out its elevations over the eons as has occurred on the near side or on Earth. The essential difference, they conclude, is that the far side has been completely water free, even deep down. "Dry as lunar dust," you might say.
To me, the most intriguing reading in the Science suite concerns striking differences in local gravity that Kaguya found between the Moon's near and far sides. Ever since the 1960s, we've known that the lunar interior is somewhat lumpy; careful radio tracking revealed that orbiting spacecraft sometimes sped up and slowed down ever so slightly as they passed over big, mare-filled basins.
The gravity signatures in and under these basins — and these mass concentrations (mascons) in particular — provide crucial clues to the state of the lunar interior some 3½ billion years ago, when most of the maria made their appearance. Geologists have argued for decades whether the mascons result from the thick "icing" of dense lava atop the basins or from dense mantle material protruding into the weakened crust lying above it.
They do agree that the far side lacks the widespread maria that fill Imbrium, Tranquilitatis, and their kin on the near side. And gravity data back there have been especially hard to come by — after all, how can you follow a spacecraft's motion when its hidden from tracking stations on Earth?
Kaguya's designers cleverly solved this dilemma by dispatching two smaller satellites, named Okina and Ouna, in separate orbits. Okina served as a radio relay that let the team track Kaguya's motion 24/7.
Alas, on February 12th Okina bit the lunar dust, figuratively and literally, ending the far-side gravity measurements.
As the examples at right show, Kaguya found that far-side basins have gravity signatures opposite those of near-side basins: they are gravity "lows," not "highs." No one quite knows what to make of this, though it might be related to the far side's thicker, stiffer crust.
Kudos to JAXA, because Kaguya has performed like a champ. Sure, later this year NASA will counter with its very capable Lunar Reconnaissance Orbiter, complete with a splashy impact in early August. But Japanese engineers plan to plunge Kaguya kamikaze-style into the Moon as well (science manager Manabu Kato couldn't tell me exactly when; "If you can wait till February 18th, I can talk about the new schedule," he teased.)
Besides, Kaguya has a secret PR weapon that NASA can't match: high-def videos shot as the spacecraft coasts 60 miles (100 km) above the lunar landscape. Check them out!