AGU: An update on Kaguya
Posted by Emily Lakdawalla
16-12-2008 10:23 CST
On Monday afternoon at the American Geophysical Union meeting there was a short session, comprising four talks, on the status of Kaguya, Japan's enormous lunar orbiter. The first presentation was an overview of the mission; the next three presentations focused on individual instruments.
The first presentation, by N. Namiki, presented a new map derived by Kaguya of the gravity field of the Moon. This is probably the most important contribution of Kaguya to lunar science. The force of gravity felt by a spacecraft is a sensitive probe of the distribution of mass at the surface and near-surface interior of a planet, and a very important clue to the history of a planet's formation. Getting a map of the gravity field of a planet usually involves two-way Doppler tracking of a spacecraft's radio signals from Earth-based radio antennas. The problem with gravity mapping of the Moon has always been that we can only see one hemisphere of the Moon from Earth; we've never been able to track spacecraft that orbit close to the far side of the Moon, because the Moon blocks our line of sight to the spacecraft. So we have pretty decent maps of the gravity field of the near side of the Moon, but generally pretty lousy data on the far side.
Kaguya sought to solve this by employing a relay satellite, so that while the main spacecraft passed over the far side of the Moon, its signals would be related to Earth via a relay satellite in a higher orbit; instead of two-way Doppler tracking, it's now four-way (Earth to relay to Kaguya back to relay to Earth). The technique seems to have been successful, because they showed dramatic improvement in the quality of the gravity map over the lunar farside. Here's a small piece of the farside gravity map; the left panel is from Lunar Prospector, the best-to-date data set, and the right panel is from Kaguya.
Gravity anomaly map of Apollo basin from Lunar Prospector data
The final presentation was on results from the Kaguya Laser Altimeter or LALT, presented by H. Araki. They're still mapping, but they already have an excellent product, with major improvement especially in the polar regions over previous topographic maps. A laser altimeter gathers many linear topographic profiles along the spacecraft's ground track, so the measurements are spaced much closer together where the ground tracks cross near the pole than they are near the equator. At the equator, there are still gaps between adjacent laser altimetry profiles of up to 10 kilometers. Araki said that the altitude measurements are highly precise, with a point-to-point accuracy of about 1 meter; due to various biases, the altimetric measurements over a whole orbit are accurate to a vertical resolution of about 12 meters. He said that the highest point on the Moon is in the south rim of Dirichlet-Jackson basin, at an elevation of 10.75 kilometers. The lowest point is within a small crater on the floor of the crater Antoniadi, which is itself within the south pole-Aitken basin on the lunar farside. Here's a comparison of the previous best map to the current Kaguya map.The differences between the two maps are not obvious at this scale, so I've picked at random an area of the lunar farside to look at in detail, comparing between the previous Unified Lunar Control Network map (top) and Kaguya (bottom). There's a lot of cool things you can do with this new topographic data. For just one example, Araki used it to calculate the area of the Moon's poles (the areas poleward of 87.5 degrees latitude) that were in permament shadow over a period of 2,000 days. He found that a cumulative area of 844 square kilometers near the north pole never saw the Sun during that time. By contrast, 2,751 square kilometers near the south pole were in permament shadow during this time. He said the results are in press in the journal Geophysical Research Letters.
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