The mission has already completed one trajectory correction maneuver and waved off a second as being unnecessary before their orbit insertion maneuver. According to a timeline from this article (with translation help from Yong-Chun Zheng), on December 10, they'll drop the periapsis to 15 kilometers, and they'll land on December 14. The rover should drive off the lander the next day, with rover and lander shooting photos of each other.
I cannot wait to see those.
The landing will occur in Sinus Iridum. Planetary cartographer extraordinaire Phil Stooke has posted this base map showing the possible area of Chang'e 3's landing site, where the Chang'e 2 orbiter achieved high-resolution imaging coverage.
Yesterday, while fixing an old blog entry that had some cool in-space videos shot by Chang'e 2, I came across this one, showing the surface of Sinus Iridum rolling beneath the orbiter as it flew only 15 kilometers above the lunar surface. Think about that for a moment. Fifteen thousand meters. Fifty thousand feet. So close. That's as close as China managed to get with Chang'e 2. With Chang'e 3, they'll land inside their highest-resolution images.
I've now received some official comment from NASA regarding the question of how Chang'e 3's impending landing will affect the LADEE mission. As I mentioned before, while it adds a little complication, in the long run it is a great addition to the LADEE experiment. In particular, "The resultant atmospheric changes will provide LADEE with a new scientific opportunity. A known quantity of propulsion products of known composition, released at a known time and location, provides an excellent active scientific experiment. For example, one of the products will be water -- and LADEE may be able to monitor how water migrates across the surface. Such measurements could shed light on how water could be cold and trapped at the lunar poles -- a topic of considerable interest to the planetary community."
LADEE has issued two updates on their status; they are now well in to their science mission. They have finished commissioning the instruments and lowered themselves into their 75-kilometer science orbit in two steps on November 11 and 21. I wasn't sure what that meant for when they really started collecting science data, so I asked LADEE program scientist Sarah Noble. She told me that the official start of the science phase was on the 21st, when they were in their final science orbit, but that the science instruments started data collection on the 11th, after the periapsis had been lowered to 75 kilometers. So they've already been collecting data for almost a full lunar day already, and will have nearly a complete lunar day's worth of data from their science orbit by the time Chang'e 3 lands.
Lunar Reconnaissance Orbiter
Meanwhile, Lunar Reconnaissance Orbiter is still doing its thing and will be performing lots of observations designed to complement the LADEE mission during the brief time that it is orbiting the Moon. And, even though there is no formal cooperation between NASA and the Chinese space agency, Lunar Reconnaissance Orbiter Camera will certainly be taking a close look at the planned landing site for Chang'e 3. Camera team leader Mark Robinson posted a lengthy article on their website speculating about this, which I'll quote below:
[The landing site for Chang'e 3 is] not yet publicly designated, but it seems likely the landing will take place in Sinus Iridum, possibly near the fresh crater Laplace A (8 km diameter). Why this particular spot on the Moon? Likely there are critical engineering constraints in terms of landing site selection as well as important science goals. And there is the dramatic grandeur of the lunar landscape!
Imagine the first rover-eye view from the crater rim - a sheer drop of 1600 meters at your wheels, and an 8 km view across to the far wall! From LROC NAC images we know rock is exposed in the upper walls and dramatic landslides streamed material down to the crater floor. Speaking of the crater floor - it hosts a now frozen lake of impact melt 2500 meters (1.5 miles) in diameter. Imagine the moments after the crater formed, the floor was a cauldron of molten rock with debris sliding down into the melt, and the crater itself was deforming as the floor uplifted after the initial pressure of the impact was relieved.
Laplace A is a fascinating scientific target for a rover. It is a great example of a very young crater formed in mare basalt. A rover traversing the crater's ejecta blanket is in essence similar to driving down into the crater (in a geologic sense). We know from studies of terrestrial impact craters (such as Meteor crater) that material ejected from deep in a crater ends up near the rim, and rocks from the pre-impact surface are thrown far from the crater (a crater radius or more). So as a rover drives closer and closer to the rim it can characterize rocks from deeper and deeper below the surface.
Some of the many outstanding questions regarding the nature of the mare basalts include: how thick are individual flows, does the composition of the erupted magma change with time and location, and are pyroclastic (explosive) eruptions intermingled with effusive eruptions? These questions can be directly addressed with the Chang'e 3 rover! No humans or robots have ever visited a fresh crater anywhere near this size on the Moon (or Mars for that matter) so the return from this mission has great potential for advancing our knowledge of the Moon.
Robinson continued (emphasis mine):
But wait, there's more! Another key question can be addressed: what is the 3D nature of large contractional ridges on the Moon? The rover is thought to have a ground penetrating radar (GPR) and it just so happens that a large wrinkle ridge (a contractional landform) lies about 10 km east of Laplace A. Although the exact mission plan is not publicly available, one potential scenario is that the lander sets down just east of the wrinkle ridge and deploys the rover. After initial testing of the lander and rover, and geologic characterization of the landing site, the rover could set off to the west towards the crater. As the rover drives up and over the wrinkle ridge the GPR would continuously probe the subsurface, slowly building up a 3D profile down to 100 meters or more (?) beneath the surface. Wrinkle ridges are complex landforms created when mare basalts are compressed, causing them to buckle and break along faults. However, wrinkle ridges have not been fully explored, and the geometry and number of faults associated with each wrinkle ridge is not known. A subsurface profile of a wrinkle ridge could tell us the number of faults, where the faults are located, and how steeply the faults dip: is it 15°, 30° or 45°? From LROC images we have mapped the location of all the mare wrinkle ridges, and measured their surface topography, but all we have for the subsurface are models! Soon we may have actual measurements providing a good first step towards interpreting these poorly understood features. Wrinkle ridges are also found on Mercury and Mars, so better understanding a lunar example will help scientists unravel the tectonic story across the inner Solar System. Since only a handful of human and robotic missions have ever landed on the Moon, the results from the Chang'e 3 mission will provide important new scientific insights into our Moon.
Once Chang'e 3 has landed, LROC should be able to spot the lander and the rover; LRO will be above Laplace A on 25 December, 22 January, and 18 February. The LROC team looks forward to posting images of the two vehicles!
Coincidentally, Lunokhod 1 landed only 250 km to the southwest of Laplace A over forty years ago (17 November 1970). This intrepid Soviet rover explored for almost a year and traveled a total distance of 10.5 km. Both the lander vehicle (Luna 17) and the rover can be seen on the surface today.
It would be so cool if the Lunar Reconnaissance Orbiter team delivered us a Christmas present of a photo of Chang'e 2 on the surface of the Moon. Stay tuned....
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