China completes world's highest-resolution 3D map of moon

Last September, China announced that it had finished what it described as the most accurate and highest resolution 3-D map ever created of the lunar surface. A seemingly unrelated event invited close scrutiny of this accomplishment.

Enter a missing Russian moon rover.

Lunokhod-2 slipped out of sight 37 years ago, and then sat quietly on the surface of the moon waiting to be found. Lunokhod-2 was a remarkable machine, and it is most famous for its 35 kilometer trip across the lunar surface in 1973. This was a great achievement at the time, and it remains a record distance today. No other rover anywhere else in space - including the US rovers on Mars - has ever propelled itself as far as Lunokhod-2.

It seems quite odd then that Chinese space mapping experts

didn't say that they were able to solve one the great mysteries of lunar exploration. They were certainly aware that a search for the precise location of this missing Russian moon rover had been ongoing for years, and that finding this missing rover as well as other spacecraft on the lunar surface is deemed important by moon researchers in general.

So why China's 3-D marvelous moon map [1] was not used for this purpose raised questions about the Chinese moon map itself, and perhaps about China's ability to perform extremely rapid imagery analysis as well.

In March, Phil Stooke, a Canadian professor at the University of Western Ontario's Center for Planetary Science and Exploration, solved the mystery that began in June 1973 when all communications between Lunokhod-2 and its Soviet controllers back on Earth ceased, after completing several months of work on the lunar surface. Stooke pinpointed its location using data and imagery generated by US National Aeronautics and Space Administration's (NASA) Lunar Reconnaissance Orbiter (LRO).

Stooke is one of the world's leading experts on lunar geography and the author of The International Atlas of Lunar Exploration, published in 2007. He made his discovery on the evening of March 15 after spending a few hours examining pictures of the moon taken by the LRO.

"The idea that the rover was lost for 37 years, as is often reported, is a bit exaggerated - its position was known approximately, just not exactly," said Stooke. "I regard what I did as finding the rover in an image, rather than finding it on the moon."

Finding man-made objects was not the mission of the team at the Chinese Academy of Surveying and Mapping (CASM) headed by Liu Xianlin, a professor and senior researcher at CASM, when they created what they described last September as the world's highest-resolution, 3-D (three-dimensional) map of the moon. Still, this team would probably no doubt be aware that all of the landing sites of these historic spacecraft are significant. An attempt to contact Liu at CASM was unsuccessful.

The CASM map was described as "covering the whole surface of the moon", and was made possible by a new stereo camera mounted aboard China's first lunar probe, the Chang'e-1, which was launched in October 2007. According to CASM, the map featured a spatial resolution of 500 meters.

Interestingly, news of this spectacular 3-D lunar map surfaced just days before the celebration of CASM's 50th anniversary, in October 2009.

And yet, given all the fanfare and the global media coverage surrounding the discovery by Stooke of the long lost Lunokhod-2, and given the completion of CASM's astounding new 3-D lunar map six months earlier, it is surprising that CASM did not find (or apparently even attempt to find) Lunokhod-2 well before Stooke.

According to NASA, "LRO was mandated to release data to the Planetary Data System beginning six months after initial operation. Some of the higher level data products require the full year of measurements and won't be released until after the end of the exploration mission ... LRO is scheduled for a one year exploration mission in a polar orbit about 31 miles [50 kilometers] above the lunar surface. During this time, LRO will produce a comprehensive map of the lunar surface in unprecedented detail, search for resources and potential safe landing sites for a potential future return to the moon and measure lunar temperatures and radiation levels." [2]

With the LRO imagery in hand, Stooke could easily observe such things as where Lunokhod-2's operator in Russia drove it back and forth in order to allow it to carry out its detailed measurement of the moon's magnetic field. And the small crater where Lunokhod-2's radiator became covered in dust was easily spotted, too. It was that dust that triggered an overheating of the rover and soon terminated the Lunokhod-2 mission as a result.

"Lunokhod-2 was easy to find - I knew roughly where to look and as soon as I did its tracks stood out easily. That's in stark contrast with the first rover, Lunokhod - 1, whose tracks were almost invisible. The Russians found that one. I probably would not have found it," said Stooke.

Mikhail Malenkov, a well-known Russian space scientist and engineer, designed the wheels for the Lunokhod-2. He and Lunokhod-2's operator, Vyacheslav Dovgan, recently discussed Stooke's discovery as well as where exactly the rover's radiator lid had come into contact with the crater wall prior to shutdown. Lunokhod-2 was able to move forward nearly 100 meters after contact was made with the crater wall, according to Dovgan. The topography of the area made it possible to drive the rover ahead without maneuvering.

The brightness and color of the LRO's images of Lunokhod-2 were discussed by these two proud members of the Lunokhod team.

"One sees Lunokhod-2 as a dark spot when the optical axis of LRO camera remains perpendicular to moon surface. In this case, the camera sees the solar panels. But if the camera sees not the top but the white sides of Lunokhod-2 than it could appear to be brighter in the photo," said Malenkov.

The total number of man-made objects which landed softly and are now parked on the lunar surface - not those which were crashed into the surface - is 26, according to Stooke.

"There are 10 Russian objects sitting on the surface, plus five US Surveyors and the six Apollo landers (descent stages). Only five rovers - two Russian Lunokhods and three Apollo rovers," he said.
Stooke described China's 3-D lunar map as being the best for only a short time.

"I am aware of it,'' he said. ''I have seen their results published as small illustrations, but I have not seen or worked with the data itself," Stooke said, adding that "most accurate" claims could be misleading. ''When they claimed their map was the best, it was indeed the best. But since then we have had other missions. LRO topographic data are already much better than the Chinese map, and the Japanese data might be better, but I have not worked with either to compare them."

So was China constrained by the technology or simply not interested in finding the missing Russian rover? Or was imagery analysis an issue in this case?

Stooke knows the reason why the Chinese were not the first to find the rover, whether they wanted to or not.

"Nobody else had the high resolution images needed for this. The Chinese mission was not a high-resolution imager (100 meter / pixel resolution). Japan and India (10 meter and 5 meter/pixel resolution respectively) also would not have had the detail," said Stooke. "The only surprise to me is that the LRO team did not announce this before their big data release, but they should have been able to if they wanted to."

Stooke emphasized that his success did not involve use of a sophisticated toolset, or any exotic imagery analysis techniques. "I just knew where to look, and chose to look at this area first. I did not use exotic tools, just Photoshop,'' he said.

''The tracks were immediately visible as thin dark lines. I just had to follow them to the end," Stooke said. "In fact, my first guess at the location was not at the end of the tracks, because I missed a faint northern extension of the tracks. I thought a certain dark spot was the rover, but it was really a mark in the soil made by the rover as it turned in place before making that last drive. The rover was actually a bright spot just outside the first image I used. My Russian friends were very pleased to be able to point out my mistake."

Malenkov never expected to hear much about Lunokhod-2 after the little rover suddenly went silent in 1973. Three years earlier, Lunokhod-1 had performed its mission successfully for several months before signing off.

"We expected to launch Lunokhod-3 which was an improved version of Lunokhod-2, but the Lunokhod-3 project was cancelled," said Malenkov. "Now, the entire Lunokhod team is very much obliged to our American and Canadian colleagues for this unexpected greeting from the moon!"

According to Stooke, despite all the attention that he has received, others may have known about the precise location Lunokhod-2 even earlier than he did.

"I now think the Russians had already seen these images, but they had not published anything about it. I had no advantage," said Stooke. "Regarding the status of the rover, its location was known to within one or two kilometers, typical for all the later Russian landers."

For example, Stooke estimates that the landing site for Luna 24 - the final Russian lunar probe which was launched in 1976 - is 1.5 km south of the location which was published in the 1970s, and the site for Luna 21, the lander which deployed Lunokhod-2 on the moon's surface, is 500 or 600 meters northwest of its previously estimated location.

Stooke did not see himself as involved in a race or in any real competition in this instance.

"Most lunar researchers knew months in advance when the large collection of new images would be released (by NASA) - on March 15th. Any of them could have looked at the images when I did, late on the 15th," said Stooke. "Other researchers actively involved in the LRO mission had the images months before the release, and they had already identified (the locations of) all the Apollo landers, for instance. The interesting thing for most people now is to know exactly where these spacecraft are, instead of only approximately."

Stooke has been looking for other man-made objects on the moon, and over the past few weeks, the LRO team has released images of many of the historic lunar landing sites. The first team that reaches one of these sites will be awarded US$1 million as part of an exploration competition arranged by Google, the Internet giant.

Cash prizes in the Google Lunar X-Prize (GLXP) competition include a US$20 million first prize for the team that is first to build, launch and land a privately funded rover on the moon by the end of 2015. In this case, "privately funded" means that at least 90% of a team's support or funding must come from the private sector.

After making a successful soft landing, the lunar rover in question needs to go at least 500 meters across the lunar surface while simultaneously transmitting video, images and data back to Earth in order to win. The second-placed team will get $5 million.

A team can take home a bonus prize if its rover transmits images of Lunokhod-2 or other man-made objects on the moon at any one of several designated "Heritage sites."

"The bonus prize for respectfully revisiting the site of Lunokhod-2 is US$1 million. This could be claimed by either the grand prize or the second place prize winner," said William Pomerantz, senior director for space prizes at the California-based X-Prize Foundation. "In all likelihood, teams pursuing the Heritage Bonus will end up moving much more than the minimum 500 meter requirement in order to avoid any risks associated with landing extremely close to these Heritage sites."

Both Malenkov and Dovgan now serve on the Advisory Board for Team Selenokhod which is the Russian GLXP team based in Moscow. And Selenokhod CEO Nikolay Dzis -Voynarovskiy confirmed that the Lunokhod-2 site is one of the potential landing sites for the Selenokhod rover. [3]

"I am not formally involved with any GLXP team, but I have discussed landing sites with several teams. I prefer to operate as an outside observer of the competition," said Stooke.

Team Selene, based in Shanghai, is the only GLXP entrant so far from China. [4]

The new LRO data might inspire new teams to join the GLXP competition, but they had better move fast because the window for the $50,000 registration closes at the end of 2010.