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Published online 23 October 2007 |
Nature
| doi:10.1038/news.2007.182
Corrected online: 24 October 2007
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The sunniest spot on the Moon
SMART-1 data indicates good spot for lunar base.
On top of a hill near the south pole of the Moon is a sunny spot that might make the ideal place for a lunar outpost, according to preliminary analysis of data from the European Space Agency (ESA) satellite SMART-1.
Ben Bussey, from the Applied Physics Laboratory in Laurel, Maryland, and colleagues found the spot by looking at images from ESA’s SMART-1 mission, which orbited the Moon for a year and a half before crash-landing there in September 2006.
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A few comments: 1) "After close analysis, the group found a hill on a ridge to the southwest of Shackleton crater at the south pole,..."; I don't think directional terms are useful so close to a pole, better to simply refer to the position with respect to the image, i.e. to the lower left of the Shackleton Crater in the image. 2) There are many locations in the north and south pole regions which get permanent illumination during their respective summer seasons. The problem is finding such locations during their respective winter seasons. 3) "But it was previously unknown whether mountains, hills or dips in this area might throw shadows that block the sunshine at certain times of day."; Using the digital elevation models derived from Earth based radar and analytical models, this has been done. For both the Shackleton Crater rim site (which Bussey/Spudis first identified from Clementine imagery a while ago) and a candidate "hill" peak site (unclear exactly where from this article), the models show no shadowing during summer (winter is quite different, see http://spaceflightsystems.grc.nasa.gov/articles/Lunar-Topographic.php ). 4) Although the article indicates the lunar scientists are searching SMART-1 imagery for the winter season, it is not clear how this can definitively ascertain permanently illuminated locations. First, the orbit period of SMART-1 is about 5 hours and shadowing by both nearby and distant terrain during worse case winter Sun angles can change rapidly. This means that snap shots at the 5 hour intervals could be fortuitously illuminated, with shadowing possibly occuring in the intervals. Second, at the low winter Sun angles, partially blocked illumination due to distant terrain and a finite Sun width occurs frequently. Partial Sun blockage would greatly affect power production at such a site. It is hard for imagery to differentiate this kind of illumination (versus terrain slope illumination differences for instance). Third, although better than Clementine's, a lot of shadowing can still occur within the SMART-1 resolution. By this, it is meant that smaller scale terrain shadows can be washed out due to the resolution. Finally, because the winter Sun angles are below horizontal, the tips of "hills" or peaks that may be very well illuminated would actually be darker than surrounding illuminated terrain. It takes many overlays of images to help deduce possible high illumination points. For solar power production during the winter, the solar array is pointed near the horizontal (perpendicular to the assumed flat surface terrain it is deployed from), so it is not necessary that the terrain it rests on be illuminated.
1. Whilst you are cartographically correct, I think that it is clear from both the context and the accompanying illustration what I meant. 2. I disagree that we knew that constantly illuminated places existed at the south pole in summer before this work. We have never before had images that covered these illumination conditions. We published information on locations of constant illumination for the north pole, but that was because we had Clementine images that covered the north pole in summer. 3. Use of topography from Earth-based radar does not produce simulations of illumination that match actual images. We therefore believe that it can not predict quantitative illumination conditions during times for which we do not have accompanying images. Observational data must always take precedence over modeling. 4. I disagree that eclipses can occur with a frequency of less than 5 hours. The Sun moves 2.5° in azimuth in that time and the solar disk itself has a 0.5° solid angle. To produce an eclipse, you would have to have a very narrow peak placed just right on a portion of the surface that was shown to be illuminated in two images taken 5 hours apart; such narrow peak topography does not occur on the Moon, where the terrain is smooth and rolling. We will not know the exact frequency of eclipses until we get higher resolution topographic data, hopefully in the next couple of years from one of the multiple international lunar mapping missions. Until then, our current image collection from the Clementine and SMART missions are the best source of information to know if a place on the Moon is illuminated for a given illumination direction.
1) I did not think it was you who stated the hill was southwest of the crater. My impression was that the author of the article had misinterpreted what you meant. It was just a minor point. 2) I agree that we could not confirm with imagery the summer illumination characteristics of sites at the lunar south pole prior to the SMART-1 imagery. My point was that digital elevation model analysis suggested that these key sites were permanently illuminated during summer. An examination of the relatively high sun angles lend credence to this. 3) While technically you are correct that digital elevation models do not create perfect replicas of actual imagery, this does not rule them out of being useful in making illumination predictions. The goal (at least the one I think is of issue here) from an illumination perspective is not to duplicate an actual image, but rather to examine how terrain blocks sun light (cast shadows) at key high terrain potential base sites. If the digital elevation model does not have some low lying terrain (since the radar digital elevation model data was gathered from Earth some lunar terrain blocks areas behind it from being examined), this is not necessarily critical since low lying terrain would not likely cast shadows on the key high potential base sites. In other words, if a digital elevation model should render an image with a number of dark areas (i.e. the missing data) which obviously to not match an image of the area, this is not important unless within those regions are very high shadow casting terrain. This has not apparently been the case in at least the various sites you have identified in prior papers. If, however, one wishes to know the illumination for every spot regardless of whether it is a high illumination location or not, then one needs either better digital elevation models from orbital spacecraft or derived (processed) illumination from imagery. I have already mentioned the problems of imagery methods in quantifying the illumination for partial sun blockage (which doesn't seem addressable without digital elevation modeling, but for best case summer illumination may not be a problem) and zero or negative sun elevation in which negligibly illuminated image pixels are at sites which likely have illumination (maybe addressable by way of image analysis but likely not a problem for the summer illumination). Also, while I agree that "observational data must always take precedence over modeling", I note that the radar data used to create the digital elevation model was based on observational data and that spacecraft imagery requires significant processing (although not as much). All data must be considered and used if possible. 4)As to eclipses during the 5 hour period, you are probably correct that at high sun elevation angles during south pole summer for the "hill" site there is little topography that can cast such "total eclipse" shadows on the site. Careful examination and animation (as you often do) of imagery would make such narrow shadows quite apparent. During times of year besides summer, the effect of fairly distant high sun blocking terrain on generating such eclipses likely occur but may not be a difficult problem for power systems to deal with. We shall have to await some higher resolution topography data to resolve this, perhaps from Earth-based radar too.