[FPSPACE] Fw: Why the Apollo laser reflectors are degrading

[LARRY KLAES]

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Date: Fri, 5 Mar 2010 13:29:01 
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 The Mysterious Degradation Of The Apollo Reflector Arrays <http://feedproxy.google.com/~r/arXivblog/~3/b3FlvFRuLFs/click.phdo?utm_source=feedburner&utm_medium=email> 
 Posted: 04 Mar 2010 09:10 PM PST 

The efficiency of the Apollo reflector arrays drops by a factor of ten during a full Moon. Now a new analysis may explain why 
   <http://www.technologyreview.com/blog/arxiv/files/38004/Lunar laser ranging.png> 
Lunar laser ranging experiments have produced a treasure trove of interesting information about the Moon, for example that it is spiralling away from us at a rate of 38 mm per year. 
The experiments are simple. Astronomers fire a laser pulse at a reflector placed on the lunar surface by the Apollo 15 mission and then use a telescope to look for the reflection, some 2 seconds later. 
The observations are challenging. Of the 10^17 photons that set out towards the Moon in each pulse, only one makes it back, on average. And only then if seeing conditions are good. 
When conditions are good, astronomers often take aim at the arrays left by the Apollo 11 and 14 missions which are only a third of the size of Apollo 15's and therefore harder to see. If the observers are feeling lucky, they might also try for the Russian Lunakhod 2 array (the Lunakhod 1 array hasn't been seen since 1971). 
All in all, astronomers have been taking observations since 1969, first from the MacDonald Observatory in West Texas and later from the Apache Point Observatory in New Mexico . This gives them a substantial database with which to analyse the behaviour of the reflectors. 
So how have these reflectors fared in the harsh conditions on the lunar surface over the years? That's the question addressed today by Tom Murphy at the University of California San Diego and a few buddies. And their analysis poses an interesting problem. 
First of all they say that the efficiency of all three Apollo reflector arrays has fallen by an order of magnitude during their sojourn on the Moon. The Lunakhod reflector has fared even worse. When it arrived on the moon in 1973, its signal was 25 per cent stronger than Apollo 15's. Today it is ten times worse. 
What's happened this gear? 
The reflectors consist of an array of cubic prisms that operate by total internal reflection. In addition, the Lunakhod prisms have silvered surfaces and are more exposed. Degradation of this silvering probably explains its relative drop in performance. 
But what has caused the degradation of the Apollo prisms? Anything that settles on or damages the optical surfaces of the prisms will reduce the efficiency of the total internal reflections. Murphy and co discuss several possibilities such as micrometeorite damage, lunar dust aggregation and the breakdown of the Teflon mounting rings which may have left deposits on the back surface of the prisms. 
Any of these mechanisms could account for the drop but its hard to pin one down. 
However, there is another more intriguing puzzle about the laser ranging data. When the Moon is full, the efficiency of all the Apollo reflectors drops by another factor of ten. Murphy and co have ruled out ground-based effects such as the saturation of their photon detectors when the moon is bright. 
So why does this happen? One clue comes from the study of returns during total lunar eclipses. Within 15 minutes of an eclipse occurring, the efficiency of the reflectors returns to its normal levels. When the eclipse ends and the Moon is full again, the efficiency immediately drops again. 
That strongly points to a thermal effect. When the Sun is low in the lunar sky, its rays cannot directly enter the prisms which are recessed in the arrays. But when the Sun is overhead (which is when the Moon appears full on Earth), its rays travel directly into the prisms. This is probably heating the prisms, distorting them and reducing the efficiency of their reflections. 
But why now? The full Moon effect was not a problem in the early days of lunar ranging. 
"Dust is perhaps the most likely candidate for the observed degradation," say Murphy and co. The sunlight is probably absorbed by dust on the optical surfaces which in turn heats the silica prisms. 
Dust is known to hover above the lunar surface because of electrostatic forces and micrometeorite impacts probably send a few puffs into the lunar atmosphere on a regular basis. 
Interesting work. And one that is of more than passing interest for many astronomers because it has implications for anybody thinking of sending gear to the Moon in future. Various astronomers want to send telescopes to the Moon, particularly the far side because of the tremendous seeing conditions there and its isolation from the Earth. Knowing how the Apollo gear has fared will be crucial when it comes to designing this stuff. 
Ref: arxiv.org/abs/1003.0713 <http://arxiv.org/abs/1003.0713> : Long-Term Degradation Of Optical Devices On The Moon 
 
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 New Model Captures Spread of Personal Information Through Social Networks <http://feedproxy.google.com/~r/arXivblog/~3/SAnD9HwKTIs/click.phdo?utm_source=feedburner&utm_medium=email> 
 Posted: 03 Mar 2010 09:00 PM PST 

If you ever share personal information with friends online you probably already use a special set of rules to determine what to reveal and what to keep secret. Now a new model of social networks attempts to capture that process 
   <http://www.technologyreview.com/blog/arxiv/files/37987/personal-info.gif> 
If you ever chat online to friends, colleagues and relatives, you probably have a set of unwritten rules about the kind of information you share with whom. The sort of thing you might take into account is the likelihood that this person will share your information with others, who these other people might be and whether this will be good or bad for you. Above all, you will almost certainly accept that divulging personal details brings you certain benefits, if only that it will strengthen the social bond between you and your correspondent. 
But this attitude to sharing personal information is probably in stark contrast to the way you think about the information you share when carrying out an electronic transaction such buying a book or signing up for a newsletter. In this case, most people feel strongly that they should minimise the information they share. 
Strangely, most studies of privacy and the spread of personal information through networks have focused on the latter scenario.Today, Jon Kleinberg and Katrina Ligett at Cornell University redress the balance. 
These guys have created a model of information sharing that attempts to capture these subtle rules. In the model, two people must agree to share information. The model assumes that they do this strategically to maximize their benefits they receive based on their expectations about what others will do. The model captures the benefit or disadvantage by allowing it take a positive or negative value. 
The result is a kind of network formation game, in which players must decide which links to maintain and which to cut so as to maximize their benefits or minimise their losses. 
Clearly, this is a game that can produce extremely rich behaviour. Kleinberg and Ligett's goal is to study the the most basic version of the model that is still rich enough to produce meaningful results. 
What they find makes for interesting reading. For example, they say that the model provides an easy way to monitor social welfare, simply by adding up the benefits that everybody gets. They can also look for networks that maximise social welfare. 
Kleinberg and Ligett look particularly at stable networks in which people are not forced to make and cut ties since these seem to best represent the ones that humans form naturally. But the new approach allows the researchers to ask an interesting question: are these the best types of network? In other words, are stable networks automatically the socially optimal ones? And if not, what is the difference between them in terms of social welfare? 
And that's just the start of what's possible. Kleinberg and Ligett have only just begun analysing these networks. The results are at an early stage but the potential is fascinating. 
All the more so because of Kleinberg's background: he has significant form when it comes to analysing networks. In the late 1990s, he developed the hubs and authorities model of websites. His idea was that some websites act as hubs directing surfers to authorities on certain topics and was hugely influential because it lead directly to an objective way of organising and therefore ranking websites . Kleinberg's work was a direct forerunner of Google's PageRank. 
The question this later paper raises is whether Kleinberg is about trigger the same kind of revolution for social networks that he did for the world wide web. Worth watching. 
Ref: arxiv.org/abs/1003.0469 <http://arxiv.org/abs/1003.0469> : Information-Sharing and Privacy in Social Networks 
 
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