An observatory on the moon
Published on Jan 12, 2006 at 7:42 pm.
4 Comments.
Filed under astronomy, moon.
I’ve been neglecting the blog for the last couple of days. This week is the week of meetings before classes begin, so I’ve been pretty busy. There is always so much to do!
A few days ago, Tom wrote about an Italian plan to construct an observatory on the surface of the Moon. I thought that I’d say a few things about this.
The idea of putting an observatory on the surface of the Moon dates back to a suggestion to do just this made by Lyman Spitzer in 1946. The idea was to get a telescope outside of Earth’s atmosphere. Only a small fraction of the light from distant objects makes it through our atmosphere. A telescope outside of the atmosphere would be able to collect data using these wavelengths of light. Furthermore, even the light that does pass through the atmosphere is distorted and attenuated, and a telescope outside of Earth’s atmosphere would not have to deal with these problems, either. Spitzer regarded the Moon as a logical place for a telescope. Remember that the way data was collected in the middle of the 20th Century was by astronomers working at a telescope reading dials or changing film plates. Automated computerized instruments were not technologically possible, and even science fiction writers seldom dreamed of such things. So, Spitzer presumed that any such telescope must be at a manned observatory. A moonbase would seem a logical place for such an observatory. Orbiting telescopes seemed totally out of the question as any observations would require time exposures, necessitating keeping a telescope trained on an object for an extended time. At that time, it seemed impossible for an orbiting satellite to maintain such stability.
However, the Hubble Telescope proves that it is indeed possible to have an automated instrument on an orbiting satellite. The technology that made HST is now even more advanced, so any new space telescope could be made even better. A telescope in low orbit would be much cheaper than one on the Moon, so why would anyone today still think of a lunar observatory? Well, one advantage is that it would be easier to build a telescope on the Moon than to build it in orbit. This means that such a telescope would not be limited to the maximum size that a complete instrument could be launched into orbit. The individual pieces could be launched and then assembled on site. This would permit a larger telescope. Construction of the International Space Station, though, has allowed some perfection of weightless construction techniques, so perhaps this is less of a concern than it used to be.
So, besides cost, what else might make a lunar observatory less attractive? Well, there are two major limitations for a lunar observatory. First, a lunar surface observatory would have half of the sky blocked off by the Moon itself. An orbiting telescope can swing and point in any direction, so more of the sky could be viewed at any time. A much more serious problem, though, would be moon dust. This is actually a serious issue. The moon dust picks up a static charge from the solar wind, and it sticks to everything. The Apollo astronauts had a terrible time with moon dust getting all over the place and it was very hard to clean up, since it stuck to everything. Even worse, you don’t have to stir up the moon dust in order for it to be a problem. The static charges cause the moon dust particles to repel each other. Dust particles then levitate above the surface of the Moon up to a height of several meters. So, the telescope’s mirror would gradually become coated in a dust that would be very hard to get rid of. This would seem to make putting an observatory on the Moon a problem. Now, if we ever go back to the Moon and set up a permanent, or semi-permanent, base of operations, then it may not be such a bad idea to have some astronomy capability at the moon base, but that should not be the primary goal of such a base.
So, I’ve given some of the problems with a lunar observatory. What might be unique advantages over orbiting platforms? For Lyman Spitzer, the stability of the lunar surface was a primary advantage. As we have seen, though, modern technology makes that much less of an issue. However, one of the disadvantages of a lunar observatory can be made into an advantage. Remember that the Moon itself blocks half of the sky as seen from the surface of the Moon. The Moon always keeps one side towards the Earth, so the other side is always facing away from the Earth. A telescope on that side of the Moon is shielded from Earth. Now, we don’t really worry about sunlight reflected from Earth, because in space there is no atmosphere to scatter light, so that is not an issue. You just look away from the Sun and Earth, and the sky is black. There is no advantage to put the telescope on the far side of the Moon if you are just interesting in blocking light from Earth. However, there is an advantage to the far side of the Moon if you want to block radio signals. Humans produce an enormous amount of radio signals, and these can (and do) interfere with radio astronomy. This interference is only getting worse. A radio telescope on the far side of the Moon would have all of this manmade radio noise blocked.
So, it would seem that the surface of the Moon would not have any advantages for situating any but a radio telescope on the lunar far side. This makes the Italian proposal, if it is true, somewhat surprising and confusing. For those who know me, you’ll find that this is a reversal of my position on a lunar observatory from 10 years ago, when I argued that the Moon would be a great place for an observatory. The technology, though, has advanced to the point that an orbital telescope would likely be much better, with the exception of perhaps a lunar radio telescope, or perhaps a VERY large telescope that would be too large to manage as an orbital instrument. This is not the Italian proposal, though.
As a footnote, though, it is interesting to note that there actually has been a telescope on the Moon. In 1972, Apollo 16 astronauts set up a small 3 inch ultraviolet telescope which was used to observe several celestial objects. To date, this was the only astronomical telescope to be used on the Moon, and it was used for such a short period of time that moon dust was not a major issue.
-Astroprof
Maz on August 28, 2006 at 6:31 am: 1
Dear Astroprof,
I thoroughly enjoyed reading your article. I have been researching the issue of lunar observatories for some time and you have summed up the arguments so well…plus given me more information about the nature of lunar dust. Do you know if anything of note was achieved by the UV telescope of Apollo 16?
Regards, Maz
Astroprof on August 28, 2006 at 8:24 am: 2
The Apollo 16 Far-Ultraviolet Camera/Spectrograph was the first instrument of its type in space, so most of what it did was significant. It was used to take far UV spectra of the Large Magellanic Cloud, map locations of far UV sources in the sky, and to study the Earth’s upper atmosphere. None of these measurements were possible from the ground due to UV extinction in Earth’s atmosphere. Really that is a lot, when you consider that the instrument was in operation for only a few days.
Maz on September 2, 2006 at 1:28 am: 3
Dear Astroprof,
I can’t find Lyman Spitzer’s paper, but every reference to it suggests that he was proposing space telescopes. Did he actually suggest the Moon or just to be out of the Earth’s atmosphere?
Thanks
Astroprof on September 2, 2006 at 12:21 pm: 4
Actually, I think that you are right. The oritinal paper (which is VERY hard to get hold of, since it wasn\’t published in a journal) only suggested telescopes outside of Earth\’s atmosphere. But, I believe that the talk quickly came to implicitly assume a lunar based observatory, an idea that I don\’t know that Spitzer readily embraced, and very quickly dropped when it became clear that satellites could be sufficiently stabalized.