Dale.Amon@H.CS.CMU.EDU.UUCP (11/22/86)
I find myself in disagreement with Paul on the value of an interstellar flyby, but he does bring up an interesting point. Let us assume there exists an earth like planet around another star with interesting structures on it's surface, be they water, mountains, cities, or what have you. What are the theoretical limits to the distance at which they can be seen by an optical scope? Are there quantum effects that will limit resolution? How much effect does intersteller dust and gas between us and the object have on theoretical resolution? What is the relationship between the scope size and the distance to the interesting surface feature? Given that a scope could be built that could detect a city or town at a distance of Alpha Centauri, what are the cost comparisons? I don't expect to find a city anywhere nearby, but I expect that things such as cratering and surface history of objects around other stars, particularly of varied spectral class, would tell us very interesting general things about planetary evolution, and would no doubt give us some surprises. Is plate tectonics common? Is it truly dependent on the presence of oceans? Is a large moon(s) helpful in driving it? My gut feel is that it might be easier to build the probe. Not to mention, it is as good a test bed for really advanced engines as just about anything, and will probably grab the 21st century imagination because it will prove that IT CAN BE DONE. Once a probe proves it, it is only a matter of time (within 50-100 years of the probe pictures coming back) before people do it. But then, the people might well pass it up, as someone noted about obsolete propulsion systems. Gary: Too bad about the solar system picture. Maybe we need a high inclination shot to get the inner solar system from 10 AU's or so over the solar pole. Maybe ISPM could get an interesting shot, if it had cameras. (Although it is not going to be all that high above the ecliptic)
Lynn.ES@XEROX.COM.UUCP (11/24/86)
In reply to Dale Amon's questions "What are the theoretical limits to the distance at which they [planetary details] can be seen by an optical scope? What is the relationship between the scope size and the distance to the interesting surface feature?" The theoretical limits for optical telescopes: resolution size = distance/(diam * 46000) where telescope diam is in inches, and the distance and size are in any units, so long as they are the same units. A single object has to be at least the resolution size to detect that it has any size at all. A smaller object that is bright is easily seen (otherwise the night sky would be blank except for the moon), but is seen as a dot that appears the size of the theoretical resolution. An object would have to be twice the resolution size to see the grossest detail (for example that the left side is black and the right white). Two objects of equal brightness have to be separated by the resolution size to even see that they are not a single object. If one object is much brighter (as in a star with an orbiting planet), then the separation may have to be tens or hundreds of times greater, else the brighter one swamps the dim one with a spread-out overexposed image. Putting real numbers in, say the largest optical telescope on earth and the nearest star, we get resolution of 2.4 million miles. But no telescope of substantial size on the ground achieves its theoretical resolution (because the atmosphere messes it up), and we haven't launched anything of substantial size above the atmosphere. The space telescope will be 2.5 times worse than these numbers (because its size is 2.5 times smaller than the big Russian telescope), but will achieve essentially the theoretical resolution, much better than the achieved resolution of bigger telescopes on earth. So what we are talking about with the space telescope is possibly seeing very bright planets hundreds of millions of miles from the very nearest few dozen stars. To be in the ballpark of seeing continents on planets of only the nearest star requires a telescope mirror of tens of miles diameter, something not likely in our lifetimes (historically, since Galileo, we have taken roughly 40 years for each doubling of the size of the largest telescope, and there is no indication yet that this rate is changing even with our present technology explosion). Remember that a mirror has to be VERY stiff; no point on it can move more than a few millionths of an inch from the correct curve, or you lose the resolution. The only outside hope I see is some form of interferometry, in which you use two or more mirrors, and use their separation in place of the diameter in the above formula. Or we could take a reasonable sized telescope and send it closer to the star in question; but it would require thousands of times closer, which is essentially a visit to that star. A visit to a star is energy-wise roughly 100,000 times harder than the TAU mission, the latter being barely within our technology soon. Other questions: "Are there quantum effects that will limit resolution?" No, the above formula is an effect of the wavelength and wave properties of visibile light. "How much effect does intersteller dust and gas between us and the object have on theoretical resolution?" Essentially no effect at the distance of planets of the nearby stars, which are already beyond our observing limits. These nearby stars are in a neighborhood that comprises less than 1/1000 the size of our galaxy. At a substantial fraction of the way across our galaxy, in the dirtiest directions, dust and gas become a problem. Also, there are a few small pockets of stuff at distances of hundreds of light years, but these are still beyond the nearby stars. Dust and gas do not degrade resolution, just contrast, and eventually completely block any view. In conclusion, optical detection of planetary detail is MANY orders of magnitude beyond what we can do. So don't hold your breath. I would guess rocket probes to stars will happen first, and I don't think we are close to doing that. Then we could always try finding the radio signals of some civilization that might be happy to describe their planets to us. /Don Lynn