DIETZ@slb-test.CSNET ("Paul F. Dietz") (11/21/86)
Additional comments on TAU and astrometric scopes... There's an astrometric scope, Hipparchos, that will be able to get down to 1 to 2 milliarcseconds accuracy (as will the HST). That should be good enough to determine distances of stars out to perhaps a thousand light years. After that, the Astrometric Telescope Facility (New Scientist, 11/13/86) will have an accuracy approaching 1 microarcsecond (!), which should, for bright objects, be able to measure distances out to perhaps a million light years, detect "Jupiters" out to thousands of light years and "Earths" about nearby stars. [Short editorial: they plan to mount the ATF on the space station. This seems to me to be an incredibly stupid idea. Do they really expect microarcsecond pointing accuracy with astronauts banging around inside, shuttles docking, spacewalking astronauts firing nitrogen gas all over the place, etc., even with a (no doubt expensive) vibration isolation system? Once again, valid scientific projects are being perverted to help support needless human activity in space. I hope the ATF is being designed so it can fly free also.] It would seem that the ATF (or a larger version thereof) could perform the major task being touted for TAU: calibration of the distance scale used in computing the Hubble constant. This should not be suprising. The idea of fly-by interstellar probes has always struck me as pretty silly. The farther the probe has to go, the bigger the advantage of stay-at-home space telescopes. A solar-system-wide microwave interferometer, for example, has the entire universe in its near field, returns data almost immediately, is cheaper, and can be used on more than one target. Also, a comment was made that, even if TAU is passed by faster spacecraft, it will give valuable experience with its propulsion system. This is true only if nuclear ion engines are not a dead-end technology. I would think that radically different systems, such as nuclear pulse rockets, would be the long term choice (Hyde would say that long term = 30 years).
Purtill@MIT-MULTICS.ARPA (Mark Purtill) (11/22/86)
<Fnord> >Also, a comment was made that, even if TAU is passed by faster >spacecraft, it will give valuable experience with its propulsion system. >This is true only if nuclear ion engines are not a dead-end technology. >I would think that radically different systems, such as nuclear pulse >rockets, would be the long term choice (Hyde would say that long term = >30 years). Nuclear ion engines will certainly be a dead-end technology if we never try to build one. And if we always wait for the "better" thing that will be around in only a few decades, we'll never get anything done, 'cause there will always be something else on the horizon. Mark ^.-.^ Purtill at MIT-MULTICS.ARPA **Insert favorite disclaimer here** ((")) 2-229 MIT Cambrige MA 02139