Lynn.es@PARC-MAXC@sri-unix (12/06/82)
Very creative idea to make parallax measurements, but it needs some more work to be practical. Parallax measurements now are practically useless beyond a few hundred light years, because the measurements get smaller than the noise. The size of the measurements is proportional to baseline (normally the distance across the earth's orbit, by which photographs 6 months apart differ) and proportional to telescope focal length (or image scale). The loss in focal length between our large telescopes (tens of meters) and Voyager (I think a meter or so) loses more than its present distance gains in baseline (about 12 AU instead of 2 that we get with the earth). In fact the loss of radio contact at perhaps 50 to 100 AU will prevent us from ever getting much accuracy gain using Voyager. At least the attitude adjustment gas should not be a factor in Voyager 1, since its electrical camera platform maneuvering mechanism (which does not use the jets) is still working. As for sending out a probe with ion rocket, let's say we can get a light year away in a reasonable time. Now we are talking gains of 30,000 in the baseline. But it will take one heck of a transmitter and antenna to get the data back to us, say a million times more effective than what is on Voyager. And the baseline improvement would barely get a marginal measurement on the nearest galaxy. We would need at least another factor of 1000 to get good distances for the Hubble constant. Now indirectly it would improve measurements by cepheid variable or other means, because they are calibrated by parallax measurements of nearby objects. But this indirectness probably means only a slight increase in cepheid distance accuracy, and still won't help the intervening material problem with it. The large space telescope may be able to get us better parallax measurements by reducing the noise (atmospheric disturbances of the incoming light) rather than increasing the measurement. I would estimate the improvement to be about a factor of 10. /Don Lynn