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View Full Version : Galileo and parallax



slang
2009-Mar-17, 10:39 PM
Galileo was active in orbit around Jupiter for quite a bit longer than half a Jovian year. This, obviously, gives a lot bigger baseline for parallax than the Earth half year. Has any effort been made to try to take advantage of this situation to get better parallax measurements to 'local' stars, or were the available instruments not precise enough to match or improve upon Earth based/orbiting instruments? Might something be done with Cassini, even though it probably won't be around for a half Saturn year?

tusenfem
2009-Mar-18, 08:35 AM
I think that except for a star sensor to check the attitude of the spacecraft there was no instrument measuring stars. It was afterall a magnetospheric/planetary mission.

slang
2009-Mar-18, 09:38 AM
Thanks, tusenfem. Earlier I found this page (http://www.mindspring.com/~feez/) on science done with the star scanner, and no mention there of parallax. I was wondering if any of the other optical instruments could have been used (even serendipitously) for that purpose, but I don't know enough about the parallax measuring process to estimate if a search of old images could be useful. Most likely not, or someone else would have thought of it already. :)

StupendousMan
2009-Mar-18, 11:09 AM
Galileo was active in orbit around Jupiter for quite a bit longer than half a Jovian year. This, obviously, gives a lot bigger baseline for parallax than the Earth half year. Has any effort been made to try to take advantage of this situation to get better parallax measurements to 'local' stars, or were the available instruments not precise enough to match or improve upon Earth based/orbiting instruments? Might something be done with Cassini, even though it probably won't be around for a half Saturn year?

The star sensors aboard these spacecraft are designed to detect bright stars (typically down to mag 5 or 6) and measure their positions to an arcsecond or so. That's sufficient to orient the spacecraft so that it can point its antenna towards Earth, put solar arrays in the proper direction, etc.

In order to make good parallax measurements, one needs to go fainter and measure positions with precisions of, say, 0.01 arcsecond. That requires a different optical arrangement and perhaps a different type of sensor.

Good idea, but too far from the main purpose of the star sensor to be practical.

For references, try

http://adsabs.harvard.edu/full/2000ESASP.425..279B
http://ieeexplore.ieee.org/iel5/7/21750/01008988.pdf

slang
2009-Mar-18, 11:03 PM
Good idea, but too far from the main purpose of the star sensor to be practical.

I was afraid of that.


For references, try

http://adsabs.harvard.edu/full/2000ESASP.425..279B
http://ieeexplore.ieee.org/iel5/7/21750/01008988.pdf

Thanks. I can access the first one, and it seems very interesting at first glance. Good old Rosetta, only a few more years to go.