Mercury Science |
Mercury Science |
Nov 16 2005, 02:28 PM
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Senior Member Group: Members Posts: 2454 Joined: 8-July 05 From: NGC 5907 Member No.: 430 |
Astrophysics, abstract
astro-ph/0511419 From: Stan Peale [view email] Date: Mon, 14 Nov 2005 22:21:56 GMT (314kb) The proximity of Mercury's spin to Cassini state 1 Authors: S. J. Peale Comments: 23 pages,12 figures, In press in Icarus In determining Mercury's core structure from its rotational properties, the value of the normalized moment of inertia, $C/MR^2$, from the location of Cassini 1 is crucial. If Mercury's spin axis occupies Cassini state 1, its position defines the location of the state. The spin might be displaced from the Cassini state if the spin is unable to follow the changes in the state position induced by the variations in the orbital parameters and the geometry of the solar system. The spin axis is expected to follow the Cassini state for orbit variations with time scales long compared to the 1000 year precession period of the spin about the Cassini state because the solid angle swept out by the spin axis as it precesses is an adiabatic invariant. Short period variations in the orbital elements of small amplitude should cause displacements that are commensurate with the amplitudes of the short period terms. By following simultaneously the spin position and the Cassini state position during long time scale orbital variations over past 3 million years (Quinn {\it et al.}, 1991) and short time scale variations from JPL Ephemeris DE 408 (Standish, 2005) we show that the spin axis will remain within one arcsec of the Cassini state after it is brought there by dissipative torques. We thus expect Mercury's spin to occupy Cassini state 1 well within the uncertainties for both radar and spacecraft measurements, with correspondingly tight constraints on $C/MR^2$. http://arxiv.org/abs/astro-ph/0511419 -------------------- "After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance. I see that such intercourse long continued would make one thoroughly prosaic, hard, and coarse. But the longest intercourse with Nature, though in her rudest moods, does not thus harden and make coarse. A hard, sensible man whom we liken to a rock is indeed much harder than a rock. From hard, coarse, insensible men with whom I have no sympathy, I go to commune with the rocks, whose hearts are comparatively soft." - Henry David Thoreau, November 15, 1853 |
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Jan 31 2011, 03:36 PM
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Member Group: Members Posts: 903 Joined: 30-January 05 Member No.: 162 |
It is interesting, now that we are on the verge of Messenger orbiting Mercury, to contemplate the Mariner 10 mission in a hypothetical 3X88 orbit.
Let's keep the geometry of the first flyby the same. One big difference would be the encounter speed. To come back after 264 days, instead of 176, Mariner 10 would need to go faster to climb further away from the sun. Mercury is well lit, so spacecraft motion won't blur the pictures, but with a faster flyby, we have less time to take all the pictures. This would probably cut the # of close range pictures, and probably reduce the extent of the mosaics. The 2nd flyby, if at a similar 50,000 km sunward direction, would have imaged the opposite hemisphere. Foreshortening along the terminator in the pictures would have made it harder to stitch the mosaics together, however, the Caloris basin would have been seen in it's entirety (in 2 strips) , and the outer ring would have helped align them. We might have had some coverage of 75 to 80 % of Mercury, but with lower resolution along the edges of the covered areas of the 2 flybys. If a second flyby could have had a geometry similar to the actual first or 3rd Mariner 10 flyby, we would have seen the west adjacent area of Caloris on approach, and on departure, we would have seen the eastern adjacent area of the first flybys approach hemisphere. We would have 4 large 'stripes' of coverage of Mercury. Stitching the 4 bands together would be tricky, all the adjoining areas on the limb would be foreshortened, and all the terminator seams would have illumination from 180 degrees around. This would have been an interesting flyby. We would have perhaps double the areal coverage, but with less of an overall global context. I would be cautiously optimistic that perhaps Mariner 10 might have still accomplished 3 flybys. Due to the loss of attitude gas, Mariner 10 used the solar panels to help control the spacecraft attitude between the flybys. This technique would still work in the 264 day orbit, so maybe, just maybe, they could have squeezed a third pass out of the old girl. Another problem however, was the thermal effects on the craft. In the 3X orbit, Mariner 10 would have been a lot further out at perihelion. The antenna feed had a temperature sensitivity problem, and this might have exacerbated that severely. This problem has the potential to drastically cut the data rate of the antenna system. Also, the on board tape recorder would still have been lost, and with the high flyby speed, this would seriously impact the amount of data returned. All in all, the mission as flown was most excellent. Given the twitchiness of the craft, I bet the controllers and flight personnel were relieved they did not have the 3X orbit, but I bet they could have still done a magnificent mission if they had. |
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