Mercury Science Options

[ljk4-1]

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

[JRehling]

A question that's nagged me for a long time...

Mariner 10 was in a solar orbit that intersected Mercury's orbit in resonance with the planet, so that it revisted Mercury multiple times (the spacecraft was actually alive for the first three; presumably more encounters, if farther ones, continued to take place after the craft died).

The rub was that Mercury's rotation is also in resonance, so that the same face of Mercury was lit at all three encounters. This is because the orbital period of Mariner 10 was 2 Mercury "years", and the 3/2 resonance of Mercury's revolution and rotation meant that it rotated precisely twice while Mariner 10 and the planet were apart.

The delta-v budget for getting to Mercury was a limiting factor, so the mission could not have gotten into any old Mercury-crossing orbit we wished, but to me, the ten million dollar question is: Why not put it into a solar orbit with a LONGER period that intersected Mercury's orbit at the same location every THREE Mercury "years"? If that had been the mission profile, the likely outcome would have been that the craft would have only survived two such flybys (as it was, it was limping into the third), but they would have shown opposite sides of the planet, giving us an almost full global map, instead of the half-planet coverage we've had to live with for the last 33 years.

In terms of pure delta-v, the longer period should have been easier to achieve. Unless the gravity assist with Venus had some stringent requirements that forbade the longer period, it seems like it would have been a win-win scenario to take the longer period and the full coverage.

This is now a moot point in many ways, but it's stuck in my craw for a long time. Any ideas?