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

[Greg Hullender]

Have a look at chapter 2 from "The Voyage of Mariner 10"

http://history.nasa.gov/SP-424/ch2.htm

"Early in 1970, Guiseppe Colombo of the Institute of Applied Mechanics in Padua, Italy, who had been invited to JPL to participate in a conference on the Earth-Venus-Mercury mission, noted that in the 1973 mission the period of the spacecraft's orbit, after it flew by Mercury, would be very close to twice the period of Mercury itself. He suggested that a second encounter with Mercury could be achieved. An analytical study conducted by JPL confirmed Colombo's suggestion and showed that by careful choice of the Mercury flyby point, a gravity turn could be made that would return the spacecraft to Mercury six months later."

Sounds as though even the delta-v to get into the 2x orbit was hard to come by.

--Greg

[JRehling]

Have a look at chapter 2 from "The Voyage of Mariner 10"

http://history.nasa.gov/SP-424/ch2.htm

"Early in 1970, Guiseppe Colombo of the Institute of Applied Mechanics in Padua, Italy, who had been invited to JPL to participate in a conference on the Earth-Venus-Mercury mission, noted that in the 1973 mission the period of the spacecraft's orbit, after it flew by Mercury, would be very close to twice the period of Mercury itself. He suggested that a second encounter with Mercury could be achieved. An analytical study conducted by JPL confirmed Colombo's suggestion and showed that by careful choice of the Mercury flyby point, a gravity turn could be made that would return the spacecraft to Mercury six months later."

Sounds as though even the delta-v to get into the 2x orbit was hard to come by.

--Greg

It definitely was. My question stems from the fact that a 3x orbit should have been easier (less delta-v, anyway) AND scientifically more desirable.

Basically, Earth (and therefore the spacecraft before it was launched) is in a 4x Mercury-period orbit. It takes more delta-v to get the probe down to 2x than just down to 3x.