QUOTE (elakdawalla @ Sep 22 2009, 10:51 AM)
I don't think so. According to my notes, it's flyby 2 and 3 that had the same geometry:
The two maps you point to are just centered on different longitudes.
Oops!
QUOTE (elakdawalla @ Sep 22 2009, 10:51 AM)
I think maybe the reason the approach crescents are different is because on flyby 2 MESSENGER was coming in from a direction farther from the Sun -- its orbit has shrunk since then. But I don't have the best instinct for orbital geometry. If I don't think of a better question to ask, I may ask this one tomorrow.
Actually, taking a look at the plots of the orbit, I think I can see it now:
This was flyby #2
http://messenger.jhuapl.edu/the_mission/ME...rthPoleFull.jpgAnd this is flyby #3
http://messenger.jhuapl.edu/the_mission/ME...rthPoleFull.jpgThe orbit "bends" a good bit more this time -- which makes sense, as Messenger is losing energy with each flyby. It's probably just a coincidence that the outbound path seems to be more or less the same (with respect to the planet surface) as it was for flyby #2. Especially since this flyby seems to be about 25 degrees further around Mercury's orbit than the last two were.
http://messenger.jhuapl.edu/the_mission/trajectory.htmlShoot. Just as I typed all this, the Messenger folks released this:
QUOTE
Because the spacecraft velocity relative to Mercury is about one-third slower at Mercury flyby 2 than at Mercury flyby 2, the gravity-assist turn angle to the spacecraft’s trajectory increases from about 27° to nearly 50°,” explains Jim McAdams, the MESSENGER mission design lead engineer. “This greater bend in the trajectory provided by the gravity of Mercury offers the spacecraft its first opportunity to view a small portion of Mercury’s surface twice with different vantage points and nearly identical lighting conditions just a few hours apart.
So I guess that's the answer. I'd still like to know exactly what percentage of Mercury will remain unimaged after this flyby . . .
--Greg