One last edit to this entry (I promise); after a lot of back and forth about whether they were going to release the image tonight, it finally did get released. Here's my take on it.
EDIT: Sorry for all my confusion -- don't expect an image until tomorrow morning.I just chatted with Louise Prockter, Instrument Scientist for MESSENGER's camera (the Mercury Dual Imaging System or MDIS) and also my former grad school officemate, about what to expect next from MESSENGER. In short, because of circumstances having nothing to do with MESSENGER, they may only get one precious image down from the spacecraft late tonight, and it probably will not be released until tomorrow. As of yesterday, they had been expecting lots of images today, but data is not going to start coming back until tonight, and there will probably be only one image in this first communications pass.
All of NASA's spacecraft communications have to go through one of three Deep Space Network stations at Goldstone (in California), Canberra (Australia), and Madrid (Spain). Each DSN station has one 70-meter dish and a couple of 34-meter dishes. The bigger the dish, the deeper in space you can talk, and the faster your communications. As you might imagine, scheduling of the single 70-meter dish is very, very tight. MESSENGER was expecting to get a 70-meter "pass" this morning, but because of an anomaly on Ulysses that required a 70-meter antenna, MESSENGER lost their 70-meter time. (I don't know much about the anomaly on Ulysses, but a helpful member of unmannedspaceflight.com posted a link to this Ulysses Monthly Operations Summary that contains the following text from the January 15 entry: "EPC 1/TWTA 1 Switch off/on Test 1 - 015.01:18 ERT. Operational test to validate future mode of operations. Failure to re-acquire X-band downlink at the expected time. Commands to switch EPC/TWTA 1 repeated without success. S/C now configured to S-band downlink.")
So, MESSENGER lost their planned 70-meter antenna time, which means they didn't get the high-data-rate pass they were hoping for. Through some quick planning and substitution they did get some time on a 34-meter antenna. Through that communications pass they managed to downlink what's referred to as "housekeeping" data, information on the current state of the spacecraft, its instruments, and its data storage. There is great news from that housekeeping data, Louise told me: it appears that the science sequence operated to its conclusion without any incident. There is the correct number of images stored on the spacecraft, and all the instruments report being "healthy and happy," so there were no anomalies during the flyby. Hooray! As impatient as I am to see pictures, I'm happy to have received that much information at least.
She said that as of now they are expecting a communications session through a 70-meter antenna beginning at 23:30 UTC, in which they will actually be able to start bringing down some of the data from the flyby. She said that her team was told to select one image, which would be given high priority and downlinked early in the pass, followed by all the other instrument data, followed by (if time permits, which Louise seemed to doubt that it would) more image data.
She said that, given the opportunity to bring down one image, they selected a wide-angle camera observation of Mercury taken as MESSENGER was departing, in which the sunlit globe of Mercury should nearly fill the field of view, providing Earth's first look at terrain unseen by Mariner 10 at a resolution of about 5 kilometers per pixel. Checking my timeline, it seems this should be the image taken at 20:17; here's a Solar System Simulator view showing you what we should see, and the MESSENGER visualization showing you where the camera should actually be pointed. Like all the wide-angle camera shots of Mercury, MESSENGER took this one through 11 color filters, but because only one image will be returned to begin with, we won't get color to start with, just a monochrome view. She said they selected their "filter G" which approximates the same wavelengths that are sampled by the narrow-angle camera (which does not have color capability).
So they should have that image on the ground sometime very late in the evening Eastern time. NASA will probably release it in the morning -- we'll have to wait and see.
Louise wasn't sure yet what the subsequent DSN schedule will be, or when to expect the rest of the images to start coming down. I imagine it's all a bit TBD as they try to get Ulysses to start talking again, and as other missions who also lost 70-meter time jockey for priority. But they should slowly trickle in over the course of the next week or two. Hopefully the trickle will be fast enough that they can release an image a day.