I'm a little late getting to this piece of news, but it's still interesting. Last week, the Mars Odyssey team announced that their mission -- the oldest spacecraft still operating at Mars, having been there since 2001 -- is being extended another two years. This mission extension will be slightly different from previous mission phases due to a planned change in the spacecraft's orbit.
First, a little background. Like Mars Global Surveyor and Mars Reconnaissance Orbiter (but not Mars Express), Mars Odyssey is in a nearly circular, nearly polar orbit. The fact that the orbit is not quite polar means that Mars' equatorial bulge exerts a pull on the plane of the orbit, slowly rotating the plane of the orbit around Mars. The inclination of the orbit is chosen to precisely match the mean motion of the Sun across Mars' sky. What this neat trick does is to time the orbit so that Mars Odyssey looks down on all parts of Mars at roughly the same local time of day (that is, the angle of solar illumination is the same for a given latitude), so it's usually called a "sun-synchronous orbit." Actually that's not quite accurate; it would be true if Mars had no seasons and had a circular orbit. The tilted axis and elliptical orbit means that there is some variation in illumination angle over the course of a Mars year.
Ignoring those effects, Odyssey has been in a "5 pm orbit" for pretty much its entire mission, meaning that it crossed the sunlit side of Mars at a local time of roughly 5:00 in the evening, and the night side at about 5:00 in the morning. 5 pm is fairly late in the day, so there are relatively long shadows on the ground at that time -- at least the shadows are much longer than they would be nearer noon. This is fine for imaging if you want to get a nice photo map of the planet, which was one of Odyssey's mission goals, because the shadows help you intuit the shape of topography from a two-dimensional image.
However, a late afternoon orbit lousy for compositional mapping. For compositional mapping, where you take images of the surface through many different color filters and see how the brightness of the surface varies from place to place and from filter to filter, the best type of illumination to have is straight overhead, so that there are no cast shadows. A good illustration of this comes from some of the recent releases from a different planet, Mercury; the MESSENGER team has posted a couple of photos comparing views of the same part of Mercury from MESSENGER and from Mariner 10, where the main difference between the images is the illumination angle.
Apparently, Odyssey has been in this 5 pm orbit for so long because one component of its Gamma Ray Spectrometer (GRS) needed a late-in-the-day orbit to avoid overheating. The Odyssey team is essentially dropping the use of that part of GRS in order to shift Odyssey eastward, to a 3:00 pm-ish orbit. The earlier orbit will decrease the shadows on the surface, but it'll have a more important benefit for Odyssey's other main instrument suite (the Thermal Emission Imaging System, or THEMIS) that I hadn't previously appreciated.
THEMIS images Mars both during the day and at night, when it still sees the surface quite well by the heat radiation that emanates from it. THEMIS is at its best when the contrast between the daytime and nighttime measurements is at its height, which is apparently true at 3 pm / 3 am. THEMIS can still gather data in the 5 pm orbit, but its principal investigator, Phil Christensen, is quoted as saying that "Many of THEMIS' most significant scientific results have come from data collected during the first six months after we arrived at Mars in late 2001," when Odyssey was in a 4 pm orbit. He goes on to say that THEMIS will just "work better" in the new orbit. So, look out -- by late next year, we should begin to see the first conference presentations from a THEMIS instrument that has a new lease on life.
Interestingly, the earlier orbit may produce some ripple effect across other Mars missions. Odyssey is the orbiter that is most often used by the Mars Exploration Rovers to relay their data back to Earth. At present, with Odyssey passing overhead near 5 pm (give or take, depending upon how far east or west the ground track of the nearest-to-overhead orbit happens to fall on a given day), the rovers currently end their working days with a communications pass. But if Odyssey passes overhead at the warmest time of day -- just when the rovers are most likely to want to be doing activities like driving -- then the rovers will have to halt activity in order to do communications sessions.
I sent a question about this to JPL, and project manager John Callas was kind enough to reply. He said that, "At some point as the [communications] pass moves earlier, we will add in activities after the pass, before the end of the rover's day." This is OK if those after-pass activities are things like imaging, but problematic for drives; if the rover were to drive after the end-of-day communications pass, then Earth wouldn't have up-to-date information on the rover's position to plan the next drive. And remember that Opportunity has just embarked on a very long driving campaign -- might the earlier Odyssey passes prevent Opportunity from reaching its maximum potential drive time?
In answer to this, John said, "the earlier passes may affect Opportunity's drive duration. Not all drives will run up against the UHF pass, but the long ones would. One option for the project is to move the morning HGA pass earlier and start any long drives earlier." The "morning HGA pass" refers to the commands that are sent to the rover, direct from Earth to its high-gain antenna, so they aren't affected by the timing of Odyssey's orbit.