MESSENGER's Final Flyby of Mercury: Old Territory, New Science

MESSENGER is fast approaching the third and final Mercury flyby during its seven-year journey to the innermost planet. On September 29 at 21:55 UTC, it will sweep within 228 kilometers of Mercury's surface. The goal of the flyby is to slow, shrink, and tilt MESSENGER's orbit to more closely match Mercury's, which will permit the smallest planet to capture MESSENGER into Mercury orbit the next time they meet, on March 8, 2011.

This time around, MESSENGER will have a nearly identical viewpoint on Mercury, with the same hemisphere sunlit, as it had during its previous flyby, on October 6, 2008. "That's why we have elected to use this flyby to do a lot of targeted observations that build on the discoveries of flybys 1 and 2," principal investigator Sean Solomon said during a telephone press briefing today. The coordinated science observations are designed to examine the composition of the surface rocks and explain some of the questions raised after the previous encounters.

Their choices of targets are based upon color images acquired during flyby 2. The MESSENGER camera system consists of two cameras: the wide-angle camera, which has a field of view of 10.5 degrees and a filter wheel containing 11 different-color filters; and the narrow-angle camera, which has a field of view of 1.5 degrees (one-seventh the width of the wide-angle view) but no color capability. Processing of the wide-angle camera color data yielded maps of Mercury that show compositional variation across the surface. But the 11 camera filters only provide very coarse compositional information.

So the science team has picked nine targets with interesting color properties as seen with the wide-angle camera, and will be targeting it with the Mercury Atmospheric and Surface Composition Spectrometer, or MASCS. As MESSENGER shoots past Mercury, it will turn to stare at each of these points for 35 seconds apiece. During that stare, MASCS will gather color information in hundreds of tiny slices of the electromagnetic spectrum from the ultraviolet to the infrared, allowing scientists to study how the composition of the surface varies from point to point. At the same time, the wide-angle camera will shoot images of the chosen targets through all eleven filters.

On approach to the encounter, they'll also gather images that will fill in the last longitudinal slice of Mercury not yet imaged by any spacecraft, amounting to about five percent of Mercury's surface. Once this third flyby is complete, "we'll be missing only some polar areas," Solomon said.

Just prior to the MASCS targeted observations, there will be another unusual observation performed by MESSENGER's Neutron Spectrometer. The Neutron Spectrometer detects neutrons emitted from Mercury's surface, which arise from bombardment by cosmic rays. Some of these neutrons are slowed by collisions with other elements. Neutrons that have interacted with iron and titanium escape Mercury at a speed of about 2.5 kilometers per second, while the spacecraft will be moving at a much higher speed, about 5 kilometers per second, from west to east. As the spacecraft flies across Mercury, it will sweep the Neutron Spectrometer back and forth, using the Doppler shifting of the observed speed of the neutrons as a sensitive probe of the relative abundance of iron and titanium. This video shows how low (blue line), medium (green), and high (red) abundances of iron and titanium in the surface would appear to the Neutron Spectrometer as MESSENGER performs its close-approach gyrations.

As MESSENGER recedes, its view of Mercury will be nearly identical to that of the second flyby. The imaging team will take the opportunity to gather a very high-resolution mosaic of the southern hemisphere; it will take a 13-by-15 grid of images to cover the map. This high-resolution mosaic will complement a high-resolution northern hemisphere mosaic gathered during flyby 2.

Fewer than six hours after closest approach, after taking one wide-angle color mosaic and two further full-disk narrow-angle camera mosaics, MESSENGER will begin returning data to Earth, beginning with selected high-priority images. Transmission of those high-priority images will be complete by 6:40 p.m. eastern time on September 29; it will likely be a long night for the MESSENGER mission team at the Applied Physics Laboratory in Maryland!

Data relay will continue as MESSENGER carries on science observations. The very last set of planned observations is a wide search for satellites of Mercury. MESSENGER will have a low-phase view of Mercury and any undiscovered moons as it leaves the scene. Three satellite searches should uncover satellites down to 100 meters in diameter.

Following is a moment-by-moment timeline of MESSENGER's science observation plans for the flyby. The times are given according to the spacecraft's clock; during the encounter, the one-way light time for transmissions to travel from MESSENGER to Earth is 6 minutes, 37 seconds. "Dynamic visualization" links take you to the appropriate moment in the MESSENGER mission's Mercury Flyby 3 Visualization Tool. That tool only has a real-time view for the near-encounter phase; for a longer-term real-time view of MESSENGER's flyby activities, visit Daniel Muller's MESSENGER Real-Time Simulation. 

MESSENGER's Mercury encounters 

Flyby 1: January 14, 2008
» Sun illuminates 95 to 275°E 
Flyby 2: October 6, 2008 
» Sun illuminates 273 to 93°E
Flyby 3: September 29, 2009
» Sun illuminates 270 to 90°E
Mercury Orbit Insertion: March 8, 2011

The encounter will slow and shrink MESSENGER's orbit around the Sun and match MESSENGER's orbit inclination to Mercury's. The previous flybys have already shrunk MESSENGER's orbit substantially. During the first and second flybys, MESSENGER's speed relative to Mercury was so fast that the trajectory only barely bent as MESSENGER sped past. This time around, Mercury's gravity will bend MESSENGER's path 49.5 degrees. One result of the greater turn angle is that MESSENGER will get two looks from two different angles of the same territory on Mercury under the same lighting conditions in one flyby.

This is MESSENGER's last encounter with Mercury before it enters orbit on March 8, 2011. Its science orbit will be an elliptical one, with its periapsis at about 200 kilometers' elevation over a point at 60 degrees north latitude and its apoapsis at an elevation of more than 15,000 kilometers. As a result, MESSENGER will get a much closer look at Mercury's northern hemisphere than its southern hemisphere during the operational orbit. The science mission at Mercury will last one year, covering slightly more than two Mercury solar days (one Mercury solar day is 176 Earth days). Much as flyby 3 will perform detailed observations that build upon the global mapping performed in flyby 2, MESSENGER will use the second Mercury solar day of observations to perform focused studies that target interesting spots observed during the first solar day of global mapping.