Emily LakdawallaSep 23, 2009

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.

Imaging plans for MESSENGER's third Mercury flyby
Imaging plans for MESSENGER's third Mercury flyby Top: On approach, MESSENGER will capture a view of a crescent Mercury, including a strip of longitude not before imaged by any mission. Middle: near closest approach, MESSENGER's spectrometer (MASCS) and the wide-angle camera will collaborate to acquire targeted observations of interesting targets. Bottom: As it departs, MESSENGER will gather a huge mosaic covering the southern hemisphere, complementing a northern-hemisphere view captured from a similar distance and lighting geometry during flyby 2.Image: NASA / JHUAPL / CIW

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.

Targeted observations during the third MESSENGER flyby of Mercury
Targeted observations during the third MESSENGER flyby of Mercury

Because the geometry of flyby 3 was so similar to flyby 2, MESSENGER scientists were able to use flyby 2 observations to plan a series of detailed observations just after closest approach on flyby 3. This map shows how the MASCS spectrometer and wide-angle camera will track across the surface to examine nine chosen interesting locations:
A: An unnamed crater with unusual bright material on its floor.
B: An unnamed crater with a set of young ejecta rays that are light blue in the enhanced-color view.
C (1 & 2): Crater Lermontov. The bright yellowish color in the enhanced-color image and the irregularly shaped depressions on its floor may be evidence of past explosive volcanic activity.
D: North of crater Homer. An area with an interesting mix of both light blue ejecta and bright orange materials.
E: Near crater Titian. Enhanced-color images show a region of comparatively deep blue material that is dark and of a different composition than the majority of Mercury's surface.
F (1 & 2): Common plains material. This target resembles a type of material that covers much of Mercury.
G: An unnamed crater with an intriguing bright yellow-orange color in enhanced-color views.
H: Ray material from a spectacular rayed crater in Mercury's north. The ray material appears bright blue in enhanced color.
I: A common area of Mercury's surface that will be viewed by MASCS very obliquely.


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.

Imaging of Mercury during MESSENGER's third flyby
Imaging of Mercury during MESSENGER's third flyby As MESSENGER approaches for its third flyby of Mercury, it will capture images covering the last major swath of the planet not previously imaged by any mission. On its way out, its view will be very similar to the one it had during flyby 2. After the third flyby is completed, only the poles will remain unimaged by MESSENGER.Image: NASA / JHUAPL / CIW

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

time (UTC)
Time with
to closest
Sep 22
-183h 35mStart approach imaging campaign
Sep 25
-108h 00mFirst camera image for public release
20:20-97h 35mStart continuous Deep Space Network coverage of encounter
Sep 27
-55h 18mComplete Gamma-Ray Spectrometer cool-down, configure for gathering science data
14:55-55h 00mSecond camera image for public release
22:52-47h 3m

Enable X-Ray Spectrometer Gas Proportional Counters

Sep 28 
-44h 9mThird camera image for public release
12:45-33h 10mFourth camera image for public release
14:24-31h 31mTurn to start encounter imaging sequence
With this turn the spacecraft's high-gain antenna will no longer be pointed at Earth, but the DSN will still be able to monitor MESSENGER through tracking of a beacon signal broadcast from its low-gain antenna.
14:55-30h 59mBegin scanning solar tail with Mercury Atmospheric and Surface Composition Spectrometer (MASCS)
Sep 29
-14h 59mEnd approach imaging campaign
10:55-11h 00mApproach wide-angle camera color imaging sequence #1
14:19-07h 36mComplete MASCS solar tail scanning; begin MASCS pole scans
15:30-06h 25mConfigure Fast Imaging Plasma Spectrometer to burst-data collection mode and Energetic Particle and Plasma Spectrometer to high sensitivity mode
15:55-06h 00mConfigure Magnetometer to high sensitivity mode, and GRS and NS to medium sensitivity mode
16:27-05h 28mComplete MASCS pole scans; begin near-distance solar tail scans
16:55-05h 00mApproach wide-angle camera color imaging sequence #2
18:35-03h 20mApproach wide-angle camera color imaging sequence #3
19:25-02h 30mApproach wide-angle camera color imaging sequence #4
19:46-02h 09mApproach wide-angle camera color imaging sequence #5
Dynamic visualization >
19:52-02h 03mConfigure X-Ray Spectrometer to medium sensitivity mode
20:36-01h 19mApproach narrow-angle camera mosaic 
A 2-by-3-footprint mosaic will cover the northern "horn" of the crescent; a 5-by-10 mosaic will cover most of the sunlit crescent; a final 2-by-3 mosaic will get the southern horn. Dynamic visualization >
20:55-01h 00mConfigure Gamma-Ray Spectrometer and Neutron Spectrometer to high sensitivity mode
21:01-00h 54mConfigure Laser Altimeter for science gathering mode
21:40-00h 15mComplete MASCS solar tail scans; start near-terminator exospheric observation sequence
21:41-00h 14mEclipse entry 
For 18 minutes, MESSENGER will be hidden from the Sun.
21:42-00h 13mStart spacecraft slew
The spacecraft will turn to point its instrument deck at Mercury's surface, simultaneously sweeping the Neutron Spectrometer across Mercury.
21:49-00h 06m

Laser Altimeter start firing laser 
Dynamic visualization >

21:51-00h 04mComplete MASCS exospheric observation sequence
21:52-00h 03mConfigure X-Ray Spectrometer to high sensitivity mode
21:55-00h 00mClosest approach to Mercury 
MESSENGER will pass within 228 kilometers of the surface, firing its laser all the time.
21:59+00h 04mEclipse exit
22:03+00h 08mOccultation entry
MESSENGER will pass behind Mercury as seen from Earth, causing a 52-minute interruption in radio communications beginning at 15:09 PDT.
22:06+00h 11mClose-approach Neutron Spectrometer sweep
22:07+00h 12mComplete laser ranging
22:10+00h 15m

Start combined surface targeting campaign 
Dynamic visualization >

time (UTC)
Time with
to closest
22:24+00h 29mStart dayside MASCS exospheric observation sequence
22:30+00h 34mNarrow-angle camera departure mosaic #1 (southern hemisphere, 210-330 m/pixel) 
This 13-by-15-footprint mosaic will complement a high-resolution northern-hemisphere NAC mosaic captured after flyby 2. Dynamic visualization >
22:52+00h 57mWide-angle camera color departure mosaic (2.25-2.6 km/pixel)
A similar observation was performed during flyby 2. Dynamic visualization >
22:54+00h 59mOccultation exit 
MESSENGER should regain contact with Earth at 16:00 PDT.
22:55+01h 00mConfigure Neutron Spectrometer for medium sensitivity mode, GRS for low sensitivity mode
23:06+01h 11mNarrow-angle camera high-resolution departure mosaic #2 (400-490 m/pixel) 
The 11-by-13 mosaic will cover the entire visible globe, resulting in the highest-res-yet full-globe image of Mercury. Dynamic visualization >
23:35+01h 40mNarrow-angle camera high-resolution departure mosaic #3 (580-600 m/pixel) 
The 8-by-10 mosaic will cover the entire visible globe. Dynamic visualization >
23:52+01h 57mConfigure X-Ray Spectrometer to medium sensitivity mode
30 Sep
+02h 05mNarrow- and wide-angle camera departure frames (0.675 and 4.75 km/pixel) 
MESSENGER is now more than 26,000 kilometers from Mercury, and the globe fits within the wide-angle camera field of view. Dynamic visualization >
03:32+05h 37mStart data playback
The spacecraft will continue to gather data even as it begins to play back the highest-priority images.
03:55+06h 00mConfigure Magnetometer, Neutron Spectrometer, and Energetic Particle and Plasma Spectrometer to low sensitivity mode
04:25+06h 00mConfigure Fast Imaging Plasma Spectrometer to normal data collection mode
04:34+06h 39mComplete playback of high-priority images 
The highest-priority data should be on the ground at 21:40 PDT on Tuesday, September 29.
08:55+11h 00mNarrow- and wide-angle camera departure frames
09:55+12h 00mStart post-flyby imaging campaign
15:23+17h 27mComplete playback of all non-imaging science data through closest approach + 17 hours 
The CA+17 hour data will be on the ground as of 08:29 PDT on Wednesday, September 30.
22:16+24h 21mStart search for Mercury satellites using wide-angle camera
Oct 1
+28h 00mStart second search for Mercury satellites using wide-angle camera
04:56+32h 00mComplete playback of all non-imaging science data through closest approach + 30 hours 
The CA+30-hour data will be on the ground as of 22:02 PDT on Wednesday, September 30.
10:55+37h 00mConfigure Gamma-Ray Spectrometer to non-science mode
22:52+48h 57mDisable X-Ray Spectrometer Gas Proportional Counter Sensor
Oct 2
+51h 24mEnd continuous DSN coverage
08:28+58h 33mStart third search for Mercury satellites using wide-angle camera

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MESSENGER's journey to Mercury MESSENGER's trip to Mercury requires a total of six gravity assists (one of Earth, two of Venus, and three of Mercury) to permit it to enter orbit at the small planet close to the Sun. This animation shows that journey and the motions of Venus and Mercury using a frame of reference that holds the Earth-Sun line fixed.Video: NASA / JHUAPL / Carnegie Institution of Washington

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.

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