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Emily Lakdawalla • Jan 14, 2008

MESSENGER has (probably) disappeared (but it's coming back)

The demands of my 17-month-old having forced me to stay in California and not travel to Baltimore to witness MESSENGER's first flyby of Mercury live from the science operations center at Johns Hopkins University's Applied Physics Laboratory, the only way I can follow the flyby is to sit at my desk and stare at the timeline, watching the minutes tick past and marking what MESSENGER should be doing now. As I write, at about 10:20 my time (18:20 UTC), MESSENGER has finished all of its approach imaging -- Mercury more than fills its field of view, for both wide- and narrow-angle cameras -- and is warming up its laser altimeter to zap Mercury's surface for one precious topographic profile through closest approach, which is now less than an hour away.

More importantly for those people following along on Earth, MESSENGER is about to disappear; at 18:22 UTC its course is going to take it behind Mercury's eastern edge as seen from Earth, and we're not going to hear from it again until after it comes from behind the western edge, at 19:30 UTC, plus the time it'll take its radio signals to travel the 170 million kilometers currently separating Mercury from Earth.

Geometry of MESSENGER's first Mercury flyby

NASA / JHUAPL

Geometry of MESSENGER's first Mercury flyby
MESSENGER's first flyby of Mercury unfolds on January 14, 2008, with closest approach at 19:04:39 UTC at an altitude of 200 kilometers. These three diagrams show three points of view on the flyby: from the north pole of Mercury (top), from the Sun (lower left), and from Earth (lower right). In all three diagrams, the ticked orange line shows MESSENGER's trajectory (the ticks are five minutes apart); the solid yellow line shows the terminator (day-night boundary); and the dashed blue line indicates the position of the terminator during all three of Mariner 10's flybys.

The top image demonstrates the new views of Mercury that MESSENGER will obtain from the flyby. The area of Mercury above and to the left of the dashed blue line is the hemisphere that was Sunlit for all three Mariner 10 flybys. The other hemisphere has only been partially imaged at much lower resolution by Earth-based radio telescopes. On this flyby, MESSENGER will bale to acquire images of not quite half of the hemisphere unseen by Mariner 10, everything above and to the right of the yellow line, which indicates position of the terminator (day-night boundary) during the flyby.

The bottom left image demonstrates that closest approach of the flyby happens on the night side of Mercury, while the spacecraft is in eclipse (Mercury's bulk blocks the Sun).

The lower right diagram shows that MESSENGER is moving generally toward Earth throughout the flyby. During the closest approach period -- including the entire eclipse period -- MESSENGER will be hidden from view, with no radio communications possible, by Mercury.

Spacecraft
time (UTC)		Time with
respect
to closest
approach		Event
Jan 13
13:04		-30h 00mWide-angle camera approach color movie
With the wide-angle camera, MESSENGER will shoot 86 frames for a movie showing Mercury's crescent growing in the spacecraft's forward view. The movie is being shot purely for public outreach purposes, through the three camera filters necessary to produce it in color; all of the rest of the color imaging during the flyby will employ all 11 of the wide-angle camera's color filters.
Jan 14
13:14		-5h 51mMagnetometer to high rate
MESSENGER's magnetometer (MAG) can take measurements of magnetic fields at a variety of rates, from once every 100 seconds to 20 times a second. For the 12 hours surrounding closest approach, it will be operated at its highest rate.
16:14-2h 51mX-ray spectrometer decreases integration times to 60 seconds
As MESSENGER approaches Mercury, the X-ray spectrometer starts taking measurements more rapidly. However, the X-ray spectrometer operates best within about 100 kilometers of Mercury, a distance it won't enjoy until MESSENGER actually goes in to orbit in 2011. Still, data from the 15 minutes surrounding closest approach will be a valuable first look at Mercury with this instrument.
16:24-2h 41mAtmospheric and Surface Composition Spectrometer (MASCS) tail sweeps
Mercury has an extended sodium tail that resembles the tails seen by comets when they are close to the Sun. As it approaches, MESSENGER will roll up and down to carry the MASCS line of sight across the tail, from north to south, in order to map its vertical extent.
17:40-1h 25mWide-angle camera approach color image (5.2 km/pixel)
Mercury will not yet fill the wide-angle camera field of view; 11 images will be taken, through all of the wide-angle camera's filters. Dynamic visualization >
18:03-1h 02mNarrow-angle camera approach mosaic (500 m/pixel)
An 11-by-5 mosaic will cover Mercury's sunlit crescent, all of which is terrain that was imaged by Mariner 10. The narrow-angle camera has no filter wheel, so all of its images are monochrome. Dynamic visualization >
18:04:00-60m 42sGamma-ray spectrometer detectors on at 60s
Similar to the X-ray spectrometer, the gamma-ray spectrometer (GRS) increases the frequency of its observations for the short time around its closest approach to Mercury.
18:19:41-45m 01sLaser altimeter to standby mode
The laser altimeter warms up in preparation for one precious altimetric profile across Mercury, to be taken around closest approach.
18:22:27-42m 15sEarth occultation entry
MESSENGER will pass behind Mercury as seen from Earth; no communications will be received from the spacecraft until it emerges from behind Mercury more than an hour later.
18:52:09-12m 33sTransition to nadir tracking
Now, close to the planet, MESSENGER will rotate so that the laser altimeter can fire downward and measure Mercury's topography.
18:53:00-11m 42sMASCS UV nightside exosphere scans
The MASCS team expects to observe differences in ultraviolet emission from Mercury's dayside and nightside; the dayside emission should be much brighter.
18:54:15-10m 27sEclipse entry
For 13 minutes, MESSENGER will be hidden from the Sun.
19:02:00-02m 42s

MLA to science mode; start firing laser
Dynamic visualization >

19:04:42-00m 00sClosest approach to Mercury
MESSENGER will pass within 200 kilometers of the surface, firing its laser all the time.
19:07:32+02m 50sEclipse exit
19:12:00+07m 18sWide-angle camera: color photometry (0.6-1.25 km/pixel)
Five sets of color images of the same area on Mercury will be snapped as MESSENGER rises quickly from behind Mercury's night side. Because of MESSENGER's motion it will see Mercury's surface illuminated at different angles, until it becomes more than half-full at a phase angle of about 55 degrees. Dynamic visualization 1 >2 >3 >4 >5 >
MASCS ultraviolet, visible, and infrared observations of the dayside surface
With a view of a largely sunlit Mercury, MASCS shifts from observations of the exosphere to measure the composition of the planet's surface.
19:18:00+13m 18sNarrow-angle camera high-resolution mosaic #1 (100-200 m/pixel)
The 4-by-17 mosaic will cover Mercury's equator, in areas unseen by Mariner 10. Because of its elliptical orbit, MESSENGER will never again get so close to this region of Mercury's equator. Dynamic visualization >
MASCS ultraviolet, visible, and infrared dayside exosphere
MESSENGER has now moved farther from Mercury than its eventual elliptical orbit will take it, and MASCS can no longer observe the surface. It will return to observing the exosphere, which should be much brighter than the nightside exosphere.
19:29:00+24m 18sNarrow-angle camera high-resolution mosaic #2 (400-500 m/pixel)
The 9-by-11 mosaic will cover the northern hemisphere, beginning with the pole and moving down to the equatorial area imaged in the previous observation. This and all other departure images mostly cover areas not seen by Mariner 10. (The westermost portion of the Mariner 10-imaged region will be on the sunlit limb.) Dynamic visualization >
19:30:33+25m 51sEarth occultation exit
Although MESSENGER has reappeared from behind Mercury, MESSENGER's radio signals will take almost ten minutes to travel the 170 million kilometers separating it from Earth, so MESSENGER mission control must wait to hear from their spacecraft.
19:31:00+26m 18sLaser altimeter back to standby mode
19:41:00+36m 18sWide-angle camera departure color mosaic (2.5 km/pixel)
A 3-by-3 mosaic of the entire visible globe of Mercury, in 11 colors. Dynamic visualization >
19:56:00+51m 18sNarrow-angle camera departure mosaic #1 (500 m/pixel)
An 11-by-9 mosaic of the entire visible globe of Mercury, at fairly high resolution, will probably be the standout image product of the flyby. The full-resolution image should require nearly 10,000 pixels to go from the north to south poles. Dynamic visualization >
20:04:00+59m 18sGamma-ray spectrometer to longer (300s) measurement interval
20:07+01h 02mNarrow-angle camera departure mosaic #2 (600 m/pixel)
A 10-by-8 mosaic. Dynamic visualization >
20:17+01h 12mWide-angle camera departure color image (4.8 km/pixel)
Mercury will have receded to the point that it fits entirely within the wide-angle camera's field of view. Dynamic visualization >
20:28+01h 23mNarrow-angle camera departure mosaic #3 (750 m/pixel)
An 8-by-6 mosaic. Dynamic visualization >
20:36+01h 31mNarrow-angle camera departure mosaic #4 (800 m/pixel)
A 7-by-6 mosaic. By the time this mosaic is captured, MESSENGER will be more than 35,000 kilometers from Mercury. The resolution of this mosaic will be only about half the resolution of the first one, but MESSENGER will be able to peek about 4 degrees farther around the sunlit limb than in the first departure mosaic. The slightly different perspective may offer the opportunity to create stereo (3D) views, when these images are combined with some of the highest-resolution observations from closest approach. Dynamic visualization >
20:42+01h 37mNarrow-angle camera departure movie (1.6 to 22 km/pixel)
At the start of this movie, Mercury will fill the narrow-angle camera field of view. A total of 288 frawmes will be shot over the next 18.5 hours at a rate of one frame every five minutes. In the final image taken during the encounter, Mercury will still be more than 200 pixels across.
22:14+03h 9mX-ray spectrometer returns to long integration time (300s)
January 15
01:04		+05h 59mNeutron spectrometer returns to low-rate mode
Fast Imaging Plasma Spectrometer returns to low-rate mode
01:14+06h 09mMagnetometer returns to low-rate mode
19:04+23h 59mEnergetic Particle and Plasma Spectrometer returns to low-rate mode
19:14+24h 09mX-ray spectrometer surface sensors off, solar sensor remains on
January 16 Gamma-ray spectrometer returns to cruise configuration
Radio science data collection ends

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