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.
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.
I'm going to repost my timeline below, to help you all follow along. There is one more new picture, too, but since I've got some time before any more images are released, I'm going to take a little time to play with it before posting.
Spacecraft time (UTC)
Time with respect to closest approach
Jan 13 13:04
Wide-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
Magnetometer 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.
X-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.
Atmospheric 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.
Wide-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 >
Narrow-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 >
Gamma-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.
Laser altimeter to standby mode The laser altimeter warms up in preparation for one precious altimetric profile across Mercury, to be taken around closest approach.
Earth 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.
Transition to nadir tracking Now, close to the planet, MESSENGER will rotate so that the laser altimeter can fire downward and measure Mercury's topography.
MASCS 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.
Eclipse entry For 13 minutes, MESSENGER will be hidden from the Sun.
Closest approach to Mercury MESSENGER will pass within 200 kilometers of the surface, firing its laser all the time.
Wide-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.
Narrow-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.
Narrow-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 >
Earth 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.
Laser altimeter back to standby mode
Wide-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 >
Narrow-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 >
Gamma-ray spectrometer to longer (300s) measurement interval
Narrow-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 >
Narrow-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.
X-ray spectrometer returns to long integration time (300s)
January 15 01:04
Neutron spectrometer returns to low-rate mode Fast Imaging Plasma Spectrometer returns to low-rate mode
Magnetometer returns to low-rate mode
Energetic Particle and Plasma Spectrometer returns to low-rate mode
X-ray spectrometer surface sensors off, solar sensor remains on
Gamma-ray spectrometer returns to cruise configuration Radio science data collection ends