April 1, 2011
Image Mission Elapsed Time (MET):
Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
7 (749 nanometers wavelength)
The radius of Mercury is 2440 km
MESSENGER captured this image while taking one of its first pictures from orbit around the planet Mercury. The frame shown is one of several images obtained of the same view through the different color filters of the Mercury Dual Imaging system (MDIS) Wide Angle Camera (WAC).
The first reaction of some on the MESSENGER team was that the feature to the left of Mercury's limb must be an imaging artifact. “It's the effect of solar neutrinos on the WAC's CCD,” pronounced Project Scientist Mack Knott. The imaging team was skeptical of this explanation, however, and all Knott could add was “I could explain it to you, but you'd have to understand Feynman diagrams.”
The imaging team brought the anomalous image to the attention of Mission Systems Engineer E. Finn Again, who immediately called an emergency gathering of the Collision Avoidance Review Board. Fortunately, the unusual object in the image did not appear to be in the immediate path of MESSENGER's next few orbits, but the fact that earlier and subsequent images of the same scene did not include the object prevented a determination of its trajectory.
One of MESSENGER's Science Team members, Prof. S. T. Rom, recognized the object immediately as Mariner 10, the only spacecraft before MESSENGER to have visited Mercury. Launched in 1973, Mariner 10 flew by Mercury three times in 1974 and 1975 before communication with the probe was lost. Prof. Rom is the only member of the MESSENGER team to have served on the science team of Mariner 10 as well.
The Science Operations Center was filled at the time with MESSENGER team members, and everyone proceeded at once to theorize on why Mariner 10 might appear in an MDIS image of Mercury.
Professor Rom suggested that Mariner 10 may have remained in place as of the time of its last signal:
“Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.”
The dead albatross hanging from Rom's neck, however, reduced the effectiveness of his argument.
Mission design lead Mick Adams quickly calculated that Mariner 10 should not be encountering Mercury on this date. “Mariner 10 and Mercury were in a resonant state that brought the spacecraft by the planet once every two Mercury years. By my calculation, this appearance is 23 days early.”
Guidance and control lead E. C. Shaughn offered that the effect of solar radiation should have substantially altered Mariner 10's orbit over the past 36 years as a result of solar sailing:
“With throats unslaked, with black lips baked,
We could nor laugh nor wail;
Through utter drought all dumb we stood!
I bit my arm, I sucked the blood,
And cried, A sail! a sail!”
Propulsion lead Brecht Engel added that some residual propellant after Mariner 10's last propulsive maneuver may have outgassed, and that multiple outgassing events may also have contributed to trajectory changes:
“A speck, a mist, a shape, I wist!
And still it neared and neared:
As if it dodged a water-sprite,
It plunged and tacked and veered.”
MESSENGER's navigation team members, all of whom are named Williams, plugged these suggestions into their codes. Minutes later they were able to announce to all assembled that Mariner 10 appeared to be in a new resonant state, one synchronous with Earth's period. The ancient spacecraft is locked into an orbit that swings it by Mercury once every Earth year, on April 1st.
As MESSENGER passed into eclipse behind Mercury, Prof. Rom had the last word:
“The sun's rim dips; the stars rush out:
At one stride comes the dark;
With far-heard whisper o'er the sea,
Off shot the spectre-bark.”
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
For information regarding the use of MESSENGER images, see the image use policy.