Mariner 10 to pay it a visit, I was a Caltech undergrad working for
the head of the mission's imaging team. So I was on hand at the Jet
Propulsion Lab when the first-ever close-ups of Mercury came in the
A few months later I left California to join the staff of Sky &
Telescope. Mariner 10 would fly past the planet two more times, in
September 1974 and March 1975, and my first-ever bylined article in
S&T was a pictorial writeup following the second flyby. (If you've got
the November 1974 issue, look it up!)
What amazed me then, as now, is how Mercury looks superficially so
like the Moon — they both have scads of craters, giant impact basins,
and broad lava plains — and yet are so very different. Scientists
estimate that Mercury's metallic core takes up 75% of its radius and
nearly half its volume. How this "iron planet" came to exist remains
one of the great mysteries of planetary science.
The control room at Johns Hopkins University's Applied Physics
laboratory buzzed with activity during Messenger's first flyby of
Mercury on January 14, 2008.
S&T: J. Kelly BeattyFast forward to yesterday. NASA's Messenger
spacecraft was making the first return visit to Mercury in nearly 33
years (a third of a century, I remind myself), and I just had to be on
hand for the reunion. So I took a predawn flight from Boston to
Baltimore, then dashed over to the mission control center at Johns
Hopkins University's Applied Physics Laboratory for a ringside seat.
I'm thrilled to report that the flyby was 100% "nominal." At 2:04:39
p.m. Eastern time (19:04:39 Universal Time), out of view from both the
Sun and Earth, Messenger swept over the planet's night-side equator at
an altitude of just 125 miles (less than a mile higher than planned).
Taken by Messenger from about 17,000 miles away, this view of Mercury
shows about half of the area not photographed by Mariner 10 in
1974–75. The heavily cratered landscape is reminiscent of other areas
previously seen. The broad circular plain at upper right, appearing
brighter than its surroundings, marks the interior of Caloris basin, a
huge impact scar more than 800 miles across. Click on the image for a
NASA / JHU-APL / Carnegie Inst. of WashingtonThree minutes later it
emerged from hiding, reestablished contact with NASA's tracking
stations, and then sped off in blazing sunlight on a slightly altered
trajectory that will bring it 'round again on October 6th and yet a
third time in September 2009. During their fourth and final meeting,
in March 2011, Messenger will fire its braking rocket and settle into
Messenger stands for MErcury Surface, Space ENvironment, GEochemistry,
and Ranging — an acronym of sorts no doubt created by NASA's
super-secret think tank for mission names. In any case, Messenger
carries an impressive array of scientific instruments — and all seven
of them were hard at work yesterday. You'll see the eye-popping
results here and at the mission's website in the days and weeks ahead.
Back in the mid-1970s, Mariner 10 left us with many unanswered
questions about Mercury. Why does it have a magnetic field? What's the
source of the tenuous atmosphere that envelops it? And are there
really deposits of ice at its poles?
Topping the list of questions is how Mercury came to be so iron-rich.
(In fact, after adjusting for compression effects, it's actually
denser than Earth.) Something happened at the dawn of solar-system
history that set this innermost planet distinctly apart from its
terrestrial siblings. As mission leader Sean Solomon confided to me
yesterday, "I want to learn how Mercury got put together."
So do I, Dr. Solomon — so do we all.
Posted by Kelly Beatty, January 15, 2008