A limb shot of Mercury’s horizon taken by the
MESSENGER spacecraft on January 14, 2008.
Photo Credit “NASA/MESSENGER”

If you can take a name like “Mercury Surface, Space Environment, Geochemistry and Ranging” and craft it into a neat acronym like MESSENGER, then you may have a future working with NASA….

And no, this blog isn’t about NASA acronymizations, but rather the heat-resistant robot behind one of them. MESSENGER is the space probe that NASA sent to Mercury to give the Solar System’s innermost planet the first up-close look since 1975, when Mariner 10 flew by.

Though MESSENGER’s main mission will begin in earnest when it returns to Mercury and finally settles into an orbit around the planet, on March 18th 2011, we were given a tantalizing peak last January 14th when the probe made its initial flyby.

What did this quick, on the fly snapshot tell us that we didn’t know before? Well-a lot, considering Mercury has been one of the least understood planets in the Solar System, and was for a long time thought to be similar in character to our own Moon. Mercury is shaping up to be a lot less like Earth’s Moon than its gray, cratered, airless appearance would mislead.

One key difference: density-how much material is packed into the planet; or how heavy a standard sized chunk of it would be. Our Moon is a lightweight on this score, with an average density of only 3.4 grams per cubic centimeter, while Mercury weighs in at a hefty 5.427 g/cc-almost as dense as Earth.

Another key difference: magnetic field. Planets like Earth and the Gas Giant worlds (Jupiter et al) generate respectable magnetic force fields, useful for everything from deflecting plasma flowing from the Sun (the “solar wind”) to properly directing magnetic compass needles. Venus, Mars, and our Moon do not possess magnetic fields worth mentioning, as it turns out.

Mercury, on the other hand, does. Planetary magnetic fields are believed to be generated by currents in a planet’s liquid outer core-like how the electric current in the wire coil of an electromagnet generates a magnetic field. Mercury’s magnetic field suggests it still has some activity in its core-molten metals circulating in currents as the core slowly cools off. And speaking of Mercury’s core, it appears to comprise 60% of the planet’s mass-about twice what is “typical” for Terrestrial (solid) planets.

I’ve often imagined Mercury to be a cosmic goldmine, with its apparent richness in metals and its density. I wonder if an astronaut could just walk along and pick up chunks of gold from its surface….

Another interesting find by MESSENGER is that some of the flat plains on Mercury may have been formed by volcanoes, long ago. In particular, MESSENGER imaged a number of volcanoes along the edge of the Caloris Basin, a large impact basin-one of the largest in the Solar System, at 1550 kilometers across.

The news coming out of the innermost region of the Solar System makes me giddy. Too bad I have to wait until 2011 for my next look at Mercury. These things take time.

NASA’s MESSENGER spacecraft at Mercury-artist concept.

Photo by: NASA

I’ve been waiting for the “whole story” on Martian ice at the Phoenix lander site to unfold more completely, but the chemical analyses have not yet run their full courses-so I’ve decided to widen the focus on this blog to give a status report on current active robotic exploration of planets going on around the Solar System.

Limiting my scope to only planetary spacecraft, the list is still respectable. In no particular order, here’s the round-up:

Spirit: Mars Exploration Rover Spirit’s activities on the Martian surface have been reduced to save on power, but the robot remains alive. With the arrival of Martian winter, Spirit spends more power running heaters to keep key electronic and power equipment healthy. Spirit remains in the giant Gusev Crater, where it will spend its entire life on Mars.

Opportunity: Exploring a much smaller crater of its own, Victoria Crater-Spirit’s twin, Opportunity, continues its investigation of the rock layers of Mars’ geological history. As of June 10, Opportunity has clocked in at 7.26 miles of total “roving” on Mars, since its landing back in 2004.

Phoenix: The brand-spankin’-new Mars Phoenix lander has been digging into one of Mars’ greatest scientific mysteries: water. Detailed chemical analysis of samples taken at Phoenix’s site near the northern polar ice cap is underway, but the big question– is Phoenix standing on frozen Martian water– has been answered: yes.

Mars Reconnaissance Orbiter: The newest orbiter in the Martian fleet continues to send back its extreme-high-resolution imagery and its revealing chemical measurements, as well as to serve as a high-speed data and communication relay for other Mars-exploring robots.

Mars 2001 Odyssey: Credited with detecting the massive amounts of frozen water in Mars’ northern hemisphere-the same ice that the Phoenix lander is now scraping at, Mars 2001 Odyssey continues its surveillance of Mars’ chemistry and atmosphere.

Mars Express: The European orbiter that launched the ill-fated Beagle II lander has continued on a respectable career of exploration in its own right. Mars Express also helped support the landing of the Phoenix.

Cassini: Saturn’s first robot-in-residence, Cassini, has concluded its initial 4-year mission and is now continuing on an extended mission. Cassini has given us unprecedented close-up images and measurements of many of Saturn’s stunning moons, its complicated ring system, and the swirling, aurora-touched cloud formations of Saturn itself.

MESSENGER: The first spacecraft to visit the little-understood Mercury since 1975 made its first flyby of that planet last January, and will settle into a permanent orbit in March 2011. Even the few pics it snapped as it hurled by gave us far more detailed images of Mercury than ever before.

New Horizons: Launched a couple years ago on its outward bound, meteoric flight to Pluto, New Horizons has already performed some exploration duty, capturing images and data of Jupiter, Jupiter’s volcanic moon Io, and Jupiter’s long magnetic “tail.” Now in “cruise mode,” this little robot will fly past Pluto (dwarf planet; king of the Plutoids) in July 2015.

Voyagers 1 and 2: Do you remember the remarkable voyages of discovery made by the Voyager spacecraft, both launched in 1977? Since completing their primary missions of flying by the Gas Giant planets (Voyager 1 at Jupiter and Saturn, Voyager 2 at all four), these two veterans have continued to operate and send information back to Earth, and are now about 3 times more distant from the Sun than Pluto.

That’s the wrap. If I missed anyone, my apologies!