At 5:55 p.m. ET today, the MESSENGER spacecraft will make its closest pass in its third and final flyby of the innermost planet.

Mercury, as seen from MESSENGER on September 28, 2009
—Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

When images from the flyby start pouring in around midnight, scientists hope the snapshots will reveal even more about the tiny world's rocky surface.

The flybys are part of maneuvers to bring MESSENGER safely into Mercury's orbit in 2011, making it the first probe to take off its proverbial coat and stay for a while in the planet's embrace.

Until 2008, only the Mariner 10 spacecraft had been anywhere close to Mercury, and that was just a drive-by back in 1974 and '75. What's more, Mariner was only able to take pictures of less than half the planet, leaving the bulk of Mercury shrouded in mystery for more than 30 years.

The first MESSENGER flyby in January 2008 showed the world a whole new side of Mercury—literally. Cameras snapped more than 1,200 pictures, imaging an additional 30 percent of the planet.

A second flyby in October 2008 added to the bounty, and now we have a pretty good idea of what 90 percent of the planet's surface looks like.

The latest flyby will take a few more shots of never-before-seen features, but it's mostly geared toward what the mission managers are calling targeted observations. In other words, this is a chance to take a closer look at interesting features spying during the previous flybys.

From the MESSENGER Web site:

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) will gather high-quality spectral data by "staring" at the chosen surface targets for ~30 seconds per target. Simultaneously, the Mercury Dual Imaging System (MDIS) will obtain many sets of high-resolution color images of the targeted regions using all 11 of its color filters. Together, the data from these targeted observations will provide a wealth of new information and insights into the nature and history of Mercury's surface.

For me, MESSENGER's results serve as an example of the information arms race between science and fiction.

I'm about halfway through the English translation of a Japanese science fiction novel called Usurper of the Sun by Housuke Nojiri. The book is all about a high school student who spots a tower being built on Mercury during a planetary transit—when Mercury passes in front of the sun as seen from Earth.

The tower, it seems, is made of nanobots that are using materials from Mercury to build a ring encircling the sun (you'll have to read the book to find out why). But the ring is in just the right spot that its shadow engulfs Earth, triggering environmental catastrophe and spurring a desperate race to make contact with the mysterious alien Builders.

Ultimately this book is not about Mercury—it's meant to be a philosophical take on the nature of aliens and what a first-contact scenario might be like [and about a beautiful, brilliant female student who is humanity's last hope for salvation, a fact that won't even faze anime fans the world over].

Trick is, the whole story hinges on us not knowing a darn thing about Mercury's backside. The book was published in 2002, two years before MESSENGER even launched. At that point, for all anyone knew, it was entirely plausible that aliens might have set up a nanobot workshop right under our noses.

Science fiction has a long history of building imaginative stories on plausible science that later turns out to be bunk. What, for example, would the Martian Chronicles have been about if Ray Bradbury had seen data from the Mars Mariner missions?

And poor Venus, once celebrated in fiction as the most likely planet to house non-Earthly life, was exposed by science in the 1960s as too hot and too under pressure for anything resembling humans to exist on its surface.

Thank goodness science in the '90s delivered unto us the first confirmed extrasolar planets, ushering in a whole new class of possible targets for the fictional (and literal) search for extraterrestrial intelligence.

On the flip side, science fiction is most frequently touted as the source of some of our modern technological advances. Star Trek tech gets closer to reality every day, it seems, and early SF masters like Verne and Wells are credited with practically inventing commonplace gadgets such as sliding doors and cell phones.

Heck, even the recent announcement that there is, in fact, water on the lunar surface just made Robert A. Heinlein's The Moon is a Harsh Mistress a touch more plausible.

Give astronomers a few decades, and I wonder what soon-to-be-published icons of SF will become either outdated whimsy or remarkable prescience?

It's been just over two weeks since the MESSENGER spacecraft swooped past Mercury during its second flyby of the innermost planet.

Since the initial fervor, the MESSENGER team has been faithfully releasing images collected during the close encounter, some of which are providing data-hungry scientists with fodder for speculation about Mercury's geologic processes.

Today's offering highlights what I think must be one of the more frustrating aspects of being a planetary explorer: naming stuff.

Where in the universe—other than your local Barnes & Noble—can you find Arabic, Swiss, Ukrainian, and ancient Roman poets sitting next to a Baroque-era French composer being cut in half by one of Captain Cook's ships?

That'd be Mercury's southern side, which is just packed with craters first seen during the original Mariner 10 flybys in 1974 and '75.

—Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Now, gone are the days when being the first person to see something and, whenever possible, stick a flag in it grants you the automatic right to bestow upon it a name. In astronomy they's got rules, and the rules for naming things can get pretty specific.

[Thank goodness for this, by the way, or the list of gas giant planets in our solar system might have been Jupiter, Saturn, Neptune, and George.]

For Mercury, the International Astronomical Union states that all craters must be named for famous dead artists, musicians, or writers. Rupes, or cliffs, are named after famous explorers' ships.

Given these constraints, creating the list of approved crater names on heavily pockmarked Mercury must have really stumped even the most ardent trivia fans.

Consider too that most of these monikers were decided before we'd even gotten to see more than 45 percent of the planet's surface.

In April the IAU added six crater names to Mercury's approved list, and more are sure to come as the onslaught of MESSENGER images gets scrutinized.

As anyone who's recently cleaned their attic can tell you, unexpectedly finding a large spider sitting in a dark, hidden part of your home can elicit excitement, consternation, and sometimes a family squabble.

Apparently it's no different if you are a planetary scientist, even when the home in question is the solar system and the "spider" is a mysterious formation sitting in a crater on Mercury.

In January the MESSENGER spacecraft beamed back images of a side of Mercury no one on Earth had seen before.

The suite of new data from the probe's first flyby of the innermost planet revealed lots of volcanism, asteroid impacts, and an odd feature the team dubbed the spider—a network of more than a hundred raised, narrow troughs radiating outward from a central structure.

—courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The whole formation sits in the middle of the Caloris Basin, a massive 3.8-billion-year-old impact crater that had not been seen in its entirety before the flyby.

Today Sean Solomon, principle investigator for the MESSENGER mission, presented at the 3rd European Planetary Science Congress his theory that the spider is the product of a meteorite impact.