In part 2 of our adventure cruise around the solar system, we visit the innermost and smallest planet, Mercury. Its severe landscape, intense environmental extremes, and brutal history provide us with a window into the violence of a region comparatively near to the Sun and deep within the solar gravity well.
The progress of our adventure so far: (current in bold)
1. The Sun
2. Mercury
3. Venus
4. Earth
5. The Moon
6. Mars + Phobos & Deimos
7. Asteroids
8. Jupiter
9. Io
10. Europa
11. Ganymede
12. Callisto
13. Saturn
14. Mimas
15. Enceladus
16. Tethys, Dione, and Rhea
17. Titan
18. Iapetus
19. Rings & Minor Moons of Saturn
20. Uranus + Moons
21. Neptune + Minor Moons
22. Triton
23. The Kuiper Belt
24. Comets
25. The Stellar Neighborhood
Mercury in true-color, as our eyes would see it if we were there:
I. Context
Aside from being the innermost planet in the solar system, Mercury is also presently classified as the smallest after the demotion of Pluto in 2006. It is additionally the closest known object of any type to the Sun, due both to the difficulty of spotting small objects in the solar glare and the long-term tendency of low-mass objects in near-stellar orbits to spiral inward. Diagrams of Mercury's solar system location and path of motion, from a receding perspective:
The tendency of low-mass objects to experience orbital decay close to the Sun occurs because bodies closer to the Sun must move through a greater density of solar wind particles, trivially decreasing momentum over time and causing bodies to migrate inward. But since momentum depends on mass, smaller objects would be subject to much more rapid decay than larger ones, clearing out the region of space inward of Mercury. The same process would occur with objects substantially further out, but their inward migration could easily place them on a collision course with the planet, in which case its heavily-cratered surface is unsurprising.
There may, however, be objects with highly eccentric orbits that pass within the planet's domain at closest approach to the Sun - a location in any given orbit known as perihelion. They would only spend very brief periods of time in this region, and thus the drag imposed by the solar wind on their momentum would be greatly reduced, allowing them to survive to this day and repeatedly cross Mercury's path. So although the frequency of collisions has radically declined since its early history, the planet is nonetheless continually bombarded by high-velocity dust and micrometeoroids from elsewhere in the solar system.
The following figure from a paper by S. Marchi et al compares impact probabilities for objects larger than 1 cm at both Mercury (at perihelion) and Earth, across a range of object velocities (in km/s), and with the solid, dotted, and dashed lines representing different time intervals as noted:
Note the much broader distribution of significant impact probabilities for Mercury, the presence of two peaks rather than one, and the fact that even the lower-velocity peak occurs at a substantially higher level than Earth's - i.e., close to 30 km/s as opposed to roughly 13 km/s. Since the meteoroid flux for Earth is roughly the same or less for the Moon (being in the same gravity well), we can see that although Mercury bears a superficial resemblance to the Moon, it lives a much harder life.
The reasons for this are twofold: For one, objects are accelerated along their path by gravity as they approach the Sun (via Kepler's 2nd Law), and the acceleration is much less significant at Earth's relatively distant orbit. And two, Mercury itself is moving much faster than the Moon both on average and at maximum, so its motion contributes a lot more to the kinetic energy of impacts. The Moon orbits Earth at between 0.97 to 1.08 km/s, and Earth orbits the Sun at between 29.3 and 30.3 km/s, so at maximum the Moon can only be traveling at 31.38 km/s. Mercury, meanwhile, has a minimum orbital speed of 38.86 km/s and a maximum of 58.98 km/s, so the kinetic energy of similar impacts can be double what they'd be on the Moon.
In addition to its other records, Mercury also has the most eccentric (i.e., elliptical) orbit of any planet in the solar system, accounting for the relatively wide spread in its perihelion (maximum) and aphelion (minimum) speeds. There are extrasolar planets known to have far more radical eccentricities, but our own planetary family appears relatively well-organized. An animation of Mercury's orbital and rotational motion:
The eccentricity is great enough that the Sun would actually appear larger in the sky at perihelion than at aphelion, although there would be no safe position from which to notice this other than through instruments and behind very heavy radiation and heat shielding. Still, there are misconceptions about the apparent size of the Sun from Mercury due to how miniscule the planet appears during transits of the Sun (when it moves in front of the Sun from our vantage point). People may incorrectly assume it would fill the Hermean (an adjectival form of Mercury) sky, but actually its apparent size increases only by a modest factor of 2 or 3:
To give a more intuitive sense of the difference, the following image is a simple scaling of the Sun in a terrestrial photograph. There are no real photographs of the Sun from Mercury (yet) because it hasn't proven worthwhile to give Mercury probes the kind of optics to survive looking at it, and dedicated solar observatory probes zoom in much more closely on the Sun than the orbit of any planet. Note that this projection does not account for brightness, temperature, or increased atmospheric scattering - it is purely a size comparison:
A good way to understand the intensity of sunlight at Mercury is to compare it with the solar constant - the amount of solar power reaching Earth's orbit per unit area. Note that this quantity, on average about 1,366 W/m^2 (watts per square meter), is substantially higher than what reaches the Earth's surface through the atmosphere, but approximately 70% of it contributes to the temperature of the terrestrial environment. On Mercury, however, not only is there no atmosphere to reflect solar radiation, but its much greater proximity to the Sun increases the total amount of power reaching it. This quantity varies between 4.6 times the solar constant (6,270 W/m^2) at aphelion and a factor of 11 (14,500 W/m^2) at perihelion. A comparison of solar intensity at Earth and Mercury:
Mercury transits the Sun as viewed from the Earth 13 to 14 times per century, at intervals determined by complex orbital rhythms. The images these events generate are often profound enough to make an impression, as per the misconceptions noted above, but the apparent relative sizes of the Sun and Mercury are due entirely to the position of the observer - the apparent size of the planet would grow much more quickly as you approached it than would the Sun:
The following clip shows Mercury in the context of huge coronal mass ejections (CMEs), illustrating the drama and intensity of its solar environment:
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II. History
Like all solid bodies in the solar system, the evolution of Mercury began when hot metallic vapors in the young Sun's protoplanetary disk became cool enough to precipitate and form droplets of fluid metal. As the temperature continued to decline, these droplets increasingly stuck together, froze into solids, and began the process known as accretion - the gradual accumulation of mass in a progressively smaller number of bodies.
It was not a pleasant scene: The vast majority of objects that accreted in the disk were eventually smashed to smithereens, devoured by the Sun, absorbed by one of the objects that exist today, or ejected out of the solar system completely. It turns out to have been rather appropriate that the planets were named after incestuous, fratricidal, and cannibalistic Greco-Roman gods.
But even in the context of such violence, the formation of Mercury is thought to have been particularly Wagnerian. It may have originally been a much bigger planet (about twice as massive), only to be impacted by an object around half its current mass and had its original crust and most of the primordial mantle blown away. In other words, the planet we see today may simply be the metallic core of that earlier world with a thin sliver of remaining mantle. This is still a developing hypothesis among researchers - although currently favored by virtue of computer modeling - but data from the MESSENGER probe that entered Hermean orbit this March will soon allow them to begin testing it.
The following animation depicts a different hypothetical impact (one thought to have occurred around another star), but is based on similarly-sized bodies and illustrates what happens in such an event:
Further, tantalizing initial results from the MESSENGER probe indicate a significantly higher amount of sulfur in the surface composition than was expected, which experts are interpreting as possible evidence that Mercury did not form from the same materials as Venus, Earth, the Moon, and Mars. If that interpretation is born out by subsequent data, it could have radical implications for theories of where in the solar system the planet formed. But even in the absence of a major revelation, the unexpected sulfur appears to suggest a significant early role for vulcanism in shaping the surface of the planet.
There is one dramatic collision in Mercury's history that is not hypothetical: Some time in a theorized period of the solar system known as the Late Heavy Bombardment - 4.1 to 3.8 billion years ago - an iron meteorite 60 miles wide impacted the Northern hemisphere, blasting a crater larger than the state of Texas with cliff-walls well over a mile high. The event was so energetic that the metallic structure of Mercury rang like a bell, the shockwaves traveled all the way around the planet to the antipode of the impact point, and their convergence caused massive disruptions in the crust called Chaotic Terrain (or "Weird Terrain"). Huge rips were torn in the crust, prior craters were uplifted into mountains, and bizarre linear features were created in this region.
The Caloris Basin, where the impact occurred, is difficult to see at first glance due to its huge size and the presence of many younger, more obtrusive craters within it, but its rimwalls can be glimpsed very clearly as the basin approaches the terminator - the line between night and day:
Notice the relatively smooth lava plains within the crater. A closer view of mountains along the rim:
Seeing the basin in its entirety is easiest in false-color:
Chaotic Terrain at the antipode of Caloris:
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III. Properties
A view of the terminator in true-color:
1. Orbital and Rotational Features
As seen in the animation of Mercury's orbit above, the planet has a remarkably slow rotation period - one Hermean day is about 58.7 terrestrial days - the second slowest rotation among solar system planets, next to Venus. This results from solar tidal forces peaking every perihelion that gradually slowed its rotation over long periods of time, and in a closer orbit would have brought Sun-relative rotation to a halt - a condition known as being tidally locked, in which a body always presents the same face to the center of its orbit. Earth's moon is tidally locked to our planet, and it was assumed for many years that Mercury was likewise locked to the Sun, but data from radar observations in the 1960s proved otherwise.
Rather, it was found that Mercury's day is slow but steady due to a 3:2 resonance between its rotation and orbit. This means the planet goes through 3 days every 2 Hermean years (each about 87.97 Earth days), causing lower and mid-latitudes to endure long bakes in the Sun followed by long nighttime freezes. However, due to the same tidal forces, the planet's rotational axis is almost exactly perpendicular to its orbital plane - unlike Earth, whose axial tilt is about 23.5° - so the "seasons" are entirely based on whether Mercury is approaching perihelion or aphelion. Its tilt is only about 2.1' (arc-minutes) - about a thirtieth of a degree.
An interesting peculiarity of Mercury's 3:2 resonance is that at certain times of the Hermean day, the Sun will appear to stop in the sky, briefly reverse course (apparent retrograde motion), stop again, and then resume its normal motion. The subsolar point (the area directly in the Sun's path) receives an extra-long dose of solar energy during these periods. An animation of the Sun's apparent motion and brightness from Mercury:
Notwithstanding this oddity, the terminator usually sweeps across the equator at a speed of about 1.73 km/h - a speed that declines with increasing latitude and virtually ceases entirely at the poles. Given the temperature extremes to which the planet is subject, the significance of this boundary's motion will soon become apparent.
2. Size & Mass Characteristics
Mercury's radius is about 38% that of Earth. Some fun size comparisons with other bodies in its class - mouse over to see the title if you don't recognize the other body:
You may notice that Mercury is smaller than both Ganymede and Titan, but the planet is much more massive than either - having 220% the mass of Ganymede, and 240% that of Titan. As a result, it packs quite a gravitational wallop in a relatively small package: Its gravity is nearly 38% of Earth's, so if you weigh 160 lbs on the terrestrial surface, you would weigh about 61 lbs on Mercury - close to what it would be on Mars.
Although Mars is both bigger and more massive, this can be true because Mercury's density is unusually high - about 138% of Martian density. This is thought to indicate an extremely large iron core, whose theorized existence has led to the giant impact hypothesis outlined above. In other words, researchers suspect that the core is atypically large because it was originally the core of a bigger planet that would have hewed more closely to the norm for density.
3. Temperatures
Day/night temperature swings on Mercury are the most extreme of any known object in the solar system, going from as high as 700 K (800 °F) - hot enough to melt lead, zinc, and tin - at the subsolar point at perihelion to 90 K (-300 °F) in some nighttime regions at aphelion. Nighttime temperatures are cold enough to freeze carbon dioxide, although this is a very rare molecule in the environs of Mercury. By comparison, the coldest temperature ever recorded in Antarctica was 255.2 K (−128.6 °F) - ironic that Earth's coldest place is a tropical paradise beside the Hermean night.
Now, there are complexities to the day/night temperature cycle based on the periods described above when the Sun appears to hover in the sky. These periods occur twice in the course of the Hermean day (once per hemisphere), and disproportionately heat two antipodal points on the surface called "hot poles." The daytime point soaks in solar energy while its nighttime counterpart radiates into space and cools. Here is a low-resolution, relative temperature map from ground-based radio spectrum data exhibiting the influence of the hot poles (note that the points labeled "pole" refer to the rotational axis, not what is being described here):
I myself am not quite clear on the details of this phenomenon - e.g., whether the hot poles migrate or if they are always in exactly the same locations on the surface. I would also be curious to know how heterogeneous nighttime temperatures are due to the influence of the night pole. Unfortunately, the information does not appear to have been broadly analyzed as yet - at least not on the web - or the data may not have been gathered yet. Hopefully researchers involved with the MESSENGER probe will produce a more detailed temperature map as its mission continues.
4. Internal Structure
One early result of the temperature data is that scientists were able to conclusively argue that Mercury is "mostly dead" in geological terms. What they found is that virtually all of the heat being radiated by the planet comes from absorption of solar energy, meaning that there is no internal process powerful enough to create significant additional heat at the surface beyond what the Sun provides. This indicates that its core and mantle are either solid or very slow-moving, which would in turn imply that the planet's relatively strong magnetic field may be a lingering remnant of a much more powerful one in the past.
However, theories abound on various dynamo processes that could produce the field as measured without violating the observed heat constraints. Tidal forces may be maintaining weak convection deep within the core, or else some other, more complex process may be occurring. Another theory is that still-slushy iron in the mantle may be freezing into "snow" and creating a dynamo by sinking downward. This is still a very active area of investigation, now energized by the data pouring in from MESSENGER. Internal structure of Mercury:
As you can see from the above graphic, the core dominates the planet, occupying 40% of its volume and extending 1800 km outward from the center - the large majority of its 2440 km radius. The mantle is only a third as thick at 600 km, but the crust is thought to be quite thick in comparison to a geologically active body like the Earth - 100 to 300 km, as opposed to the 35 km depth of the terrestrial crust. Some apparently buckled surface terrain is thought to have been caused as early internal processes wound down, the planet cooled, and the crust began to thicken and shrink.
Due to the dominance of its core, Mercury is composed of about 70% metallic elements, and only 30% silicates - in other words, it is more a ball of metal than a ball of rock. This will, I think, play a strong role in shaping its long-term significance to humanity, as I describe below.
5. Surface Features
Mercury's surface has a light bronze or coppery tint, as seen in the image at the very top of the diary, so photos that give it similar colors to the Moon are either monochrome or false color to emphasize features. Since its true appearance is not especially appealing to the public, there are few reasons for scientists to take a substantial number of true-color photographs, so the overwhelming majority are either black and white, false color, or represent spectra outside the human visual range.
The first probe to visit the planet, Mariner 10, was only able to image 45% of its surface due to being on a flyby trajectory - it did not have the fuel to enter orbit - so it was not until subsequent flybys and then orbits of Mercury by MESSENGER beginning in 2008 that the map of the surface was completed. However, this has created a backlog of features that have yet to be named by the International Astronomical Union (IAU) - the global scientific organization recognized as the naming authority for celestial objects and their features. The process for assigning names is described here.
In order to maintain diversity, the IAU assigns different categories of names to different types of features, and these categories vary from object to object. In the case of Mercury:
Craters...............Famous deceased artists, musicians, painters, authors.
Dorsa (ridges)......Astronomers who made detailed studies of the planet.
Fossae (long, narrow, shallow depressions)......Significant works of architecture.
Montes (mountains)......Words for "hot" in various languages.
Planitiae (plains).....Names for Mercury (either planet or god) in various languages.
Rupēs (escarpments)........Ships of discovery or scientific expeditions.
Valles (valleys).........Radio telescope facilities.
An exhaustive list of feature names for Mercury is available here, via the US Geological Survey. About 355 names have been approved so far, with only three rejections, although this particular source does not list proposals or unnamed features pending assignment. Some high-level (incomplete) maps from Mariner 10 - note that in the spelling of some names, the letter 'y' has been replaced with 'j' for whatever ethnolinguistic reason (e.g., Tolstoy is "Tolstoj") :
Some horizon shots and then close-ups from MESSENGER:
Remembrandt Basin (in enhanced color) at distance and close-up:
Northern polar terrain:
South pole:
Matisse crater:
Machaut crater:
Camoes crater:
Spitteler and Holberg craters:
Mickiewicz crater (its rim is the arc at left):
Occasionally an impact will shoot ejecta at a direct tangent to the surface, causing chunks of debris to "skip" like stones across water and leave behind tracks like these:
Shotgun-blasted ejecta terrain outside Abedin crater:
Verdi crater:
Endeavour Rupes:
Brahms crater:
Atget crater:
Belinskij crater (in the upper left):
Degas crater - note the cracked lava plain inside:
Bernini crater (at left):
Beagle Rupes:
Some yet-to-be-named features:
6. Gases
Mercury has no atmosphere per se, but it does have an exosphere - very diffuse gases and ions in vacuum that are only weakly bound to the planet, and can easily escape. Hydrogen, helium, oxygen, sodium, calcium, potassium, and magnesium have been observed, along with simple ionic compounds involving these elements. Since the planet's gravity is insufficient to keep its exosphere in such a high temperature environment, it can only come from ongoing processes that continuously replenish it.
To be specific, the hydrogen and helium are largely particles captured from the solar wind, while the remainder are created both from chemical interactions of the solar wind with surface materials and the vaporization of surface materials by micrometeorite impacts. In terms of "weather," concentrations of heavier vapors behave differently according to latitude and disposition to the terminator: Sodium, potassium, and magnesium concentrate at the poles, and occur in greater profusion near the dawn terminator than the dusk terminator. Calcium vapor, however, occurs in higher concentrations near the equator.
A visualization of seasonal sodium fluctuations in the Hermean exosphere:
Because the planet is constantly being blasted by the Sun, both radiatively and in terms of the solar wind, and because its exosphere is so weakly bound, the gases are constantly being blown away in the direction opposite to the Sun. As can be seen in the above video, this creates a particle tail similar in behavior to a comet's, although much smaller, more diffuse, and invisible to the human eye.
7. Magnetic Field
Mercury has a global, dipole magnetic field about 1% the strength of Earth's. Its ability to somewhat divert charged particles from the solar wind defines the presence of a magnetosphere. As a result of the field, solar ions are selectively channeled to the poles where they may contribute to the latitude differences in exospheric composition. However, the field is asymmetric from North to South.
Aside from being much smaller than Earth's, the Hermean magnetosphere is also much less reliable: It will at times, in various locations, spontaneously reconnect with the interplanetary (i.e., solar) magnetic field, opening vortices in the magnetosphere and allowing the solar wind to enter unhindered. On average, the field maintains a shape typical of one moving through a flow of charged particles - compressed in the forward-facing region (the bow shock) due to the abrupt deceleration of the solar wind, and extended in the other direction due to being dragged along with it. Shape of Mercury's magnetic field:
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IV. Past Relevance to Humanity
Although Mercury has played a role in the mythologies of most ancient cultures, the fact that it is never seen on Earth more than 28 degrees from the Sun, and its motion appears relatively simple, means that it failed to develop the rich occult traditions surrounding bodies with more complex apparent motions such as the Moon, Mars, and Jupiter. Its movements in the sky were not interpreted as omens, because they were too predictable and usually difficult to observe.
However, it was still a point of curiosity to ancient philosophers, and there is some poetic symmetry to the fact that the Greeks had given it two names based on its two separate appearances during the day: Hermes (hence, "Hermean"), and Apollo - the latter of which Americans would eventually name missions to the Moon, and find to their surprise a few years later (via Mariner 10) that Mercury had a similar appearance to that body. The planet is additionally known in Chinese, Japanese, and Vietnamese via mythology of the Five Elements as the "Water Star" - a label whose irony the ancients would surely have considered an omen of something or other.
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V. Modern Relevance to Humanity
Despite public indifference (it's largely been written off by laymen as a hotter version of the Moon), Mercury has proven far more relevant in the modern era than it ever was for the ancients. The planet is deep enough in the Sun's gravity that relativistic effects have a nontrivial influence on its orbital motion - an effect inexplicable to Newtonian mechanics - so once science had the benefit of Einstein's theory, the anomalous measurements of Mercury could finally be explained. In other words, it was one of the lines of evidence confirming the validity of general relativity. In addition, the interaction of solar flares and CMEs with the exosphere and magnetosphere of Mercury may prove educational as we contend with these phenomena on Earth.
There hasn't been a lot of fiction associated with it, especially in visual media, but just for your enjoyment here is a very beautiful scene from the 2007 science fiction film Sunshine, depicting (somewhat convincingly) a transit of Mercury across the Sun - embedding is disabled, but I think it's fully worth opening a new tab:
http://www.youtube.com/...
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VI. Future Relevance to Humanity
Getting to Mercury is very hard, and not just because of the heat, radiation, and micrometeorite bombardment: Its depth within the Sun's gravity well means that it takes a tremendous amount of energy to reach its solar orbit, and then even more energy to slow down in order to orbit or (hypothetically) land on the planet itself. This was the reason Mariner 10 was designed as a flyby mission rather than an orbiter, and why MESSENGER had to go through years of flybys and gravitational assist maneuvers to slow down enough to enter Mercury orbit. And that's just to get there, nevermind if you intend to leave.
In fact, despite its apparent proximity to Earth in the broader context of the solar system, in energy terms Mercury is actually "farther away" (i.e., requires more fuel to reach) than Neptune - at least for a one-way trip. In fact, it takes less energy to leave the solar system entirely from Earth's location than to enter orbit around Mercury. Now, this does not mean it is easier in absolute terms (at least for a probe) to reach other bodies than Mercury - some of them are far enough away from the Sun that a probe requires a nuclear power source to operate, and that carries additional costs compared to the high-temperature solar panels that can be used for a Mercury mission. But it does impose constraints on how long a mission takes, and/or how much fuel must be brought along.
If for some reason you wanted to leave having gotten there, it would take almost as much energy to get back to Earth - only somewhat less because escaping Mercury's planetary gravity is less expensive than leaving Earth. But in returning to our planet from Mercury, it's the Sun whose gravity must be fought at great cost. In other words, in light of how costly it is to even orbit the planet, something like a sample return mission would be extremely difficult compared to a similar mission for Mars.
For this and other reasons, it seems extremely unlikely there will be human exploration of Mercury in this century, and probably not for several centuries. For the sake of illustrating the difficulty, let's briefly walk through the broad (and not at all technically precise) requirements of a hypothetical first manned mission to the surface of Mercury:
A. Leave the Earth's surface into orbit (energy input 1).
B. Construct spacecraft, landers, and surface habitats shielded as thickly as nuclear reactors to withstand both the temperatures, the radiation, and the meteorite impacts of the Hermean environment.
C. Move to an Earth-to-Mercury Hohmann transfer orbit (energy input 2), taking the spacecraft from Earth's solar orbit to that of Mercury.
D. Slow the spacecraft down sufficiently to enter orbit around Mercury (energy input 3).
E. Because of the temperature extremes at lower latitudes, the most survivable areas (and the only possible deposits of accessible ices) would be near to the poles, so to land there the spacecraft would have to change its equatorial orbit around Mercury to a polar one (energy input 4).
F. Land on Mercury (energy input 5).
G. Once the mission is complete, ascend back into Mercury orbit (energy input 6).
H. Change the spacecraft's polar orbit back to an equatorial one (energy input 7).
I. Escape Mercury orbit (and the planet's position in the solar gravity well!) into a Hohmann transfer orbit back to Earth (energy input 8).
J. Enter orbit around Earth (energy input 9).
K. Return to the surface (energy input 10, but assisted by the atmosphere).
With current technology, the mission would either be unfeasibly long or require bringing an impractically large amount of fuel for all these orbital changes. The fuel requirements would be exacerbated by the massiveness of the required shielding, and even if it was all accomplished, there would not be much to say about the human experience of being on Mercury: Apollo-style spacewalks out in the open would probably not be feasible even at the poles, so the environment would be glimpsed only through instruments or - at enormous cost for relatively little gain - under a thickly-shielded, enclosed "porch" with artificial lighting.
Now, this is not say it will never happen: Quite the contrary - it will inevitably happen. Just not in the next few centuries. Humans will have to become much more comfortable with and technologically adept at manned interplanetary operations before this becomes feasible, and a lot more than that would have to change before it's considered worthwhile. Still, I would find it plausible to speculate that a crewed flyby mission could occur by 2100 - the logistics of that are far simpler, albeit still onerous in absolute terms.
In the very distant future, however, humanity is likely to become Mercury's nemesis: It is a giant ball of metal with substantial gravity in a solar energy-rich environment, and the resources of its nearest neighbor (Venus) are absurdly difficult to access even by Hermean standards. If we as a species survive long enough to migrate into space and build solar infrastructure out there, I don't see the process having any natural endpoint - Mercury would be swept away over millennia, and turned into the energy infrastructure of civilizations vast and complex beyond our imagination. Absent some voodoo physics to take us to other stars in short periods of time, I see no way around it - if humanity flourishes, Mercury is doomed.
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VII. Future of Mercury
Of course, there is no guarantee we will flourish; no guarantee that currently-understood laws of physics will constrain our flourishing; and no guarantee that non-rational motives will fail to constrain it. If humanity goes extinct, or does realize some voodoo physics, or preserves the inner solar system as a museum for sentimental reasons, or some other scenario, there are two alternative futures for Mercury. Chaotic changes in its orbital eccentricity over millions of years due to perturbations from other planets produce a 1% probability that Mercury will crash into Venus at some point in its expected lifetime. And if, as is 99% likely, that does not happen, it will simply be swallowed by the Sun as it expands into a giant a few billion years from now.
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VIII. Catalog of Exploration
Past & Current Spacecraft:
Mariner 10 (USA, 1973-1974)
MESSENGER (USA, 2004-)
Future Spacecraft:
BepiColombo (Europe/Japan, planned to launch in 2014) (Tragic note: It was originally intended to carry a lander, but the lander was cancelled due to budgetary constraints)
