Astrobiology Magazine presents another 'Gedanken', or thought experiment - musings on scientific mysteries in a series of "what if" scenarios. Gedanken experiments, which have been used by scientists and philosophers to ponder thorny problems, rely on the power of one's imagination to project these scenarios to logical conclusions.

They usually do not involve lab equipment or even experimental data. They can be thought of as focused daydreams. Yet, as in the famous case of Einstein's Gedanken experiment about what it would be like to hitch a ride on a light wave, they have often led to important scientific breakthroughs.

Ray Villard has been a popular astronomy writer for the past 35 years and currently is News Director for the Hubble Space Telescope. In part two of his essay, he follows a future mission to investigate a terrestrial planet beyond our solar system that may have life.

After hurtling across interstellar space for a century or longer, a comparatively small, low-mass probe decelerates into orbit around a planetary system that had been carefully selected over decades of Earth-based observations.

Upon arrival the mothership's first priority is to forage for resources. The architecture of the entire planetary system is scrutinized. The mothership seeks out asteroid-size bodies as a home to "make a nest."

Ideally it can find asteroids that share an orbital path around the star with a super-Earth planet in this system, much as the asteroidal Near Earth Objects do with us. This would minimize communications travel time as well as physical travel time between the mothership and its probes.

Asteroids are rich in the elements needed for building the survey team robots and probes. What's more, an asteroid's gravity is so weak that very little energy is needed to launch probes off its surface.

At the heart of this strategy is nanobot technology: Von Neumann's "Universal Replicators." These self-replicating microscopic machines behave like living cells.

They can reproduce themselves and diversify into individual units designed to perform specific tasks inside the larger machine "organism" they are constructing. Like our acorn that reads its genetic code to grow into a tree, the nanobots are pre-programmed to build the necessary exploration architecture.

The first order of business is building a solar collector for boosting power to the site. The next task is building a communications device, perhaps a laser transmitter, to "call home" and let those of us waiting on Earth know that the mission has set up base camp.

The "I arrived OK" message doesn't arrive at Earth for a few decades, depending on the star system's distance. The eternally patient sentry tracking facility at Lagrange Point 2 receives the long-awaited signal.

It immediately downlinks to autonomous ground receivers at legacy stations on Earth, and perhaps also to stations set up in other solar system colonies. News reports announce that a robotic Ulysses has reached an Earth-like planet beyond the solar system, and discoveries should be forthcoming.

However, the mission timeline depends on the decisions made by the probe's artificial intelligence.

The mothership's next task is overseeing construction of planetary probes. A nanobot factory is set up on the asteroid. The mothership's artificial intelligence is at the adult human level, and it follows a set of pre-launch directives to analyze the data and to make decisions about further exploration.

Interplanetary orbiters are then launched off the asteroid's surface to explore all the planets and moons in the system. This task takes many years, since the probes follow long trajectories on small reaction motors. Terabytes of data are relayed back to Earth.

The prime objective of the mission is to collect biological data on life in the system. As probes survey the rest of the planetary system, the super-Earth is intensively studied by orbiting reconnaissance probes, much as NASA's EOS satellites survey Earth. The artificial intelligence aboard the mothership has no idea what biological life is. It must follow the criteria established by its builders.

Once the most promising biohabitants on the super-Earth are identified, the mission's next task is to explore the surface. This will be carried out over many years by an armada of landers, rovers, hydrobots for the ocean, aerial balloons and/or heavier-than-air vehicles. These are kept as miniaturized as feasible to conserve resources.

The probes are disposable, and the mothership's nanobot factory can make replacements as long as the asteroid resources hold out.

A freshly-built lander parachutes down through the dense atmosphere of the super-Earth. It establishes a base camp for dispatching numerous small insect-like robots to investigate the local terrain. Flying and crawling robots have their own onboard intelligence to look for interesting sites for biology.

The surface probes are instructed to bring back biological samples to the lander for intensive chemical and metabolic tests. Balloon-borne aerial probes act as a flying command center, directing the expeditionary mission from above.

This type of mission would be extremely challenging to design and put into action. The exobiology experiments carried to Mars by NASA's Viking landers in the mid 1970s were faulted for being premature, because at the time we didn't have a good enough understanding on the Mars surface environment.

Washington State University scientist Dirk Schulze-Makuch recently published a study proposing that the methods the Viking landers used to search for life would have missed detecting microbes, and even could have killed them.

The Viking landers experiment was designed to search for salt water-based cells, while the strong probability is that life in the dry, cold martian environment would have evolved using water and hydrogen peroxide.

Viking teaches us that for our imagined future mission to search for life, the mothership's artificial intelligence needs to be so advanced that it can learn as the mission proceeds, and reprogram the mission according to what its surface probes find.

The super-Earth target planet in our scenario is several billions years old. It has been selected with the expectation that Darwinian evolution is in high gear. Much of the survey robots' tasks are to visually record the behavior of macro-organisms.

These robots may be seen as a threat by advanced life forms, and no doubt some probes will be snagged and destroyed by predatory creatures. The astrobiology base camps on the super-Earth may need to be camouflaged, have active defense mechanisms, or otherwise be stealthy to avoid detection by predators.

Introducing these robots might be problematic, because a captured robot might influence the native culture in any number of unpredictable ways. Would the mission planners have a Star-Trek type "Prime Directive" to avoid leaving any evidence of extraterrestrial origin? Or would the robots have protocols to attempt to communicate with and test the intelligence of the organism it encounters?

This is even more problematic if the mission encounters a space-faring civilization that went undetected on Earth. If we wanted to avoid detection, the mission would come to an abrupt halt if the native species came upon the asteroid lair.

To avoid this, the mothership could be programmed to avoid detection. This defense response would be automatically triggered if the first orbital reconnaissance probe detects other artificial satellites orbiting the super-Earth.

Without interference from native alien species, the mothership could dutifully collect, organize and relay the findings from the robot landing teams. Scientists on Earth would receive progress reports decade after decade.

In all likelihood, the mission would go on indefinitely. The mothership is immortal for all practical purposes. For redundancy, the mothership could assemble and train a duplicate of itself to serve as a backup command center in case of unforeseen failure.

Though this scenario sounds like wildly imaginative science fiction, if anything it is too conservative in making predictions as to how we'll visit exoplanets in the next century. Less than a century ago, the New York Times was making fun of Robert Goddard's rocket experiments and schemes to put a payload on the Moon.

In fact, science fiction writers of his time predicted that a manned Moon landing was centuries away. Likewise, sending artificial intelligent life to be our proxies in exploring exoplanets is only a matter of when, not if.

This leaves one unsetting thought. If our civilization can do this within a few generations from now, then extraterrestrial societies many thousand of years ahead of us have already done these experiments. But we don't have any evidence of alien von Neumann machines prowling around our solar system. Either we have never been visited, or they are out there among the asteroids - being stealthy.