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 one of his essay, he ponders how we'll study the first terrestrial planet that we discover outside our solar system that may harbor alien life. (Part two of this essay will appear next week).

First Contact: Interstellar Mission to an Inhabited Planet

The discovery of planets around other stars is going through an "inflation era" of rapidly expanding new knowledge. Beginning in 1995, the first decade of exoplanet observations involved simply doing an inventory. In the second decade we are rapidly characterizing the physical properties of these remote worlds, and by the third decade well will be cataloging inhabited Earth-like planets.

The first potentially inhabitable exoplanet we find will likely be a super-Earth several times the mass of Earth. Super-Earths are probably more abundant in the local stellar neighborhood than puny Earth-mass worlds. Several have already been discovered. A super-Earth could have very deep oceans (if not be totally water-covered) and very active plate tectonics.

Odds are that it may not have an atmospheric composition that is exactly a clone of Earth's. A planetary atmosphere in disequilibrium - where gases interact and inputs are needed to keep the atmosphere in its present state - could be a sign of life, since life processes can play a role in maintaining such an atmospheric composition.

Sometimes, however, geological processes or other non-living factors provide all the necessary atmospheric contributions. Years of debate and analysis will take place before experts reach the conclusion that an alien planet with an atmosphere in disequilibrium got that way because life processes have modified it.

Photometry from space telescopes will measure light and color variations on the super-Earth. This will show if the planet's surface is variegated by the presence of oceans and continents. When scientists develop a fantastically larger space optical interferometer, this could map the planet's geography and atmospheric features in coarse images only tens of pixels across.

One design requires combining light from five clusters of four 8-meter telescopes flying in precision formation with a baseline of nearly 4,000 miles.

But there will never be any telescope big enough to yield clues as to what's living there.

Certainly there will be SETI follow-up observations to monitor the planet for signs of artificially produced electromagnetic radiation - radio and television - leaking off the world. It's extremely unlikely that the first inhabited planet we discover will have a civilization relatively close to our level of scientific and technical evolution. Given the 12 billion year age of the galaxy, such developmental synchronicity between far-flung civilizations is improbable.

The discovery of a true exo-Earth will be the most compelling reason to identify and ultimately explore nearby exoplanets. The staggering technical challenges of interstellar spaceflight aside, such exploration could at last address one of the most fundamental questions in astrobiology: how does life begin and evolve on a planet with a completely different set of initial conditions?

A multi-billion year old exoplanet with a complex biosphere would provide an extraordinary set of insights to the adaptation of life on an alien world.

Short of receiving a radio or optical transmission from a technological civilization, we will never know anything about alien biology without traveling to exoplanets and conducting experiments on macro-life such as plants, animals and even unknown types of observable life forms.

It's reasonable to anticipate that, by the next century, we will realize breakthroughs in engineering and physics that will make it feasible to send probes to the stars. today, speculative papers are being published about schemes for interstellar travel by harnessing the energy from the vacuum of space, modifying gravitational or inertial forces, or pushing against or distorting the fabric of space-time itself.

If we conservatively assume conventional rocket propulsion is the only foreseeable option for an interstellar mission, this endeavor would span many generations. The scientists receiving the data would be the great-great-great-great-grandchildren of the mission's designers.

This would the longest science/engineering project ever conceived by humankind, making the construction of the Egyptian pyramids seem like a weekend Home Depot project.

An autonomous tracking station would need to be set up to ensure the project endures over centuries. It would need to be isolated and self-sufficient in order to survive natural disasters, wars, political shifts and social upheavals on Earth.

An ideal place for such a mission tracking station is the Earth-Sun Lagrangian point 2, basically a gravitationally-balanced parking lot for spacecraft positioned between Earth and the Sun.

The facility caretaker would be an artificial intelligence supercomputer capable of self-repair and self-reprogramming. Soaking up energy from perpetual sunlight, this computer would survive for the duration of the mission and obediently relay information down to Earth.

In the event that humans abandoned the interstellar mission prior to completion, the artificial intelligence's prime directive would be to dutifully archive all the data from the expedition for some future manifestation of humanity to review.

This idea superficially resembles the 1970 science fiction film, Colossus: The Forbin Project, where an autonomous national defense supercomputer was buried in a mountain and isolated from all human intervention. Unfortunately in this tale, the supercomputer starts playing God!

Post-apocalypse scenarios aside, let's assume that a future society is stable enough to conduct a multi-generation mission to a nearby exoplanet. In other words, there isn't a governing body that has to revisit and vote on project funding every few years.

The mass of the payload is composed of everything needed for conventional interstellar travel using classic action-reaction propulsion. The more massive the probe, the more fuel has to be carried to accelerate it to a fraction the speed of light. Even worse, more fuel has to be carried along so that the probe can decelerate and enter orbit around the target star system.

If you were a pioneer in the 1800s and you wanted to transport an oak tree across the country to the West Coast, you wouldn't carry the whole tree in your covered wagon. You'd take an acorn. Likewise, to minimize mass and weight, a very tiny payload will be needed for interstellar travel. Former NASA Administrator Dan Goldin even speculated about sending a payload to the stars no bigger than a tomato soup can!

The mini-probe would be a full-blown artificial intelligent supercomputer able to direct the mission. In the absence of ground controllers, its neural net would need to branch out from a set of fundamental directives for exploration. The idea is that this tiny "mothership" would be programmed to use resources at the target star system to build and launch smaller probes for reconnoitering the system's planets.

The notion of an intelligent self-replicating machine is credited to the early 20th century Hungarian-born mathematician and physicist John von Neumann, who rigorously studied the concept of what he called "Universal Assemblers." Now they are often referred to as "von Neumann machines."

As outlandish as this might sound, it would be far cheaper and more efficient than launching all the needed freight from Earth, accelerating this mass to perhaps one-tenth the speed of light, and then decelerating it.