If the development of human crew-carrying starships proves to be an impossible technical feat during the next few centuries, the human race can still search other star systems for alien life and spread terrestrial, carbon-based life around the galaxy with the help of sophisticated robot spacecraft. A modest effort might even be started at the end of this century with a decision by humans to launch one or more interstellar probes. Then, sometime in the 22nd century, our descendants might make innovative use of self-replicating system (SRS) technology in robot spacecraft based "life-detecting" and "life-spreading" exploration campaigns that extend eventually across the entire galaxy.
An interstellar probe is a highly automated robot spacecraft sent from one solar system to explore another star system. Most likely this type of probe would make use of very smart machine systems that are capable of operating autonomously for decades or centuries.
Once the robot probe arrives at a new star system, it would begin a detailed exploration procedure. The target star system is scanned for possible life-bearing planets, and if any are detected, they become the object of more intense scientific investigations. Data collected by the probe (which also serves as a mother spacecraft) and any miniprobes (deployed to explore individual objects of interest within the new star system) are transmitted back to Earth. After years of travel, these signals eventually are intercepted and analyzed by terrestrial scientists. Over time, such probes provide a continuous stream of interesting, unpredictable discoveries that enrich knowledge of the nearby star systems and any life forms that might share the galaxy with the human race.
The robot interstellar probe could also be designed to carry a payload of specially engineered microorganisms, spores, and bacteria. If the robot probe encounters ecologically suitable planets on which life has not yet evolved, then it could make the decision to "seed" such barren but potentially fertile worlds with primitive life-forms. In that way, human beings (in partnership with their robot probes) would not only be exploring neighboring star systems but would be participating in the spreading of life itself through some portion of the Milky Way galaxy.
Long-range strategic planners in the aerospace field have examined some of the engineering and operational requirements of the first interstellar probe, as might be launched at the end of this century to a nearby star—most likely one within 10 light-years distance or less. Some of these challenging requirements (all of which exceed current levels of technology by one or two orders of magnitude) are briefly mentioned here. The interstellar probe must be capable of sustained, autonomous operation for more than 100 years. The robot spacecraft must be capable of managing its own health—that is, being able to anticipate or predict a potential problem, detect an emerging abnormality, and then preventing or correcting the situation. For example, if a subsystem were about to overheat (but has not yet exceeded thermal design limits), the smart robot probe would redirect operations and adjust the thermal control system to avoid the potentially serious overheating condition.
The first interstellar robot probe must have a very high level of machine intelligence and be capable of exercising fault management through repair, redundancy, and workarounds without any human guidance or assistance. The smart robot must also be able to manage its onboard resources carefully, supervising the generation and distribution of electric power, allocating the use of consumables, deciding when and where to commit emergency reserves and a limited supply of spare parts and components. The main onboard computer (or machine brain) of the probe must exercise data management skills and be capable of an inductive response to unknown or unanticipated environmental changes. When faced with unknown difficulties or opportunities, the robot probe must be able to modify the mission plan and to generate new tasks.
During one such hypothetical mission, long-range sensors on board the probe might discover that the hot-Jupiter-type extrasolar planet within the target star system has a large (previously unknown) moon with an atmosphere and a liquid-water ocean. Instead of sending its last miniprobe ahead to investigate the extrasolar planet, the smart robot mother spacecraft makes a decision to release its last miniprobe to perform close-up measurements of this interesting moon. Since the mother spacecraft is more than eight light-years from Earth when the (hypothesized) discovery is made, the decision to change the mission plan must be made exclusively by the robot spacecraft, which is less than a few light-days away from the encounter. Sending a message back to Earth and asking for instructions would take more than 16 years (for round-trip communications), and by then the interstellar probe would have completely passed through the target star system and disappeared into the interstellar void.
Similarly, instruments on board the interstellar probe (regarded here as the mother spacecraft) and its supporting cadre of miniprobes must be capable of deductive and inductive learning so as to adjust how measurements are taken in response to unfolding opportunities, feedback, and unanticipated values (high and low). Some of the greatest scientific discoveries on Earth happened because of an accidental measurement or unanticipated reading.
So, the instruments on board the robot probe must be capable of exercising some "artificial thinking" level of curious inquiry and then be able to respond to unanticipated but quite significant new findings. The robot probe should also have a level of machine intelligence that is capable of knowing and appreciating when newly acquired data is very significant. This is a difficult task for human scientists, who often overlook the most significant pieces of data in an experiment or observation. To ask a robot's mechanical brain to respond "eureka" (I've found it) at a moment of a great discovery is pushing machine intelligence well beyond the technical horizon that is projected for the next few decades. Yet, if the human race is going to make significant discoveries using robot interstellar probes, that is precisely what these advanced exploring machines must be capable of doing.
From a purely engineering perspective, the interstellar robot probe should consist of low-density, high-strength materials to minimize propulsion requirements. Remember that to keep a mission to the nearby stars within 100 years or so duration, the robot spacecraft should be capable of cruising at about one-tenth the speed of light (or more). Any less than that and a star-probe mission to even the nearest stars would take several centuries. The great-great-great-grandchildren of the probe engineers would somehow have to remain interested in receiving the signals from the (probably long-forgotten) probe. So, these early interstellar probe missions (using advanced, but nonreplicating technology) will most likely take a 100 years or less.
The materials used on the outside of the robot probe must maintain their integrity for more than a century, even when subjected to harsh, deep space conditions—such as ionizing radiation, cold, vacuum, and interstellar dust. The structure of the robot spacecraft should be capable of autonomous reconfiguration. The power system must be able to provide reliable base power (typically at a level of 100 kilowatts-electric up to possibly one megawatt-electric) on an autonomous and self-maintaining basis for more than 100 years. Finally, the star probe must be capable of autonomous data collection, assessment, storage, and communications (back to Earth) from a wide variety of scientific instruments and onboard spacecraft state-of-health sensors.
Some of the intriguing challenges in information technology include the proper calibration of instruments and the collection of data during a period of years after decades of sensor dormancy. The robot probe must be able to transmit data back to Earth for distances ranging from 4.5 to 10.0 light-years, or more. Finally, after decades of handling modest levels of data, the spacecraft's information systems must be capable of handling a gigantic burst of incoming data as the robot probe and its miniprobes encounter the target star system.
While this section and the remaining sections of the chapter discuss robot spacecraft that are launched by the human race to explore nearby star systems, the reader should not lose sight of the fact that the reverse circumstance is also a distinct possibility. An alien civilization, perhaps 10 to 15 light-years distant from Earth and about a century ahead of the human race in technology, could right now be in the process of sending its own robot probes to investigate humans' solar system and home planet.
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