Are We all Alone, or could They be in the Asteroid Belt?

Summary

The observations that life has a natural tendency to expand into all available space, that advanced technological civilizations should be able to engage with relative ease in interstellar travelling, and that once this threshold is crossed the complete colonization of the entire Galaxy will be accomplished in a very short interval relative to the age of the Galaxy, lead us to the following dilemma: either the entire Galaxy is teeming with intelligent life and hence our solar system must have been colonized hundreds of millions of years ago, or there are no other inhabitants in our solar system and hence most probably neither anywhere else in the Galaxy. Before accepting, however, the bleak verdict that we are all alone in the Galaxy, we must search carefully throughout the solar system for any signs of other technological civilizations. The most logical place to look for them seems to be the asteroid belt because of the many advantages it offers to a galactic society living in space colonies.

A Change Of Heart

The euphoric optimism of the sixties and the early seventies that communication with extraterrestrial civilizations seemed quite possible (Sagan 1973), is being slowly replaced in the last couple of years (Hart 1975; Jones 1976; Shklovskii 1977) by a pessimistic acceptance that we might be the only technological civilization in the entire Galaxy. This change of heart has been happening as a result of the following observations:

1. Life seems to possess a natural tendency to expand like a gas to occupy all available space. This is evident, e.g. when algae rapidly take over an unattended swimming pool, and certainly has been the characteristic of man who after conquering the entire planet is now ready for new ventures in outer space.
2. Interstellar travelling seems easily attainable, especially in the 0.01-0.1 c range, by advanced technological societies. The building of permanent colonies in space, as envisioned by O'Neill (1974), will make it possible for people living all their lives in these colonies to disengage emotionally from the mother planet. Such colonies would have the emotional strength and coherence to undertake trips to the nearby stars that would last for several generations. Nuclear fusion, with a 0.007 mc2 yield, can become a most attractive energy source for such trips even at an efficiency Eta as low as 10-20 per cent. As seen from the relation
   
   1/2MV2 = 0.007 Etamc2 mc2 (1)
   
   a spaceship of mass M will be able to reach speeds V = 13 x 10-2c with a fuel load m not larger than M.
3. In the last 100 yr or so, the velocities of long, non-stop voyages (trains to spaceships) have increased from ~ 103 to 106 cm/s. It seems reasonable, therefore, to anticipate an additional increase by a factor of 3001000 in the next 100200 yr, especially with the use of nuclear fusion. One can be optimistic also about self-sufficient space colonies, which according to the computations of O'Neill could be a reality even before the turn of the century. It appears, therefore, that with steady technological progress and without the need of any new major discoveries, we should be able to undertake stellar missions in a few centuries. This is an interval in cosmic terms as brief as a few minutes in the life of a man, which means that our civilization is extremely close to this critical moment.
4. Once the threshold of interstellar travelling is crossed the entire Galaxy will be colonized in only a few million years, which is a very short period relative to the 10-15 billion year age of our Galaxy. Even by assuming an expansion rate of one light year per century (say 500 yr for a 10 lt yr trip to a suitable nearby star, and 500 yr for the building of the new colony before it can undertake further stellar missions), we see that the entire Galaxy can be conquered in less than 10 M yr. It is also clear that as new colonies join the colonization wave, there will be so many interstellar travellers that there will be no reasonable place in the Galaxy that will remain unoccupied.
5. The many attractive features of our solar system (a single, well-behaved, long-lasting, hot star surrounded by a multitude of diverse planets, moons and asteroids) could have not been missed by the colonizers, and therefore our solar system could have not been bypassed as the colonization wave swept through the Galaxy.
6. The likelihood is that the extraterrestrial colonizers of our solar system, especially after their long interstellar voyages, will have become accustomed to space living. As a result, not only will they not need a habitable planet to settle on, but most probably they would prefer to continue living in space colonies. The orbits of their choice would obviously be these that provide the most efficient access to material and energy resources.
   

From the above it follows that if hundreds of millions of intelligent civilizations did evolve in our Galaxy over the past several billion years, as suggested by the integration of the DrakeSagan probability formula over the entire history of the Galaxy (Freeman & Lampton 1975), then it seems inevitable that some of these galactic civilizations would have achieved interstellar travelling and the whole Galaxy, including our solar system, would have been teeming with advanced technological societies. Conversely, if we are the only technological inhabitants of our solar system, then most likely we are also the only ones of the entire Galaxy. This deduction implies that the values commonly used for one or more of the probability factors of the Drake formula (Shklovskii & Sagan 1966; Kreifeldt 1971; Sagan 1973; Oliver 1975) must have been grossly over-estimated (Papagiannis 1978).

Could They Be Around?

We have reached, therefore, the stage where the acid test for our dilemma seems to be whether or not our solar system is inhabited by an advanced extraterrestrial society. Of course there are in the literature several reports of UFO sightings and even stories of dramatic encounters with extraterrestrials. There are also several popular books, such as those of von Da:niken (1969), in which the intervention of extraterrestrials in this planet is envisioned on countless occasions. Still, however, there is no convincing proof to any of these stories or suggestions; in accordance with the Shklovskii principle that "all events should be considered natural unless proven otherwise' the scientific community remains unconvinced about visits to Earth by extra-terrestrials. As a result, and in accordance with the presently available evidence, we tend to believe that we arc the only advanced civilization inhabiting our solar system.

Absence of evidence however, should not be taken as evidence of absence. Before we resign therefore, to a pessimistic acquiesence that we might be the only technological inhabitants of our Galaxy, we have the responsibility to search exhaustively in our solar system for other advanced societies. The supposition that we are alone in the solar system is based essentially on the assumption that if others were here they would have already made contact with us, or at least we would have become aware of their existence. Neither of these assumptions, however, is necessarily true, though it is possible that some of the thousands of UFO sightings might deserve some further consideration, as suggested by Hynek (1972).

The most intriguing question in the whole problem is the following:

if our solar system is indeed inhabited by extraterrestrials, where are they most likely to be found? In earlier days people had tried to identify one of the other planets of our solar system, most frequently Mars (Lowell 1908), as the abode of an extraterrestrial society. From the above discussion however, it follows that the colonizers of our solar system are likely to continue to live in space colonies, probably at reasonably close distance to the Sun so as to have a sufficient supply of solar energy for their needs, and most likely near celestial bodies of weak gravity from which they would obtain all the natural materials needed for the continuous prosperity of their civilization.

The Asteroid Belt Choice

Within this framework, it seems that the asteroid belt would be an ideal place for the extraterrestrials to set up their space colonies. Not only would they have an easy access to all natural resources by mining the asteroids, but they would also be close enough to the Sun to have ample solar energy for their needs. Some years ago this suggestion would have sounded unreasonable, mainly because we used to think that the asteroid belt must be full of debris which can be very hazardous for any spaceships permanently stationed in their vicinity. The Pioneer 10 and 11 missions, however, (Kinard et al. 1974) have found that the density of meteriods in the asteroid belt hardly differs from any other place in the solar system, and therefore the colonization of the asteroid belt seems quite feasible. There are, of course, also the Kirkwood gaps in the asteroid belt which are practically free of any asteroids. A spaceship could remain there almost indefinitely by simply compensating against the tidal effects of Jupiter with its own propulsion system.

One can even consider the possibility that the large fragmentation of the components of the asteroid belt might be the result of mining projects by the extraterrestrial colonies. It is even conceivable that they have tried to keep the region clean of free floating debris for their safety. The identification of space colonies 110 km in size hidden in the asteroid belt would not be an easy task for a terrestrial observer because from a long distance these colonies would be practically indistinguishable from the thousands of natural asteroids. They would also follow the same orbits around the Sun as the asteroids, which would be much more numerous and therefore it would be almost like searching for a needle in a haystack. Still with careful observations in the radio domain we might be able to detect some leakage of radio noise, infrared observations might reveal a higher effective temperature than that which is justified by their distance from the Sun and finally, properly planned space missions to the asteroid belt might do some successful eavesdropping and might even return some direct photographic evidence. The search project, therefore, though admittedly quite difficult, is still within the capabilities of our present technology and in view of the far-reaching consequences of either positive or negative results, should be given a serious consideration.

Why Are They Silent?

As to why they have not yet made contact with us, one can think of several answers, including the zoo hypothesis of Ball (1973). The simplest explanation, however, and hence, maybe the most probable one, might be that of confusion and indecision. Our hypothetical neighbours were probably acquainted for millions of years with a lethargic Earth inhabited by life forms not worth any effort of communication. Suddenly, in the last 50 yr or so, which probably is a very short interval for a well-settled galactic society, they have been confronted with an exponentially mushrooming technological society. (aeroplanes, radio-communications, nuclear bombs, spacecraft) which undoubtedly must be causing them some serious concern. It is possible, however, that faced with such a sudden technological explosion, a serene cosmic civilization would be perplexed and undecided as to how to handle the situation. They might be debating on whether to crush us or to help us, and therefore they might be simply postponing their decision, waiting to see what we are going to do with ourselves. Meanwhile, the asteroid belt provides a natural hide-out where they can remain inconspicuous for a long time until we decide to search for them.

Conclusions

In conclusion, though the idea that the asteroid belt might be harbouring a number of extraterrestrial colonies sounds like science fiction, the arguments presented above suggest that if there are any extraterrestrial colonies in our' solar system then the asteroid belt seems to be the most logical place to look for them. Before accepting, therefore, the bleak verdict that we are all alone in the Galaxy, we have an obligation carefully to investigate this possibility, remote as it might seem.