1963 00 00 Planetary and Space Science - Vol 11 No 5 - Sagan

Magazine Overview

This document is a reprint from 'PLANETARY AND SPACE SCIENCE', Volume 11, with pages 485-498. The article, titled 'DIRECT CONTACT AMONG GALACTIC CIVILIZATIONS BY RELATIVISTIC INTERSTELLAR SPACEFLIGHT*', is authored by Carl Sagan and was received on December 16, 1962. It was presented at the American Rocket Society's 17th Annual Meeting on November 15, 1962. The publisher is Pergamon Press, with locations in Oxford, London, New York, and Paris.

Direct Contact Among Galactic Civilizations by Relativistic Interstellar Spaceflight*

Abstract

The paper estimates the number of advanced technical civilizations in our Galaxy by considering factors such as star formation rate, the frequency of planets, the probability of life, intelligence, technical civilization, and their lifetimes. The estimate suggests around 10^6 such civilizations, with the nearest being several hundred light-years away. Relativistic interstellar spaceflight is presented as a viable method for contact, offering advantages over electromagnetic communication. The article discusses technical challenges and potential solutions, concluding that interstellar travel is a feasible objective for humanity.

Introduction

The article begins by noting the resurgence of interest in the possibility of civilizations on other worlds. It highlights that recent work on stellar statistics and the origin of life suggests that life, intelligence, and technical civilizations may be common in the universe. The prospect of contact with other galactic communities is raised, with radio emission and relativistic interstellar spaceflight being considered as communication channels. The paper aims to explore the likelihood and consequences of the latter.

Distribution of Technical Civilizations in the Galaxy

Sagan employs a modified Drake Equation (N = R* f_p n_e f_l f_i f_c L) to estimate the number of extant advanced technical civilizations. Key parameters discussed include:

• R* (mean rate of star formation): Estimated at ~10 stars/year.
• f_p (fraction of stars with planetary systems): Assumed to be high, with later spectral type stars likely forming massive planets.
• n_e (mean number of habitable planets per system): Estimated at ~1, considering Earth and Mars, and potential for life on Jovian planets.
• f_l (fraction of habitable planets where life develops): Assumed to be ~1, as life appears to arise rapidly under suitable conditions.
• f_i (fraction of inhabited planets with intelligent life): Estimated at ~10^-1, acknowledging the improbability of the specific evolutionary path to intelligence.
• f_c (fraction of planets with technical civilizations): Estimated at ~10^-1, due to the concatenation of improbable events required for technical development.
• L (lifetime of a technical civilization): This is the most uncertain parameter. Two extreme cases are considered: L < 10^2 years (self-destruction) and L > 10^8 years (long-term survival). Sagan favors the latter, estimating L ~ 10^7 years.

Using these estimates, the number of advanced technical civilizations (N) is calculated as approximately 10^6. This implies a distance of several hundred light-years to the nearest such community.

Feasibility of Interstellar Spaceflight

Electromagnetic communication over interstellar distances is deemed too slow, with query-response times of up to 1000 years. Relativistic interstellar spaceflight is presented as a more effective method for direct contact. The article discusses two basic approaches:

1. Slowing down metabolic activities: For very long flight times.
2. Relativistic interstellar spaceflight: Achieves time dilation, allowing journeys within a human lifetime as measured on the spacecraft.

Sagan analyzes a flight plan involving uniform acceleration to the midpoint and uniform deceleration thereafter. At 1g acceleration, it takes a few years to reach nearby stars, 21 years to the Galactic Center, and 28 years to the nearest spiral galaxy. Time dilation significantly reduces the perceived travel time for the crew. For example, a round trip to the nearest stars would take a few decades on Earth.

Technical Challenges and Solutions

The paper addresses several technical difficulties of relativistic spaceflight:

• Mass Ratios: Enormous initial vehicle masses are required, with liftoff weights potentially 2000 times the payload for speeds near the speed of light. Round trips require even higher ratios.
• Energy Conversion: Complete conversion of mass to energy is limited by baryon charge conservation.
• Antimatter: Containment and production of antimatter are significant problems, and its gamma-ray exhaust could be lethal.

Potential solutions discussed include:

• Staged Fusion Rockets: Offer some relaxation of mass ratios but may not achieve relativistic velocities alone.
• Bussard Interstellar Ramjet: This concept uses the interstellar medium as both a working fluid and energy source via thermonuclear fusion. It requires large frontal area loading densities and may involve seeking trajectories through H II regions where the medium is ionized.

Conclusion

Despite the significant engineering challenges, Sagan concludes that a combination of technologies, including staged fusion boosters, ramjets, and potentially superconducting flux pumps for magnetic field generation, makes interstellar spaceflight at relativistic velocities a feasible objective for humanity. He quotes Bussard's assessment that while the undertaking is grand and magnitudes more difficult than interplanetary travel, the expansion of human horizons would be proportionate, and 'nothing worthwhile is ever achieved easily.'

Recurring Themes and Editorial Stance

The recurring themes in this article revolve around the scientific estimation of extraterrestrial intelligence, the feasibility of interstellar travel, and the potential for direct contact between civilizations. Sagan's stance is one of optimistic scientific inquiry, grounded in current astrophysical and biological understanding, while acknowledging the significant uncertainties and challenges. He advocates for continued research and exploration, suggesting that interstellar spaceflight is not only possible but a worthy goal for humanity's future.

This document consists of pages 494-498 from a publication, likely a magazine or journal, featuring an article by Carl Sagan titled "DIRECT CONTACT AMONG GALACTIC CIVILIZATIONS". The article delves into the theoretical aspects of interstellar communication and contact between advanced civilizations in the Milky Way galaxy.

Frequency of Contact Among Galactic Communities

Sagan posits that if interstellar spaceflight is technically feasible, it will inevitably be developed by advanced civilizations, even if it is expensive and difficult from our current perspective. The scientific advantages are immense, including direct astronomical sampling of stars and planets across various evolutionary stages, and the study of diverse biologies and societies. He suggests that these undertakings could inspire even long-lived civilizations. For a civilization's lifetime, L, interstellar spaceflight to all points within the Galaxy, and even to other galaxies, is theoretically possible. Voyagers would return in the distant future, but a record and community interested in the results would persist. To avoid duplication, communicative societies would pool information and act in concert, as noted by Bracewell. Direct contact and exchange of information and artifacts would be common among spacefaring societies with relativistic starships. Over large distances, starship communication would be nearly as rapid and more reliable than electromagnetic radiation.

The article estimates the mean time interval between contacts for a given planetary system. While shipboard transit times at relativistic velocities are similar across the Galaxy, the elapsed time on the home planet is proportional to the voyage distance. Thus, contact would be greatest among neighboring communities, though occasional long journeys would occur. If N planets are in the communicative phase and launch q relativistic starships per year, with each journey resulting in at least one contact and lasting 10^3-10^4 years, then there are approximately qN contacts per year for the Galaxy. A value of q = 1 yr^-1 is considered modest. Each civilization would make about 1 contact per year, and an average of 10^7 contacts during its lifetime. The total number of contacts per year for the Galaxy is estimated at 10^6, with a significant fraction occurring between two advanced communities. The mean number of starships on patrol from each technical civilization at any given time is estimated to be around 10^3-10^4.

Direct Contact Among Galactic Civilizations

If contacts are made on a purely random basis, each star would be visited about once every 10^5 years. Advanced civilizations would have excellent knowledge of which planetary environments are most likely to develop intelligent life. With an average contact frequency per planet of 10^-5 yr^-1, the origin and evolution of life on every planet in the Galaxy could be monitored efficiently. The development of metazoa, cooperative behavior, tool use, and primitive communication schemes would be noted, leading to increased interstellar sampling frequency. If the frequency of contact with intelligent pretechnical planetary communities is around 10^-4 yr^-1, this frequency would increase once technical civilization is established and enters the communicative phase, potentially reaching 10^-8 yr^-1. Planets of extraordinary interest would be visited more frequently. Under these assumptions, each communicative technical civilization would be visited by another about once every thousand years. The survey vehicles of each civilization would return to the home planet about once a year, and a sizable fraction would have had contact with other communities. This commerce of goods, information, arguments, artifacts, concepts, and conflicts would sharpen curiosity and enhance vitality.

This discussion has an application to Earth. Based on the assumptions, about one or two million years ago, with the emergence of Proconsul and Zinjanthropus, the rate of sampling of our planet should have increased to about once every ten thousand years. At the beginning of the most recent post-glacial epoch, the development of social structure, art, religion, and elementary technical skills should have further increased the contact frequency. If the interval between samplings is only several thousand years, there is a possibility that contact with an extraterrestrial civilization has occurred within historical times. This possibility was famously raised by Enrico Fermi with the question, "Where are they?"

Possibility of Extraterrestrial Contact with Earth During Historical Times

Sagan notes that there are no reliable reports of contacts in the last few centuries, when critical scholarship is widespread. Earlier contact stories are likely encumbered by fanciful embellishment due to prevailing views at the time. He cites Brailoiu's account of a Rumanian folklore incident where a story became embellished with mythological material.

A more relevant incident is the native account of the first contact between the Tlingit people of the Northeast Coast of North America and European civilization in 1786, led by the French navigator La Perouse. The Tlingit kept no written records, but a century later, the verbal narrative was related to Emmons by a principal Tlingit chief. The story, though overlaid with a mythological framework (e.g., sailing ships described as immense black birds), preserved the true nature of the encounter. A blind old warrior boarded a French ship and exchanged goods, reasoning that the occupants were men. This interpretation led to active trade.

The Tlingit encounter suggests that under certain circumstances, a brief contact with an alien civilization can be recorded in a reconstructable manner. This reconstruction is aided if the account is written down soon after, if a major change occurs in the contacted society, and if the contacting civilization does not disguise its exogenous nature. However, reconstructing contact with an extraterrestrial civilization is fraught with difficulties. The guise of contact myths can vary; a simple account of a strange being performing marvelous works in the heavens is insufficient. Peoples need to understand their environment, and attribution to nonhuman deities is common. When different groups have different deities, each may claim extraordinary powers for its god. Residence in the sky is not necessarily indicative of extraterrestrial origin. The article suggests that a credible contact legend would require a description of the morphology of an intelligent non-human, a clear account of astronomical realities, or a transparent presentation of the purpose of the contact.

In the Soviet Union, Agrest and others have pointed to biblical incidents that might reflect contact. Agrest considers the incidents in the apocryphal book "Slavonic Enoch" to be an account of visitation by extraterrestrial cosmonauts. However, the "Slavonic Enoch" fails to meet criteria for a genuine contact myth, as it is molded into supernatural frameworks, lacks transparent extraterrestrial motivation, and contains largely incorrect astronomy.

Other legends that more nearly satisfy contact criteria include the Babylonian account of the origin of Sumerian civilization by the Apkallu, representatives of an advanced, nonhuman, and possibly extraterrestrial society.

Direct Contact Among Galactic Civilizations (Continued)

Sagan acknowledges that a completely convincing demonstration of past contact may not be possible based solely on textual and iconographic grounds. However, other sources of information exist. Statistics suggest Earth may have been visited by various galactic civilizations many times (~10^4) during geological time. Artifacts of these visits might still exist, or a base might be maintained automatically within the solar system. Such a base would likely not be on Earth's surface due to weathering and potential detection, making the Moon a reasonable alternative. High-resolution photographic reconnaissance of the Moon's far side might reveal such possibilities. Other locales in the solar system could also be of interest. Contact with such a base would provide the most direct check on the paper's conclusions.

Alternatively, the abundance of advanced civilizations in the Galaxy could be tested by detecting intelligible electromagnetic signals of interstellar origin. In the coming decades, humanity will have the capability to transmit signals over hundreds of light-years. Receiving and returning such a signal would announce our presence as a technical civilization, potentially leading to a special contact mission if the paper's conclusions are valid. Even if an intelligible signal were received and returned today, it would take several hundred years for a contact mission to arrive on Earth. Sagan hopes a thriving terrestrial civilization would still exist to greet these visitors.

Acknowledgements and References

The research was supported in part by grant NsG-126-61 from the National Aeronautics and Space Administration. The author thanks J. Finkelstein, J. Lederberg, L. Sagan, M. Schmidt, C. Seeger, I. S. Shklovsky, C. Stern, and W. Talbert for their discussions, and fellow members of the Order of the Dolphins, especially F. D. Drake and P. M. Morrison.

A comprehensive list of 41 references is provided, citing works on topics such as the origin of life, stellar evolution, planetary science, radio astronomy, and interstellar communication.

Резюме (Summary in Russian)

The Russian summary reiterates the main points: the number of advanced technical civilizations depends on star formation rates, planetary distribution, probabilities of life and intelligence, and civilization longevity. The article estimates 10^6 such civilizations in the Galaxy, with the nearest community being several hundred light-years away. Interstellar relativistic flights have advantages over electromagnetic communication due to time dilation. Technical challenges for spacecraft construction are discussed, with conclusions that energy problems are not insurmountable with nuclear stages, fusion reactors, and Bussard ramjets. The article hypothesizes a unified galactic society. If each advanced civilization launched one starship per year, the average interval between testing a star would be 10^5 years, a planetary system with life 10^4 years, and another advanced civilization 10^3 years. This implies a statistical probability that an advanced extraterrestrial civilization has visited Earth at least once in historical times. Proving such contact through ancient documents and iconography is difficult, but legends may be useful. Bases or artificial objects of interplanetary civilizations might exist in the solar system, such as on the far side of the Moon. The article's conclusions are presented as preliminary.

Recurring Themes and Editorial Stance

The recurring theme is the scientific and statistical exploration of the likelihood and nature of extraterrestrial contact. Sagan adopts a rational, evidence-based approach, using scientific principles and estimations to discuss complex hypothetical scenarios. The editorial stance appears to be one of open inquiry, encouraging further research into the possibility of extraterrestrial intelligence and contact, while maintaining a critical perspective on the interpretation of historical and mythological accounts. The article emphasizes the vastness of the universe and the potential for advanced civilizations, urging a scientific approach to understanding our place within it.