How Far Human Civilisation is Detectable in Space 

Earth’s most detectable techno-signatures are the planetary radar transmissions from the erstwhile Arecibo Observatory. The Arecibo message could be detected up to about 12,000 light-years away from the Earth which is nearly halfway to the galactic centre.  However, since Arecibo message was broadcasted in 1974, it has travelled only about 50 light-years so far. Given the right timing and orientation of the receivers, the Arecibo message would be detectable to an ETI with “Earth level technology” up to a distance of about 12,000 light years from earth when the signal reaches that distance in future. The next are atmospheric technosignatures such as nitrogen dioxide emissions which are detectable as far as 5.7 light-years away. Other techno-signatures such as city lights, lasers, heat islands, and satellites are detectable closer to earth only.   

Does life exist elsewhere in the universe? The probability of existence of extraterrestrial intelligence is high given observable universe has about 200 billion to 2 trillion galaxies, and our home galaxy Milky Way alone may have between 1000 and 10 million planets with civilizations (as per Drake’s original estimate). These estimates have been refined over the years. A study published in 2020 suggests there should be at least 36 communicating civilizations within our home galaxy based on the assumption that the average lifetime of a communicating civilization is 100 years. Paradoxically, there is no evidence of existence of extraterrestrial intelligence yet. For the last seven decades, there have been consistent scientific efforts to search for extraterrestrial intelligence (SETI) which largely focussed on detecting extraterrestrial signals or transmissions from extraterrestrial worlds to our home planet.  

However, there have also been initiatives to reach out to intelligent life beyond earth by sending messages to alien civilizations. For instance, Pioneers 10 and 11 missions launched in 1972-73 contained gold-anodized aluminium plaques depicting a naked man and woman and diagrams showing the location of the Sun and Earth relative to pulsars to serve as cosmic landmarks. The purpose was to enable extraterrestrial intelligent life to contact humanity on earth if the metal plates were ever found by them. Similarly, Voyager 1 and Voyager 2 spacecrafts, launched in 1977, each carried a gold-plated copper disk containing sounds and images portraying the diversity of life and culture on Earth. The aim was to communicate the story of life on earth to advanced space-faring civilizations in interstellar space. Both the Voyager spacecrafts are now in interstellar space exploring the outermost edge of the Sun’s heliosphere as Voyager Interstellar Mission (VIM).  

In addition to sending metal plates inscribed with messages, narrowband radio signals have also been deliberatively transmitted. For instance, “Arecibo message” was an interstellar radio message that carried basic information about human civilisation and our home planet which was broadcasted by Frank Drake about 50 years ago in 1974. This single radio broadcast was directed to the globular cluster Messier 13 situated about 25,000 light years away from Earth and was meant to be a demonstration of human technological achievement. Nonetheless, as a techno-signature (viz., technology including signals or patterns that are developed by intelligent life and cannot be explained by natural phenomena) of humankind on earth, the Arecibo interstellar radio message has already travelled about 50 light years in interstellar space.  

Would the Arecibo message be detectable to extra-terrestrial intelligence (ETI) out there in the space? Would various present-day Earth techno-signatures such as radio transmissions, atmospheric techno-signatures, optical and infrared signatures, and objects in space or on planetary surfaces be detectable to the ETIs?  

Detectability of various techno-signature signals of earth in interstellar space depend on several factors including timing, location/orientation of the receiving body, signal strength, technological advancement of the civilisation receiving the signal, etc. Of the various variables, level of technological advancement of extra-terrestrial intelligence (ETI) is crucial in determining detectability of Earth’s signals. Given high probability of existence of ETIs which may be at different level of technological advancements, just anything is possible. However, it may help to put things in perspective if we consider the special case of a hypothetical extra-terrestrial civilisation at the same level of technological advancement as the present-day earth. What would be the maximum distance of detectability of various present-day Earth techno-signatures in such a case? This has been recently investigated by researchers in a recent paper titled “At what distance could earth’s constellation of technosignatures be detected with present-day technology?”.   

Using a theoretical, modeling-based method, the researchers assessed the maximum distance of detectability for various present-day Earth techno-signatures such as radio transmissions, atmospheric signatures, optical and infrared emissions, etc using only present-day level Earth instruments. It was found that Earth’s most detectable techno-signatures are the planetary radar transmissions from the erstwhile Arecibo Observatory.  

The Arecibo message could be detected up to about 12,000 light-years away from the Earth which is nearly halfway to the galactic centre (our home galaxy Milky Way is 105,700 light-years in diameter and the solar system is 26,000 light-years from the galactic centre). However, since Arecibo message was broadcasted in 1974, it has travelled only about 50 light-years so far (for a comparison, the Voyager spacecrafts which are now in intergalactic space have travelled only about 0.0026 light years so far). Given the right timing and orientations, the Arecibo message would be detectable to an ETI with “Earth level technology” up to a distance of about 12,000 light years from earth when the signal reaches that distance in future. The next are atmospheric technosignatures such as nitrogen dioxide emissions which are detectable as far as 5.7 light-years away (for comparison, Proxima Centauri, the nearest star to Earth after the Sun is located at a distance of 4.25 light-years). Other techno-signatures such as city lights, lasers, heat islands, and satellites are detectable closer to earth only.  

Since the Arecibo message, the most powerful techno-signature of Earth has travelled only 50 light years so far since its transmission in 1974, the maximum distance of detectability at present is about 50 lights years. For other techno-signatures, the maximum distance of detectability is much less. None of Earth’s techno-signatures may have reached beyond 50 light years in the space although radio transmissions are used for about a century. This is Earth’s presentation to the outer world.  

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References:  

1. Westby T., and Conselice C.J., 2020. The Astrobiological Copernican Weak and Strong Limits for Intelligent Life. The Astrophysical Journal, Volume 896, Number 1. DOI: https://doi.org/10.3847/1538-4357/ab8225 

2. NASA. The Golden Record. Published 5 Nov 2024. Available at https://science.nasa.gov/mission/voyager/voyager-golden-record-overview/  

3. NASA. What are the contents of the Golden Record? Published 14 Aug 2024. Available at https://science.nasa.gov/mission/voyager/golden-record-contents/  

4. SETI Institute. Arecibo Message. Available at https://www.seti.org/seti-institute/project/details/arecibo-message  

5. SETI Institute. Press release – Earth Detecting Earth. Posted 2 February 2025. Available at https://www.seti.org/press-release/earth-detecting-earth 

6. Sheikh S. Z., et al 2025. Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures Be Detected with Present-day Technology? The Astronomical Journal, Volume 169, Number 2. Published 3 February 2025. DOI: https://doi.org/10.3847/1538-3881/ada3c7  

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Related articles: 

• Extra-Terrestrial: Search for Signatures of Life (15 July 2019) 

• Exoplanet Science: James Webb Ushers in a New Era (3 September 2022)  

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