Nikolai Kardashev, who was one of the earliest, and most consequential, SETI practitioners, passed away on August 3, 2019. At a time when a search for extraterrestrial intelligence was more a thought experiment than one requiring actual hardware, clever physicists in the Soviet Union formulated many of the seminal ideas for this nascent discipline. Of these, Kardashev was among the best known.

Coming of age in the postwar era, Kardashev studied under Josef Shklovskii, the celebrated author of the first general treatise on how we might search for technologically capable beings in the cosmos. Shklovskii’s book was translated (and added to) by Carl Sagan, and became the inspiration for many in SETI after its 1966 publication in the West. His early exposure to the fundamental premises of SETI led Kardashev to turn his inventive mind to this new field, one that was open to big ideas.

A famous example of his early involvement occurred following the development of practical aperture synthesis, a radio astronomy technique that allowed the precise determination of positions of sources on the sky. The increased precision soon led to the discovery of radio galaxies and quasars. One of the latter, known by its catalog designation CTA 102, was claimed by the Soviets to vary in intensity, and Kardashev famously suggested that its irregular emissions might be a deliberate message from extraterrestrials. This was a sensational claim (it even led to a hit song by The Byrds), but one that was eventually ruled out when it was learned that the quasars were intrinsically variable.

Perhaps the most well-known of Kardashev’s contributions to SETI was the Kardashev Scale, a handy rubric for categorizing putative civilizations. A society (such as our own) that commands the energy resources of its own planet is described as a Type I civilization. Type II civilizations can avail themselves of the total energy output of their home star, and a Type III society is able to use the total energy of its own galaxy.

Kardashev was one of the important early thinkers in SETI. His supple mind and gentle manner will be missed by all who knew him.

- Seth Shostak, Senior Astronomer