Here’s a look at how scientific experts over the past decades have been attempting to find out if humans are well and truly alone in this universe.

The search for intelligent life elsewhere in the universe first began in 1959. Giuseppi Cocconi and Philip Morrison of Cornell University proposed in a published article that the microwave region of the electromagnetic spectrum was the best to use for interstellar communication since signals in this spectrum can travel very long distances. They surmised that the aliens would most likely use a "universal" frequency for their transmissions – their suggested frequency of 1420 MHz is still the most popular frequency used in SETI projects to this very day.

Around that time, young astronomer Frank Drake used the 85-foot West Virginia antenna to perform the first search for extraterrestrial signals in a project called “Project Ozma”. The staff member from the National Radio Astronomy Observatory in Green Bank, West Virginia, had also chosen the microwave region of the spectrum and settled on the same frequency Cocconi and Morrison had chosen – 1420MHz. This frequency corresponded to a wavelength of 21cm, which is similar to the wavelength at which neutral hydrogen atoms periodically emitted a photon – Drake reasoned that any extraterrestrials who wanted to attract attention would associate with the most abundant atom in the universe. “Project Ozma” kicked off on April 8, 1960, searching the two closest Sun-like stars of Epsilon Eridani and Tau Ceti for about a month. While the search ultimately proved fruitless, “Project Ozma” became the model for most SETI projects. Drake was to later develop the famous Drake Equation, which attempts to estimate the number of extraterrestrial civilizations in our galaxy with which we might come in contact. It allowed scientists to quantify the uncertainty of the factors which determine the number of extraterrestrial civilisations.

In the 1970s, the National Aeronautics and Space Administration (NASA) became interested in SETI. Bernard Oliver, an electronics expert from Hewlett Packard and a leading advocate of SETI who subsequently became chief of NASA's SETI program in the 1980s, co-developed the ambitious “Project Cyclops”. The plan called for a forest of about one thousand 100-meter dishes, occupying an area of about 10 kilometres in diameter, and used to sweep the sky for extraterrestrial signals. The project never came to pass – with a required investment of US$10 billion, it was far more than NASA would or could sanction.

In later years Oliver added another search frequency – 1662 KHz. This was the emission frequency of another very common molecule, OH, or hydroxyl. Hydrogen and hydroxyl combine to form H2O – or water – the basic component of life. Oliver believed that the band between 1662 KHz and 1420 MHz held a unique promise for detecting alien signals. He termed this band around the hydrogen emission frequency as the “water hole”, and from then on SETI searches concentrated on this narrow range of frequencies.

The Russians had also developed their own SETI program, which was initially modelled after the work of Cocconi, Morrison and Drake. However, Nicolai Kardashev of the Sternberg Institute of Moscow created a framework for SETI which showed far greater imagination than the Americans. He proposed the classification of civilisations into three categories. Type I civilisations – of which Earth was one – had technology levels which mastered energy from their star incident on their planet. Type II civilisations are able to harness all the energy of their star, while Type III ones could utilise energies on the scale of their own galaxies. Kardashev argued that it was far more practical to identify Type II and III civilisations, as opposed to the U.S. approach of searching for Type I ones.

While SETI searches had been mostly modest affairs so far, in 1992 NASA launched the largest SETI program ever undertaken. NASA adopted a two-pronged approach – its Ames Research Center would target 1,000 Sun-like stars and observe them, while the Jet Propulsion Laboratory would conduct an all-sky search. Tragically, the United States Congress terminated its funding for the project after less than a year.

This targeted search was later salvaged and resurrected as Project Phoenix by the SETI Institute, a privately funded organisation set up to conduct and sponsor research on SETI and life in the universe which continues to search for extraterrestrial signals. Unlike the SETI Institute which concentrates its efforts on a search in one highly-sophisticated area, The Planetary Society supports various SETI initiatives experimenting with different techniques, and largely following the all-sky search previously adopted by the Jet Propulsion Laboratory. Despite the earlier setbacks by NASA, today the SETI program is still well and truly alive.