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One of the most fascinating conversations I’ve had about my book Extraterrestrial was with a group of high school kids. They were genuinely curious and did not carry the baggage of prejudice or self-importance. At the end of our chat, they brought up the most consequential question: “What are the primary goals of our civilization?”

I explained that the two biggest tasks on the agenda of the human species are extending the longevity of our civilization and exploring the universe.

The first objective includes curing pandemics, avoiding wars, limiting climate change, shifting threatening asteroids away from Earth, and ultimately spreading our “eggs” in multiple baskets by traveling to space. The exploration objective has been pursued so far with telescopes or by sending spacecraft to destinations within the solar system.

But we can do better and reach the stars, literally speaking. The Starshot project, for which I chair an advisory board, aims to launch a probe that would visit the nearest star system, Alpha Centauri, within decades. This requires motion at a fraction of the speed of light, an improvement by a factor of a thousand in speed relative to chemical rockets, similar to the jump in speed from a Model T Ford to the New Horizons spacecraft.

The Starshot technology, a lightsail pushed by a powerful laser beam, was already envisioned long ago in a paper written by Robert Forward in 1962, my birth year. Starshot attempts to realize the concept imagined by Johannes Kepler in a letter to Galileo Galilei from 1610: “Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse.” Traveling to new worlds around other stars could be even more revolutionary than the expeditions that revealed the Americas to Europeans.

The kids followed up with the question: “Should we expect extraterrestrial civilizations with similar goals?” I answered “yes” out of a sense of cosmic modesty. The latest data from the Kepler space telescope imply that roughly half the sunlike stars have an Earth-size planet at about the same separation from them. Having similar temperatures and chemicals on the surfaces of tens of billions of Earth-like planets in the Milky Way galaxy, could have led to multiple technological civilizations capable of launching Starshot-like probes. Most stars formed billions of years before the sun, allowing such probes the opportunity to traverse the Milky Way galaxy many times, long before we came to exist.

The next question was obvious: “Could we detect probes of interstellar origin whizzing through the solar system at a fraction of the speed of light?” Fortunately, I had already studied this question quantitatively. In a paper with my colleague Thiem Hoang, we calculated that the James Webb Space Telescope (JWST), scheduled for launch on October 31, 2021, could detect the thermal infrared emission from nearby probes larger than a football field and moving faster than a tenth of the speed of light. Even without artificial lights on board, the unavoidable heat generated by friction with interplanetary would be detectable all the way out to a few times the distance to the Kuiper belt, at a hundred times the Earth-Sun separation—roughly where the two Voyager spacecraft are located.

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