On blind dates, we search for others that resemble us, at least at some level. This is true in our personal life but even more so on the galactic dating scene, where we have been seeking a companion civilization for a while without success. While developing our own radio and laser communication over the past seven decades, the Search for Extraterrestrial Intelligence (SETI) focused on radio or laser signals from outer space—two kinds of electromagnetic “messenger” that astronomers use to study the cosmos.

Over the same period, we have been also launching probes, like Voyager 1 and 2Pioneer 10 and 11 and the New Horizons spacecraft, towards interstellar space. These could eventually reach alien civilizations, passively announcing our existence. But in 1960, at the dawn of the space age, Ronald Bracewell noted in a Nature paper that a physical space probe could also search for technological civilizations across interstellar distances. SETI should therefore explore this technique as well—a timely notion in the era of multimessenger astronomy, ushered most recently by the detection of gravitational waves.

This sort of exploration obviously could work both ways. Thanks to data collected by the Kepler space telescope, we now know that about half of all sunlike stars host a rocky Earth-size planet in their habitable zone. Within this zone, the planet’s surface temperature can support liquid water and the chemistry of life. The famous Drake equation quantifies (with large uncertainties) the likelihood of receiving a radio signal from another civilization in our Milky Way galaxy. But it does not apply to physical probes that might arrive at our doorstep. The distinction resembles the difference between a cell phone conversation at the speed of light and the exchange of letters through surface mail.