What does it look like, our history of trying to find alien life? How far back does it go?
Nikola Tesla, the Serbian-American innovator who helped advance radio communication and the adoption of electricity, was tinkering with a radio-wave receptor in his lab when he picked up strange repeating beeps. He thought they came from aliens attempting to reach us. It turned out they were just cosmic noise; chirps from the magnetic fields disturbed by large planets in our solar system. Already, though, the idea that we might someday "meet the neighbours" was generating excitement.
* 1924: America's "Big Listen" for Mars
The eccentric astronomer David Todd somehow convinces the US Navy to help him listen for signals from our neighbouring planet, Mars. As its orbit swung it closer to Earth than it had been in 120 years, the Navy orders a National Radio Silence period. For five minutes every hour, from August 21 to 24, Americans turned off their radios so instruments could listen for alien signals. There are, of course, no messages. But excitement about explorations off-planet is growing.
* 1958: One small step...
NASA, which will go on to perform giant leaps for mankind, is set up.
* 1960: SETI, or the Search for Extra-Terrestrial Intelligence, begins
The non-profit initiative is launched in 1960, by Cornell University astronomer Frank Drake. His Project Ozma uses radio telescopes to scan the stars for artificial signals. He draws hope from the Fermi paradox (which posits that, in a universe so vast, other sentient life must exist, and wonders why we haven't found it yet).
By 1984, the SETI Institute is set up. It continues to run listening programmes and collate data from citizen-science efforts.
* 1971: Project Cyclops
NASA launches an initiative that invites engineers to create sketches of what it will take to listen better. The engineers draw up a mega-array of 2,500 giant radio dishes. Given the astronomical estimated costs, the idea remains on paper.
* 1976: Viking's soil tests on Mars
Earthlings have tempered their expectations by this point. We now know that any life we encounter will likely be microbial, and we'll be lucky to find that.
Could there be microbes on Mars, or even proof that microbial life once existed there? NASA's twin Viking landers are the first to collect and study Martian dirt. It is humanity's first direct contact with another world.
* 1982: The Soviets land on Venus
The Venera 13 probe transmits the first colour images of Venus's surface: a barren, volcanic landscape of layered rocks under a mustard-yellow sky. The probe operates for just over two hours, in the planet's 457-degree-Celsius heat and 90-atmosphere pressure.
* 2007: The Allen ears
The sketches from the 1971 Project Cyclops inspire the Allen Telescope Array, set up in California in a joint venture between the SETI Institute and University of California Berkeley, with funding from Microsoft co-founder Paul Allen. The telescope remains operational.
* 2012: More dirt on Mars
NASA's car-sized rover, Curiosity, lands on the Red Planet. Mars, about 75 million km further from the Sun than Earth, is an icy place, with temperatures ranging from 20 to -153 degrees Celsius. But missions like Curiosity prove that it was warm once, and likely held liquid water.
Could there still be water flowing far below its rocky surface? Ten missions have landed on Mars so far (nine of those by NASA), in attempts to uncover more clues.
* 2014: Lower than Gravity
The Indian Space Research Organisation's Mars Orbiter Mission makes it into the Red Planet's orbit, on a budget so low ($74 million) that it famously cost more to make the 2013 set-in-space movie Gravity. ISRO has since contributed steadily to efforts to identify signs of microbial life or liquid water in promising sites such as the lunar poles and on Mars.
* 2018: An asteroid grab
The Japan Aerospace Exploration Agency (JAXA) launches Hayabusa2, which lands on the asteroid Ryugu, millions of km away, in 2018, after a journey of four years. It scoops up bits of rock and flies them home. The dark rock is found to hold evidence of liquid water, and organic molecules such as amino acids and hydrocarbons.
Could this kind of asteroid be where the building blocks of organic matter on Earth originated? This is the first probe to retrieve pristine subsurface samples from an asteroid and return them to Earth.
In 2021, China's Zhurong rover successfully lands on Mars, making it only the second country to do so. The new space race continues, with the US, India, China, Japan and the European Space Agency vying to lay claim to new discoveries and undiscovered resources.