Everything we know about the space beyond our own patch comes from observing the light that has made it at least 40 trillion km (25 trillion miles) – the distance from the next nearest Solar System, Alpha Centuri – to our planet. The rest is a series of educated guesses, based on the chemistry and physics of our own neighbourhood. We can only imagine the exotic compounds these remote territories contain. At least, for the moment.  

A rare event 

At first, IM1 was just a handful of numbers in an online database, under the label "CNEOS 2014-01-08".  

Though space is constantly under watch from the roughly 10,000 professional astronomers that inhabit the Earth, together with thousands more amateur enthusiasts, meteorites are easily missed. The sky is simply too big to be monitored in its entirety, all of the time – and most telescopes aren't sensitive enough to detect small objects.  

So, when IM1 slammed into the Earth, no one noticed. The only record of its existence came from the US government, whose sensors recorded its trajectory, speed and altitude as it streaked through the atmosphere over the Atlantic Ocean near Portugal. Any further details, if they exist, are found in classified documents – not because it was a UFO, but because making them public would reveal too much about the capabilities of the military equipment that found it.  

But there were enough crucial details in the database to pique Loeb's interest. For a start, IM1 was hurtling along at an uncanny speed.  

All the stars in the Milky Way are moving – gradually orbiting its centre, though in the case of our Sun a single revolution can take about 230 million years. As they travel, they take the contents of their Solar Systems with them. This means that any object that entered our solar neighbourhood would already be bringing the speed set by its own star. As it got closer to the Sun's gravitational pull it would "fall" towards it, increasing its pace yet further. As a result, scientists expect that interstellar meteorites would be moving faster than regular ones. 

Loeb's analysis suggests that not only was IM1 moving more rapidly than our own Solar System, it was also travelling faster than 95% of the stars nearby. This, he believes, suggests that it was interstellar. However, even factoring this in, it's not clear how it achieved such momentum.   

Secondly, the meteorite was extremely tough – instead of breaking up in the Earth's upper atmosphere, IM1 held on until it reached the lower atmosphere. Exactly what it was made of remains a mystery – but it was more robust than steel.  

"We found that its material strength must be at least a few times bigger than all other space rocks, 272 of them [at the time] in the same catalogue," said Loeb, in an interview with the BBC a couple of weeks before the expedition started.  

Together with a colleague from Harvard, Loeb calculated with 99.999% confidence that IM1 was an interstellar visitor. This would have made it only the third ever discovered, after the comet 2I/Borisov, which was discovered in August 2019, and Oumuamua – only this time, it had ended up within reach. But there was a catch.  

When the team wrote up their findings, the paper was initially rejected for publication in a scientific journal, partly because the experts reviewing it felt they needed more detail. In need of urgent access to classified documents, Loeb's mission stalled.