The Great Fishing

Preparing to visit our very first interstellar meteor.

The final decision has been reached. Within the coming month, I will be leading an expedition to collect the fragments of the first interstellar meteor in May 2023.

This meteor is the first near-Earth object ever detected by humans from outside the solar system. In anticipation of meeting it, I would not mind sleeping on the open deck of a boat and taking risks associated with a trip to the Pacific Ocean. Elon Musk dreams of dying on Mars; I feel content with staying on Earth, as long as I have had the opportunity to hold an interstellar fragment in my hands.

The Galileo Project expedition has received more than a million dollar in funding. We have a boat. What’s more, we have a dream team, including some of the most experienced and qualified professionals in ocean expeditions. We have complete design and manufacturing plans for the required sled, magnets, collection nets and mass spectrometer. And, most importantly, we have received the green light to go ahead.

But let’s back up a bit. What is all this fuss about?

On January 8, 2014, an object from interstellar space collided with Earth at a speed of 45 kilometres per second. As a result of its friction with air, this fiery intruder disintegrated into tiny fragments about a hundred kilometres off the coast of Manus Island in Papua New Guinea.

The fragmentation increased the collective surface area, and hence the friction with the atmosphere. This in turn accelerated the release of heat and ended up generating a runaway fireball. The resulting explosion liberated a few percent of the energy associated with the Hiroshima atomic bomb within a fifth of a second.

The bright flare that ensued was detected by US Government cameras. The location was listed in the CNEOS fireball catalog of JPL/NASA to one significant digit after the decimal point, in longitude and latitude.

After hearing about the object in 2019, I wrote a paper with my student, Amir Siraj. Looking at the way it entered the atmosphere and the trajectory it was taking, we identified it as an interstellar meteor—the first one ever observed! Eventually, it was given the official title of IM1 or Interstellar Meteor 1.

The interstellar origin was confirmed with a 99.999% confidence level in 2022, in an official letter from the US Space Command under the Department of Defence to NASA. The confirmation letter was accompanied by the light curve of the fireball, which showed three distinct explosions separated apart by a tenth of a second. This fireball data allowed us to conclude in a follow-up paper that the meteor was tougher than all other 272 meteors in the CNEOS catalog.

When this confirmation came out, I wished there was a way I could get my hands on the fragments of IM1.  Analysing the composition of the fragments could allow us to determine whether the object is indeed natural or artificial in origin, and also provide valuable insights into the object itself. The only problem? Fortunately for most people, but unfortunately for science, IM1 had fallen not on land but into the ocean—so while CNN designated it as one of 2022’s most extraordinary cosmic revelations, there was nothing more that could be done about it.

We needed to conduct and expedition.

And so it was that I established a team, and designed a two-week expedition to search for the meteor fragments at a depth of 1.7 kilometres on the ocean floor.

Going into the ocean is all very well, but the ocean is a vast place. How does one even begin to search there?

Of course, we had some leads. The published coordinates defined the fireball location to within a 10-kilometer radius. This is too large for an efficient search, but we were in luck. The blast wave from the meteor explosion, it turned out,  generated a high-quality signal in a seismometer located at Manus Island. The sound signal includes two broad peaks separated by about a minute, each lasting for tens of seconds.

Sound speed in air is much smaller than in water or ground—and therein lay our clue. The first peak on the Manus Island graph begins with a sound path going through air from the explosion straight down to the ocean surface, and then through the water and ground to the seismometer.

The shortest path through air goes directly from the explosion to the seismometer, and defines the beginning of the second peak in the seismometer signal. The envelope of that second peak involves the sum over paths where the spherical blast wave in air reflects off the ocean surface in circles of different radii at different times, and with an amplitude that declines inversely with distance from each reflection point.

By using a simple geometry of a spherical blast wave bouncing off the ocean surface, Amir and I were able to reproduce the timing of the first peak and the shape of the second peak. Altogether, the model provides many more constraints than free parameters and measures tightly the explosion elevation and distance. We constrained the meteor path to a narrow line within the original 10-kilometre localization box, narrowing down the search area by nearly two orders of magnitude.

Now, we only had a strip along the meteor path, which is roughly a kilometre wide, to search through.

Our “fishing expedition” may collect fragments of different sizes. The meteor size is inferred to be half a meter based on its speed and explosion energy.

The huge explosion melted the object into tiny droplets. Of these, the smallest fragments were stopped quickly by their friction on air owing to their large surface area per unit mass: they would have fallen straight down from the explosion site as hot rain, raised steam from the ocean surface, and sunk down to the ocean floor. Larger fragments, on the other hand, would have continued farther along the original path of the meteor.

As a result, we expect to have a strip of fragments on the ocean floor, oriented along the original path of the meteor, with the smallest fragments marking the beginning of the strip straight below the initial explosion site and larger fragments farther along.

How many fragments should we expect of different sizes? This was the focus of a recent paper that I wrote with Amir and an intern, Amory Tillinghast-Raby. Our forecast depends on composition. For an iron meteorite, we predict about a thousand fragments bigger than a millimetre, whereas for a stainless-steel composition we expect larger sizes, with tens of fragments bigger than a centimetre.

Unusual material strength is not a rare finding within the interstellar meteor population. Recently, I wrote another paper with Amir that identified a second interstellar meteor, IM2, which was detected near Portugal on March 9, 2017 and was also extremely tough.

Both interstellar meteors, IM1 and IM2, collided with Earth from a trajectory that was gravitationally unbound to the Sun. In other words, the objects arrived to the Solar system from interstellar space and were moving faster than the escape speed from the Sun when they were collected by the “fishing net” of the Earth’s atmosphere.

The second interstellar meteor was ten times more massive and roughly a meter in size. Meteor speeds are calculated relative to the Local Standard of Rest, the local frame of reference of the Milky Way that averages over the random motions of all the stars in the vicinity of the Sun. By this measure, IM2 was moving at a speed of 40 kilometres per second, compared to the 60 km/s for IM1.

Remarkably, despite their unusually high speeds, both IM1 and IM2 disintegrated low in the Earth’s atmosphere. The inferred yield strengths of 194 Mega-Pascals (MPa) for IM1 and 75 MPa for IM2, imply that both were tougher than ordinary iron meteorites, which have a maximum yield strength of 50 MPa.

IM1 and IM2 ranked number 1 and 3 in the distribution of material strengths among all 273 meteors in the CNEOS catalog. The probability of drawing the material strength of the first and second interstellar meteors out of the familiar population of Solar system rocks is roughly the square of (3/273), or a chance of one in 10,000. This means that the population of interstellar meteors is different from Solar system meteors at the 99.99% confidence level. This conclusion is corroborated by fitting the distribution of CNEOS meteors with a Gaussian shape in the logarithm of material strength. Both IM1 and IM2 lie on the ‘far tail’ of the distribution, making their combined likelihood less than a part in a million in this context.

This tantalising conclusion about the extremely rare material strength of IM1 and IM2 implies that interstellar meteors may not be rocks from planetary systems like the Solar systems.

But in that case, what could be their origin?

The Earth collides with interstellar objects along its orbit around the Sun. The simplest assumption to make is that these are natural objects that arrive into the Solar system on random trajectories in the Local Standard of Rest. Based on the detection rate of IM1 and IM2 in the CNEOS catalog, roughly once per decade, one finds that a up to a third of all refractory elements in the Milky-Way galaxy must be locked in meter-scale interstellar objects if IM1 and IM2 are natural in origin. This extraordinarily high abundance again seems to defy a planetary system origin.

Supernovae have been observed to produce iron-rich bullets. For example, X-ray imaging of the Vela supernova remnant revealed bow shocks from objects flying out of the explosion site, a discovery that I attempted to explain three decades ago. It is possible that IM1 and IM2 have unusually high material strength because they were produced in the ejecta of an exploding star or in collisions of two neutron stars. These explosive events produce the heaviest elements, but the ejecta must be slowed down to the speed of tens of kilometres per second, characteristic of IM1 and IM2, before making these objects.

Alternatively, it is also possible that IM1 and IM2 are tough because they are artificial in origin, resemblingour own interstellar probesbut launched a billion years ago from a distant technological civilization. The advantage of an artificial origin is that it reduces the inferred abundanceof interstellar objects from nearly 10 to the power of 24 (a trillion trillions) per star like the Sun to a much more reasonable number.

In case we recover a sizeable technological relic from the Pacific ocean, I promised the curator of the Museum of Modern Art, Paula Antonelli, that I will bring it for display in New York. Even if it is a relic of ancient history for the senders, it would represent modernity for us, being of great interest not only to art collectors but also to entrepreneurs from Silicon Valley.