According to solar system formation models, most of the unused primitive material was ejected to the interstellar space by gravitational instabilities shortly after the formation of gas giants. Similar processes are expected to occur in other, countless planetary systems, populating the Galaxy with myriads of wanderers. Although expected for decades, the first interstellar interloper in the Solar System, ‘Oumuamua, was identified in 2017. Looking purely asteroidal, the object presented multiple peculiarities, of which the most intriguing was non-gravitational acceleration detected in its movement and unique rotational light curve, exhibiting an enormous amplitude, as well as small irregularities revealing rotational tumbling.Two years later, in 2019, another interstellar interloper, comet 2I/Borisov, was found as it passed through the Solar System. It turned out to be strikingly similar to our local comets, both in terms of morphology and chemical composition. Early observations of 1I/‘Oumuamua did not reveal any traces of cometary activity. Even the deepest images presented a completely star-like object. The obtained limits on dust production rates were as low as 2×10-3 kg/s or even 2×10-4 kg/s depending on the assumed dust particle size distribution. The search for gaseous emissions did not reveal any detections, though it was not comprehensive due to discovery circumstances. The object was discovered a few weeks after perihelion, around the closest approach to Earth, and faded extremely quickly, receding from both Earth and Sun. A search for the OH 18 cm radio emission using the Green Bank Telescope resulted in the limit of Q[OH] < 1.7×1027 molec/s, equivalent to water production rate < 500 kg/s. However, this limit was obtained a few weeks after the discovery, when the object was already much farther from both the Sun and the Earth. Spitzer Space Telescope IR observations were carried out even later. No signal from ‘Oumuamua was found, leading to a crude estimate of the water production limit < 1.8 kg/s, assuming a 5:1 dust-to-gas ratio.
The mysterious non-gravitational forces discovered in the astrometric data several months later, when 1I/‘Oumuamua was already too far to be observed, could be explained by water outgassing at the level of ~10 kg/s, however, this order-of-magnitude estimate required either dust production rates inconsistent with the observations or bizarre, never observed cometary dust size distribution lacking any particles smaller than a few hundred μm. The lack of any apparent cometary activity combined with the unsolved mystery of non-gravitational motion sparkled numerous hypotheses about the nature of 1I/‘Oumuamua. For example, it was speculated to be either a molecular hydrogen iceberg, N2 ice fragment of an exo-pluto surface, a piece of dark matter, or even an artificial object (e.g. solar sail, fragment of Dyson sphere). The second interstellar visitor, 2I/Borisov, was completely different from 1I/‘Oumuamua. Already initial imaging observations revealed its striking similarity to native solar system comets. Thanks to the fact that it was discovered a few months before perihelion and relatively bright, it was studied much more thoroughly than 1I/‘Oumuamua. Spectroscopic studies showed most of the typical cometary species, and their relative abundances were similar to what we observe in solar system comets. As it is often in the case of our native comets observed with ground-based optical telescopes, the first chemical detected in the spectrum of 2I/Borisov was CN, followed shortly by C2.
Subsequent detections of OI, NH2, OH and HCN were also consistent with our knowledge about solar system comets. The somewhat high abundances of CO also fall into the broad range of values observed earlier for carbon monoxide-rich comets. Even the unexpected detection of gaseous atomic nickel in the coma of 2I/Borisov turned out to be a common, but overlooked feature of comets native to the solar system. Until now, no more interstellar minor bodies were observed in the Solar System, but many more discoveries are expected soon after the commencement of LSST. Each such body is a unique piece of material coming from another planetary system, which can be investigated from up close with the well established methods of planetary astronomy. However, there is a burning need for a dedicated instrument able to rapidly observe another interstellar object once it has been discovered, and detect even small amounts of gases.