The second interstellar object, found in 2019 by an amateur on a telescope he built himself: the boring one, which is exactly why it matters.
Documented
The garage observatory
Gennadiy Borisov, an amateur in Nauchnij, Crimea, spotted it on August 30, 2019 with a 0.65-meter telescope he designed and built. Guzik et al. (Nature Astronomy 2020) found it pristine: a comet never before heated by any star, arriving untouched from deep freeze.
Assessment: Documented: discovery circulars and the characterization papers are public.
Stories
The alien chemistry
Hubble's COS spectrograph (Bodewits et al., Nature Astronomy 2020) measured carbon monoxide against water at at least 173 percent, more than triple any comet recorded inside 2.5 au; ALMA (Cordiner et al. 2020) confirmed the extreme CO abundance. The reading: it condensed in a disk around a cold red dwarf, chemically unlike our nursery.
Assessment: Documented: the first direct chemistry sample from another planetary system, and it does not match ours.
Interesting claims
The control group
In March 2020 its nucleus fragmented on the way out, dying the way ordinary comets die. Because Borisov matched the textbook so closely, it became the baseline: whatever ʻOumuamua and 3I/ATLAS are, they are not this. The open reading is that our comets, not the visitors, might be the odd sample.
Assessment: Interesting claims: the normal one is what makes the other two look broken.
Sources and further reading
Guzik et al., Initial characterization of interstellar comet 2I/Borisov, Nature Astronomy (2020)
Bodewits et al., The carbon monoxide-rich interstellar comet 2I/Borisov, Nature Astronomy (2020)
Cordiner et al., Unusually high CO abundance of the first active interstellar comet (ALMA), Nature Astronomy (2020)
Minor Planet Center circulars for C/2019 Q4 (Borisov), discovery August 30, 2019
Minor Planet Electronic Circulars, March-April 2020: the fragmentation of 2I/Borisov
Worth asking
The one visitor that behaved perfectly turned out to be chemically alien; which is stranger, the stranger that acts strange or the stranger that acts familiar?