More than 2.7 billion miles from the sun—29 times farther than Earth treads—a tiny sliver of sunlight reflected off something plummeting toward our home star. Something icy. Something unimaginably old. Something big.

About four hours later, in the predawn hours of October 20, 2014, a telescope in Chile’s Atacama Desert turned its gaze toward the heavens and snapped an enormous picture of the southern night sky, capturing hints of this reflected light.

However, it would take nearly seven years for researchers to identify that strange dot of light as a huge primordial comet—possibly the biggest ever studied with modern telescopes. Called Bernardinelli-Bernstein, the comet was announced in June, and researchers have now compiled everything they know about it in a