EARLY IN 1986, a visitor from deep space graced Earth’s skies, and for a few breathless weeks human eyes were snatched from their daily labors and fixated on the heavens. The object of this transitory public veneration was Halley’s Comet — the most famous, easily recognizable, and beloved of celestial wanderers. This icy interloper, officially known as 1P/Halley, revisits the inner solar system every 72 to 80 years.
Aggregated lumps of frozen water and gases mixed with smaller amounts of rock and dust, comets originated in the solar nebula, from which the Sun and its planetary retinue formed, some 4.5 billion years ago. Their highly eccentric orbits periodically carry them to the outer solar system and back toward the Sun. There they undergo drastic heating and outgas chemical volatiles, which…