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24 july 2021

Asteroids shade at one definitional border into comets. If a comet is a dirty snowball, an asteroid is a dirty clump of rock, and some clumps of rock are a bit icy. Hence some asteroids have tails, so there must be relatively comety asteroids and relatively asteroidish comets.

Asteroids were pretty big news when people first saw them – in 1801, from Sicily. Traditionally it was astronomer Giuseppe Piazzi, of Palermo, who discovered Ceres, asteroid #1. But Clifford Cunningham says that Piazzi must share credit with his assistant Niccolò Cacciatore, who actually saw the thing; Piazzi reported the sighting and then promptly lost track of the new object for some time before locating it again. Probably a lot of findings that a principal investigator gets associated with have really been the work of a longsuffering staffer.

Ceres caused a stir because an astronomer named Johann Bode had predicted in general terms that a little planet ought to be located somewhere between Mars and Jupiter, and Ceres fit the bill. But before long, lots of other little items circling in the same general area came to light, and Ceres was downgraded to just another in their ranks. The term "asteroids" came quite early after some brief support for "aorates," which would have been a terrible name for an arcade game and was judiciously discarded.

Johann Bode sounds like a dreamy crackpot, but much of the science of asteroids, to this day, makes use of Bodelike ideas about resonances and harmonies of orbits. Asteroids tend to cluster in belts at various intervals between planets, because the gravity of the bigger objects "polices" them into formation.

For a while in the 1800s, Ceres and some of its early kin like Pallas, Juno, Circe, and Thalia were thought of on a par with the much larger true planets. Then people started finding too many of the little things to bother giving them names. Hundreds of thousands of asteroids have been observed, almost all of them mechanically rather than by human observation, and most are lucky if they get an official number. Though of late there has been an attempt, Cunningham says, to interest the public by having people vote on the names of newfound large asteroids. By fan vote, one was recently named Gonggong, after a Chinese deity. It's a nice name, though one suspects the people who ran the poll had to throw out votes for "Rocky McRockface."

Gonggong lies way the hell out beyond the orbit of Neptune, and indeed a lot of larger and more interesting asteroids are very distant from the Sun and from the traditional "asteroid belt." The sadly demoted Pluto is basically a far-out asteroid, and though smaller than several moons including our own, it remains the largest thing in the Solar System aside from planets and moons. Asteroids come in a vast range of sizes, and just as with comets at one end, asteroids grade into minor planets at their largest and "planetesimals" at their smallest. Meteors are just tiny asteroids burning through our atmosphere, and sometimes punching through to land as meteorites, as in Flensburg in northernmost Germany a couple of years ago.

There are so many asteroids zooming around that they occasionally bash into each other, forming larger objects. Little is known even today about how planets form, but asteroids clumping together is one candidate for a process. If two well-matched objects collide they sometimes do so at pretty low speeds and just sort of nudge and nestle themselves together, as with weird-looking faraway Arrokoth and funny little Itokawa.

So asteroids don't necessarily bash each other to smithereens; but when an asteroid hits Earth, it gets huge acceleration from Earth's gravity and becomes the stuff of summer blockbusters. Cunningham reports that we are getting better at spotting potentially dangerous objects, and that the US Space Force at least has contingency plans to go Bruce Willis on them and blast them out of our way. Unfortunately, though we can certainly hit an asteroid with a rocket, the biggest bombs we have are unlikely to impress a true killer asteroid.

Cunningham includes a couple of pages (89-90) on the Younger Dryas Impact, 12,800 years ago, which seems to have hit several places on Earth, probably because a large object splintered when it hit our atmosphere. People were around to witness Younger Dryas, and they were perhaps around to be incinerated by it; one archeological site in Syria, Cunningham says, shows evidence that a huge fireball destroyed an entire community. And you thought Sodom and Gomorrah was just a teaching story.

Cunningham, Clifford J. Asteroids. London: Reaktion, 2021.