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Review: Asteroids


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Asteroids
by Clifford J. Cunningham
Reaktion Books, 2021
hardcover, 190 pp., illus.
ISBN 978-1-78914-358-4
US$40

The strength of Asteroids lies in its historical studies. The primary thrust of the author’s previous scholarship has also been similarly situated. Thus, the personalities and quirks of 19th and 20th century astronomers take center stage.

A study of the fits and feuds of white-haired men over discoveries and competing theories may not be the average reader’s cup of tea, but assuredly, Clifford Cunningham’s narration is compelling reading. Along the way, he also manages to summarize the more significant scientific discoveries relating to asteroids and recent developments such as appearances of asteroids in popular culture, the interstellar asteroid/comet ’Oumaumua, contemporary asteroid mining ambitions, asteroids as testbeds for gravity studies, NASA’s Lucy space probe (scheduled to launch this Saturday) and Europe’s Extremely Large Telescope, with its 39-meter mirror, slated to go online in 2025.

The rivalries of 19th century scientists had a direct impact on the development of scientific knowledge concerning asteroids, including the evolution of the name “asteroid.” For a time, “planetoids” vied with “pocket planets” and “Aorates.” In the end, “asteroids” (meaning star-like) won out. Clifford also describes the controversy over the (ultimately disproven) assertion in the 19th century that asteroids are left over fragments of planetary explosions.

The rivalries of 19th century scientists had a direct impact on the development of scientific knowledge concerning asteroids, including the evolution of the name “asteroid.” For a time, “planetoids” vied with “pocket planets” and “Aorates.”

At times, the text is quite whimsical. Only the eye of an historian would uncover a gem like Edmund Ledger’s 1895 suggestion of asteroid sporting events while speaking at Gresham College (London): “[W]ith a good jump we should be absent for about an hour before we came down again, which fact present[s] the idea of very interesting athletic contests on the minor planets.”

Clifford also does an admirable job of explaining abstract, technical, and complex scientific phenomena to the lay person—a rare and important skill for scientists. He explains how it was the American astronomer Daniel Kirkwood who first noted that asteroids were not evenly distributed throughout the solar system. Rather, certain patterns could be observed. In 1868, Kirkwood posited that the zones of concentration and chasms might be caused by Jupiter. Clifford explains the theory of resonance quite nicely:

The gaps are a result of the phenomenon of resonance, a term taken from acoustics where it was observed that a note struck on a piano can set a nearby violin string vibrating. Considering Jupiter as a celestial piano, and the asteroids as violin strings, a series of resonances were created in the early solar system.

Thus, for example, in the orbital path where a satellite would circle the Sun precisely three times faster than Jupiter (a 3:1 ratio), an empty space or vacant groove in the asteroid belt is observed. Likewise, concentrations of asteroids occur in mathematically consistent patterns (e.g., the Hilda group of asteroids, positioned at 3:2, with an orbital period of two-thirds of one Jupiter year). Explaining the causation of orbital resonances would tax the patience and abilities of most lay readers, but Clifford’s analogy to piano and violin strings is both apt and instructive.

Asteroids is a readable and attractively bound monograph on quality paper with excellent illustrations. It features an index, spare notes, and a selective bibliography. It also contains an appendix titled “Engaging with Asteroids” in which Clifford outlines backyard astronomy projects for observing and studying asteroids in more depth.


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