The New Asteroid “Bible” Reviewed
In the world of planetary science, the University of Arizona Press has a long heritage of publishing incredibly valuable academic volumes on a wide array of topics. The publisher’s “Space Science Series” consists of lengthy tomes that may not be the stuff of late-night, light entertainment reading, but they are essential for detailed background on the subjects they cover. And they’re priceless as reference books, containing the definitive word on various aspects of the objects they treat and holding up pretty well for years between updates.
The series last treated asteroids in 2002, when it produced Asteroids III, edited by William F. Bottke, Jr., Alberto Cellino, Paolo Paolicchi, and Richard P. Binzel. Before that, the series included Asteroids II (1989) and Asteroids (1979). The series also included Hazards Due to Comets and Asteroids (1995).
And so comes another exciting moment for asteroid experts. At year’s end 2015, the Press published Asteroids IV, the latest reworking in this series (edited by Patrick Michel, Francesca E. DeMeo, and William F. Bottke, Jr., 785 pp., hardcover, University of Arizona Press, Tucson, 2015, $75; ISBN 978–0–8165–3213–1).
The coverage, as you might guess from the book’s length, is not only authoritative but also is sweeping. An introduction by the editors precedes a lengthy section on physical properties of asteroids, and other sections treat asteroid space missions, evolutionary processes, ground-based surveys, hazards, and future exploration.
The contributors form a who’s-who in the asteroid field, comprising several dozen scientists from a wide range of institutions. Numerous photographs and diagrams illustrate particular sections where appropriate. Altogether, some 43 research summaries appear written by a collective 148 planetary scientists. Among the names familiar to popular science readers are Rick Binzel, Bill Bottke, Clark Chapman, Paul Chodas, Dan Durda, Matthieu Gounelle, Alan Harris, Peter Jenniskens, Amy Mainzer, Harry McSween, Patrick Michel, Alex Parker, Vishnu Reddy, Faith Vilas, and Don Yeomans. And among the contributing authors is the chair of the Asteroid Day Expert Panel, Mark Boslough. Asteroid experts held a 2014 conference in Helsinki, Finland, “Asteroids Comets Meteors 2014,” in association with the book project.
In short, this is the collected best view of asteroid science we have at this particular moment in time.
Among the key questions the book treats as well as we now can include:
• Which classes of meteorites come from which classes of asteroids?
• Did asteroids melt to form magma oceans during their evolution, or just partial melting?
• What are the internal structures of Jupiter’s Trojan asteroids?
• Do we have meteoric samples from the zones of the innermost planets?
• How did Earth get its water — how significant were icy asteroids in this process?
• Which classes of asteroids participated in the Late Heavy Bombardment in the inner solar system?
• What are the sources of the asteroid groups yet to be explored by spacecraft?
Of particular interest to followers of Asteroid Day are the sections that cover Near-Earth Objects (NEOs) and impact hazards. The editors emphasize that asteroids crossing Earth’s orbit of course pose a threat to humanity. The Chelyabinsk event of 2013, which produced more than 1,000 injuries from shattered glass resulting from the atmospheric shock waves, proves how hazardous even small objects can be. If a large asteroid strikes Earth, it could have catastrophic consequences for life on Earth. But the nearness of some asteroids to our planet also offers the opportunity for human exploration of an asteroid. The book also explores the improvements in surveys, which have increased the numbers of known Near-Earth Objects dramatically. But we have a long way to go in this respect, and thus the reason for Asteroid Day.
Planetary scientists believe we have found 90 percent of Near-Earth Objects greater than 1 km in diameter. Now the challenge is to do the same for smaller objects. Among NEOs, more than 9,000 are now known smaller than 1 km in diameter. Amazingly, large amounts of Chelyabinsk were recovered, as well as substantial amounts of Almahata Sitta, a fireball that exploded over the Nubian desert in 2008. This object was the first to be observed on its way in, as asteroid 2008 TC3, observed as it exploded in Earth’s atmosphere, and then recovered in fragments by scientists and enthusiasts searching northern Sudan.
Micrometeorites also rain in on Earth’s surface. As much as 100 tons of this material cascades into our atmosphere each day, and the largest particles span 1 mm across. A great deal of it comes from disrupted Jupiter-family comets. Material from a true asteroid arriving onto Earth’s surface is best represented by 3200 Phaethon, an Apollo asteroid with a comet-like orbit that moves closer to the Sun than any other space rock. If you run a magnet through your house’s gutters, carefully sifting out all of the residual Earth debris, you can recover some of this micrometeoritic junk.
Three detailed sections of Asteroids IV will be of greatest interest to supporters of Asteroid Day. The first is “Orbits, Long-Term Predictions, and Impact Monitoring” by Davide Farnocchia, Steven R. Chesley, Andrea Milani, Giovanni F. Gronchi, and Paul W. Chodas.
This lengthy paper includes a state of the art assessment of impact hazards. Discussion of various threats and keyholes, the small regions of space that could gravitationally alter an asteroid’s orbit, causing an impact on a future orbital pass, is extensive.
Also of interest will be “Modeling Asteroid Collisions and Impact Processes,” by Martin Jutzi, Keith Holsapple, Kai Wünneman, and Patrick Michel. This paper describes the numerical simulations planetary scientists have conducted to better understand collisions and impacts than would be possible by observations alone. To understand collisions between asteroids, the modelers examine the effects of gravity, strength, porosity, shapes, and structural properties. The authors of this paper also address the momentum transfer from a kinetic impactor for asteroid deflection.
Of greatest interest to Asteroid Day supporters will be “Asteroid Impacts and Modern Civilization: Can We Prevent a Catastrophe?,” by Alan W. Harris, Mark Boslough, Clark R. Chapman, Line Drube, Patrick Michel, and Alan W. Harris. (And yes, there are two asteroid experts named Alan W. Harris, an Englishman at the German Aerospace Center and an American retired from the Space Science Institute and JPL.)
This detailed and fascinating paper explores what we’ve learned from Chelyabinsk, describes the current understanding of impact hazards, surveys potential deflection techniques, and examines current priorities, what we don’t yet know, and international efforts and politics. The bottom line is that, on average, a 10-meter asteroid ought to enter Earth’s atmosphere every 5 years. A 30-m object (like Chelyabinsk) should cause an airburst every 150 years. A 50-m object that could cause a violent, Tunguska-type airburst should enter the atmosphere every 1,000 years. A 100-m asteroid, which would produce an impact crater, should enter the atmosphere about every 10,000 years.
And of course with larger objects, the stakes are correspondingly higher. A 300-m asteroid will strike Earth on average every 70,000 years. A 500-m space rock will smack into our planet about every 140,000 years, and would cause a 10-km crater and potentially millions of deaths. A 1-km object will strike on average every 500,000 years and produce global effects, with a partial disruption of human civilization. A 10-km asteroid, akin to the K-Pg impactor that killed off the dinosaurs, will strike Earth on average every 100 million years, potentially ending our civilization.
This book, Asteroids IV, is a heavy tome. Parts of it laden with equations and long histories of research projects and analysis will be of little interest to general interest readers. And yet this book is the “bible” of asteroid studies, unwaveringly the definitive source of all things asteroid. I have found it to be unceasingly fascinating as I can dip into numerous areas, privileged to peek into the current best thinking on many aspects of what we know about these small bodies in the solar system. For those of you dedicated to knowing the most about asteroids, NEOs, impact hazards, and what might be done that could one day save humanity, I heartily encourage you to read at least parts of this definitive book.