Asteroids and comets have a bad reputation. Looking back over the books I have reviewed, they usually come up in the context of impact and destruction. But there are other important reasons to study them and geologist and cosmochemist Natalie Starkey here steps up as their enthusiastic spokeswoman. Whether as frozen time capsules, possible vehicles dispersing the basic chemicals required for life, or even future mining quarries, Catching Stardust champions the importance of scientific research on these celestial objects.

Astronomers largely agree on how our Solar System formed, and Starkey runs the reader through the basics of the nebular hypothesis: giant gas clouds condensing and contracting to form nascent stars with protoplanetary discs of dust swirling around them that start to congeal into planets. Plate tectonics on Earth has erased most traces of our planet’s very deep history and only a few places, such as Greenland, host rocks older than 3 billion years (see my review of A Wilder Time: Notes from a Geologist at the Edge of the Greenland Ice). The beauty of nearby comets is that they are effectively 4.6 billion-year-old frozen time capsules of the early nebular cloud from which our Solar System formed.

But what of asteroids? After reviewing Cosmic Impact: Understanding the Threat to Earth from Asteroids and Comets, I complained being left puzzled by what the differences are between the two. Starkey here takes a bit more space to explain it all. The classic model classified comets as fluffy, icy, fragile bodies that would have formed in the cold reaches of outer space, and asteroids as hard, dry, solid objects that would have formed closer to the Sun. But data from recent space missions shows that this neat dichotomy is not the whole story, and there is a spectrum of chemical and structural compositions possible in between these two extremes. Further complications arise from models suggesting that the early Solar System saw some dramatic rearrangements of planetary orbits, with the contents of the distant Oort Cloud possibly having formed closer to the Sun than the currently closer Kuiper Belt. As Starkey is keen to point out, there are still many, many unresolved questions here.

Catching Stardust is largely a book of the how and the why of studying space rocks and alternates between these questions. Other than Solar System formation, there is also the question of how Earth formed, where its water came from, and whether comets and asteroids seeded our planet with the chemical precursors of life. Although hydrogen and carbon are found throughout space, I was fascinated to learn that amino acids, one of life’s basic building blocks, are too. But there remain interesting questions. Why do lifeforms only use one variant of amino acids (the left-handed one)? And do comets and asteroids contain equal mixtures of both, as you might expect, or not? (There is titillating evidence they don’t.)

“The beauty of nearby comets is that they are effectively 4.6 billion-year-old frozen time capsules of the early nebular cloud from which our Solar System formed.”

Throughout these chapters, Starkey gently introduces the reader to the more complicated underlying scientific concepts. Whether isotopes, why isotope ratios of hydrogen and deuterium (a slightly heavier variant of hydrogen) matter and what they reveal about the origin of water, or the chirality of amino acids (the left- or right-handedness I mentioned just now) – she is an enthusiastic communicator with a knack for keeping the reader both informed and entertained.

Equally fascinating is the how. The easiest way to study asteroids and comets is to start right here on Earth with meteorites. But scientists are also studying much smaller interplanetary dust particles gathered from our planet’s stratosphere. What is really pushing the envelope technically is to study comets and asteroids in situ, in space. The Rosetta mission to the comet 67P/Churyumov–Gerasimenko, which had a very unexpected shape, captured the public imagination, and was well publicised by the European Space Agency. Starkey takes the reader through the trials and tribulations of this mission, but is equally infectious about NASA’s Stardust mission. And she shortly details other missions, less well known outside of the circles of astronomy-aficionados.

What I really appreciated here was that Starkey gave very clear-cut reasons for why we mount such expensive and high-risk missions where much can go wrong. We cannot really send sensitive scientific instruments that need careful calibration and constant human attention into space unattended, so there is a really good reason to try and develop missions that can return samples to Earth, as has been done a few times now. But it also forces us to develop new technologies so that the instruments we do send out can operate remotely in the harsh conditions of outer space. There have been some real boons and bonuses as a consequence of developing new technology detailed here. Who would have thought there could be a link between spacecraft technology, bedbugs and perfume?

“Who would have thought there could be a link between spacecraft technology, bedbugs and perfume?”

The final two topics Starkey tackles are space mining and, of course, the threat of impact and countermeasures. I always thought of the former as sci-fi, but Starkey takes this topic very seriously, giving a clear overview of how it could be done and what the hurdles are. With rare earth elements being uneconomical to extract and humanity’s high-tech society being very dependent on them (see The Elements of Power: Gadgets, Guns, and the Struggle for a Sustainable Future in the Rare Metal Age), I am starting to see why there is such interest in this. Starkey describes how it could even be the stepping stone for expanding out into space.

The danger of impact was recently discussed in my review of Cosmic Impact and is not the focus of this book. Starkey nevertheless does not ignore the topic altogether and gives a very good overview of the inevitability of impact in the long run, the consequences for life, recent examples such as the Tunguska Event and the Chelyabinsk meteor, and the various strategies we could employ to counter this threat.

The highlight of Catching Stardust for me was how Starkey champions the relevance of scientific research, and the wider, real-world benefits “astronomers-having-fun” brings. Since she is a host on Neil deGrasse Tyson’s StarTalk Radio podcast, you might have already guessed Starkey is a good science communicator. And she brings this flair to what is, unbelievably, only her first book. Another excellent addition to the Bloomsbury Sigma imprint and a great example of well-written popular science.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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