J. Brit. Astron. Assoc., 105, 6, 1995

The Great Comet Crash: The Collision of Comet Shoemaker–Levy 9 and Jupiter

by John R. Spencer & Jacqueline Mitton (Eds.)

Cambridge University Press, 1995. ISBN 0-521-48274-7. Pp ix + 118, £16.95 (hbk).

reviewed by Iwan P. Williams

Before the event, the collision of comet Shoemaker–Levy 9 with Jupiter was described as the astronomical event of the century. Rather remarkably for astronomical phenomena, after the event it was still the event of the century. It is thus very pleasing that a book describing this remarkable comet has appeared so soon after the collisions. What is equally pleasing is that the book has generally got things right. It is very easy to go for a fast publication and hang the quality but in this case this has not happened.

Even at discovery it was clear that this comet was unusual and the story of its discovery is appropriately told by Carolyn and Gene Shoemaker. As if having a comet in 21 bits was not unusual enough, when more observations came in the orbit calculators started predicting a collision with Jupiter. This part of the story is told by Brian Marsden who keeps track of all solar system orbits on behalf of the International Astronomical Union. Once the collisions were accepted as a certainty, attention switched to the target. Both theoreticians and mystics had a field day predicting the outcome, which at the time varied from 'we will see nothing' to 'wear sunglasses in July'. I suppose that it was inevitable that the book has concentrated on the middle of the road predictions, these were broadly correct, but the incorrect ones were more fun and it would be nice to see them recorded somewhere.

In the midst of all this predictive activity, we must not forget that observations of the cometary fragments continued unabated and this is superbly described by Hal Weaver and Dave Jewitt. It is fair to say that we learnt more about the comet, though not about other things, through this phase.

Of course inevitably, July 16th came and the world and all its observatories waited. So many images were taken that week that now, hardly a year afterwards, it is difficult to remember many of them. Nevertheless I shall never forget the excitement of waiting for our first image to appear on the computer screen. Both for those who have forgotten those images and those who haven't, this book is a must and John Spencer and Jacqueline Mitton have done a great job of assembling a selection of the best. I suppose that one of the very pleasant surprises from the event was that amateur observers were able to see the aftermath of the collisions and their contribution is not overlooked in the book.

The bravest contributor is Jay Melosh who attempts to summarise what we learned from the event. He was faced with an impossible task since the real science is only emerging in detail now. However, he does a good job of the material available at the time of writing and the book benefits from having that chapter even though a much better one can be written in twelve months time.

The final chapter is 'What if ?'. It is actually the wrong title: we all know the answer to 'what would happen if SL9 had hit the Earth?'. The real question, and the one discussed, is 'What are the chances of something hitting the Earth?'. Clark Chapman is an expert in this area and you will have to buy the book to find out the answer!

I thoroughly recommend that you buy the book even if you do not want to know the answer to that question. It is a worthy record of a very momentous event.


Iwan Williams is Professor of Astronomy at Queen Mary and Westfield College, University of London. He was part of a UK team that had observing time on both the 1m Jacobus Kapteyn telescope and the 2.5m Isaac Newton telescope on La Palma to observe this event. He is a past President of the Junior Astronomical Society (now the Society for Popular Astronomy).

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