Authors of books on speculative history or archaeology face the same problem as pop stars: having to find a quick follow-up to cash in on their last success. In the case of Graham Hancock and Robert Bauval (authors of several fascinating and bestselling books on the pyramids), their latest offering is - like too many follow-ups - something of a turkey.

The Mars Mystery retreads old ground with its examination of a couple of blurry photographs of Mars taken in the 1970s, purportedly revealing a mile-long, human-like face and a bunch of pyramids. Rarely can so little evidence have inspired so much speculation. Proponents of "life-on-Mars" theories claim the "face" must be artificial, and evidence of a lost Martian civilisation; NASA claims it's simply a natural rock formation.

Hancock, Bauval and Grigsby link the "face" with Egyptian mythology, the state of the surface of Mars, and the large comets and asteroids whizzing around the solar system. They conclude that the "face" could be a deliberate message to us, "a warning that a Mars-like doom lies in wait for the Earth unless we take steps to avert it."

The logical sloppiness of this book is breathtaking. The authors build much of their case on detailed geometric comparisons between measurements of Egyptian and Mexican pyramids - accurate to fractions of an inch or degree - and measurements of the supposed Martian pyramids. These come from photos in which each pixel (or dot) of the image covers over 2,000 square metres. Whatever one's views on extra-terrestrial life, this is just bad, bad science.

As for the famous "face", the authors and publishers must be congratulated on keeping the book right up-to-date. The latest photograph of this area was taken this April. Surprise, surprise: the new close-up is quite different from previous computer-enhanced, long-distance photos. The "face" no longer looks at all artificial. Surely this destroys the whole thesis? But no: "face" proponents now claim that in the close-up shot, "a well-weathered sculpture would actually look less like a face the closer one got".

This is rather reminiscent of the rapid back-pedalling of religious prophets when their prophecies fail to come true. Amazingly, the faith is rarely destroyed. Indeed, although the authors say at the end that they "have remained dubious throughout" about the Martian "monuments", their faith in their theory - "the most staggering revelation of the millennium" - is not dented one whit.

The one very worthwhile point they make, in a book full of repetition and padding, is that astronomical study of all the lumps of rock flying around us does need far more funding. If even a medium-sized chunk hits Earth, mankind could follow the dinosaurs (not to mention the Martians). Last year, Britain spent pounds 6,000 on the problem.