Over the Moon and back again

Of all the current breakthroughs in human knowledge, our new awareness of the solar system is the one publishers love best. It produces better pictures than the Human Genome Project or the internet, it is easier to explain than cloning, and it attracts terrific writers to put words around the images.

These two books approach today's torrent of information about space in very different ways, but they agree about one thing. As Serge Brunier puts it in his big picture book, translated from the French by Storm Dunlop, the real heroes are machines, not people: the dozens of robots, ranging billions of kilometres from Earth that have told us far more about the place than any ground-based telescope could. As for sending people to take a look: December 14 2002 was the 30th anniversary of Apollo 17 , the last time a human set foot on another world, and the technology to do it again is not available.

Nigel Hey agrees that machines have been ideal delegates for their human creators. He quotes the physicist Freeman Dyson, who claimed that the cost of the Apollo programme set space science back 20 years. And he follows through better than Brunier, giving insights in a skilfully constructed book into how discoveries have been made, as well as what they mean.

The argument continues because the International Space Station, now half-built 380km above our heads, is set to cost $60 billion (£36 billion), far more than all the unmanned space missions described in these two books. The scientific returns are disputed and last Saturday's destruction of the Columbia space shuttle will mean lengthy delay because there is no other vehicle capable of lifting major space-station components into orbit. With machines, the costs and risks are far lower than when people are involved. And the scientific returns can be immense.

A feature of Hey's book is his high-risk tactic of bringing in the work of other writers. These include luminaries such as the scientist Carl Sagan and novelist Arthur C. Clarke, as well as humbler figures, including engineers involved in major space projects. His book is the place to find out just how a machine gets to somewhere 1 billion km away with an accuracy of 100km, and on time to within a few minutes. Sometimes it goes wrong, but the fact that it usually works is a major achievement of human creativity.

Hey also describes space techniques that sound far-fetched but are in routine use, such as "gravity assist", using a fly-by of a planet to accelerate a spacecraft to speeds that would be unfeasible with a conventional rocket. This method cut years off the time needed to get the Galileo spacecraft, one of the most successful machines in the history of science, to Jupiter. It got there via a visit to Venus - towards the Sun rather than away from it, the right direction for Jupiter - and a return fly-past of the Earth.

Hey does not let the technology overwhelm the science it has uncovered.

People had been talking about the solar system long before the accuracy of the term became apparent. His book, a hardback beautifully presented in a small-page format, explains the many ways in which the Sun, its clutch of planets and satellites and its millions of comets and asteroids, relate to each another. Some solar system bodies such as the Earth are re-engineered continuously by air, water and life. By contrast, Jupiter's satellite Io is rebuilt rapidly by volcanoes (if you go there, leave the map behind - it will be out of date before your spaceship arrives), while others such as the Moon are fossils that change little over billions of years. This sounds dull but is handy for scientists wanting to see relics of the early solar system. Perhaps the biggest change in our knowledge has been in our understanding of planetary satellites. We once saw them as little more than points of light, but we now see them as worlds in their own right. Some have battered, rocky faces, others icy surfaces, and yet others dense atmospheres, such as Saturn's satellite Titan, which is reminiscent of the early Earth.

Hey is also a handy guide to the many speculations about the solar system that our new knowledge has fed. We know that Europa, another satellite of Jupiter, has an icy surface; we can speculate with reasonable certainty that it has liquid water beneath; we can speculate with extreme uncertainty that this vast ocean contains life. He writes well about some of the big ideas, such as "terraforming" other planets for human habitation, that are now being discussed by scientists and engineers.

He shows another advantage we have gained from our understanding of the other objects of the solar system: a better awareness of our own world, just at the time we need it because of human-induced climate change.

Knowing how the greenhouse effect works on other planets, and on their icecaps, frost layers, oceans and atmospheres, helps our own thinking and planning, and has been obtained with instruments similar to those we use to look at the Earth itself.

Hey's account is judicious, well written and full of lively characters.

Brunier's book, by contrast, is bigger and relies on its pictures far more.

Some of the text is plain annoying (no galaxy is a "fragile jewel"), and some of the images need more explanation.

But Brunier agrees with Hey that anything said about the solar system before we distributed our robots far and wide is now of interest only to historians of science. He refers to the pre-space age as the "classical era" of planetary astronomy. Being European, he is careful to stress that it is not only the US taxpayer who has funded the advance from classical to modern; he gives various plugs to the European space effort.

Solar System Voyage is more spectacular than Solar System. But it also has intellectual strengths, including good material on small objects in the solar system. Comets and asteroids were once seen as fuzzy blobs or dots, but have become places that are visited by spacecraft and - in the case of the asteroid Eros - even landed upon.

Brunier also provides useful tables on solar system objects, space missions, eclipses and other topics, and spectacular images from space specials such as Comet Shoemaker-Levy's 1994 collision with Jupiter. His approach is more whimsical - each chapter begins by imagining that one is standing on the object in question - but well worth buying for the words as well as the pictures.

Martin Ince is contributing editor, The THES.