A Murder Mystery in Reverse

Deep Impact - the Nasa mission that notched a hole in a comet called Tempel 1 on Monday - was a stunning exercise in human cooperation, celestial sharpshooting and cosmic curiosity.

In January, using laws of heavenly mechanics proposed by Isaac Newton 350 years ago, a consortium of Nasa engineers and university scientists lobbed a spacecraft on a 268 million mile journey around the sun. After 172 days, and precisely according to a timetable calculated years in advance, the spacecraft let go of a little passenger roughly a meter across, and then watched it fly along an orbit that would leave it exactly in the path of the onrushing comet.

And at 6.52am BST on July 4 - America's birthday - everything happened as planned. The advancing comet overtook and ran over the little copper-plated projectile at 6.3 miles a second, or 23,000mph. As planned, the projectile vaporised in the kinetic energy of the impact and created a burst of heat of about 2,000 C. But the collision also blasted a crater in the comet, and released a vast jet of ice, dust and complex chemicals which flared in the distant heavens.

As it did so, the mother ship 300 miles away started taking pictures. So did cameras aboard Hubble and six other telescopes in orbit about Earth. They studied the burst of energy 83m miles away in the ultraviolet, infrared, x-ray and optical spectra. Earth-based telescopes, at first in Arizona and Hawaii - because the collision occurred during America's night - and then right round the world watched the comet as the celestial flare exploded into space at twice the speed of a jet airliner, soared about 1,200 miles, brightened, grew dim and faded in the heavens. The data they collected will keep the planetary science community busy for years. "They say a picture can speak a thousand words," said Deep Impact's project manager, Rick Grammier, of the Jet Propulsion Laboratory in Pasadena. "But when you look at some of the ones we captured in the early hours of July 4, I think you can write a whole encyclopedia."

What happened on Monday was more than a lesson in slick engineering and celestial calculation. It was part of a worldwide attempt to answer some of the most fundamental questions: who are we, where did we come from, and how did we get here? Comets are lumps of primordial fabric that spent most of the last few billion years parked in the cold and dark far from the sun, far beyond the furthest of the nine planets. They are the leftovers, untidy bundles of stuff from which the planets must first have been formed, 4.6bn years ago. Comets are at least 50% water ice: was it from this primordial water that the Earth's oceans formed?

Comets contain 20% soot, and a suite of complex organic chemicals forged in the furnace of stars that lived and died far away and long ago. Was it from these that life on Earth somehow organized itself, more than 3bn years ago? Comets routinely hit the Earth during infancy, and still do. If they helped life into existence, did they also alter the course of evolution from time to time? And could they do so again, by wiping out humankind, as they may have obliterated the dinosaurs?

These questions are not simple. The water in the world's oceans has been altered, muddied, scrubbed clean and recycled many times. It has been buried in the planet's crust and spat out by volcanoes, sucked up by plant roots and transpired through leaves, consumed by gullets and filtered by kidneys. Life, too, is a kind of murder mystery in reverse: life emerged, and then proceeded to trample around and erase the evidence of its own origins. But locked in the heart of comets such as Tempel 1 - a lump of matter almost nine miles long and three miles thick, slowly tumbling through space - are the answers. Deep Impact, one of four missions so far to explore the comets, was a daring and successful bid to intercept one of these strange visitors, and then literally get under its skin.

It could be years before convincing answers emerge. Tempel 1 is just one comet among billions: is it representative? Are there different classes of comet? So the Europeans have launched Rosetta to visit another comet, and ride along with it as it falls to the sun, in 2014. Nasa scientists sent a mission called Stardust through the tail of a comet to collect some of its dust: these delicate fragments will fall to Earth in 2006. It may be decades before scientists begin to put together the big picture. But the Nasa engineers on this week did settle one point: if indeed there is a comet with the Earth's name on it - a monster the size of Manhattan speeding towards the planet at 30 miles a second - it will be very difficult to stop. Deep Impact cost £188m and its copper bullet hit Tempel 1 so hard that it triggered a 1,200 mile explosion from a grubby ice lolly a mere 9 miles long.

But did Tempel 1 slow, falter or alter course? Not so that anyone would notice. Deep Impact was more than a triumph of human patience and cooperation. It was a lesson in humility as well. We should be grateful for both.

· Tim Radford is the Guardian's science editor