WITH THE accuracy of a speeding bullet hitting another speeding bullet, a projectile launched from a NASA spacecraft slammed into a comet 83 million miles from Earth on Monday, right in time for the Fourth of July. The collision was intended: NASA scientists wanted to measure the effect of the impact and the size of the subsequent crater, and, if possible, to examine some of the resultant debris. The mission was perfectly timed and perfectly executed -- so much so that it ought to get many Americans wondering whether this Independence Day success might not point a way forward for the U.S. space program.

True, the mission did not include any astronauts, and it did not advance the cause of human space travel. But it did advance the cause of human knowledge, in the most profound and mystical sense. The ultimate objective of the mission, after all, was no less than the discovery of the origins of the universe. Since comets are thought to contain the same ice, dust and gases that first formed the solar system, analysis of their composition may help scientists better understand the origins of life itself. Yet the mission could have some practical applications, too. Many scientists now believe that the era of the dinosaurs came to an end after an enormous comet hit Earth, radically changing the planet's climate. Someday, NASA's expanded knowledge of comets might help prevent another, equally devastating planetary catastrophe.

For those who persist in believing that human space travel is the only worthwhile goal of the American space program, this amazing achievement offers a perfect counterargument. Space exploration needn't be costly -- the price of this mission was $333 million, a fraction of NASA's $16.5 billion 2006 budget -- to expand the boundaries of knowledge. Nor is it necessary to take the risks and pay the exorbitant price of human space travel to carry out a project that may help ensure the survival of life on Earth.