The idea of life vagabonding through the cosmos has been around for millennia, but scientists first considered it seriously in the mid-19th century. In 1871, British physicist William Thomson Kelvin told his colleagues in Edinburgh: "We must regard it as probable in the highest degree that there are countless seed-bearing meteoritic stones moving about through space. If at the present instant no life existed upon this earth, one such stone falling upon it might ... lead to its becoming covered with vegetation. "
　　Three decades later, Swedish chemist and Nobel laureate Svante Arrhenius agreed, but he took issue with part of Kelvin's scenario. The fiery trauma of a meteoroid ejected from a planet or out of the solar system, he argued, would incinerate any cells it harbored. Instead of hitching rides within rocks, Arrhenius said, life could travel unaided. In 1903, he proposed that spores of plants and germs might drift through space propelled by the gentle pressure of starlight. He called this idea panspermia ( from the Greek for "seeds everywhere") .
　　When astronomers later grasped the true distances between.stars and the vast size of the Milky Way, panspermia fell out of favor. Chemists and biologists devised other credible theories for how life might have arisen on Earth, such as the "warm little pond" of gentle chemical reactions envisioned by Charles Darwin.
　　Now panspermia is gaining credence again, but with more caveats. Planetary geologist Jeffrey Moore of the NASA Ames Research Center says that if panspermia simply means exchanges of life among bodies in our solar system, Kelvin's "seed-bearing meteoritic stones" could be spot on. "Panspermia redefined is perceived as reasonable by vutually everybody," Moore explains. "Say you have several. places in the solar system where organisms could multiply. Once one gets it, all the planets and moons with suitable environments come down with life. It's the day-care effect. They infect each other." The inner solar system, he adds, with its friendly temperatures and hard surfaces, is the most likely place for such exchanges. Still, migrating microbes face significant obstacles. Until recently, no researchers had evaluated every stage of the scenario. Then a Swedishscientist rounded up a team to do just that.
　　Curt Mileikowsky, 78, works at the Royal Institute of Technology in Stockholm. A nuclear physicist by training, he is the former chief engineer for development of nuclear power reactors in his country. For 12 years he served as chief executive officer of Saab Scania, known for itshigh-performance planes, missiles, and rockets. His company also helped develop the Ariane rocket for the European Space Agency. "All of this got me very interested in space, " he says.
　　Mileikowsky's academic career also included work with medical devices, such as neutron-beam guns for cancer treatment. That expertise in the study of radiation, combined with his knowledge of projectiles, perfectly prepared him to study life whizzing through space.
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