If there are other planets like Earth out there, at least one in three probably harbors life, according to Charles Lineweaver and Tamara Davis of the University of New South Wales in Sydney, Australia. If life can arise on planets unlike ours, then the odds on finding life are even more favorable.

We can get important knowledge from the fact that life on Earth seems to have evolved very quickly, say the researchers. According to the earliest fossil records, life took no more than about half a billion years to gain a foothold, once the planetary conditions were amenable. This time scale might actually have been much less – even instantaneous in geological terms.

This rapidity tells us that the probability of life developing on an Earth-like planet is high. If a gambler wins the lottery within the first three days of buying tickets, it is likely, although not certain, that the chance of winning is high. The fact that “we won soon after life became possible on Earth”, say the researchers, points to there having been a good chance of life developing.

Several unknowns might demolish the researchers’ statistical argument. For example, life may have to develop rapidly if it is to develop at all. Or Earth may be more finely tuned to nurturing life than we think – a truly habitable planet may have to be not just similar to Earth but virtually identical. It?s also possible that life might appear on planets that are radically unlike Earth.

Lineweaver and Davis used the geological record of life on Earth to pin down a key unknown in a famous formula that calculates the likely number of stars in our galaxy on which life has appeared. Called the Drake Equation, the formula was devised by astronomer Frank Drake in 1961. It summarizes all the factors involved in the likelihood of our detecting signals broadcast by an intelligent alien civilization.

Lineweaver and Davis considered a simpler version of the equation, which asks merely what proportion of stars have evolved any kind of life, not necessarily intelligent life. This question can be broken down into three parts: what fraction of stars have planetary systems, what fraction of those planetary systems contain a habitable planet, and on what fraction of those habitable worlds life has actually appeared.

In the past decade, astronomical observations of planets around stars other than our own sun have started to tell us about the first two terms in the equation. NASA has just announced a new program called the Terrestrial Planet Finder (TPF), which will use Earth-based astronomy to scan for Earth-like planets outside our Solar System. The design of the TPF mission will be finalized in 2006.

But it is the mysterious third factor that Lineweaver and Davis have put a figure on – the number of habitable worlds on which life has actually appeared. Some scientists argue that it is extremely small; others contend that life is virtually inevitable on any potentially habitable world.

For Earth-like planets that are older than about a billion years (Earth is about 4.5 billion years old), the likelihood of life is almost 100%, the researchers think. They think there is a high probability that at least 1/3 of the planets harbor life.

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So let?s figure out how to travel to other planets. Rodney Bartlett and his colleagues at the University of Florida think we can use a version of table salt for rocket fuel. Nitrogen atoms will be linked in groups of five, and half of these groups will be positively charged and half negatively charged, like the sodium and chlorine atoms in salt.

Nitrogen has a long association with rocket fuels and explosives, such as nitroglycerine and TNT (trinitrotoluene). This is because nitrogen gas, N2, is extremely stable. Compounds with more than two nitrogen atoms per molecule readily decompose to N2, releasing a lot of energy as they do so. Hydrazine – a blend of nitrogen and hydrogen – works this way.

N5+N5- would contain twice as much energy as the same volume of hydrazine, the fuel that propels many spacecraft today, so N5+N5- fuel could be packed into a smaller tank, allowing smaller rockets to be used. Right now, much of their space is used for carrying fuel.

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Or we could use the energy of the Sun. The first attempt to fly a craft pushed by the light of the Sun is scheduled for an autumn launch. The project is a joint venture of the Planetary Society and Cosmos Studios, a group of film-makers and writers set up by the widow of writer Carl Sagan. The craft will begin its journey on board a rocket fired from a submarine in Russian waters. If all goes as planned, the solar sail spacecraft will separate from the rocket where it will unfurl and fly for a few weeks or months around the Earth pushed by the Sun. Experts at NASA and the European Space Agency will be watching closely to see if the low-budget mission succeeds.

The experiment could pioneer a new generation of space travel using spacecraft that are propelled by the our star’s energy in the same way that ships on the sea are pushed by the wind. The technology relies on ultra-thin, mirror-like solar sails that trap individual particles of light from the Sun. In theory, the photons should transfer their energy to the sails, pushing the spacecraft along.

It?s possible that a spacecraft might be able to travel, say to Mars, propelled by conventional fuel. To travel further, however, would stretch the limits of chemical propulsion. The idea is to use energy from the Sun to power spacecraft travelling to the outer planets and beyond. Sunlight would become too weak beyond the realms of Jupiter but one theory for interstellar travel is to direct space lasers at the sails.

Planetary Society executive director Dr. Louis Friedman says, “Solar sailing is the technology that can take us to the stars. That’s the dream of solar sailing.”

Other applications for solar sails include deflecting an asteroid that?s on a collision course with the Earth. Colin McInnes, who heads a program at Glasgow University in Scotland that is investigating solar sail technology, says, “Because the solar sail propulsion is open-ended you could use the same spacecraft – the same set of instrumentation – to hop from one asteroid to the next, doing comparative science.”

There is also strong commercial interest in developing solar sailing ships. By skimping on rocket fuel, they could make the most expensive form of transportation ever invented One U.S. company – Team Encounter – plans to send a solar sail loaded with photographs, messages and DNA into interstellar space. You can include your own message for only $50.

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Former astronaut Buzz Aldrin and scientists at Purdue University are working on a shuttle they hope will one day take people from Earth to Mars and back. This interstellar bus line would involve two massive spacecraft that could house up to 50 people on their six-month shuttles between the two planets. The craft would continuously cycle between Earth and Mars, using gravity as their primary power source, with an occasional shot from a booster rocket. “If we’re going to go to Mars with human beings, we need to do it in an evolutionary way, so that we can continue doing it,” says Aldrin.

NASA’s Mars program has been set back in recent years, particularly in 1999 when two spacecraft failed upon reaching Mars, one burning up in the planet’s atmosphere and the other disappearing after a software failure. The program got back on track earlier this year when the Mars Odyssey spacecraft entered an orbit around Mars and began mapping the mineral and chemical makeup of the surface. NASA will spend about $500 million a year over the next 10 years on Mars exploration. “We’re in a decade of discovery for Mars,” says Jim Garvin, NASA ‘s lead scientist for Mars exploration.

Aldrin first came up with the idea of an Earth-to-Mars cycler in the mid-1990s, and he’s been working on it ever since, along with researchers at Purdue, the Massachusetts Institute of Technology, and the University of Texas. The earliest they can imagine cyclers being used is 2018. The craft would never actually stop at either planet. A smaller “taxi” vehicle would dock with the cycler on its sweep past, unloading passengers and transporting them to the surface. “These cyclers, they just whiz on by,” says James Longuski, professor of aeronautics and astronautics at Purdue. “It’s like a bus that never stops. Passengers just have to jump on as it’s going past.”

Once a cycler is launched on a specific trajectory, gravity would keep it circling the planet, in what’s known as a gravitational “slingshot” effect. As a spacecraft travels near a planet, the gravity of that planet pulls the craft toward it, then whips it around the other side, boosting its speed. This is the same phenomenon that in 1970 helped NASA safely bring home the crippled Apollo 13, using the moon’s gravity to slingshot the craft and crew back to Earth.

The trick is finding the right path for these cyclers to follow. The orbits of Earth and Mars are out of sync ? Earth moves around the sun in an almost circular path, while Mars’ orbit is more of an oval. That makes finding the right trajectory a complicated mix of celestial mechanics and mathematical theory. “It’s like a game of celestial billiards,” Longuski says. “Tell me which shot will knock all the balls into the pockets.” Students at Purdue are using complex computer simulations to calculate the right paths. Researchers at Texas and MIT are considering other details, like how to get the cycler positioned just right in space to begin its journey, and how to design the taxi craft that will ferry people to land.

Aldrin said the push toward Mars is inevitable, both for economic and scientific reasons, and it’s imperative for America to lead the way. Russia already has started capitalizing on space tourism, something Aldrin believes could become a major industry and a primary source of funding for Mars exploration. “I think the leadership will come to the nation that develops a strong space transportation system,” Aldrin says. “The nation that develops that will have an absolute clear advantage over others, economically and in many other ways.”

Longuski sees the colonization of Mars more as a necessity for survival ? 10,000 or 100,000 years down the road ? given the Earth’s limits in room and natural resources.”It gives people great ideas to ponder,” he says. “Great dreams.”

Can?t get into space the conventional way? Try going out-of-body. Skip Atwater learned how from the U.S. government. Read about it in ?Captain of My Ship, Master of My Soul? (which includes a free CD-Rom),click here.

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