The US will launch the Kepler spacecraft Friday to look for other Earths.

The United States is scheduled to launch on Friday an orbiting telescope designed to help answer one of the oldest and deepest questions of astronomy: Are we - or at least our planet's microbes - alone in the galaxy?

The Kepler spacecraft will stare at a patch of sky - the same 100,000 stars near the northern constellation Cygnus, all at once - for at least 3-1/2 years. The goal is to detect Earth-like planets orbiting their host stars at distances thought to be sweet spots for life.

Dubbed habitable zones, these are orbits where a planet is bathed in light that is strong enough to permit liquid water to collect and remain a persistent feature of the planet's surface.

If all goes well, Kepler's journey will start with the launch from Cape Canaveral in Florida at around 10:49 p.m. Eastern time.

For planet-hunting astronomers, the $591 million Kepler mission promises to cross what they've dubbed the "Great Divide" that separates the decidedly uninhabitable planets they have found so far with the Earth-like ones they seek.

During the past 15 years, ground-based telescopes have detected more than 300 so-called exoplanets - planets that are beyond our solar system. But these discoveries have involved a bizarre menagerie of objects ranging in size from 10 times Jupiter's mass to nearly Earth-scale planets.

They often follow highly elliptical orbital paths, subjecting them to enormous temperature swings. Some are more dense than lead; others would float like foam on water, notes William Borucki, a scientist at National Aeronautics and Space Administration's Ames Research Center in Sunnyvale, Calif.

Some orbit so close to their host stars that one "year" in those planets is less than one Earth day.

The ultimate goal, however, is "to someday take a picture of a pale blue dot orbiting a nearby star," says Debra Fischer, an astronomer at San Francisco State University. To decide how to do that in the most efficient and cost-effective way, it's vital to know how many of these planets are likely to be out there for a given number of stars. The frequency with which Earth-like planets are found will be a key driver for future planet-hunting missions, she says.

And if Kepler finds only a tiny handful of such planets in habitable zones?

"That would be another profound discovery," says Dr. Borucki, the mission's lead scientist. "It will mean that Earths must be very rare. We may be the only extant life," at least in our galaxy.

No aliens? Alien chemistries will do
Even if Kepler detects few Earth-like worlds in Earth-like orbits, the number of other planets it finds is likely to explode. Some solar-system experts would be content to give up the prospect of alien life forms for alien geology and chemistry.

The pressure at the center of the Earth is some 3.5 million times higher than the atmospheric pressure at the surface, but Jupiter's core is squeezed to some 70 million higher, notes Raymond Jeanloz, a planetary scientist at the University of California at Berkeley.

At these pressures, helium is no longer a gas incapable of carrying electricity; it's a fluid metal that has no problem conducting electricity.

On some of the larger exoplanets, pressures at the cores are estimated to reach billions of times Earth's surface pressure.

"By the time you get to billion-atmosphere pressures, it's not just the chemical bonds between atoms" that change. "Atoms themselves are crushed," Dr. Jeanloz said during a briefing at the American Association for the Advancement of Science's annual meeting in Chicago last month. "This is a regime of a new kind of chemistry."

The one-ton Kepler observatory boasts a 1.4-meter (4-1/2 foot) diameter mirror and a push-the-envelope camera, according to James Fanson, project manager for the Kepler mission at NASA's Jet Propulsion Laboratory in Pasadena, Calif. A typical digital camera has roughly eight million to 10 million individual picture elements, or pixels, on its light detector. Kepler's camera boasts 95 million pixels.

The camera won't take pictures of the stars it monitors, however. Instead, it will measure changes in starlight as an orbiting planet slips in front of its host star. It's a tough measurement to make - akin to measuring the dimming effect of a flea passing in front of a car's headlight, Dr. Fanson says.

The amount of dimming yields the planet's radius. The elapsed time between each dimming episode yields its orbital period. From this data, and from a knowledge of the parent star's traits, astronomers can calculate the planet's mass, its density, and whether it falls in the habitable zone.

Not all solar systems will be oriented in ways that allow the telescope to detect planet "transits" across the faces of stars. So Kepler's "transit" technique for picking out planets has dictated the large number of stars the telescope must monitor.

Tracking 100,000 stars
Picking the targets, 100,000 sun-like stars, was no cake walk. Astronomers spent five years methodically measuring traits of 4.5 million stars in Kepler's planned field of view.

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