From a sliver of a coastal island, a multistage rocket is set to blast off into the Florida sky tonight, on a quest to answer one of astronomy's great mysteries: Are there other planets like Earth out there, or are we alone in the universe?
New insights into that question might come following the scheduled launch this evening of the Kepler space telescope.
For the next three to four years, Kepler will trail millions of kilometres behind our planet, staring unblinkingly at a star-rich patch in the Cygnus-Lyra constellations. By recording the tiniest shifts in the brightness of those stars, scientists are hoping to detect planets that are like Earth.
"It's the Holy Grail of astrobiology," said University of Washington astronomer Donald Brownlee, one of the researchers involved in the mission.
Many scientists have held that the universe is so vast that, statistically, there have to be planets elsewhere similar to ours. However, since 1995 astronomers have identified more than 300 planets outside the solar system - none of them similar to Earth.
The planets the Kepler mission is seeking have to be the same size as Earth and orbit within a habitable zone from their stars, so that they are neither too hot nor too cold.
This would ensure that any water present on those planets wouldn't be entirely frozen or vaporized, a pivotal condition for the formation of life, said one of the mission's co-investigators, Gibor Basri, an astronomer at the University of California at Berkeley.
The first planets discovered beyond the solar system were Jupiter-scaled celestial bodies, gaseous behemoths 10 times the size of Earth, orbiting extremely close to their sun and far too hot to support life. "It was completely unexpected," Prof. Brownlee said.
Other extra-solar planets turned out to be equally uninhabitable, having highly elliptical orbits, so that part of the year they are too hot and at other times too cold.
"One of the main issue remains, 'Is our solar system common or not?' " Prof. Brownlee said.
Some time between 10:48 and 11:17 p.m. ET, lifting off from Cape Canaveral Air Force Station, a Delta II rocket will hoist the telescope, a one-tonne payload about the size of a compact car. (The telescope is named after Johannes Kepler, the German astronomer who discovered the laws of planetary motion in the 17th century.)
Kepler will take two days to settle into a solar orbit, slightly further from the sun than the Earth's orbit. The telescope will trail the Earth so that it will take 373 days to circle around the sun.
Using sensors similar to those of digital cameras, Kepler will detect planets indirectly as it monitors about 100,000 stars. It will look for planets' transits - that is, the moment a planet passes in front of its sun, temporarily dimming the star's brightness. If the size of the star is known, then "the dip tells us the size of the planet," Prof. Basri said.
The change would be so minute that some researchers compare it to spotting an insect flying in front of a car's headlights. "The drop in brightness is extremely small so it's best done from space," Prof. Brownlee said.
Since Kepler must measure at least three transits to confirm the validity of a planet, it would take three years to do it. Planners caution that the first possible announcement of Earth-size planets in a habitable zone won't happen until 2012.
"We all expect to find many [similar to Earth]. Everything we know about planet formation tells us that's what we should find," Prof. Basri said. "But if we don't find many, that would be earth-shaking as well. That would really mean that we're really a special planet."
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Searching for habitable planets
Kepler is designed to measure tiny brightness variations in stars that might indicate orbiting planets. NASA hopes to find Earth-like planets orbiting stars at a distance that would permit liquid water, potentially allowing them to support life. Kepler will be placed in an orbit around the sun, surveying more than 100,000 stars for three years.
KEPLER
4.7 m tall
HUBBLE
13.3 m tall
SMALLER COUSIN
Considerably smaller than the Hubble Space Telescope, Kepler is the largest telescope ever to be sent beyond Earth orbit.
Thermal control
Blankets of teflon and other materials (not shown) help maintain operating temperatures.
Reaction wheels
Allow minor changes in orientation, without burning fuel.
Star trackers
Help orient the telescope in space.
Primary mirror
Made of ultra-low expansion glass coated with silver, it measures 1.4 m in diameter.
Solar panels
Provide up to 1,100 watts of electrical power and shield the telescope from the sun's direct heat.
HOW IT WORKS
Kepler will hunt for planets by measuring tiny drops in a star's brightness as a planet passes in front of it, or transits. Three or more transits will be required to rule out other causes, such as sunspots.
Focal plane
Kepler's 95-megapixel camera holds 42 light-sensitive modules. It is the largest camera NASA has flown in space.
The camera will focus on one region of space, monitoring more than 100,000 stars every half hour for three years.
FRANK O'CONNELL/THE NEW YORK TIMES/THE GLOBE AND MAIL; SOURCES: NASA; BALL AEROSPACE & TECHNOLOGIES CORP.; CARTER ROBERTS
