Yay!

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Presently perched on a Delta 2 rocket at Cape Canaveral is a one-ton spacecraft called Kepler. If all goes well, the rocket will lift off about 10:50 Friday evening on a journey that will eventually propel Kepler into orbit around the Sun. There the spacecraft’s mission will be to discover Earth-like planets in Earth-like places — that is to say, in the not-too-cold, not-too-hot, Goldilocks zones around stars where liquid water can exist.

Kepler’s strategy is, in effect, to search for the shadows of planets. The core of the spacecraft, which carries a 55-inch-diameter telescope, is a 95-million-pixel digital camera. For three and a half years, the telescope will stare at the same patch of sky about 10 degrees, or 20 full moons, wide, in the constellations Cygnus and Lyra. It will read out the brightnesses of 100,000 stars every half-hour, looking for the telltale blips when a planet crosses in front of its star, a phenomenon known as a transit.

To detect something as small as the Earth, the measurements need to be done with a precision available only in space, away from the atmospheric turbulence that makes stars twinkle, and far from Earth so that our home world does not intrude on the view of shadow worlds in that patch of sky. It will take three or more years — until the end of Barack Obama’s current term in office — before astronomers know whether Kepler has found any distant Earths.

If Kepler finds the planets, Dr. Borucki explained, life could be common in the universe. The results will point the way for future missions aimed at getting pictures of what Carl Sagan, the late Cornell astronomer and science popularizer, called “pale blue dots” out in the universe, and the search for life and perhaps intelligence.

But the results will be profound either way. If Kepler doesn’t come through, that means Earth is really rare and we might be the only extant life in the universe and our loneliness is just beginning. “It would mean there might not be ‘Star Trek,’ ” Dr. Borucki said during a recent news conference.

We might answer the question "Are there other worlds like ours?" relatively soon.