UPDATED: Story first posted 2:34 p.m. EDT, June 10, 2003

CAPE CANAVERAL, Fla. -- A Boeing Delta 2 rocket has successfully sped NASA's Mars rover Spirit out of Earth orbit to begin a seven-month journey to the Red Planet.

Finding a hole in cloudy skies over Florida's Space Coast, the three-stage booster lifted off at 1:58:47 p.m. EDT (1758.47 GMT) Tuesday.

Afternoon thunderstorms, so common over the Florida peninsula during the summer months, had forced NASA to call off launch attempts on Sunday and Monday. But conditions remained clear enough Tuesday to allow the shot.

During the next 39 minutes the rocket stepped through its launch sequence, much of it broadcast live on NASA TV thanks to presence of a rocketcam bolted to the side of the vehicle.

Exactly as planned, Spirit separated from the Delta 2's upper stage having already achieved Earth escape velocity.

Even with this victory behind them, Mars mission managers know they still have a long way to go. Only 12 of humanity's 30 attempts so far to unlock Mars' secrets using robot probes have succeeded.

"In Mars exploration we have learned to celebrate success one step at a time," said Orlando Figueroa, NASA's Mars exploration program director. "We know there is much work ahead of us."

At NASA's Jet Propulsion Laboratory, which built the $800 million pair of robots, hundreds of scientists and their families watched the launch on television. They cheered as the rocket lifted off.

Less than an hour later, mission controllers received the first signal from the spacecraft as it sped outward from Earth to Mars.

"It's like a birthing process ... hearing the baby's first cry,'' mission science manager John Callas told the Associated Press.

Mission members said they were relieved to watch Spirit leave Earth, but cautioned the most difficult portion of the mission was yet to come.

We can't get too excited about launch. A Mars mission is all about landing. But we'll take our successes as they come,'' said Firouz Naderi, manager of JPL's Mars exploration program.

Spirit is due to arrive at Mars on Jan. 24, 2004. Its twin rover spacecraft, now called Opportunity, is to be launched June 25 and arrive at the Red Planet on Jan. 25, bouncing onto the sandy surface with the help of parachutes, retro rockets and inflated balloons.

As now planned, Spirit is targeted to land at Gusev Crater, 15 degrees south of Mars' equator. Opportunity is to touch down at Meridiani Planum, about two degrees south of the equator and halfway around the planet from Gusev.

Total cost of the two missions, including launch services, spacecraft hardware and mission operations is about $800 million.

NASA's pair of Martian probes will join the already enroute spacecraft of Japan's Nozomi and the European Space Agency's Mars Express, which includes the Beagle 2 lander.

Already in orbit over Mars are NASA's Mars Global Surveyor and Mars Odyssey spacecraft.

"The present Mars exploration program represents the most intensive scientific assault of any heavenly body since the Apollo-era. We are indeed in the midst of an unprecdented chapter in robotic exploration," Figueroa said.

While each of the probes now at Mars and on their way go about their studies in different ways, all are interested in helping to answer the same basic questions.

"Getting to answer whether life ever took hold there, understanding its evolution for clues of our evolution perhaps, and our destiny, and setting the stage for possible human exploration makes a compelling and worthy case for the continued exploration of the Red Planet," Figueroa said.