A Boeing-built Delta II "Heavy" rocket lifted off from a launching pad at an Air Force base here at 11:18 pm (0318 GMT Tuesday), after a defective valve discovered in the first stage of the rocket postponed the launch by an hour.

A camera mounted on the rocket allowed experts to monitor the probe's night-time ascent into the dark Florida sky.

The launch went without a hitch.

Four minutes after liftoff, the Delta's main engine shut down and the first stage of the rocket separated. The engine of the second stage then took over, carrying the probe into space until it also shut down nine minutes into the flight.

Mission controllers will be able to breathe a sigh of relief only about 80 minutes into the flight, after the separation of the rocket's third stage.

At that point, Opportunity will begin its 491-million-kilometer (about 300-million-mile) journey to the Red Planet where it is expected to arrive on January 25, 2004.

Opportunity was the second of two robot probes designed to explore the planet's surface.

The spacecraft was originally set for launch June 25, but its flight has been repeatedly delayed by weather and technical problems.

The first of the twin probes, MER-A, or "Spirit," was launched on June 10, beginning a three-month, 500 million kilometer (310 million mile) voyage, intended to end on January 3, 2004 in the Gusev Crater, 15 degrees south of the equator of Mars.

The second probe is heading for the Meridiani Planum, a zone containing an concentration of ferrous oxide situated two degrees south of Mars' equator.

Although both probes will set down relatively close to the Martian equator, they will be on virtually opposite sides of the planet, some 6,000 miles (9,600 kilometers) apart.

NASA is investing some 800 million dollars in two six-wheeled vehicles, which are expected to look for three months for sites considered geologically important.

However, NASA associate administrator Ed Weiler said late last month these missions were not designed to find water on Mars.

"We know Mars has water, we know it had it in the past and may have it in the present," he explained. "What we don't know is how long the water persisted in any given place. If it stayed there for tens of millions of years, then there is a good chance that life might have evolved."

NASA had until July 15 to launch Opportunity. After that date, the Red Planet is expected to move out of range for an Earth launch.

The US mission comes on the heels of a European mission begun June 2 with the Mars Express probe, now on its way to the Red Planet.

SPACE.WIRE