A National Aeronautics and Space Administration statement gave no reason for the delay in the launch of the Opportunity probe, which had been scheduled to blast off at 10:51 pm Saturday (0251 GMT Sunday) from Florida's Cape Canaveral.

NASA said a new attempt would be made for a launch on Sunday night.

"Launch of the Mars Exploration Rover Opportunity aboard a Boeing Delta II heavy launch vehicle has been rescheduled for no earlier than Sunday, July 6 -- with two launch opportunities available at 10:43 pm and 11:26 pmand 0326 GMT Monday)," NASA said in its statement.

Spirit, the first of the twin probes in the US mission, took off on June 10 for its 500-million-kilometer (300-million-mile) journey to Mars.

Opportunity was originally set for launch on June 25 but was delayed by technical problems with the cork insulation on its Delta II rocket. Another attempt was made on June 28, but strong winds over the Florida launch pad forced NASA officials to postpone it again.

A day later, scientists found another insulation problem, just hours before liftoff.

NASA had set a new launch date for Wednesday, but announced a further delay to allow more time for tests.

Spokesman George Diller said previously that the additional time was needed "to perform tests on the process used to bond the cork insulation to the surface of the Delta II launch vehicle."

A band of cork about 60 centimeters (24 inches) wide is meant to insulate the rocket from the incredible heat generated by the rocket boosters. NASA is concerned that the cork insulation is not sticking to the rocket properly.

NASA has only until July 15 to send Opportunity on its journey. After that date, Mars will have moved too far away for the probe to complete its trip from Earth.

Opportunity is bound for a seven-month journey through space. Its touchdown target, 491 million kilometers (305 million miles) away, is the Meridiani Planum, a zone containing a concentration of ferrous oxide situated two degrees south of Mars's equator.

Its twin, Spirit, is headed for the Gusev Crater, 15 degrees south of the Red Planet's 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.

SPACE.WIRE