NASA has postponed the launch of its first Mars Exploration Rover at least three days to give managers more time to complete pre-flight engineering reviews.

The liftoff, which had been scheduled for June 5, is now targeted to occur at 2:05:55 p.m. EDT (1805:55 GMT) June 8 aboard a Boeing Delta 2 rocket from Cape Canaveral Air Force Station in Florida.

"They just needed more time to finish all the reviews. It just didn't look like they could get through it all and make the 5th. So they are going to build in an extra couple of days to get all that done," NASA spokesman George Diller said.

Worker prepare to enclose the Mars Exploration Rover in its transport canister at the processing facility for the move to launch pad 17A. Photo: NASA

The delay was announced Monday, just hours after the rover was transported from its Kennedy Space Center processing facility to the air station's pad 17A for attachment atop the Delta 2 booster.

Enclosed in a protective canister, the rover and already-attached Delta 2 third stage left the clean room at 2:20 a.m. Once at the pad, a crane hoisted the cargo into the launch pad tower and maneuvered the package into place atop the Delta 2's second stage. Mating to the rocket was successfully completed around 9:30 a.m., Diller said.

Launch preparations for the payload remain on schedule with no significant issues being addressed.

"It is just the engineering review process. It's so extensive and there's so much to it," Diller said of the cause for delay.

June 8 is only a target date. A new official launch date will be established early next week, once managers have a better feel for the amount of time needed to complete the reviews.

"They are going to wait until next Monday, June 2, and say for sure," Diller said.

NASA has until June 19 to launch the first Mars Exploration Rover. Two instantaneous launch times are available each day during the planetary alignment window. See the complete launch time chart here.

The mission was originally slated for May 30. But it was pushed back a week to repair suspect circuit boards on the spacecraft.

The twin Mars Exploration Rover-B remains slated for launch June 25 from the Cape.

The rovers will land in separate locations on the Red Planet next January to serve as field geologists, studying rocks and searching for clues about past water on Mars.