An exotic satellite mission to test Albert Einstein's theories of space and time, scheduled for blastoff in early December, is being bumped into 2004 by a technical bug.

NASA's $650 million Gravity Probe-B mission, in the planning stages for 40 years, was finally headed to the launch pad this week at Vandenberg Air Force Base, California, for liftoff December 6 aboard a Boeing Delta 2 rocket.

But on Monday, mission leaders concluded that a glitch, originally thought not to be a problem, needed to be corrected.

"Data obtained during spacecraft pre-launch testing shows electronic noise on an output channel associated with the No. 1 experiment gyro. This could compromise the quality of data received from it," space agency officials announced.

The Gravity Probe-B mission logo. Credit: NASA

Gravity Probe-B carries four gyroscopes that serve as the heart of its experiment to test Einstein's universe. Over the course of several months in space, scientists will look for very precise changes in the direction of spin of the four gyros as they provide an almost perfect space-time reference system.

The mission will test two predictions of Albert Einstein's general theory of relativity that he advanced in 1916: the geodetic effect -- how space and time are warped by the presence of the Earth -- and frame dragging -- how Earth's rotation drags space and time around with it.

Each gyro has two channels that send out data. Launching the spacecraft with a known fault in one-of-two paths to get information from a gyro was too risky, NASA acknowledged.

"At first they felt they could develop a plan to work around the way they manage the data coming down from that gyro. But the more they looked at it, they felt like if anything happened to that other channel, they would not have reliable data from that gyro," agency spokesman George Diller said. "It was something that we really needed to go fix. So that is why they reached this decision before they went out to the launch pad."

The Gravity Probe-B spacecraft. Credit: Lockheed Martin/Russ Underwood

The problem has been traced to a component in the satellite's Experiment Control Unit -- a device that interfaces with the gyroscope. Engineers are not yet sure if an electronics card or some other item inside the unit will have to replaced or if the entire ECU should be swapped out, Diller said.

"They are going to need about a week to work through this, determine the approach to fix this and then determine a time frame for launch," Diller said.

As of Monday, there was no consensus on when the launch might occur. But NASA said Gravity Probe-B would not fly this year.

"We want to have the greatest chance of success," Diller said. "Launching knowing we had a noisy channel on one of the gyros was just not going to give us that assurance."

At the Space Launch Complex 2-West pad, the two-stage, Boeing-built Delta 2 rocket has successfully completed its scheduled pre-flight preparations to this point, officials said.

"There are no issues or concerns with the Delta 2. The current plans are for it to remain at the pad enclosed within the gantry-like mobile service tower until the spacecraft arrives," NASA said.