Since 1959, Gravity Probe B has overcome a half-dozen attempts at cancelation, countless technical hurdles and several delayed launches. The NASA-funded, university-developed spacecraft is now scheduled to begin its 16-month mission following an April 17 liftoff.
"It's the longest-running physics experiment in history," Richard Packard, a University of California, Berkeley, experimental physicist who participated in a 1995 review of the mission, said Friday.
The unmanned, Earth-orbiting probe is designed to test two of Einstein's predictions about the nature of space and time, and how the Earth and other bodies warp and twist the fabric that combines the two.
If it works, everybody expects the Lockheed Martin Corp.-built satellite to prove Einstein right, said Ed Weiler, the associate administrator for space science at the National Aeronautics and Space Administration.
If the satellite's results contradict Einstein, that could throw science's understanding of the structure of the universe into flux, he said.
"If it produces a positive result, ho-hum. If it doesn't, it could get a Nobel Prize," said Weiler, who over the years called for several cancelation reviews of the project, NASA's longest-lived mission.
"It's great to be beyond the point of building, the point of actually launching," said Stanford physicist Francis Everitt, the mission's main scientist, whose persistent lobbying long kept the project alive.
Scientists marvel at the simplicity of the spacecraft's premise and the complexity of its execution.
"Of course, the more simple something is, the more difficult it is. And I would say that is the case with Gravity Probe B," said Anne Kinney, director of NASA's division of astronomy and physics.
At the spacecraft's heart are four pingpong-sized balls of quartz, the most perfect spheres ever made. Each is just a few dozen atoms shy of being perfectly round.
To ensure accuracy, the balls must be kept chilled to near absolute zero in the vacuum of the largest thermos ever flown in space, and isolated from any disturbances in the quietest environment ever produced, Kinney said.
Once in space, lined up with a distant star and set spinning, nothing should disturb the orientation of the balls - unless Einstein was right.
In 1916, he proposed that space and time form a structure that can be curved by the presence of a body, like the Earth. The warping of space-time is like the dimple created by a bowling ball resting on a soft mattress. That distortion accounts for gravity.
Two years later, others suggested that the rotation of such a mass should drag space-time with it, twisting the structure of the fabric.
If theory holds and the 3.5-ton satellite works as designed, the mass and rotation of the Earth, 397 miles below the probe while in orbit, should throw the alignment of the spinning balls off kilter in subtle but measurable ways.
The warping effect has been measured before. The twisting effect, called frame-dragging, has never been directly detected. Gravity Probe B aims to detect both.
"The principle goal of Gravity Probe B, in my mind, is to see that (frame-dragging) effect, to measure it with high precision," said Kip Thorne, a California Institute of Technology physicist.
Gravity Probe B spun off dozens of Ph.Ds, as well as nine new technologies before it made it to the launch pad, following decades of work and delays.
"It always seemed when we got to a year from launch there would be a major problem and we'd be two years from launch," Weiler said.
Gravity Probe B is scheduled to lift off from Vandenberg Air Force Base, Calif., aboard a Boeing Co. Delta II rocket.
Gravity Probe A was a separate mission, launched in 1976, that looked at the warping of time.