The U.S. space agency (NASA) has launched what is thought to be the most technologically difficult experiment ever undertaken aboard a spacecraft. A satellite called Gravity Probe B goes into orbit from Vandenberg Air Force Base, California to determine if German physicist Albert Einstein was correct when he theorized that large celestial bodies like our planet bend space and time.

Approximately 45 years after it was first conceived, Gravity Probe B is finally getting its chance to make the subtlest measurements of the cosmos ever attempted. With that data, scientists hope to show that Einstein's theory of how gravity works is correct.

The eminent 17th century English scientist Isaac Newton thought of gravity as a force between two objects that acts instantaneously, no matter how far the distance between them, but Einstein rejected Newton's theory because nothing, not even gravity, is believed to be faster than light, and light takes time to travel between two objects.

Instead, Einstein conceived of gravity in an entirely different way, as Stanford University physicist Francis Everitt points out. "To cut a long story short, Einstein realized that no longer could one look at gravity as a force in the way Newton had, but as something that distorts space and time," he said.

Imagine a bowling ball on a mattress. In a simple analogy to gravity, the weight of the ball causes the bed to sag so that another object placed on the mattress tends to roll toward it.

Einstein also suggested that a rotating object has another affect on space and time, as explained by California Institute of Technology physicist Kip Thorne.

"The spin of the Earth or the sun or any other body drags space into motion around itself like the air in a tornado," he said. "That we have never seen in any definitive way. There have been hints of observations of it, but nothing definitive and absolutely nothing that is quantitative."

To measure the predicted twist and curvature of space and time, physicists Thorne and Everrit have helped design Gravity Probe B so that it will remain as steady in space as possible.

To accomplish this, the probe has four of the most accurate gyroscopes ever made. Gyroscopes are devices that spin and are used to keep spacecraft in a stable position. The basis of these gyroscopes is four nearly perfect spheres the size of ping pong balls, made of glass coated with a thin layer of the metal nobium. They are so smooth and round that if Earth was as smooth, it would have no mountains and the highest point would be less than three meters.

Because of the near perfection of their shape and balance, their spin will maintain a steady spacecraft as it is initially pointed at a guide star. That way, any drift of the gyroscopes away from the guide star is due not to their structure, but to the warping and the swirl of space around the spinning Earth.

"The whirling motion of space around the Earth grabs the gyroscope to change its spin axis like a straw floating on the surface of a river," Kip Thorne explained. "Where the river flows faster in the center and slower near the bank, the straw will turn. It's precisely the same thing going on here. Space moves faster near the Earth and slower farther away and the spin axis of the gyroscope turns."

However, the drift of the gyroscopes' axis is expected to be extremely subtle. Over one year, it it should be about one-tenth the width of a human hair, as measured by extremely sensitive detectors aboard Gravity Probe B.

It has taken 45 years and seven project cancellations to move from concept to flight. This is the longest development time for any U.S. space mission. Its goals may seem esoteric and impractical, but Stanford University engineer Brad Parkinson said so did Scottish physicist James Maxwell's laws of electricity and magnetism in the 19th century, until the world was electrified.

"Physics finds fundamental laws and it is rare that the generation you're currently in has a direct benefit, but it is the sons and grandsons," he said. "So I think it is for our grandchildren."

The Gravity Probe B satellite is to circle Earth for 16 months. If its detectors show no shift in the direction the gyroscopes point, theorists might have to begin developing new concepts of gravity and the shape of space.