Will gravity theory hold up in space?

A controversial space mission that will try to provide the most precise test yet of Einstein's ideas about gravity and space is set to launch next week.

The spacecraft, the size of a delivery van, will try to take direct measurements of two predictions from Einstein's theory of general relativity. First, it will gauge how a massive object - Earth - warps space around it. Second, it will test for the first time whether Earth's rotation drags the fabric of space nearby, much like a top twirling on a small cloth drags the cloth into a pinwheel pattern.

The $700 million mission is one in a growing number of experiments designed to unravel the secrets of gravity - one of nature's most fundamental and least understood forces. Scheduled for launch April 17, it is the most technologically demanding science effort NASA has ever undertaken, scientists say, and, surprisingly, the oldest.

If Gravity Probe B delivers the expected results, it could help provide a deeper understanding of black holes and enormous jets of matter that hurtle into deep space from the cores of young, active galaxies. If the spacecraft delivers the unexpected, the results could point to a new realm of physics beyond the picture of forces, particles, and interactions widely accepted today, researchers say.

At first blush, past experiments confirming Einstein's theory of general relativity would appear to have rendered it a done deal, says Stanford University physicist Francis Everitt, lead investigator for Gravity Probe B. Yet "we don't know it all," he says. "General relativity is much less well established experimentally than special relativity," a theory that includes concepts such as the speed of light as a constant.

The mission's design is deceptively simple. It uses four gyroscopes to measure Earth's impact on spacetime. If warping and dragging occur, the gyros' spin axes should drift over time in specific directions as the spacecraft orbits Earth and constantly aims itself at a distant star.

Using gyros to test general relativity was first proposed in the 1930s by British physicist Sir Arthur Eddington, but the technology didn't exist to pull it off. To make this project feasible, the team had to build the world's most accurate gyros.

At the heart of each gyro is a spinning fused-quartz sphere the size of a Ping-Pong ball. Covered with a thin layer of niobium, the sphere is so smooth that if it were the size of Earth, the highest and lowest points would vary by no more than eight feet from the surface.

With no way to visually detect the direction of the gyros, researchers instead will measure the orientation of the magnetic fields they create as they spin. To set up the magnetic fields, researchers immerse the experiment in 645 gallons of liquid helium, which chills the spheres to -455 degrees F. and turns their niobium skins into superconductors. The experiment is housed in what Anne Kinney, NASA's director of astronomy and physics, calls the largest thermos bottle ever flown.

The year-long mission is a Methuselah among NASA projects, dating back 45 years. The project's long life, technical demands, and high price tag have earned it a number of opponents.

In 1995 when the National Academy of Science reviewed the project's value, the majority of the 12-member review panel gave it a thumbs up. But the panel also noted that "a significant minority" felt the mission's goals were too narrow compared with competing projects. Seven years later, NASA axed missions such as the Pluto-Kuiper Express and two other solar-system missions to help pay for "problems" with the Gravity Probe B. Pluto-Kuiper was restored only after intense lobbying.

Yet Gravity B will be important, argues Kip Thorne, a physicist at the California Institute of Technology in Pasadena. The mission should yield for the first time a precise value for the relationship between the rotation rate of Earth and the degree to which it drags the fabric of space. It will play a critical role in helping researchers interpret data from an ambitious US-European gravity-wave detector planned for orbit in 2011.