The mission's data is shedding light on Einstein's theory of relativity

It will take another eight months to determine whether he got the other correct say scientists analysing data from Nasa's Gravity Probe B satellite.

The spacecraft was launched into orbit from California, US, on 20 April 2004.

The mission's chief scientist presented details at a physics meeting in Jacksonville, Florida.

Gravity Probe B uses four ultra-precise gyroscopes to measure two effects of Einstein's general relativity theory.

A scientist starts with a bulldozer, follows with a shovel, and then finally uses dental picks and toothbrushes to clear the dust away from the treasure. We are passing out the toothbrushes now

William Bencze,

Stanford University

One of these effects is called the geodesic effect, the other is called frame dragging. A common analogy is that of placing a heavy bowling ball on to a rubber sheet.

The bowling ball will sit in a dip, distorting the rubber sheet around itself in much the way a massive object such as the Earth distorts space and time around itself.

Minute measurements

In the analogy, the geodesic effect is similar to the shape of the dip created when the ball is placed on to the rubber sheet.

Einstein is now being tested under highly controlled conditions

If the bowling ball is then rotated, it will start to drag the rubber sheet around with it. In a similar way, the Earth drags local space and time around with it - ever so slightly - as it rotates.

Over the course of a year, these effects would cause the angle of spin of the gyroscopes to shift by minute amounts.

The mission's principal investigator, Professor Francis Everitt, from Stanford University, discussed preliminary results at the American Physical Society meeting in Jacksonville at the weekend.

The data from Gravity Probe B's gyroscopes clearly confirm Einstein's geodesic effect to a precision of better than 1%.

The scientists from Stanford are still trying to extract its signature of frame-dragging from the data.

They plan to announce the final results of the experiment in December 2007, following eight more months of data analysis.

Larger puzzle

Professor Tim Sumner, a physicist at Imperial College London, told BBC News: "Having an announcement at this stage, on the way to the final result, is very encouraging. I'm very pleased to see that the result has now been released.

There is an expectation that at some level we will expose a departure from pure general relativity as envisaged by Einstein

Professor Tim Sumner,

Imperial College London

"Most individual measurements are part of a larger puzzle. But general relativity is one of the big branches of physics and it is poorly tested at the moment because of the relative weakness of gravity as a force."

"I would see this as a piece of solid verification to underpin general relativity, which occupies a special place in physics."

William Bencze, Gravity Probe B programme manager at Stanford University in California, said: "Understanding the details of this science data is a bit like an archaeological dig.

"A scientist starts with a bulldozer, follows with a shovel, and then finally uses dental picks and toothbrushes to clear the dust away from the treasure. We are passing out the toothbrushes now."

Unified theory

Tim Sumner said few physicists were expecting to see a deviation from Albert Einstein's predictions in this experiment.

But he said that other tests could start to reveal cracks in general relativity, suggesting where modifications might be made.

The gyroscopes were called the most perfect spheres ever made

Physicists have been unable to incorporate gravity into a unified theory to describe all that is known about the fundamental forces between elementary particles in nature.

Modifications to general relativity could be important steps towards a unified theory.

"There is an expectation that at some level we will expose a departure from pure general relativity as envisaged by Einstein," Professor Sumner said.

"One of the areas of general relativity that is less well founded is when you get into very intense gravitational field interactions. Some astrophysical objects will be in very high field situations such as pairs of massive black holes orbiting one another."

A joint mission between Nasa and the European Space Agency called Lisa (Laser Interferometer Space Antenna) will study gravitational waves coming from binary systems such as these.

General relativity is not expected to break down in these situations. But Lisa should help scientists understand how the theory works in "high field" gravitational regimes such as pairs of massive black holes.

Other experiments are due to test the equivalence principle, one of the foundation stones of general relativity. This principle stems from the observation that when two objects are dropped, they will accelerate at the same rate.

"Here there is a theoretical framework where one might expect to see a departure from the equivalence principle," said Professor Sumner. "This might give us pointers as to the way forward."

The Imperial College physicist is involved in two mission concepts to test the equivalence principle. One is the Satellite Test of the Equivalence Principle (Step), which has been proposed by some of the same scientists involved in the Gravity Probe B mission. Another is the GrAnd Unification and Gravity Explorer (Gauge).

Gravity Probe B was launched from Vandenburg Air Force Base in California on 20 April 2004. It transmitted its data for exactly 50 weeks, from August 2004 to August 2005.