He told us about his goals for the project.

It's nearly 100 years since Einstein developed his first theory of relativity, surely these things are now established and confirmed?

Einstein developed two theories of relativity. His first theory of relativity we now call special relativity. The object was to reconcile Maxwell's theory of electromagnetism with Newtonian mechanics. And Einstein succeeded in doing that.

His theory amongst other things gives the formula that perhaps everybody has heard of, E = mc² (energy equals mass times the speed of light squared), and is a very well established experimentally.

If Einstein's theory turned out to be incorrect, many things would have to be rethought

When Einstein had completed that step, he realised he had a new problem on his hands which was to understand gravitation. And this resulted in a 10-year search which ended with Einstein's general theory of relativity, or theory of gravitation.

Unlike special relativity, surprisingly few tests have been made quantitatively of the results of this theory. This is what Gravity Probe B will be endeavouring to strengthen.

The project itself was first suggested 45 years ago and you've been involved with it for many years. Why has it taken so long to get to the launch pad?

The fundamental truth is that doing quantitative tests of Einstein's theory of gravity is extraordinarily difficult because there is almost only one place where you can do it and that is around the Earth or the Sun.

And all the predicted effects from Einstein's theory are small - for example, in the GPB mission, which tests Einstein's theory by making very precise gyroscopes the size of a ping-pong ball and comparing the angle of spin at a line of sight to a star.

That experiment requires a gyroscope that's more than 10 million times as accurate as any ground-based gyroscope. So it's taken enormous technological development, and that development takes time.

In a nutshell, how is this spacecraft going to test Einstein's predictions?

Picture a star and a telescope pointing at that star and have not one spinning sphere, but four independent ones, almost pointing towards the star at the beginning of the year.

As the satellite orbits the Earth, two effects will happen, two separate equally important but separate tests of general relativity.

In the plane of the orbit, the direction of spin of the gyroscope will tilt by a certain predicted amount because space and time are curved. We expect to test better than a part in a thousand.

At right angles to the plane of the orbit, just parallel to the direction in which the Earth is rotating, space and time will be pulled around by the Earth - according to Einstein's theory. Our gyroscope will observe that strange pulling of space and time around by the rotating Earth.

The probe will measure if the Earth distorts space-time as Einstein predicted

It's a very complex thing to grasp, is there any simple analogy you can use that can help people understand?

Imagine the Earth immersed in a kind of liquid - a viscous liquid that can be dragged around like honey. It's rather easy to see that as the Earth rotates, it will drag that liquid around with it.

The amazing consequence of Einstein's theory is that an identical thing happens to space and time. That picture is actually even better than one would think because it works out exactly the way a rotating liquid does.

So that is the description of one effect. In terms of the other effect, I like to talk about the problem of the missing inch. Imagine drawing a circle in space.

Remember the circumference of a circle is two pi times the radius. If this circle is the satellite orbit - radius and circumference - and if you tried to measure the circumference, one inch would be missing.

It wouldn't quite equal two pi times the radius, it would be a little less. And that is because space is curved. Curved space sounds very mystical, but it isn't as mystical as it sounds.

It all comes back to that problem with the missing inch. That's what we'll be looking for and measuring.

What would you expect to find from your experiments to prove that Einstein was right?

People sometimes ask whether we will prove Einstein right or prove him wrong. And my answer to that is it's not our job to decide.

Our job is to make sure we get the right answer whatever it is.

If any inconsistencies were found, what would it mean for physicists?

Einstein's theory of gravitation deals with the interaction of space, time and gravity. It is the theory for which we have the best understanding of the large-scale Universe and objects such as black holes.

If Einstein's theory turned out to be incorrect, many things would have to be rethought. In many senses, we know the theory does agree with experiment. What Gravity Probe B is doing is pushing it into new areas.

Going back before Einstein, we had Newton's theory of gravity and for many purposes we still use that. Einstein took a step beyond Newton - a very fundamental step. We might find, but I'm not predicting one way or the other, that one has to take a further step beyond Einstein.