What is gravity?
This is an age-old question that mankind has been trying to answer for millennia. We understand that gravity is a purely attractive force - it can only pull, never push - and that it is generated by any object with mass.
Galileo was one of the first scientists to investigate the way gravity causes objects to move but it was not until Isaac Newton that mankind began to understand this facet of the Universe. In 1687, Newton published his Universal Law of Gravitation. It provided mathematical expressions that would precisely describe the way objects moved in a gravitational field. Not only did the expressions work for objects falling to the ground on Earth, they also explained why the planets moved through the night sky. Unsurprisingly, this is heralded as one of the greatest works of science. There was only one observation his work could not explain. Mercury's elliptical orbit was gradually moving around the Sun, a phenomenon known as precession.
Much later, in the early twentieth century, Albert Einstein developed the Theory of General Relativity in which he described gravity as a deformation in space, caused by the presence of massive objects, similar to the way a heavy ball would warp a sheet of rubber. This deformation 'told' smaller things how to move through space, so they either went into orbit or fell onto the larger celestial object.
This was a very different way to visualise space. In the past, it had been thought space was filled with a fluid known as ether. When no one could prove the existence of the ether, people began to think of space as simply an empty nothingness. So, Einstein's idea that space was like a fabric stretched across the Universe was revolutionary. He called it the space-time continuum.
General Relativity not only explained the precession of Mercury, it made a number of surprising predictions that, over the subsequent decades, have been observed to be true. Among them was that light passing by a massive object would be deflected from its original path and that light escaping from a gravitational field would lose energy. (In fact, satellite-based navigation systems such as GPS have to take this second effect into account, in order to pinpoint precisely the location of their users.) The only prediction of general relativity yet to be tested is that certain celestial objects will emit gravitational waves, which will ripple through the fabric of space. The joint ESA-NASA mission LISA will attempt to detect these.
Now, however, the clues are mounting that General Relativity itself is incomplete. Certain observations suggest that there is more gravity in our Universe than can be explained by General Relativity. For example, the Pioneer anomaly, in which the spaceprobes Pioneers 10 and 11, Galileo and Ulysses are being slowed down for a still unknown reason. Certain galaxies (collections of hundreds of billions of stars) spin as if they were generating more gravity than appears possible. One explanation may be that vast quantities of exotic `dark matter' exist in the Universe or it may be that General Relativity is not the final theory of gravity.
Only by testing the predictions of general relativity, to the limits possible in space, will scientists be able to gain clues about what the next breakthrough in our understanding of gravity might be.