The Martian sphinx

By the twenty-first century the world was drowning in its own population. A solution had to be found quickly - and it was, in 'gravi-power'. Wonderful, unending source of power - only the scientists knew that its use was reducing the Earth's distance from the sun at a dangerous rate. But if another planet's gravi-power could harnessed... An expedition was launched to Mars, known to be uninhabited - except that a woman was wandering around its surface who claimed she had come from another galaxy to warn Earth of a terrible menace, and there was a huge poly-hedron of metal emanting a force very much like that of gravipower. Someone else had discovered it! A thousand years ago, or now? Friend, or enemy?