The Hubble Space Telescope (HST) has shut itself down, preventing astronomers from making observations of the Universe.

It is a safety precaution triggered by the failure of another of the space observatory's gyroscopes.

HST has six gyros onboard, half of which must be operational to point the observatory accurately.

Hubble needs at least three gyros to operate

But one by one, three have failed and there have been indications for some months now that a fourth gyro would also cease working.

This is apparently what happened on Saturday.

Fortunately, Hubble's third service and repair mission is just weeks away. Nasa has put together a very experienced crew, including the British-born Mir veteran Michael Foale, who are due in orbit in early December to sort out all of Hubble's problems.

See how the shuttle will grab hold of Hubble

The replacement of the gyroscopes will be one of the key tasks of their mission. The shuttle crew will also replace a computer and install a collection of new components, including so-called voltage improvement kits and a fine guidance sensor.

The upgrade, together with two further servicing missions in 2001 and 2003, will help extend the observatory's life well into the next decade.

The astronauts have been practising the removal and replacement of the faulty gyros in giant water tanks on Earth. The gyros are encased in easy-to-fit boxes.

The gyros are in easy-to-fit boxes

The safe mode into which Hubble has gone does not require gyros, so even if another gyro should fail in the next few weeks, the observatory will remain safe, according to project managers.

The aperture door has been closed to protect the optics, and the spacecraft is aligned to the Sun to ensure adequate power is received by Hubble's solar panels.

"It's [Hubble] quite safe," said HST programme manager John Campbell. "We're not doing science, so the power load has been reduced. But everything is quite OK."