Hubble is having problems with its gyroscopes. These are used for stability and for fine control when pointing the telescope at astronomical objects such as stars and planets

The US space agency will send up a shuttle in October so that astronauts can replace the problem equipment.

Hubble has a total of six gyroscopes. Two have failed and another is acting abnormally. If another gyroscope goes down, the telescope will put itself into a "safe mode", bringing to a halt all observations.

"The Hubble Space Telescope is the crown jewel of Nasa's space observatories, and we need to do everything within reason to maintain the scientific output of this national treasure," said Dr Edward Weiler, Associate Administrator for the Office of Space Science at Nasa headquarters in Washington DC.

Mir veteran

A service mission had already been scheduled for June 2000 - some of the work on this mission will now be brought forward.

Hubble will go into "safe mode" if another gyroscope fails

A team of veteran astronauts has already begun training to install the new instruments and upgrade the telescope's systems. It includes the British-born astronaut and Mir veteran Michael Foale.

Just as in previous missions to Hubble, spacewalks will be required to make the repairs.

"When Hubble reached the point of having no back-up gyros, our flight rules said we must look at what we term a 'call-up mission' to correct the situation," said Dr John H. Campbell, the telescope's Project Director at Nasa's Goddard Space Flight Center.

"Since we are already involved in preparations for the scheduled third servicing mission next year, we essentially decided to divide the planned mission into two flights and reduce the workload on each."

New computer

In addition to replacing all six gyroscopes on the October flight, the shuttle crew will replace a guidance sensor and the spacecraft's computer. The new computer will reduce the burden of flight software maintenance and significantly lower costs.

Saturn: Huble has taken stunning pictures of many astronomical objects

A voltage/temperature kit will be installed to protect spacecraft batteries from overcharging and overheating when the spacecraft goes into safe mode. A new transmitter will replace a failed spare currently aboard the spacecraft, and a spare solid state recorder will be installed to allow efficient handling of high-volume data.

Both missions will replace telescope insulation that has degraded. The insulation is necessary to control the internal temperature on the Hubble.

The later servicing mission will focus on installing the Advanced Camera for Surveys. With its new imaging capabilities, this camera will be 10 times more powerful than the present Faint Object Camera.

Hubble was launched in 1990. It has produced the most stunning pictures ever taken of the cosmos.