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On borrowed time

On borrowed time AFTER seven years in space, the Hubble space telescope's efficiency is coming down. Radiation has degraded some of the telescope's electronics, while micrometeoroids have occasionally slammed into its mirror, damaging the surface. The craft also suffers from the effects of extreme temperature changes as it passes in and out of the Earth's shadow (New Scientist, Vol 153, No 2068).

Astronauts will remove and replace some of Hubble's ageing cameras, sensors and data recorders, giving the spacecraft a new lease of life. If all according to goes plan, a revamped Hubble will again be peering into the depths of the universe by the beginning of March. Further servicing missions in 1999 and 2002 should keep the spacecraft operating until 2005.

But Hubble can't last forever. So the National Aeronautics and Space Administration (NASA) and American aerospace firms are designing a replacemerit known as the Next Generation Space Telescope (NGST). Three competing designs were unveiled last August. The NGST will be bigger and better than Hubble but lighter and less costly to build and operate. it will sit some 1,5 million kilometres from earth, at a point in space where the earth and the Sun's gravitational fields cancel out. And even though the NGST would be closer to the Sun than Hubble, its lonely post would help make it easier to control, as it would be at a constant temperature -and relatively unaffected by gravity.

But winning the funds and developing the technology for a successor to Hubble will not be easy. Although highly successful, Hubble has faced criticism over its enormous budget. The telescope cost over US $1 billion to build and NASA was forced to spend a further US $800 million to make good the defects in its mirror. By the time it finally stops functioning, Hubble will have cost a further US $4 billion to operate. "The Hubble is terrific, but at a quarter of a billion a year (to operate), we must plan a replacement," says NASA chief Dan Goldin, who is leading the effort to build a "cheaper, better and faster" spacecraft.

NASA has imposed a strict limit on the amount of money that can be spent on a new telescope. The NGST Must Cost no more than US $500 million to build and US $400 million to operate for a decade. Despite the reduced costs, some scientists say that a replacement would be too expensive, and that Hubble should be patched up one last time in 2005 and left to soldier on until it finally wears out.

The teams competing for the contract to build the NGST are TRW Space and Electronics and Lockheed Martin, two American aerospace companies, and a third team from NASA's Goddard Space Flight Centre. At the heart of their designs are ultra-lightweight mirrors Lip to eight metres across. Hubble's glass mirror is 2.4 metres across and weighs 826 kilogrammes - one of the reasons the 11.6 tonne telescope had to be put in orbit by the space shuttle, at a huge cost. The NGST Will Weigh only 2,500 kg and will be launched by a much cheaper expendable rocket.

One of the NGST's major goals will be to look at young galaxies and how they form. These galaxies can be seen at the edge of the visible universe, and light from events that took place there billions of years ago is reaching us only now. The young galaxies are moving away from us at a colossal speed and this Ired shifts' their light in much the same way as that of the changing frequency of a train whistle as it passes by. The NGST will be optimised for infrared imaging although it will be able to capture visible light.

But infrared imaging introduces another problem, The telescope's infrared cameras must be cooled to about 30 kelvin (-240oC) even though the spacecraft will be in the constant glare of the Sun. This needs a whole new generation of technologies capable of operating reliably at such low temperatures. "There's a whole nature of what I call cryogenic engineering that has to be brought to space. It's not black magic, just engineering," says Peter Stockman, NGST project scientist at the Space Telescope Science Institute (STSI), whose primary site is at the Johns Hopkins University campus in Baltimore.

To shield the telescope and its cameras from the Sun will be the first priority. Both Goddard and TRW have opted for sunshades that will unfold to about the size of a tennis court. These shields and most of the spacecraft's electronics will operate at around 300 kelvin (30oC) insulated from the telescope by a short mast. Once in shadow, any excess heat should radiate into space.

Ultimately, cost win be the deciding factor,and with falling budgets there is a very real chance that the NGST could fall by the wayside. "The laws of physics say it's possible, though the laws of humanity might not," says John Mather, a scientist at the Goddard Space Flight Centre, US. Goldin is more optimistic. He says new technology will allow a cheap and reliable replacement.

Either way, Hubble is eventually going to stop functioning, says Robert Williams, director of the STSI. And although he supports the idea of extending its life, he admits: "This and that will break, and it will go brain dead."

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