Leading astronomers from Europe, North America, Asia and Australia have signed an agreement jointly to plan a huge new radiotelescope, the Square Kilometre Array (SKA), which will come into operation in the middle of the next decade. The signing ceremony will take place during the triennial General Assembly of the International Astronomical Union being hosted by the University of Manchester. The SKA will be a uniquely sensitive instrument. Its collecting area will be almost 100 times larger than today's biggest radio imaging telescope and 200 times larger than the pioneering Lovell Telescope, which is in day-to-day operation at the Manchester University's Jodrell Bank Observatory.

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The idea of the SKA sprang from astronomers' desire to detect the faint emission from hydrogen gas in structures forming soon after the Big Bang, and in the galaxies which developed from these structures. Hydrogen is the commonest constituent of the Universe and, as University of Manchester Professor Peter Wilkinson says:

"One square kilometre is not just a convenient round number -- it arises naturally from a desire to image the hydrogen gas in very distant galaxies".

Radio astronomy has been crucial in discovering phenomena such as quasars, pulsars, gravitational lenses, superluminal motion and the cosmic microwave background. It has led to three of the five Nobel prizes awarded for work in astrophysics, including all those awarded for observational work. Major advances in knowledge can be expected from a new radio telescope with the sensitivity of the SKA.

Radio telescopes have a big advantage over those operating at most other wavelengths because they can see through cosmic dust. This dust often prevents optical telescopes seeing into the regions where stars are forming and into the centres of galaxies; dust can even obscure an entire galaxy at visible wavelengths. Radio telescopes have another advantage in that they can be combined in arrays to produce images with the highest resolution in all astronomy. On completion the SKA will, therefore, be the world's premier instrument for astronomical imaging.

The SKA's superior resolving power and exceptional image quality will also provide crucial new information on the formation and early history of stars, galaxies and quasars, unaffected by obscuring dust. Its enormously high sensitivity will mean that, for the first time, an object detected by any other telescope can also be studied in the radio. The SKA is thus the perfect scientific complement to the next generation of large optical, infrared and millimetre wave telescopes. It will also play a major role in the search for extraterrestrial intelligence.

In order to achieve its ambitious astronomical goals the design of the SKA will integrate computing hardware and software on a massive scale in a revolutionary break from current radio telescope designs. The SKA is a challenging project and as Prof. Ron Ekers of the Australia Telescope National Facility says:

"Designing, let alone building, such an enormous technologically-advanced instrument is beyond the scope of individual nations, or even small groups of nations. The SKA is therefore being planned from the outset as the first truly-global radio telescope."

At present 24 leading institutions in 10 countries have agreed to pool their research and development efforts, with individual institutions concentrating only on a part of the overall design. The shared goal is to reach agreement on the fundamental design of the SKA and its location by 2005 and to begin construction in 2010.