By BBC News Online Science Editor Dr David Whitehouse

An electronic link-up of telescopes in space and on Earth has produced the most detailed close-up of quasars, superbright exploding objects at the edge of the Universe.

The technique, known as Very Long Base Interferometry (VLBI), utilises this combination of satellite and ground-based radio telescopes to create a telescope more than two-and-a-half times the diameter of the Earth.

The project is called Space VLBI and, because it is the largest astronomical instrument ever built, it has given astronomers one of their sharpest views yet of the Universe.

"These images probe some of the most distant, ancient, and energetic objects in the Universe, giving us a glimpse of quasars as they existed billions of years ago," said Dr Robert Preston, US Space VLBI project scientist at Nasa's Jet Propulsion Laboratory in California.

Triple power

By taking the VLBI technique into space for the first time, astronomers have tripled the power previously available with only ground-based telescopes.

The space-based component of the network is a small radio dish called Halca. It was launched in 1997 on a Japanese rocket. Although it has experienced some problems with its transmitter, it has returned valuable data to Earth.

"The Space VLBI satellite system has more than 100 times greater resolving power at radio wavelengths than the Hubble Space Telescope has at optical wavelengths." said Dr Preston. "In fact, its resolving power is equivalent to being able to read a newspaper headline in Tokyo all the way from Los Angeles."

Quasars are enormously bright point-like optical objects, often shining with an intensity many hundreds of times brighter than that of an entire galaxy. However, they are so distant that they appear only as very faint points of light to optical telescopes on Earth.

Astronomers believe that quasars are powered by gas spiralling into black holes at the centres of galaxies. Most of this in-rushing matter is captured forever by the black hole but some of it is ejected at enormous speeds to form narrow radio-emitting jets. By studying these jets, which are usually visible only at radio frequencies, astronomers hope to learn more about the black holes that power them.

Space VLBI observations have resolved individual components in the observed quasars' jets. Perhaps the most significant single result of the Space VLBI mission so far is the detection of a number of radio sources associated with quasars that are brighter than theory generally allows for a stationary source.

They appear brighter because of a strange prediction of Einstein's theory of relativity that radiation from an object moving at near light speed will be amplified in the direction of motion.

This effect allows some sources to appear much brighter than they really are, solving the conflict between the observed and theoretically allowed brightness of the radio-emitting quasars.