Earth's ear in space

BLACK HOLES are the darkest objects inthe Cosmos and in our understanding.Scientists conjecture that these compactobjects are formed when a dying starexplodes and the matter collapses in avery small region of space. The matterdensity is so high that the gravity of theobject is strong enough to prevent lightfrom escaping out.

Our understanding of black holescomes from theories of gravitation andcosmology. Astronomers have extensively studied these by looking at theireffect on other objects. One of the mostimportant instruments in studyingblack holes is the radio telescope.

A new international initiative hasreported the first results from an orbiting radio telescope. These may go on torevolutionise our understanding ofmany of the Universe's 'crazy' objects,including black holes (Science, Vol 281,No 5384).

Radio observation of the Cosmosbegan when Karl Jansky detected radiowaves from space in 1933. Several powerful telescopes were built over theyears. But the big break came in the1950s with the development of radiointerferometers, which use wave interference to make precise measurementsof length or displacement. These consistof two or more widely separated antennae connected by transmission lines.With their greatly increased resolution,they can be used to determine the position or diameter of a radio source or toseparate two closely spaced sources.

Another significant step in thisdirection is the phase-array telescope,which consists of a large numbers of relatively small antennae elementsarranged over a relatively large area,yielding the effective sensitivity and resolution of an antenna much larger thancan practicably be built.

An example of such a system is the27-antenna Very Large Array nearSocorro, New Mexico, USA, one of theworld's largest and most sensitiveradio telescopes. Use of atomic clocksand magnetic tapes has allowed scientists to do away with transmission linesand to use individual telescopes whichare very far away as parts of a radiointerferometer.

Once the hurdle of transmission lines was overcome, it was suggestedthat a large telescope be used in space. For this purpose, a radio telescope with a diameter of 8 meters was put into orbit by Japanese scientists in 1997.

This telescope, called the HighlyAdvanced Laboratory for Communications and Astronomy (HALCA), isused complements ground-based telescopes to give a telescope with an effective diameter of over 25,000 km. Its resolution is phenomenal - comparableto reading this page from a distance of 2,000 km.

The first results from the space observatory have been reported recently. These indicate the presence of massive black holes in the centre of Virgo A,a very strong radio source which has twogiant radio lobes emanating from it.

The success of this mission has led to astronomers planning evenmore ambitious missions like the us National Aeronautical and SpaceAdministartion's 25-metre orbiting telescope. With these new eyes on theUniverse, we would be closer to explaining some of the outstanding conundrums of the Cosmos.