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Tuesday, July 20, 1999 Published at 07:33 GMT 08:33 UK


Sci/Tech

Shuttle carrying great X-ray observatory

Artist's impression of Chandra in space

By BBC News Online Science Editor Dr David Whitehouse

One of the most powerful space observatories ever will launch on the space shuttle Columbia.

The Chandra X-ray Observatory will spend at least five years observing X-rays from the hottest and most violent parts of the cosmos, looking in particular at superheated matter being pulled into black holes.

Chandra weighs over 25 tonnes and is the largest payload ever carried on the space shuttle.


[ image: Impression of superhot matter being sucked into a black hole]
Impression of superhot matter being sucked into a black hole
It should herald a revolution in X-ray astronomy. It is 10 to 100 times more powerful than any of the dozen or so X-ray telescopes previously placed in orbit.

If printed letters gave off X-rays then Chandra could read a newspaper from 800 metres (half a mile) away.

The astronauts, led by Nasa's first woman space shuttle commander, Eileen Collins, will release Chandra with the flick of a switch seven hours after launch. However, it will be another month before the telescope's eagerly awaited observations begin.

Hot spots

X-rays in space are produced by the superheating of gas, so Chandra will have a view of the most energetic and violent parts of the cosmos.


Animation of Chandra deployment
It will observe the stormy cores of distant galaxies called quasars where it is believed that matter is being dragged into supermassive black holes. It will also watch the remnants of exploded stars by tracing the dispersal of hot gas.

Matter being sucked into a black hole can be heated to millions of degrees as it swirls in a so-called "accretion disc" around the black hole.

Astronomers will be especially looking for short bursts of X-rays as matter is pulled beyond a black hole's "event horizon", tearing it from our Universe.


[ image: Chandra prepared for launch]
Chandra prepared for launch
Ordinary mirrors do not work with X-rays so a special design has been developed. Chandra uses conical mirrors and the X-rays focus by glancing off the sloping surface at a shallow angle, like pebbles skipping across a pond.

"Superlatives are used in everything from sports to politics, probably a little bit too much," says Martin Weisskopf, a Nasa scientist who has been working on the project for two decades. "But I can't help but use those superlatives with this observatory. They are really well based."

"We will redefine our understanding of how the Universe works and of basic atomic physics," says Professor Weisskopf. "And X-ray astronomers are the geographers of the Universe and with Chandra we are going to make some fabulous interesting maps."

Big science

The telescope, named after the late Nobel prize-winning astronomer Subrahmanyan Chandrasekhar, cost $1.5 billion to develop. Add the launch and five years of orbital operations, and the price jumps to $2.8 billion, making it one of Nasa's most expensive science projects ever.

The mighty Chandra Observatory is the third of Nasa's four "Great Observatories". The first was Hubble and the second was the Compton Gamma Ray Observatory, launched in 1991.


[ image: Chandra's mirrors]
Chandra's mirrors
Chandra should have flown last summer, but was delayed by faulty electronics and the late delivery of crucial detectors.

Another worry for Nasa was the rocket motor attached to Chandra. It is similar to one that malfunctioned on an Air Force satellite in April and left that spacecraft in a useless orbit.

The solid-rocket motor is needed to boost Chandra from the relatively low altitude of the shuttle into an elliptical orbit with a low point of 10,000 kilometres (6,200 miles) and a high point of 140,000 km (87,000 miles) - one-third of the way to the moon.

Nasa officials say they are confident the thoroughly-checked motor - and the telescope itself - will work.

In 1990, the Hubble Space Telescope was launched with a flawed mirror that blurred its vision. It took millions of dollars and a space shuttle mission to repair Hubble's view.

But rigorous testing of Chandra's fully assembled optics uncovered no flaws, says Nasa's chief space scientist, Ed Weiler.





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