Science: X-ray eye on the invisible Universe

A TELESCOPE designed to observe the invisible parts of the Universe should now be on its way into its final "parking orbit" around Earth. The 94th Shuttle mission - the first to be led by a female commander, Air Force Colonel Eileen Collins - aims to make the Chandra X-ray telescope our latest eye in the sky, giving astronomers the sort of detailed images of the X-ray part of the spectrum that the Hubble telescope provided for the visible light region.

"Chandra brings X-ray astronomy within spitting distance of the best optical instruments," said Dr George Fraser, an investigator on Chandra's high-resolution camera at Leicester University. "It will do for X-ray astronomy what the Hubble Space Telescope did for visual astronomy."

Everything about Chandra is big. The five-tonne observatory is the size of a railway carriage. Its two sets of mirrors and advanced instruments will enable scientists to observe X-ray sources 20 times fainter than those seen by previous observatories, and produce images with 50 times more detail.

The orbit is big, too. After deployment from the Shuttle's cargo bay, a small rocket motor is designed to fire Chandra into an elliptical orbit to carry it one-third of the way to the Moon. This unusual path allows the observatory to be used continuously for 55 hours out of each 64-hour circuit, and, with the telescope's sensitive optics, will enable astronomers to discover and study millions of celestial X-ray sources extra to the 120,000 known today.

Most of us are unaware that our world is continually struck by cosmic X-rays. Normally, we come into contact with X-rays only when visiting a hospital to check on possible broken bones. Large doses can lead to damaged cells and cancer. But high-energy X-ray photons not only have the capability of penetrating flesh. They can also reach Earth from the furthest reaches of the visible Universe.

Fortunately for our health, the planet's atmosphere protects us from the bombardment of X-rays. The down side is that scientists wanting to discover the secrets of the X-ray Universe must put their instruments into outer space.

This is not the only obstacle facing astronomers. Though satellites have been studying the X-ray sky for more than 30 years, advances have been modest because of the peculiar nature of this type of radiation. X-rays pass straight through ordinary mirrors, so special "grazing incidence" telescopes have had to be developed.

Chandra carries two sets of four cylindrical mirrors, arranged, one inside the other, like open-ended Russian dolls. These mirrors, each coated with highly reflective iridium, have the most accurately figured surfaces ever made for an X-ray observatory. To give an idea of this, if the surface of the Earth were as smooth as Chandra's mirrors, Mount Everest would be just two metres high.

Incoming X-rays are barely deflected as they glance off these mirrors, like stones skipping over a pond. They travel down a 10-metre tube and are focused to enter Chandra's two scientific instruments: a CCD imaging spectrometer and a high-resolution camera.

These will reveal, in unprecedented detail, objects that glow in X-rays as they are heated to more than 1 million degrees Celsius. Among the high- priority targets are supernova remnants (from exploding stars), clusters of galaxies where dark matter seems to hold the star systems together, and black holes where gas and dust are heated as they spiral around the void at thousands of kilometres per second.

Apart from recording the positions of such objects with high accuracy, Chandra will record the energy levels of the incoming X-rays. Scientists can thus find out the composition of the X-ray-emitting objects, and how the X-rays are created.

Chandra is expected to operate for at least five years. It will soon be joined by two more, highly advanced X-ray observatories. The European Space Agency's XMM (X-ray Multi-Mirror mission), carrying clusters of 58 curved mirrors for its three telescopes, will collect five times as many X-rays as Chandra. Japan's Astro-E has revolutionary new detectors to give the best energy resolution of all.

These state-of-the-art facilities will dominate high-energy astronomy for a decade, until the next generation of X-ray telescopes is ready to take over. The European Space Agency is already studying a concept called Xeus, which may fly on the International Space Station, while Nasa intends to fly a cluster of four identical satellites known as Constellation X. The golden age of X-ray astronomy is at hand.