RULER OF THE UNIVERSE

At $1.6 billion, its snaps are hardly a snip. But as Charles Arthur reports, Hubble's pretty pictures are actually measuring the beginning of time

Sunday, May 20, 1990 should have been a memorable date in the history of astronomy: the first day of operation for the orbiting Hubble Space Telescope (HST), a $1.6 billion piece of the most sophisticated viewing equipment in the world. In theory, it would be able to spot a 5p piece from 20 kilometres away, and see the light of a firefly 16,000 kilometres distant.

Instead, what the scientists had hoped would be a glowing testament to their skills turned to ashes. The first pictures relayed from the system floating 600 kilometres above us were badly blurred. A few teething problems in the systems of the telescope - a cylinder the size of a single-decker bus - were sorted out, but technicians realised something more serious was wrong. Finally, they deduced a flaw in the three-metre main mirror: the curves of the edges had been ground two-millionths of a metre too flat.

Cartoonists and satirists vied with each other to make sharp jokes about Hubble's dull vision. Instead of being the best telescope available to humanity, the HST was only as good as a ground-based one. NASA had put a 12-tonne turkey into space.

Looking at the pictures here, it's hard to remember those days. This elevation from useless lump to inspiring example of modern technology was achieved in December 1993 by an equally inspiring piece of repair work, consisting of some of the longest spacewalks ever by the crew of the Space Shuttle. The process was described by one astronaut as "like fixing your car with oven-mittens while hanging upside-down". Essentially, they added specially designed components to correct for the aberration in the mirror. On December 18, the ground-based scientists gathered again to see whether the work had succeeded. To their delight, it had.

Hubble's transformation was soon complete: the pictures began to pour in, and so did the plaudits. It was named among the wonders of the modern world, along with the scanning electron microscope, the Human Genome Project (which aims to map all of the 100,000 human genes) and the pyramids of Giza.

The HST's place in the list is well-deserved and not only because it can take (as its inventors would say) some neat pictures. Like the pyramids for the Egyptians, the HST is an example of the state that our control over technology has reached. But it also represents something deeper: our urge to understand what is out there, in the same way that the scanning electron microscope lets us gaze at individual atoms, and the Human Genome Project lets us gaze at the biological "atoms" that constitute our genetic recipe. The HST lets us see light that has come from stars almost as old as the universe itself.

It also reveals images of spectacular beauty within the galaxies surrounding us, that are invisible from our positions on the ground because of the atmosphere and pollution. Astronomers today face increasing problems with ground-based observation. It is bad enough that the atmosphere absorbs so much of the incredibly faint signals that reach us, but now, terrestrial interference from street lights, television, radio and mobile-phone antennae is worsening daily.

The HST has none of these problems. Up there, the stars do not twinkle and street lights do not impinge on its work, though the brightness of the Sun, Earth and Moon means it cannot observe objects that also bring them in the field of view, and a region of high-energy particles over the South Atlantic - caused by a warp in the earth's magnetic field - make observations there unreliable. Apart from these limitations, the sky's the limit.

The HST is distinguished from ground-based observatories by its capability to observe light in the ultraviolet and near infra-red wavelengths (some stellar processes emit light at these frequencies) a resolution and order of magnitude better than is possible from earth. Light entering its axis and focused by the internal mirrors falls on to "charge-coupled devices" (CCDs) that turn light into electrical signals; these are passed to a computer and combined to generate an electronic image. It is the digital equivalent of photographic emulsion, but sensitive to a far wider range of light than the human eye. It also takes much longer exposure times to gather enough light for the pictures (all the pictures here come from exposures lasting hours at a time) so the HST has extremely sensitive gyroscopes to keep it pointed towards the same part of the sky during such work.

While the HST is a testament to our ability to engineer to enormous precision, part of its mission during its 15-year operational life is to give us a better ruler with which to measure the universe. This follows on from the pioneering work first carried out by its namesake, Edwin Hubble.

Born in Kentucky in 1889, Hubble joined the staff of the Mount Wilson Observatory in Pasadena, California, in the summer of 1919. Another astronomer observed of him: "He wants to find out about the universe; that shows how young he is."

Ten years later, Hubble announced his discovery that every other galaxy appeared to be moving away from us. It was the result of painstaking observation and calculation, which he was able to express as a simple mathematical expression: Ho = v/d, where v is the galaxy's radial outward velocity (in other words, motion along our line-of-sight), d is its distance from earth, and Ho is the "Hubble Constant". From this, it is possible to deduce that everything must once have been closer together; rewinding the film of the universe would take us back to the Big Bang, the moment of creation.

For this reason, it is of prime importance that Ho is correct, and that the distances to stars being measured is also correct. Astronomers have argued bitterly for years about the exact value of the Hubble Constant; estimates vary by a factor of two. If the higher estimate is correct, then the universe might be younger than its oldest observed stars - a logical contradiction which would require substantial revision of many astronomical theories. The HST should be able to help calculations of both velocities and distances.

So far, the HST has helped two teams to converge on the value of the constant: using it, one puts the age of the universe at between nine and 12 billion years; the other puts it at 11 to 14 billion years. We are closing in on one of the most profound questions of our existence - when did the universe begin? It must be to the Hubble Space Telescope's credit that, while helping to answer that with a scientific precision, it can also offer us pictures from the sky that please the artist inside us. !