An Eye to Greatness: The Hubble Space Telescope
By Khee Gan Lee
It was an unexpected place to find the nerve-centre for what has been considered the most important telescope ever built. Orbiting 570 km above the Earth, the Hubble Space Telescope (HST) has, as its ground control centre, an unassuming office building in a wooded suburb right outside downtown Baltimore, USA. Behind, a small river flows serenely past, and tree branches sway almost within touching distance of the windows. In such unlikely surroundings, the scientists and engineers of the Space Telescope Science Institute operate humanity’s eye in the sky.
Deep inside the building, engineers sit in the dim lighting of the mission operations room, keeping a vigilant eye on computer screens displaying the status of the HST. The orbiting telescope’s images and scientific data stream continuously into the Institute, before being studied by scientists within the Institute and beyond.
Several floors above, Steven Beckwith, the director of the Institute, was more circumspect about the HST’s place in history. “I would actually rank it as the third most important telescope ever”, he said when asked about the assessment of the HST by an expert committee of the US National Academies (the umbrella body for science and intellectual activities in the USA). The committee, tasked with assessing the fate of the HST in the wake of the Space Shuttle Columbia disaster, had referred to the HST was “arguably the most important telescope in history”. He continued, “I personally think the most important telescope in history was Galileo Galilei’s telescope, and the second was Edwin Hubble’s”.
In 1609, Galileo first pointed his telescope at the heavens, making him the earliest person to put the then-new invention to astronomical use. Although crude by modern standards, Galileo’s telescope transcended the limits of the naked eye and opened totally new possibilities in astronomy. More importantly, his discoveries helped to dethrone the Earth from its position at the centre of the geocentric, Ptolemaic, Universe, and demolished the contemporary view of the Heavens as a mystical place, unsullied by terrestrial imperfection. Our modern view of the Universe can be traced directly back to this first telescope.
Centuries later, in 1923, Edwin Hubble used the massive 100-inch diameter Mt. Wilson telescope in California to measure the distances to diffuse objects in the sky then known as nebulae. He did this by measuring the changes in the apparent brightness of a type of star known as Cepheid variables, which fluctuated with a period that was directly related to its true brightness. By measuring the period of a Cepheid variable in the Andromeda nebula, Hubble estimated its true brightness, and by comparison with its observed brightness, its distance from Earth. He found that Andromeda is 2 million light years away (a light-year is the distance traveled by light in a year, or about 9,500 billion kilometres), proving beyond doubt that it had to be outside the Milky Way. This showed that ‘nebulae’ were in fact galaxies similar to our own, but separated by unimaginably vast distances. This literally pushed back the boundaries of the known Universe, and settled a contemporary debate regarding whether or not our galaxy was the entire extent of the Universe.
In 1929, Hubble proved that all galaxies were in fact speeding away each other, showing that the Universe was expanding. This was done by measuring how the light from distant galaxies was red-shifted (an analogous phenomenon to the change in pitch of an ambulance moving away). He discovered that all galaxies were speeding away from each other, which meant that the Universe was expanding. This was yet another astonishing discovery which shook the entire scientific community to its core, as it implied that at some point in the distant past, there was a time when the entire Universe clumped together. This finding had long-lasting repercussions, and provided the foundation for modern cosmology.
In homage to Hubble’s achievements, NASA (the US National Aeronautics and Space Administration) named the HST after him before it was launched in 1990. Since then, from its vantage point above the obscuring murk of Earth’s atmosphere, the HST has been one of the most important workhorses of the astronomical community.
As for the HST’s own contributions, Beckwith said, “I think the Hubble Space Telescope has made many more contributions, but maybe individually not as profound as those of Galileo’s or Hubble’s. However, if I was to choose the most important discovery, I would say that it was when working in collaboration with ground-based telescopes, the HST showed that the expansion of the Universe is in fact accelerating, instead of slowing down as expected previously”.
When this discovery was made in 1998, accepted cosmological theories were again overturned, and scientists are still racing to come up with new theories to explain this phenomenon. “The HST is without a doubt one of the greatest scientific instruments ever created by mankind, and will continue to make important discoveries for years to come, especially if improved by new instruments”, continued Beckwith.
The HST was designed to be serviced by the Space Shuttle astronauts in space, replacing crucial parts and improving its capabilities by installing new cameras and instruments. Four Shuttle servicing missions have already been carried out on the HST since its launch, extending its life-span and adding new instruments.
However, after the tragedy of Space Shuttle Columbia in 2003 that killed seven astronauts, NASA’s upper echelons cancelled the next servicing mission to the HST, scheduled for 2004. Without this mission, one of the HST’s vital components could fail by 2007, leaving it a lifeless hulk drifting in space.
This decision created a major uproar by astronomers all over the world. In his capacity as the director of the Space Telescope Science Institute, Beckwith has been one of the foremost advocates of the campaign to save the HST, canvassing support from both the astronomical community and the general public.
In addition to performing necessary maintenance work on the HST, the cancelled servicing mission would have installed two new scientific instruments: the Cosmic Origins Spectrograph (COS) and the Wide-Field Planetary Camera-3 (WFPC-3). Assuming the servicing mission does go ahead, not only would the HST have its operational lifetime extended to well beyond 2010, but it would have its already unique capabilities expanded as well.
Once COS and WFPC-3 are in place, there are four areas in which scientists expect the improved HST to make major contributions. The first area is the study of extra-solar planets and planetary systems. Explained Beckwith, “The HST has an unparalleled ability to detect the slight dimming of a star caused by the passage of a planet across its face (known as a transit). We hope that the HST will discover over 10 new extra-solar planetary systems in the next few years, and also to be able to measure the chemical composition of these planets by studying their spectrum. This would be a profound contribution to the field, as it is something in which the HST has unparalleled ability”.
“The second area is a major effort in the next five to 10 years to study the acceleration of the Universe’s expansion, and the nature of dark energy”, said Beckwith. Dark energy is a term coined by scientists to describe a mysterious force, accounting for over 70% of the total mass and energy in the Universe, which is pushing galaxies away from each other in spite of the fact that they would tend to fall towards each other through their mutual gravitational attraction.
Beckwith continued, “This would involve measuring the brightness of a class of exploding stars known as Type-1A supernovae in extremely distant galaxies, at distances of over a billion light-years away. As the true, or intrinsic, brightness of the supernovae can be calculated, measuring their apparent brightness as seen by us would be a measure of their distance. Combined a measurement of how their light has been red-shifted from the expansion of the Universe, scientists can measure how quickly the early Universe was expanding compared with now”.
Another area of study is the creation of the earliest galaxies, back when the Universe was only a fraction of its current age. As light travels at a finite speed, looking at distant objects means looking back in time. “The HST has the unparalleled ability to look back into the early Universe of billions of years ago, when the earliest galaxies were being created. These are clumps of matter spanning less than 1 arc-second (the angular width of a coin from a distance of several kilometres) as seen from Earth. This is too small to be seen clearly by ground-based telescopes, so the improved HST would be crucial in this area of study”, explained Beckwith.
“Finally, the HST will help to study the nature of ordinary, or baryonic, matter in our Universe (i.e. protons, neutrons and electrons of which we and all known elements are made)”, said Beckwith. Most of baryonic matter is in fact in the form of hydrogen, and the clouds of hydrogen gas lying in the space between galaxies make up a significant proportion of this. By observing the dimming of ultra-violet light from distant objects due to absorption by these clouds, the amount of matter in these clouds can be tallied.
Besides all these, there will definitely be more. Many important discoveries in science have been made unintentionally. “The HST will almost certainly make serendipitous discoveries beyond our expectations, as it had already done so on several occasions. So, we can expect the unexpected”, said Beckwith.
However, ground telescopes are not standing still, and ever more gigantic telescopes with phenomenal light-gathering power are under construction, like the 30m diameter California Extremely Large Telescope. Nevertheless, even though the 2.5m diameter main mirror of the HST is small in comparison with those of ground telescopes, the ability to resolve images to within fractions of an arc-second remains the exclusive domain of the HST. The enormous ‘light-bucket’ telescopes on the ground can gather more light from faint objects compared with the HST, but they are not able to resolve them with the detail possible on the HST. It is obvious that despite its age, the HST remains a vital scientific tool that cannot be replaced. Nor is a viable replacement possible for at least another decade or two, given the long incubation time for major space missions. The James Webb Space Telescope, scheduled for launch in 2011, will observe mainly in the infra-red rather than the HST’s combination of near-ultraviolet, optical and near-infra-red, and will not be able to take over the HST’s role.
Over the past 15 years, the HST has pushed back the boundary of the visible Universe, made breathtaking discoveries of exotic astronomical phenomena, and shed light on known objects with unprecedented clarity. More importantly, it has been the layperson’s window to the heavens for an entire generation, taking spectacular and breathtaking images which have captured the imagination of the public. If given a new lease of life, it might make discoveries that will put it firmly on the pedestal as the most important telescope ever built. For the moment, it is still surveying the stars silently and unflinchingly, its days seemingly numbered by the squabbles of men.
The author thanks Dr. Steven Beckwith of the Space Telescope Science Institute for his kind assistance in consenting to an interview.