杏吧原创

THREE miles outside the small town of Cloudcroft in New Mexico, astronomers are putting the final touches to an unusual telescope. Instead of a large glass mirror to collect and focus light, NASA鈥檚 Orbital Debris Observatory has a spinning bowl of mercury at its heart. As it turns, the liquid metal forms a curved surface like a cup of tea that has been stirred. At 3 metres across, it is the largest liquid mirror telescope in the world. In September it will start searching the skies for the remains of old satellites and the upper stages of the rockets that launched them. Monitoring this orbiting junk will help scientists decide how best to protect the space station from its impact.

Although the idea of liquid mirrors is not new, building them is a formidable challenge. In 1909, the eccentric physicist Robert Wood, based at Johns Hopkins University in Baltimore, photographed the Moon using one. Although the image was not bad, he quickly realised that these telescopes have severe limitations. Any vibration produces waves that ruin the image while changes in the speed of rotation alter the focal length of the mirror. Even worse, liquid mirrors cannot be tilted, preventing them from following stars as they move across the sky as conventional telescopes do. Eventually, Wood abandoned his work and liquid mirror telescopes were quietly forgotten. Until now.

The big advantage of a liquid mirror is its cost. 鈥淎 3-metre conventional instrument would cost in the region of $10 million. Ours has cost $500 000,鈥� says Andrew Potter, the astronomer who leads the project. The most expensive part of a conventional telescope is the mirror, which is usually cast in glass and then painstakingly polished into shape 鈥� a process that can take several months. By contrast, NASA鈥檚 liquid mirror can be formed easily. 鈥淭he current record is 45 seconds,鈥� says Potter. The trick is to spin the dish until the mercury covers most of its surface, then suddenly increase the speed of rotation by yanking the dish. 鈥淚t seems to stretch the layer into shape,鈥� he says. The result should be a film of mercury only 2 millimetres thick but often small holes form and spread quickly across the mirror due to the metal鈥檚 strong surface tension. When this happens, the astronomers have to start again. The astronomers get plenty of practice 鈥� every few days, the mirror must be reformed to remove dust and insects that collect on its surface.

Mercury is highly toxic, so the atmosphere at the observatory is constantly monitored for mercury vapour. The most dangerous time is just after the mirror has been formed and the team don protective masks if levels become dangerous. After a few hours, however, an oxide layer forms on the surface and this greatly reduces the vapour that boils off, says Potter.

Solving the problems that bedevilled Wood鈥檚 work has not been easy. One of the challenges is to minimise vibrations which can ruin the image. One source is the observatory itself, which vibrates when the wind blows or when people move around inside. To protect against this, the telescope is mounted on a concrete plinth that is isolated from the building and has its own foundations in the bedrock beneath.

Friction free

The dish sits on a cushion of air that could support the weight of a small car. There is so little friction that a single push would set the dish spinning for more than an hour, says Mark Mulrooney, who manages day-to-day work at Cloudcroft. The friction from conventional ball bearings, on the other hand, would prevent more than a couple of revolutions. Even the surface tension of a thin layer of mercury helps to damp out any waves that are created.

The dish must be special, too. It must be light enough to be handled easily but strong enough to support up to 300 kilograms of mercury. It must also be resistant to changes in shape caused by temperature fluctuations. To meet these requirements Paul Hickson, an astronomer at the University of British Columbia in Vancouver, found a way of roughing out a dish from Kevlar, a strong synthetic fibre. To create a more accurate shape, the Kevlar is coated with a layer of polyurethane approximately 1 centimetre thick, which is poured into the spinning dish and left to harden into a natural parabola. This contains and supports the mercury layer. The entire assembly is a tenth of the weight of a glass mirror.

The rate of rotation is also crucial because it determines the focal length of the mirror. The dish spins ten times each minute and is controlled to an accuracy 100 times greater than the best record player turntables. Any change in this rotation rate sends the image out of focus.

Although liquid mirrors cannot be tilted, stars can be tracked electronically. The technique is the electronic equivalent of moving a film at the same rate as the star to create a point-like image rather than a streak. As the image passes across a light-sensitive chip, like the ones in camcorders, the data is downloaded in a way that mimics this process. In the early 1980s, Ermanno Borra, an astronomer at Laval University in Quebec City, used this technique to look for faint celestial orbits. An individual star takes about a minute and a half to pass through the field of view but pictures can be built up by superimposing images from successive nights. With this method, a liquid mirror can cover a strip of sky 1 degree wide, twice the apparent size of the Moon, although the one at Cloudcroft has half this coverage.

Junk overhead

Despite their limited coverage, liquid mirror telescopes are also ideal for spotting debris because the Earth rotates beneath the orbiting remains. 鈥淚f we wait long enough, almost every piece will pass overhead,鈥� says Potter. The telescope will have a resolution capable of spotting pieces as small as 2 centimetres across at an altitude of 500 kilometres.

The debris is the result of satellites and rockets exploding in space due to malfunctions or unburnt fuel detonating. 鈥淭here are about three or four explosions each year,鈥� says Potter. The space junk shows up as a trail against the background of stars. But if the scientists know the approximate orbit of the debris, they can set up the telescope to produce images in which the debris appears as points and the stars as trails and this gives them greater resolution. The team uses brightness to determine each lump鈥檚 approximate size and the speed across the sky to work out its altitude. Potter says these methods are not perfect 鈥� not all materials reflect light in the same way, for example, and the speed is only a good indication of altitude if the orbit is circular. 鈥淏ut they give us a pretty good idea of what鈥檚 up there,鈥� he adds. Within a year or so Potter鈥檚 team will have built a healthy statistical picture of the amount of debris at different altitudes.

Using radar NASA has already found more than 130 000 pieces larger than 1 centimetre across. But certain materials such as plastic and foam do not reflect radar signals, so NASA will look for the sunlight they reflect instead. The results will help in calculating the risk that debris poses for the International Space Station and what shielding will be necessary to protect it. At an altitude of 500 kilometres the average collision speed is 10 kilometres per second. 鈥淭his would a give a 1 centimetre object the same kinetic energy as a bowling ball travelling at 100 kilometres per hour,鈥� says Potter.

For the moment, Potter is interested in objects at altitudes between 300 and 1000 kilometres where the space station will fly. But these objects are hidden in the Earth鈥檚 shadow for most of the time, reflecting light only in the two hours immediately after sunset and before dawn. As a result, Potter estimates it will take more than a year to complete the survey.

Also, the small but significant proportion of debris that orbits at the equator cannot be seen from Cloudcroft. This includes the geostationary orbits at 39 000 kilometres in which the orbital speed matches the ground speed so that satellites perch over the same spot of land. Satellites are being launched into this region at a rate of 20 a year but nobody is sure how much debris has accumulated there. At lower altitudes, the debris burns up in the upper reaches of the atmosphere. But this does not happen at geosynchronous altitudes. 鈥淥nce the debris is there, it is there forever,鈥� says Potter. 鈥淚magine what it will be like in a century or more.鈥�

Hallowed ground

Potter hopes to move the entire telescope to a site on the equator where these orbits will be visible. But equatorial sites are difficult to come by. Most of the globe at equatorial latitudes is covered in water and much of the land area suffers from poor weather or unstable political conditions. One of the best options were the Gal谩pagos Islands. 鈥淏ut these are an ecological holy grail,鈥� he points out. 鈥淭hey wouldn鈥檛 have let us stir poisonous substances around.鈥� Now, one of the most likely candidates is Ecuador but Potter says there is a lot of negotiating to be done before a move could take place.

In the meantime, astronomers are queuing up to use the telescope for stargazing. For example, Hickson wants to use the hours when debris cannot be seen to look for galaxies in deep space. Liquid mirror telescopes could revolutionise optical astronomy where the limit in viewing time is set by the small number of telescopes that are available. 鈥淵ou can build ten of these telescopes for every glass telescope,鈥� says Mulrooney.

Hickson and his colleagues from Laval University and the Institute d鈥橝strophysique de Paris are designing a 5-metre model that will be ready in two years. In theory, there is no limit to the size of liquid mirrors. In practice, the size is limited by the weight that can be supported by air cushions and the breeze that builds up around the edge of the mirror as it turns. The bigger the mirror the greater this wind, which can kick up waves on the mercury鈥檚 surface like a breeze over a lake. Mulrooney says that air cushions could support mirrors up to 8 metres across and Borra believes that a transparent plastic sheet suspended just above the mirror could reduce the amount of wind.

Although factors like wind could eventually limit the size of such telescopes, astronomers like Borra and Mulrooney expect to see them take off. One day, cheap liquid mirrors telescopes could outnumber their glass counterparts.