Far sighted

European academics are working on a project to develop extremely large, lightweight space reflectors. If successful, the reflectors may be deployed on a 20m-class space telescope equipped for grand astronomical discoveries.


A team of European academics is working on a project to develop extremely large, lightweight space reflectors. If successful, the reflectors may be deployed on a 20m-class space telescope equipped for grand astronomical discoveries, such as terrestrial (Earth-like) planets, outside our solar system.

The research team, which includes members from Cambridge University, will spend the next three years working to control the shape and stability of shiny, thin-film membranes that could be used as primary mirrors for future ultra-light space telescopes. A primary mirror is the principal light-gathering surface of a reflective telescope.

The results of the programme will be of interest to space agencies, such as NASA, which are trying to reduce the weight of their telescopes while increasing the diameter of their apertures to obtain better resolutions of more faraway images.

Since the early 1960s, the astrophysics community has considered the problem of launching 20m-class space telescopes.



Weight constraint

One concern has been how to launch such huge mirrors, given the area and weight constraints in launch vehicles. For example, the active Hubble Space Telescope’s primary 2.5m-diameter mirror occupied about four per cent of the volume and weight capacity of its vehicle. The yet-to-be-launched James Webb Space Telescope’s 6.5m-diameter mirror is expected to occupy 15 per cent of the volume and weight capacity of its launch vehicle.

Another factor is cost, since it is more expensive to launch heavier payloads. The Hubble’s mirror accounted for less than two per cent of the cost of launching the Space Shuttle. The James Webb mirror is expected to account for more than 50 per cent of the launch expenditure.

It is little wonder there has been interest in gossamer telescopes. They are extremely lightweight, with aerial densities of less than 1kg/m, and would use thin, membrane optics coupled with active wave-front control. But this technology is immature, which is why the team hopes to develop it further.

Their co-ordinator, Prof Andre Preumont, of the Mechanical Engineering and Robotics department at the Université Libre de Bruxelles, said the central theme of the research will be developing a system to control the quality and shape of the membranes after they are deployed. He said the film shells will be made of a type of piezoelectric polymer that will be controlled with myriad actuators.

The membranes will need to be rolled up and stowed at launch but the release of strain energy will unfurl them during deployment. Preumont imagines the membrane inflating like a soap bubble until it stiffens into shape.

The researchers will demonstrate their ability to control the films by experimenting on a piezoelectric polymer in laboratory tests. However, Preumont said that their experimental material has not been tested in a space environment and will probably only be used to demonstrate the concept of controlling the shape of a membrane.



Better materials

‘We assume that by the time we finish this work, there will be better materials on the market,’ he said.

If the researchers’ concept proves viable, thin-filmed membrane optics might cut launch costs and space constraints for large space telescopes in the future.

When the James Webb Telescope launches next decade, it is expected to be able to detect faint light from the first stars and galaxies formed after the Big Bang.

However, scientists predict that it will take a telescope mirror with an aperture double its size to directly image terrestrial planets outside our solar system. To date, the majority of planets found have been gas giants that are larger and easier to see or infer from observation. However, a number of extra-solar planets are known or suspected to be terrestrial.

Yet there are plenty of other applications for lightweight telescopes, said Preumont.

‘These telescopes are not only looking at the stars,’ he said. ‘They could also look at the Earth for military applications or they could survey the planet.’