Sunlight set to corral telescopes

Scientists at the University of Arizona in Tucson hope to harness the power of sunshine to position and stabilise future space telescopes.

Sunshine exerts a weak force on spacecraft, which is said to have given space scientists problems for years because of the way it gently turns spacecraft off target or off-orbit. But there has long been the idea of harnessing solar pressure with huge, gossamer solar sails to propel spacecraft.

‘While most people think of solar sails for pushing with the pressure of light to accelerate and move spacecraft, our thought is to use that force to point the telescope and keep it in position,’ said Roger P. Angel, founder and director of the UA Mirror Lab.

Currently, space telescopes are rotated, pointed and steadied by motors, gyroscopes and thrusters. As space telescopes become increasingly lighter, the vibrations and oscillations created by these devices can blur images. They also require using finite fuel supplies, whereas sunshine is an inexhaustible source of steering force as well as energy.

Angel and UA researchers Blain Olbert and Paul Calvert want to take advantage of the solar shield that is already needed for space telescopes. This shield sits between the telescope and the sun to keep the cryogenically cooled telescope from heating up.

The UA scientists visualise a shield shaped like a pyramid, with its sloping surfaces covered with hinged, reflecting tiles. Normally the tiles lie flat against the surfaces. When the tiles on one side of the pyramid are raised, however, the pressure balance is disturbed and the sunshade is pushed to one side. By raising tiles like ailerons on the right pyramid faces with electrical energy collected from solar cells, the solar pressure could be used to hold the spacecraft stable or to change its orientation or angular momentum.

The key to all of this is developing suitable lightweight tiles that bend when a voltage is applied to them.

UA engineers are exploring the feasibility of building these tiles under a $100,000 grant from NASA. Professor Paul Calvert and graduate student Blain Olbert, both of the Materials Science and Engineering Department, are working on the project.

They are constructing tiles from piezoelectric polymer films that bend when a voltage is applied to them.

‘We are looking for polymer films that already are produced commercially and want to see if any of them are suitable for this application,’ Olbert said.

The tiles will reportedly lie on the solar shield surface with one edge of each tile glued to the shield. This glued edge acts as a hinge on which the tile rotates. Each glue joint and tile must survive hundreds of thousands of cycles during the spacecraft’s 10-year life span without detaching or delaminating.

In order to evaluate the films and adhesives, Olbert and others are building test tiles in a Steward Observatory laboratory. This is said to be a difficult, tedious and time-consuming task because the films are like thin plastic wrap. Static charges make them stick to everything and are ruined once they’re creased.

The adhesives also have to be carefully squeezed out from between the films as they’re sandwiched together, making low-viscosity adhesives essential.

Currently Olbert’s work is funded under Phase I of NASA’s Gossamer Spacecraft Initiative. NASA envisions gossamer spacecraft as large, ultra-light vehicles that can reconfigure themselves or evolve in response to changing mission conditions.