Scientists at the University of Arizona in Tucson hope to harness sunshine
to point and stabilize future space telescopes.
Sunshine exerts a weak force on spacecraft. This has given space scientists
headaches for years, gently turning spacecraft off target or off-orbit. But
there has long been the idea of harnessing solar pressure with huge,
gossamer solar sails to push spacecraft like high-tech clipper ships.
"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," says 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 lighter and 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 already is 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 visualize 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 upset 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.
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. Olbert also is a staff engineer at UA¹s Steward
Observatory.
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 says.
The tiles will lie on the solar shield surface like shingles on a roof. One
edge of each tile is 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 spacecraft¹s 10-year life span without
detaching or delaminating.
"We are evaluating commercially available films and adhesives and the
technologies for sticking them together," Olbert says. "Then there¹s the
whole problem of lead technology. How do we attach the electrical leads to
each tile that are needed to energize it? And we also have to think about
redundancy because there are micro-meteorites out there that are constantly
punching tiny holes through the tiles."
In order to evaluate the films and adhesives, Olbert and others are building
test tiles in a Steward Observatory laboratory. This is a difficult, tedious
and time-consuming task because the films are like thin plastic wrap. Static
charges make them stick to everything, and once they¹re creased, they¹re
ruined.
The adhesives also have to be carefully squeezed out from between the films
as they¹re sandwiched together, which means low-viscosity adhesives are a
must.
Currently Olbert¹s work is funded under Phase I of NASA¹s Gossamer
Spacecraft Initiative. NASA envisions gossamer spacecraft as large,
ultra-light vehicles that can reconfigure themselves or evolve in response
to changing mission conditions.
Phase I is basically for evaluation of ideas. If NASA thinks the UA work
shows promise, the project could be funded for Phase II, which calls for
manufacturing and testing complete solar shield roof panels.
"We are in the weeding-out phase right now," Olbert says. "There are lots of
ideas that need to be turned into working solutions."
