THERE鈥檚 big money to be made if you can come up with a killer design for a
space telescope. So with the major space agencies close to choosing a
$500-million design for the Next Generation Space Telescope, which will
succeed Hubble from the end of this decade, thoughts are turning to what will
replace the NGST some decades hence.
Ivan Bekey, former director of advanced space flight research at NASA,
reckons he has the answer. He envisages a distributed telescope in which all the
essential parts, like mirrors and image sensors, will fly freely in space. The
idea, revealed in the current edition of the journal Smart Materials and
Structures (vol 10, p 1145), is that the sum of these separate parts will
create a telescope which performs beyond the dreams of today鈥檚 designers. Its
sensitivity should be up to 10 000 times that of Hubble and 100 times that of
the NGST.
On Earth, the main job of a telescope鈥檚 casing is to keep the optical
components鈥攕uch as the main mirror, the imaging aperture and the focal
point鈥攍ined up and accurately spaced. But a heavy, bulky casing is
obsolete outside Earth鈥檚 strong gravitational field, says Bekey.
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His idea eschews the casings and heavy structural trusses that normally keep
a spacecraft together. Like most telescopes, primary and secondary mirrors focus
the light from distant stars onto an image sensor that sends pictures back to
Earth. But these mirrors would be free-floating, up to 250 metres away from each
other in space, but kept in position by thousands of millimetre-sized
microrockets attached to their edges.
A radio and laser network will monitor the floating pieces and tell the
rockets when to fire, keeping the pieces aligned to within a tenth of a
millimetre (see Diagram).
Just like a high-tech burglar alarm, sensors monitor a
web of laser beams between the components. If the distances between the beams
alter, computers tell the rockets to fire and bring them back into line.
Bekey, now a consultant aerospace engineer based in Annandale, Virginia, says
the microrockets exert a force measured in micronewtons
(快猫短视频, 28 April 2001, p 24).
The microrocket technology is being tested by The
Aerospace Corporation in Los Angeles and the University of California at
Berkeley. The rockets have a tiny cavity filled with indium metal. Solar power
is used to ionise and propel the metal through a nozzle, exerting a small
thrust.
Bekey predicts that鈥攕hort of major collisions with space junk鈥攁
few kilograms of fuel will be enough to keep his optical components in place for
many years.
The heaviest part of most optical telescopes is the primary mirror, which has
to be carefully transported into space. Bekey wants to make the job easier and
cheaper by launching a folded 25-metre parabolic mirrored membrane which can be
unfurled in space. This membrane would be made of a piezoelectric material that
stiffens when a charge is applied to it. An electron beam projector floating
behind the membrane would then keep the parabola in shape by charging it up once
a minute.
Bekey reckons the total payload, including the 25-metre mirror, could weigh
as little as 200 kilograms, compared with more than 11 000 kilograms for Hubble
and a projected 5000 kilograms for the NGST. But with respective mirror
diameters of 2.4 metres and 6 metres, they are far less sensitive.
Although a lot of the technology for Bekey鈥檚 idea doesn鈥檛 exist yet, that鈥檚
no reason not to start making mission plans. 鈥淚f you don鈥檛 look 10 or 20 years
ahead in this business, you鈥檒l be out of a job,鈥� says Mark Schattenburg, a space
scientist working on MAXIM, a new space-based X-ray Interferometer at MIT.
Microrockets are also being considered for use on MAXIM, he says.
Bekey鈥檚 major challenge will be lining everything up once the components get
into space. 鈥淭hey can get out of control,鈥� he says. 鈥淪o you need a computer to
talk to the microrockets, power and communications鈥攐therwise, before you
know it, your optical component has become a satellite.鈥�