ASTRONOMERS will never forget the near miss they had with the Hubble Space
Telescope. Soon after its launch in 1990, researchers discovered that its mirror
was misshapen and produced virtually useless blurred images. Over the following
two years, NASA analysed the problem, built an optical device to correct the
defect, and installed it with the help of astronauts who went up on the space
shuttle. 鈥淚t would have been a bag of bolts from day one had we not been able to
go up and fix it,鈥� says Roger Angel, an astronomer at the University of Arizona
in Tucson.
With Hubble鈥檚 planned successor, the Next Generation Space Telescope, on-site
repair will not be an option. The NGST will focus on infrared wavelengths, so it
will have to travel 1.5 million kilometres into deep space to escape the warming
rays of the Sun. NGST will extend the frontier of telescope technology by leaps
and bounds. Its giant 8-metre mirror鈥攎ore than 10 times the area of
Hubble鈥檚鈥攚ill have to be folded during transport and assembled in space.
NASA will only get one shot at putting this together, and that makes the agency
nervous.
So NASA wants a test run. In 2005, it plans to launch Nexus, a $200
million prototype that will fly to the same place in space and will test the
folding mirror idea with a scaled-down, 2.5-metre version. Great, say
astronomers, we could do a lot with a new 2.5-metre telescope. But NASA is not
playing ball. Nexus, it says, is a test bed for the technology. It won鈥檛 do
science.
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That鈥檚 a waste, astronomers say. 鈥淚 think it鈥檚 an overwhelming feeling in the
astronomical community that to do Nexus and not do science with it would be a
tragedy,鈥� says Rodger Thompson, an astronomer at the University of Arizona.鈥�
NASA researchers don鈥檛 see it that way. The real tragedy, they say, would be
to let the Nexus project grow and crowd out NGST. Funds for the two telescopes
come out of the same pot, and NASA researchers worry that if Nexus does more
than test the technology, it will gobble up resources and put NGST over budget.
This will delay the NGST launch date, which is currently set for 2009.
The debate over Nexus highlights a continuing tension between NASA, which
shies away from potentially humiliating risks, and academic astronomers who
would rather gamble for scientific glory.
The $2 billion NGST will be the most ambitious space telescope ever.
Hubble has revealed delights ranging from stars newly born from enormous gas
clouds to the galaxies that populated the Universe when it was just a few
billion years old. 快猫短视频s plan to use NGST to look even deeper into
space鈥攁nd hence further back in time. With it they hope to see the first
stars and galaxies switching on.
But NGST is so big that scientists will have to send it into space boxed up
like a complicated Christmas present. No current launcher is wide enough to
accommodate an 8-metre mirror in one piece. So NGST will have a segmented mirror
that will automatically unfold in space.
NGST developers hope to test this cutting-edge mirror technology with Nexus,
whose 2.5-metre mirror will have fewer segments. When it reaches its deep space
orbit, Nexus will snap pictures of the cosmos with two digital cameras
specifically designed to test image quality and map out tiny imperfections in
the mirror. Everyone agrees that this dress rehearsal is the best way to ensure
the folding-mirror scheme will work. 鈥淵ou鈥檙e in the real environment,
looking at real stars, and facing real problems,鈥� says Chuck Perrygo of NASA
contractor Swales Aerospace in Beltsville, Maryland.
But while the technological tests may be indispensable, the academics want
Nexus to have a third camera with coarser pixels that will yield a larger field
of view and capture more of the dim light from distant galaxies. Such a camera
could look in the spectrum of cosmic radiation that no one has yet explored,
Angel says.
Astronomers have problems viewing distant stars and galaxies because of the
dust in the Solar System. Light with wavelengths shorter than roughly 3
micrometres reflects off the dust, while the dust itself emits light with
wavelengths longer than 3 micrometres. This tends to obscure the faint signals
from deepest space. In the infrared, with wavelengths at around 3 micrometres,
however, the reflected and radiated light fall away, providing a gap through
which the light of the heavens shines more clearly.
Hubble can鈥檛 see this light. It orbits a mere 600 kilometres above the Earth,
and is repeatedly warmed by the Sun. This keeps it at roughly 290 kelvin (17
掳C), which means that Hubble itself generates enough infrared to overwhelm
any distant infrared light. 鈥淪cientifically, it鈥檚 extremely rich to look at
[this light] with a Nexus-size telescope,鈥� Angel says.
But Peter Stockman, NGST project scientist at the Space Telescope Science
Institute in Baltimore, Maryland says: 鈥淭here鈥檚 a very narrow niche for which
Nexus would be unique, and developing Nexus to fit that niche wouldn鈥檛 be simple
or inexpensive.鈥� Some NASA researchers even question whether Nexus is needed at
all. NASA should scrap it altogether, they say, to ensure NGST stays on schedule
and ahead of potential competition.
That competition may come from the ground. There are plans for several
enormous ground-based telescopes, such as the 100-metre Overwhelmingly Large Telescope
(快猫短视频, 20 February 1999, p 36).
鈥淭he longer you wait
to launch NGST, the more you risk launching an obsolete instrument,鈥� says Eric
Smith, an astronomer at NASA鈥檚 Goddard Space Flight Center in Greenbelt,
Maryland.
But other astronomers say telescopes on the ground can鈥檛 begin to compete for
the big prize, the sight of the Universe鈥檚 first stars and galaxies. All the
light from these distant objects gets shifted to the infrared, which get swamped
by the glow of the atmosphere and the ground-based telescopes. The telescope鈥檚
sunshield coupled with its position in deep space, would cool NGST to a mere 30
kelvin and so it would give off little infrared. 鈥淎 telescope in orbit that鈥檚
relatively cold can whip any telescope on the ground no matter how big,鈥� says
John Mather, NGST project scientist at Goddard.
If the agency chooses to go ahead with Nexus, Thompson thinks it should set
aside $50 million to build a scientific instrument. Some academic
astronomers would like NASA to abandon its tight control over the construction
of scientific instruments. 鈥淚 believe it鈥檚 possible for university groups to cut
a lot of corners and still make an excellent instrument,鈥� says John Tonry, an
astronomer at the University of Hawaii at Manoa. He says NASA should provide a
weight allowance and power for an instrument on Nexus鈥攁nd then let
academics do what they think best.
But this approach still leaves researchers with millions of dollars to find
for their payload, says Anne Kinney, director of NASA鈥檚 Origins programme, which
oversees NGST and Nexus. 鈥淚 think it would be hard to find the money,鈥� she says.
And this approach wouldn鈥檛 make NASA any less wary. Even cheap missions cost
NASA plenty in terms of public perception when they fail, Kinney says.
The debate over Nexus has vexed NASA so much that the agency has decided it
needs more time to think the matter through. This week, NASA鈥檚 Office of Space
Science pushed back a review of the Nexus project from October to January to
allow the NGST team to determine which tests they can do on Earth and which must
be done in space. Officials such as Kinney are leaning toward a purely
technological mission. 鈥淚 certainly would be very worried about a science
component,鈥� she says. 鈥淣ever say never, but it would take a lot of convincing.鈥�