Public release date: 7-Jun-2006
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New Scientist

How NASA will avoid double Hubble trouble

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NASA has had its share of foul-ups, but few can match the embarrassment of launching the multi-billion-dollar Hubble Space Telescope only to find that it could not focus properly. Determined not to repeat the mistake, designers of the James Webb Space Telescope (JWST), Hubble¡¯s successor, are already testing the new observatory even though its launch is seven years away. This time astronauts will not be able to reach the telescope to fix any mistakes, as they did for Hubble.

¡°We have made a lot of the investments in the technology up front to try to lessen the risk,¡± said Mark Clampin, JWST project scientist at NASA¡¯s Goddard Space Flight Center, speaking at the International Society for Optical Engineering¡¯s conference in Orlando, Florida, last week. ¡°The real critical item in this programme is the mirror.¡±

It was a surprise flaw in Hubble¡¯s primary mirror that horrified NASA when the first test images came back from space. The 2.4-metre glass reflector was slightly misshapen, an error eventually traced to an improperly assembled test device. Spacewalking astronauts installed corrective optics and Hubble became the flagship observatory of its time.

The JWST¡¯s 6.5-metre primary mirror has six times the surface area of Hubble¡¯s, but weighs only one third as much. Since no launch vehicle is big enough to carry the fully assembled mirror, it will be folded up like an origami figure and deployed in space. However, if there are any Hubble-like mistakes, there can be no Hubble-like rescue. The JWST will be positioned more than 1.5 million kilometres from Earth, at a point where its single shade can block the heat and light from the Earth, sun and moon. That is much too distant for astronauts to reach in today¡¯s spacecraft.

Last week, the second piece of the telescope¡¯s 18-segment beryllium primary mirror arrived at Tinsley Laboratories in Richmond, California, for grinding and polishing. By mid-June, the first polished mirror segment will undergo cryogenic testing at NASA¡¯s Marshall Space Flight Center to make sure it can operate at 40 kelvin. The frigid temperature is needed to allow the telescope¡¯s instruments to detect the faint infrared glow from the very first galaxies, as well as dust-shrouded stars and planet-forming discs.

To catch any manufacturing errors, the new telescope will be fully assembled and tested on Earth well before its launch, unlike Hubble. ¡°The basic problem with Hubble was that we only did component-level testing. We didn¡¯t do an end-to-end test,¡± says Clampin. ¡°This time, the entire telescope is going to be tested on the ground. And we¡¯re going to do it early so that there are no surprises at the end.¡±

In addition, all critical technologies, including the mirrors, the deployment mechanisms and the massive five-layer shade, are expected to be successfully demonstrated by January 2007. Meanwhile, the telescope¡¯s four scientific instruments are already under construction.

Contractors have also spent $6 million building what they call the JWST ¡°mini-me¡± ¨C a scaled-down working model of the optical system. And engineers are already developing algorithms to operate the mirror segments. ¡°We plan on making hundreds of mistakes on this one, so that we don¡¯t make any up there,¡± says Scott Acton of mirror subcontractor Ball Aerospace in Boulder, Colorado.


Author: Irene Klotz ¡ñ


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