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Hubble Successor Under Way, Will See Even Farther

John Pickrell
for National Geographic News
June 8, 2004
 
Launched in 1990, the orbiting Hubble Space Telescope has captured the public imagination by beaming back unprecedented images of countless astronomical wonders. Left alone, however, the Hubble's battery and gyroscopes will likely fade by 2007, making the telescope useless.

After much public outcry over the telescope's planned demise, NASA is now considering a space shuttle-based or robotic tune-up mission. Perhaps even more important, the U.S. space agency has something much more impressive up its sleeve: the next major space telescope, already in the early stages of construction.


Hubble vs. The Next Generation

Scheduled for launch in 2011, the James Webb Space Telescope (JWST) could be powerful enough to "see" objects 400 times fainter than those visible with Earth-based telescopes—potentially snagging a peek at objects 15 billion light-years away.

By contrast, the Hubble can see objects 60 times fainter than those visible with Earth-based telescopes.

Objects 15 billion or so light-years away are so distant that light emitted from them shortly after the big bang is only now reaching us. This means that the JWST should be able to see even farther "back in time" than the Hubble can.

To get an idea of how far 15 billion light-years is, consider that light from the sun—93 million miles (150 million kilometers) away—takes just eight minutes to arrive at Earth.

The JWST (named for NASA's second chief administrator) is a joint project of NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). Though the project is NASA led, its most important imaging instrument is being built in Europe. The consortium is scheduled to launch the telescope from French Guiana in one of the ESA's Ariane 5 rockets.

Astronomers will use the telescope to observe the birth of galaxies, the physics of star and planet formation, and even the early evolution of the universe.

The JWST will be more powerful than the Hubble in a number of ways. Its 21-foot-wide (6.5-meter-wide) main mirror will have ten times the light-collecting power of the Hubble's.

The JWST will also carry three different types of cameras, all tuned to detect infrared light, which is invisible to the human eye. In contrast, the Hubble was equipped to capture mostly visible light and ultraviolet parts of the spectrum.

All objects produce infrared light, or heat radiation. This means that scientists can use infrared cameras to detect cool, faint objects, which emit very little of their light in the visible parts of the spectrum.

"One of the great strengths of infrared is that you can use it to look into the huge clouds of dust and gas found in space [where star formation occurs]," said Alistair Glasse, an astronomer at the Royal Observatory in Edinburgh, Scotland. Glasse is the instrument scientist behind the development of MIRI (Mid Infrared Instrument). MIRI is said to be the most sophisticated of three cameras to be aboard the JWST.

Another space telescope—the less powerful Spitzer, launched last year—already collects infrared light, but only has a 0.85-meter-wide (33-inch-wide) mirror.

Seeing the Oldest Objects

Due to unusual physical properties of the universe, the farther astronomers look back in time, the redder light from objects appears to be, Glasse said.

The phenomenon is called red shift, and it means that light from the most distant (and therefore oldest) objects is deep in the infrared part of the light spectrum—where the Hubble is unable to see.

In fact, the top goal for the JWST will be to capture infrared light from the very first stars and galaxies formed after the big bang. These objects are currently invisible to all telescopes, Glasse said.

However, capturing infrared light presents a unique set of challenges.

All objects emit infrared light as heat radiation—including the space telescopes themselves. To keep the JWST's own heat radiation from interfering with its infrared imaging of distant objects, the telescope will have to be supercooled to an incredible -266 degrees Celsius (-447 degrees Fahrenheit)—versus the Hubble's 20 degrees Celsius (68 degrees Fahrenheit).

To help ensure the JWST's low temperature, an unmanned craft will release the telescope in an orbit 930,000 miles (1.5 million kilometers) away from Earth. (The Hubble sits just 372 miles [600 kilometers] away.)

Next, a light shield the size of a tennis court will unfold to hide the JWST from the light of the sun. Finally, powerful cooling systems will help the JWST to reach its working temperature.

This distance puts the telescope beyond the reach of space shuttle servicing missions, and therefore it is expected to have a shorter life than the Hubble. The JWST will help answer a number of other questions, said University of Arizona astronomer George Rieke, co-leader of the telescope's MIRI instrument team.

The telescope should be able to fill a gap in our knowledge of the "dark ages" immediately following the big bang, he said.

"We know almost nothing about this time, after the universe was hot and uniform but before it was filled with stars and galaxies," George Rieke said.

The JWST might also provide answers about how other solar systems form and evolve.

The Greatest Challenge

However, the greatest challenge will be getting the telescope into space and functioning, Rieke said. The entire telescope will have to be fitted into a rocket 16 feet (5 meters) wide—a task that the engineers liken to designing a ship in a bottle.

The main mirror, for example, will be folded up like the pedals of a flower during launch.

"Any space launch is very tense. You have spent 10 or 15 years of your life on something and it is sitting there on a pile of explosives," Rieke said. "You never quite know what's going to happen, and this telescope is way beyond any other built before."

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