The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. Its 5 year mission is due to be launched in 2012. Initially known as the Next Generation Space Telescope, it was renamed in honour of James E. Webb, NASA's second administrator, on September 10th, 2002. It is however slightly more specific than Hubble, since its main goal is to observe the first stars and galaxies. More precisely it is designed to help find answers to the following questions :

  1. What is the shape of the Universe?
  2. How do galaxies evolve?
  3. How do stars and planetary systems form and interact?
  4. How did the Universe build up it present elemental/chemical composition?
  5. What is dark matter?
Radiation from these objects is strongly redshifted, so JWST is designed to observe wavelengths between 0.6 and 28 microns (far visible to medium-infrared). Many of these events happened when the Universe was between 1 million and 1 billion years old, a period we know little about. To accomplish this goal, a design requirement was that JWST would be able to see objects up to 400 times fainter than those observable by current ground or space based observatories.

Facts and Figures

Some basic data about JWST. Some of this may be subject to change.

  • Primary mirror diameter: 6.5 m
  • Mass: 11,000 lbs
  • Cost: $824.8 million
  • Orbit: 1.5 million km, at the L2 Point
  • Operating temperature: <50K

Lightweight optics and sunglasses

If you compare with the data for the Hubble Space Telescope, you will notice that JWST weighs half as much and costs little more than half what Hubble cost, yet its mirror diameter is 2.5 times bigger. This is possible through the use of advanced techniques not available at the time of Hubble's design. Hubble's mirror is in one large piece. Making mirrors of this size is expensive, difficult and slow. Hubble's mirror as we know contained imperfections. Large mirrors are also very heavy which increases launch costs significantly. By contrast, JWST's main mirror will of 36 segments. What more, they will not be rigid, but controlled by computers so that the mirror always has the optimum shape. To add to the technological challenge, the mirror and the actuators controlling it will have to function at very low temperatures.To enable the telescope to fit onboard the launcher, the mirror will actually be folded up when the telescope is launched, only unfolding when it reaches its destination.
A number of companies and universities are currently competing to design the mirror.

The sunshield used by JWST will be approximately 22 m x 10 m, in other words far too big to fit into any current launcher. For this reason, the sunshield will be stored folded, and deployed when the telescope arrives at its destination. Some movies of suggested deployment can be found at http://www.jwst.nasa.gov/Hardware/text/deploy.html

Far from home

JWST will operate at the L2 Lagrange point. Although this will make any servicing missions impossible, there are very good reasons for doing to place the telescope there.

JWST is primarily an infrared tool, which means that its instruments must be kept cool. Bodies like the sun and the earth emit large amounts of infrared radiation that are blocked by JWST's sunshields. This shielding operation is vastly simplified if all of these 3 objects are lined up. The L2 Lagrange point is a point about 1.5 million km beyond the earth fulfilling this condition and which is advantageous because it is stable: an object left there will stay there with very little effort (i.e. fuel) required on the part of the telescope. The other main reason for placing the telescope there is that the L2 Lagrange point is a fairly chilly place. This of course will make life much easier for the various cooling systems on board, that maintain a temperature of 7K for the mid-infrared instruments and 30 K for the near-infrared instruments (space may be cold, but electronics generate heat and in the vacuum of space there's no atmosphere to convect it away).

Instruments

Partners and contractors.

The JWST is a joint project between NASA, the European Space Agency and the Canadian Space Agency, although as you might guess, the main partner is NASA. The JWST Prime Contractor is Northrop Grumman Space Technology. It has not yet been decided what the launch vehicle will be, but Ariane V, Atlas V and Delta IV are candidates.

Sources:
http://www.jwst.nasa.gov/
http://sci.esa.int/home/ngst/index.cfm

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