Tuesday, May 19, 2009

How does JWST contrast with HST?

The James Webb Space Telescope (JWST) has been called the successor to the Hubble Space Telescope (HST). But what does this really mean? How will JWST be different than HST? There are some similarities - both telescopes are (or will be) in space. They both seek to improve our understanding of processes like star birth and the evolution of galaxies. However, there are many differences between HST and JWST.

For starters, JWST will primarily look at the Universe in the infrared, while HST studies it at optical and ultra-violet wavelengths. JWST also has a much bigger mirror than HST. This larger light collecting area means that JWST can peer farther back into time than HST is capable of doing. HST is in a very close orbit around the earth, while JWST will be 1.5 million kilometers (km) away at the second Lagrange (L2) point.

Read on to explore some of the details of what these differences mean.


JWST will observe primarily in the infrared and will have four science instruments that can take images and spectra of objects. These instruments will provide wavelength coverage from 0.6 to 28 micrometers (or "microns"; 1 micron is 1.0 x 10-6 meters). The infrared part of the electromagnetic spectrum goes from about 0.75 microns to a few hundred microns.. This means that JWST's instruments will work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range.

The instruments on HST can observe a small portion of the infrared spectrum from 0.8 to 2.5 microns, but its primary capabilities are in the ultra-violet and visible parts of the spectrum from 0.1 to 0.8 microns.

EM Spectrum and  satellites

Orion  Nebula It is very important to make observations at different wavelengths as we get different information by looking at different wavelength bands. For example, stars and planets that are just forming lie hidden behind cocoons of dust and cannot be seen in visible light. The same is true for the very center of our Galaxy. However, infrared light can penetrate this dusty shroud and reveal what is inside. An example is the image of the Orion Nebula at left that combines Infrared and visible-light data from both the HST and the Spitzer Space Telescope.

Orion Nebula Other objects may not emit visible or infrared light and may only emit X-rays. Or different regions of an object might emit light of a different wavelength than another region. We then need a telescope that can detect X-rays. Thus data obtained at different wavelengths can be combined to provide a more complete picture. For example, the image on the left shows the same two patches of sky, as viewed by an X-ray telescope (Chandra), a visible-light telescope (HST), and an infrared telescope (Spitzer). Each observation shows us something different (in this case, scientists were looking for black holes) - but combining these observations can give us a more complete (and more accurate) picture.


size comparison HST is 13.2 meters (43.5 ft.) long and its maximum diameter is 4.2 meters (14 ft.) It is about the size of a large tractor-trailer truck. By contrast, JWST's sunshield is about 22 meters by 12 meters (72 ft x 39 ft). A Boeing 737-200 is 100 feet long!

JWST and Hubble mirror  comparison JWST will have a 6.5 meter diameter primary mirror, which would give it a significant larger collecting area than the mirrors available on the current generation of space telescopes. HST's mirror is a much smaller 2.4 meters in diameter and its corresponding collecting area is 4.5 m2, giving JWST around 7 times more collecting area! JWST will have significantly larger field of view than the NICMOS camera on HST (covering more than ~15 times the area) and significantly better spatial resolution than is available with the infrared Spitzer Space Telescope.



The Earth is 150 million km from the Sun and the moon orbits the earth at a distance of approximately 384,500 km.

earth sun distance  graphic

The Hubble Space Telescope orbits around the Earth at an altitude of ~570 km above it.

JWST will not actually orbit the Earth - instead it will sit at the L2 Lagrange point, 1.5 million km away! Because HST is in earth orbit, it was able to be launched into space by the space shuttle. JWST will be launched on an Ariane 5 rocket and because it won't be in earth orbit, it is not designed to be serviced by the space shuttle.

HST, JWST distance  graphic

A Lagrange point is one of the five positions in interplanetary space where a small object (like a satellite) can be relatively stationary with respect to two larger objects (like the Earth and the Sun). It is analogous to an earth satellite in a geosynchronous orbit that allows it satellite to stay stationary over one spot on the Earth. At a Lagrange point, a satellite can stay "fixed" in space, rather than orbiting the Earth.

lagrange  diagram JWST will sit at the L2 point, with its solar shield blocking the light from the Sun, Earth, and Moon. This is very important as it will help JWST stay cool, which is very important for an infrared telescope. As the Earth orbits the Sun, JWST will orbit with it - but stay fixed in the same spot with relation to the Earth and the Sun, as shown in the diagram to the left.


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