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.
Wavelength
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.
|
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.
|
|
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.
|
[top]
Size
|
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 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. |
[top]
Orbit
The Earth is 150 million km from the Sun and the
moon orbits the earth at a distance of approximately 384,500 km.
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.
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.
|
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.
|
[top]