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How does the Webb Contrast with Hubble?

Webb often gets called the replacement for Hubble, but we prefer to call it a successor. After all, Webb is the scientific successor to Hubble; its science goals were motivated by results from Hubble. Hubble's science pushed us to look to longer wavelengths to "go beyond" what Hubble has already done. In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared. Thus observations of these distant objects (like the first galaxies formed in the Universe, for example) requires an infrared telescope.

This is the other reason that Webb is not a replacement for Hubble is that its capabilities are not identical. Webb will primarily look at the Universe in the infrared, while Hubble studies it primarily at optical and ultraviolet wavelengths (though it has some infrared capability). Webb also has a much bigger mirror than Hubble. This larger light collecting area means that Webb can peer farther back into time than Hubble is capable of doing. Hubble is in a very close orbit around the earth, while Webb 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.

You might also like this interactive feature: http://www.nasa.gov/externalflash/webb_hubble/


Wavelength

Webb will observe primarily in the infrared and will have four science instruments to capture images and spectra of astronomical 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 Webb's instruments will work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range (in particular in the red and up to the yellow part of the visible spectrum).

The instruments on Hubble 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
Credit: ESO/NASA/JPL-Caltech/S. Kraus

Why are infrared obsevations important to astronomy? Stars and planets that are just forming lie hidden behind cocoons of dust that absorb visible light. (The same is true for the very center of our galaxy.) However, infrared light emitted by these regions can penetrate this dusty shroud and reveal what is inside.

At left is an infrared image from the Spitzer Space Telescope of the dust and gas that has served as a stellar nursary for the bright, young star IRAS 13481-6124 (visible in the upper left of the image). Dust in this image is red-orange, and stars are blue, but ones that are deepy embedded in dust like IRAS 13481-6124) can take on a greenish-yellow tint.

 

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Size

size comparison

Hubble 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, Webb's sunshield is about 22 meters by 12 meters (69.5 ft x 46.5 ft). It's almost as big as a Boeing 737 airplane!

size comparison

Webb and Hubble mirror 
comparison

Webb 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. Hubble's mirror is a much smaller 2.4 meters in diameter and its corresponding collecting area is 4.5 m2, giving Webb around 7 times more collecting area! Webb will have significantly larger field of view than the NICMOS camera on Hubble (covering more than ~15 times the area) and significantly better spatial resolution than is available with the infrared Spitzer Space Telescope.

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Orbit

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.

Webb will not actually orbit the Earth - instead it will sit at the Earth-Sun L2 Lagrange point, 1.5 million km away! Because Hubble is in Earth orbit, it was able to be launched into space by the space shuttle. Webb 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.

Hubble, Webb distance 
graphic
(Note that these graphics are not to scale and are just meant to illustrate distance!)

lagrange 
diagram

At the L2 point Webb's solar shield will block the light from the Sun, Earth, and Moon. This will help Webb stay cool, which is very important for an infrared telescope. As the Earth orbits the Sun, Webb 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. Actually, satellites orbit around the L2 point, as you can see in the diagram - they don't stay completely motionless at a fixed spot.

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How Far Will Webb see?

Because of the time it takes light to travel, the further away an object is, the further back in time we are looking.

This illustration compares various telescopes and how far back they are able to see. Essentially, Hubble can see the equivalent of "toddler galaxies" and Webb Telescope will be able see "baby galaxies". One reason Webb will be able to see the first galaxies is because it is an infrared telescope. The Big Bang caused the universe (and thus the galaxies in it) to expand, so most galaxies are moving away from each other. The most distant (and thus youngest) galaxies are moving away so quickly that the light they emit gets shifted towards the red end of the spectrum. This is very similar to listening to a train whistle shifting from higher to lower frequency as it passes by. Because visible light from faraway, quickly moving, “high redshift” galaxies is shifted to the infrared, infrared telescopes, like Webb, are ideal for observing these early galaxies.

Seeing back into the cosmos
Credit: NASA and and Ann Feild [STScI]

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What About Herschel?

Herschel image of M31
Credit: ESA/Herschel/SPIRE/PACS/HELGA ; ESA/XMM/EPIC/OM

The Herschel Space Observatory is an infrared telescope built by the European Space Agency - it is currently located at the L2 point (where Webb will be).

The primary difference between Webb and Herschel is wavelength range: Webb goes from 0.6 to 28.5 microns; Herschel goes from 60 to 500 microns. Webb is also larger, with a 6.5 meter mirror vs. Herschel's 3.5 meters.

The wavelength ranges were chosen by different science: Herschel looks for the extremes, the most actively star-forming galaxies, which emit most of their energy in the far-IR. Webb will find the first galaxies to form in the early universe, for which it needs extreme sensitivity in the near-IR.

At left is an infrared image of the Andromeda Galaxy (M31) taken by Herschel (orange) with an X-ray image from XMM-Newton superposed over it (blue).

 

 

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