Why Send Spitzer into Space?


The amazing variety of objects in our Universe send us light all across the electromagnetic spectrum. However, much of this light (or radiation) does not reach us at ground level here on Earth. Why? Because we have an atmosphere which blocks out certain types of radiation, while letting other types through. Fortunately for life on Earth, our atmosphere blocks out harmful high-energy radiation like x-rays, gamma rays and most of the ultraviolet rays. The atmosphere also absorbs most of the infrared radiation which reaches the Earth from space. On the other hand, our atmosphere is transparent to visible light, most radio waves, and small windows within the infrared region.


Most of the infrared light coming to us from the Universe is absorbed by water vapor and carbon dioxide in the Earth's atmosphere. Only in a few narrow wavelength ranges, can infrared light make it through (at least partially) to a ground based infrared telescope.

The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter).

So the best view of the infrared universe from ground based telescopes are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground. The only way to observe these wavelengths is to get high above the atmosphere.

From the table below, you can see that only a few of the infrared "windows" have both high sky transparency and low sky emission. These infrared windows are mainly at infrared wavelengths below 4 microns.


Infrared Windows in the Atmosphere

Wavelength
Range
Band
Sky Transparency
Sky Brightness
1.1 - 1.4 microns
J
high
low at night
1.5 - 1.8 microns
H
high
very low
2.0 - 2.4 microns
K
high
very low
3.0 - 4.0 microns
L
3.0 - 3.5 microns: fair
3.5 - 4.0 microns: high
low
4.6 - 5.0 microns
M
low
high
7.5 - 14.5 microns
N
8 - 9 microns and 10 -12 microns: fair
others: low
very high
17 - 40 microns
17 - 25 microns: Q
28 - 40 microns: Z
very low
very high
330 - 370 microns
 
very low
low


For the most part, everything we learn about the Universe comes from studying the light (or electromagnetic radiation) emitted by objects in space. To get a complete picture of any object in the Universe, we need to examine it in all of its light, using the information sent to us at all wavelengths. This is why it is so important to send observatories, like the Spitzer Space Telescope, into space, to get above our atmosphere which prevents so much of this valuable information from reaching us. Spitzer's instruments will study radiation between wavelengths of 3 and 180 microns. Most of this infrared radiation is blocked by the Earth's atmosphere and cannot be observed from the ground.