Earth's atmosphere

The Earth's atmosphere is our best defense against harmful radiation from the Universe. This is great for life on Earth, and we are certainly not complaining. However, this great defense system just happens to absorb out all of the X-rays and gamma-rays from space, so we have to get above the atmosphere to see them.

The image below shows what energies of light can get through the Earth's atmosphere. The top of the atmosphere is at the top of the image; for the different energies of light, the color indicates how far into the atmosphere that light can get.

This image is from Imagine! the Universe EM spectrum page

Notice that visible, radio and a portion of microwave radiation get through very easily. However, as you move above optical energies, the light has trouble getting very far into the atmosphere.

The upshot of this is that all X-ray and gamma-ray astronomy must be done above a good portion of the Earth's atmosphere - either with a sounding rocket, balloon or satellite. Since these technologies were not well established until the mid- to late-1960s, X-ray and gamma-ray astronomy were not feasible until then.