Page "X-ray astronomy" Paragraph 40
from
Wikipedia
Usually observational astronomy is considered to occur on Earth's surface ( or beneath it in neutrino astronomy ).
As soon as the observer leaves the cozy confines of Earth, the observer becomes a deep space explorer.
Except for Explorer 1 and Explorer 3 and the earlier satellites in the series, usually if a probe is going to be a deep space explorer it leaves the Earth or an orbit around the Earth.
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