Complex sentence: Sputnik 2 detected the Earth's outer radiation belt in the far northern latitudes, but the significance of the elevated radiation was not realized because Sputnik 2 passed through the Van Allen belt too far out of range of the Russian tracking stations.
Split into simpler sentences: Sputnik 2 detected the Earth's outer radiation belt in the far northern latitudes, but the significance of the elevated radiation was not realized. This did not happen because Sputnik 2 passed through the Van Allen belt too far out of range of the Russian tracking stations.