The imagery is free of licensing fees
NASA requires that they be provided a credit as the owners of the imagery
Visible Earth Addendum
Beyond the NASA Terms, the Visible Earth team requests, but does not require:
The Visible Earth be provided a credit as the location that the imagery was found at
A URL be provided, either to the Visible Earth http://visibleearth.nasa.gov/ or to the page providing the link to the used image.
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