Why did the Spanish government need to grant Alexander von Humboldt permission to visit its American colonies?

The Spanish government required Alexander von Humboldt to obtain official permission to visit its American colonies because those territories were strictly closed to the outside world at the time. Access was restricted exclusively to Spanish officials and members of the Roman Catholic mission.1

Because the colonies were effectively shut off from foreign influence to protect Spanish interests and maintain a monopoly over the region, any scientific explorer seeking entry needed a royal permit. Humboldt was able to secure this rare authorization through his high social standing and the support of the Spanish prime minister, Mariano de Urquijo, who recognized the value of his scientific mission.12