What challenges did 15th- and 16th-century explorers face?

It has been established that people came from Europe to the Americas before Columbus—the Vikings, for example. Why then is there a Columbus Day? Why is he given the credit for the discovery?

·         How did contact with the East enhance Western exploration and the development of civilization?

What challenges did 15th- and 16th-century explorers face? How were such challenges met? Reference at least one explorer from the text in your answer.