Spirit Opportunity Mars rovers dust solar panels

Two Mars rovers worked for years

NASA expected the solar panels on Spirit and Opportunity to fail after about 90 days as dust built up. But the dust sometimes cleared from the panels, and both rovers lasted for years.

No known society relied mainly on barter

Historians and anthropologists have not found a society where barter was the main everyday system of exchange. Societies without ordinary money have generally relied more on gifts, debts, and social obligations.

Dropping the R sound went from fashionable to unfashionable in America

In parts of England, pronouncing words such as 'car' without a final R became associated with prestigious speech. Similar pronunciation spread through some American coastal cities and upper-class communities. During the twentieth century, however, pronouncing the R gained prestige in much of the United States. The same accent feature can signal high status in one era and lose it in another.

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Spirit Opportunity Mars rovers dust solar panels

Two Mars rovers worked for years

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