President Trump's missile defense project, called the "Golden Dome for America," aims to protect the U.S. from foreign threats. But it has a steep price and has been criticized by China as threatening to increase the risks of militarizing space and a global arms race.

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An initial sum of $25bn (£18.7bn) has been earmarked in a new budget bill - although the government has estimated it will end up costing much more than that over decades.