Asystem proposed by u. s. president reagan to use ground- and space-based systems to protect the u. s. from strategic nuclear ballistic missiles.

Strategic Defense Initiative--The SDI program or "Star Wars" was a military program focused on protecting the US from USSR attack. 

The SDI program consisted of weapons and shields both on the ground and in space. This was a costly program and by all accounts was not possible at the time to achieve. In theory, it would have created a defense shield preventing missiles from being used in space.