US space agency NASA has selected four US-based sub-orbital space companies to carry payloads near the "boundary of space".Payload is the amount of goods or material that can be carried by an aircraft and is usually measured in terms of weight.The selection is in line with NASA's effort to foster a commercially viable, sub-orbital platform that would allow testing of new space technologies within Earth's atmosphere."These proven flight service providers will allow payloads from organisations including NASA, industry, academia, and other government agencies to be tested on flights at the edge of space before being committed to demonstration in the harsh environment of space itself," said Michael Gazarik, an associate administrator at NASA.The four selected companies are Masten Space Systems, Mojave, California; Paragon Space Development Corp., Tucson, Arizona; Up Aerospace Inc., Highlands Ranch, Colorado; and Virgin Galactic, New York City.The three-year contracts have two-year extension options and a minimum value of $100,000."Through these new awards, selected companies will receive an indefinite-delivery, indefinite-quantity contract for integration and flight services," NASA said in a statement.