CAPE CANAVERAL - The first test launch of NASA's new deep space capsule, Orion, was delayed Thursday due to wind gusts, technical issues with the rocket and a misplaced boat off the coast.
The capsule is meant to carry humans to an asteroid or Mars in the coming years.
No astronauts were on board the spacecraft, poised atop a Delta IV Heavy rocket at Cape Canaveral, now set to blast off at 8:26 am (1326 GMT), an hour and 21 minutes after the initial target time.
The launch window ends at 9:44 am (1444 GMT).
Tourists and space enthusiasts lined the area known as Florida's Space Coast to see the take-off at sunrise, and 27,000 guests were at the Kennedy Space Center for a close-up look.
The capsule's four-and-a-half hour test flight is due to carry the spacecraft around the Earth twice before splashing down in the Pacific Ocean.
The launch is the first of a US spacecraft meant to carry people into deep space in more than four decades, since the Apollo missions that brought men to the Moon.
With no American vehicle to send humans to space since the space shuttle was retired in 2011, some at NASA said the Orion launch has re-energized the US space programme, long constrained by government belt-tightening and forced to rely on costly rides aboard Russian Soyuz spacecraft to reach the International Space Station in low-Earth orbit.
Potential future missions for Orion, which is designed to fit four people at a time, include a trip to lasso an asteroid and a journey to Mars by the 2030s.
"We haven't had this feeling in a while, since the end of the shuttle programme, (of) launching an American spacecraft from America's soil and beginning something new," said Mike Sarafin, lead flight director at NASA's Johnson Space Center in Houston.
Two laps around Earth
The launch was due to propel 1.63 million pounds (739,000 kilograms) of spacecraft, rocket and fuel straight to space, where the capsule was due to make two laps around the Earth before splashing down in the Pacific Ocean around 11:30 am (1630 GMT).
The first orbit was to be about as high as the International Space Station, which circles at an altitude of about 270 miles (430 kilometers), but the second would soar 15 times higher, to an apogee of 3,600 miles above the Earth.
The chief contractor of the Orion capsule is Lockheed Martin. The spacecraft was first designed to take humans to the Moon as part of NASA's Constellation programme, which was cancelled by President Barack Obama in 2010, in favour or seeking new destinations in deep space.
The goal is both nebulous and costly, and NASA has already spent $9.1 billion on Orion and the powerful rocket meant to propel it with crew on board, the Space Launch System (SLS).
Another unmanned test flight is slated for 2018. The first Orion test flight with people on board is scheduled for 2021, and costs are projected to reach $19-22 billion.
As NASA looks beyond the Moon, safety for human explorers is another key problem that has yet to be solved.
"Radiation is one of the biggest challenges for us," NASA Administrator Charles Bolden told an audience of NASA enthusiasts gathered at Kennedy Space Center for a social media event.
The primary objective of the test, according to Orion programme manager Mark Geyer, is to see how the heat shield performs as it reaches temperatures of 4,000 degrees Fahrenheit (2,200 Celsius) on its high-speed plunge back to Earth at a velocity of 20,000 miles per hour.
"A part of me hopes that everything is perfect. We land, have high-fives and everybody has a great time," Geyer told reporters.
But he added that the test is designed to find things that go wrong before precious lives are at risk.
"We want to discover things that are beyond our modeling capability and beyond our expertise so we can learn it and fix it before we put people on board."