'Reusable' rocket launch aborted
An attempt by a private aerospace company to land a rocket on a platform in the ocean was postponed today as the mission to the International Space Station was postponed.
The targeted landing is part of attempts by US business SpaceX to make rockets re-usable.
Nasa said the countdown had been aborted and the next possible launch attempt would be on Friday if the issue - unexplained so far - was resolved.
Countdown was halted with only a minute remaining.
Lift-off was due at about 11.20am British time, with SpaceX's Falcon 9 rocket carrying its Dragon cargo spacecraft into the sky on the way to the space station.
After detaching, the rocket would aim for a barge floating in the Atlantic off Florida.
The custom-built platform is known officially as the "autonomous spaceport drone ship".
The founder of SpaceX has said being able to use rockets again would cut the cost of space travel.
Elon Musk said: "If one can figure out how to effectively reuse rockets just like airplanes, the cost of access to space will be reduced by as much as a factor of a hundred. A fully reusable vehicle has never been done before. That really is the fundamental breakthrough needed to revolutionise access to space."
The company admitted that the odds of success in the attempt were not great - perhaps 50% at best.
But the test was the first in a series designed to deliver a fully reusable rocket.
It said on its website: "Returning anything from space is a challenge, but returning a Falcon 9 first stage for a precision landing presents a number of additional hurdles. At 14 storeys tall and travelling upwards of 1,300 m/s (nearly 1 mi/s), stabilising the Falcon 9 first stage for re-entry is like trying to balance a rubber broomstick on your hand in the middle of a wind storm."
It added: "To complicate matters further, the landing site is limited in size and not entirely stationary. The autonomous spaceport drone ship is 300 by 100 feet (91.4m by 30.5m), with wings that extend its width to 170 feet (51.8m). While that may sound huge at first, to a Falcon 9 first stage coming from space, it seems very small.
"During previous attempts, we could only expect a landing accuracy of within 10km (six miles). For this attempt, we're targeting a landing accuracy of within 10 metres (33ft).
"The concept of landing a rocket on an ocean platform has been around for decades but it has never been attempted. Though the probability of success on this test is low, we expect to gather critical data to support future landing testing.
"A fully and rapidly reusable rocket - which has never been done before - is the pivotal breakthrough needed to substantially reduce the cost of space access.
"While most rockets are designed to burn up on re-entry, SpaceX is building rockets that not only withstand re-entry, but also land safely on Earth to be refuelled and fly again.
"Over the next year, SpaceX has at least a dozen launches planned with a number of additional testing opportunities. Given what we know today, we believe it is quite likely that with one of those flights we will not only be able to land a Falcon 9 first stage, but also re-fly."