The space shuttle Discovery and its seven astronauts safely returned to Earth on Friday after some last-minute suspense over which landing site to use, closing out a year in which Nasa finally got construction of the international space station back on track.

Nasa officials decided a crack in Discovery‘s fuel tank insulating foam was not enough of a threat to stop the countdown on Tuesday to their first Independence Day holiday shuttle launch and ”a nice fireworks display” for the United States. Inspectors spotted the crack on Monday.

Over the past three months, workers at the Kennedy Space Centre have tripped, dropped things, banged into sensitive equipment and started fires in a baffling string of accidents that have left one person dead. ”There’s enough going on that we’re very, very concerned,” said Bill Parsons, deputy director of the Kennedy Space Centre.

Nasa scientists hope the third time is the charm for their -million unmanned mission to Pluto. The space agency planned to make a third attempt to launch the New Horizons probe on Thursday, a day after a storm knocked out power at the Maryland-based laboratory that will command the mission.

Nasa prepared to launch an unmanned, piano-sized probe that will fly by Pluto, the solar system’s last unexplored planet, and also study a mysterious zone of icy objects that surrounds the frosty planet at the outer edges of the planetary system. The launch has drawn protests from anti-nuclear activists because the spacecraft will be powered by 11kg of plutonium.