Nasa is set for a fresh attempt to launch the US shuttle Discovery for a mission to the International Space Station (ISS).

美国航空航天局重新计划将发现号航天飞机送上国际空间站。

Discovery's mission is scheduled to include three spacewalks

Two previous attempts to launch the orbiter have been postponed¹ by a mix of bad weather and a technical glitch²（小故障，小失误） affecting its main propulsion system（推动系统，动力装置）.

Lift-off from the Kennedy Space Center in Florida is now scheduled for 1159 EDT Friday (0359 GMT Saturday).

Discovery's mission will be the 30th flight dedicated³ to ISS maintenance（维护，保持）.

It is transporting materials including science equipment and a freezer to store research samples.

It also carries a new sleeping compartment⁴, an air purification system and a treadmill⁵（踏车，跑步机） to help maintain astronauts' health.

Six sorties

Nasa plans an additional six sorties to the orbiting platform before retiring its re-useable spaceship fleet at the end of next year or early in 2011.

Managers had been discussing an early Friday flight but decided⁶ to give their engineers an extra 23 hours to study the hydrogen stop-drain fuel-valve problem that thwarted⁷（反对，阻碍） the second launch effort on Wednesday.

Engineers now believe the unexpected signals they were getting from the propulsion system while filling the orbiter's giant external tank on Tuesday were related to a errant（错误的） sensor⁸.

Heavy rain and lightning had intervened（插入，调停） at the first launch attempt on Tuesday morning.

Nasa has until 30 August to get Discovery away before standing⁹ down the mission to allow the US Air Force to conduct military satellite operations in the Cape¹⁰ Canaveral area.

If that happens, the next flight opportunity for the shuttle would not be until 17 October.

The big gap is due to congestion¹¹（拥挤） at the space station, with Japan's new cargo¹² ship due to make its maiden¹³ voyage（处女航） to the ISS in the middle of September, and a Russian Soyuz craft expected to carry a new crew and space tourist a couple of weeks later.

Discovery's 13-day mission is scheduled to include three spacewalks to replace exterior¹⁴（外部的，表面的） science experiments, prepare the platform for the arrival of a new module¹⁵ next year, and to install a new ammonia（氨水） storage tank and return the used one.

Ammonia is used to move excess heat from inside the station to the radiators¹⁶ outside.

Discovery will also drop off US astronaut Nicole Stott for a three-month stay on the ISS, and pick up colleague Tim Kopra for the ride home. Kopra has been living on the platform for the past six weeks.

The mission is commanded by Rick Sturckow.