CAPE CANAVERAL, Fla. — SpaceX returned to the launch pad Friday in hopes of finally delivering supplies to the International Space Station, but stormy weather threatened to interfere.
NASA's commercial supplier was foiled by a leaky rocket valve during Monday's launch attempt. The valve was replaced, and the company aimed for a liftoff at 3:25 p.m. Friday. Forecasters put the odds of acceptable weather at 40 percent.
The unmanned Dragon cargo ship contains 2½ tons of station supplies, including material originally intended for urgent spacewalking repairs coming up in just a few days.
A critical backup computer failed outside the space station last Friday. Astronauts will replace it as early as Sunday; the timing for the spacewalk depends on when the Dragon flies.
The primary computer is working fine, but numerous systems would be seriously compromised if it broke, too. A double failure also would hinder visits by the Dragon and other vessels.
"It's imperative that we maintain" backups for these external command-routing computer boxes, also called multiplexer-demultiplexers, or MDMs, said flight director Brian Smith. "Right now, we don't have that."
NASA decided late this week to use the gasket-like material already on board the space station for the repair, instead of waiting for the Dragon. Astronauts trimmed the thermal material Friday to fit the bottom of the replacement computer, and inserted a fresh circuit card.
Much-needed food is also aboard the Dragon, along with a new spacesuit and spacesuit replacement parts. NASA wants these things at the space station as soon as possible.
The shipment is already more than a month late. Initially set for mid-March, the launch was delayed by extra prepping, then damage to an Air Force radar and, finally on Monday, the rocket leak.
As the countdown entered its final few hours, NASA's space station program manager Mike Suffredini said an investigation continues into the reason for last summer's spacesuit failure. The helmet worn by an Italian astronaut filled with water from the suit's cooling system, and he nearly drowned during a spacewalk.
Routine U.S. spacewalks are on hold until engineers are certain what caused the water leak. The upcoming spacewalk by the two Americans on board is considered an exception because of its urgent nature; it will include no unnecessary tasks, just the 2½-hour computer swap.
NASA is paying the California-based SpaceX — Space Exploration Technologies Corp. — and Virginia's Orbital Sciences Corp. to keep the orbiting lab well stocked. The Dragon also can bring items back for analysis. Russia, Japan and Europe also make periodic deliveries.
In case the Dragon remains grounded Friday, better flying weather is anticipated Saturday. The company could try again to launch next week as well.
Online: NASA: http://www.nasa.gov/mission_pages/station/main/index.html SpaceX: http://www.spacex.com/