Hours before the space shuttle Endeavour was expected to take off on its final mission, NASA scrubbed the launch to no earlier than Monday afternoon due to technical problems with heaters in the shuttle's auxiliary power unit.
"Shuttle Endeavour's launch now no earlier than Monday at 2:33 p.m. EDT," NASA tweeted at 1 p.m. ET.
An hour earlier, NASA tweeted, "Endeavour's launch has been scrubbed for at least 48 hours because of an issue with Auxiliary Power Unit 1 heaters.
The shuttle was scheduled to launch at 3:47 p.m. out of the Kennedy Space Center in Cape Canaveral, Florida. President Barack Obama, the First Lady, and recovering Congresswoman Gabbrielle Giffords were scheduled to attend. Giffords' husband, Mark E. Kelly, is the Endeavour's commander, and today's launch would have marked her first public appearance since she was shot in the head on January 8.
On Thursday NASA officials said that if, for any reason, the launch did not go as planned, the agency had four more attempts to launch, up until May 4. There was also a small concern about inclement weather that increased the chance of canceling the launch from 20 to 30 percent.
Among the items Endeavour will carry to the International Space Station during its two-week mission is the Alpha Magnetic Spectrometer-2 (AMS). It will be used to measure cosmic rays to gain a better understanding of cosmic radiation, a challenge for long-duration spaceflight. It might also help uncover the mysteries involved in dark matter or antimatter, NASA said. The 15,000-pound AMS is expected to be operational for the rest of the station's life, at least 10 years.
NASA is holding a press conference about the delay at 4 p.m. ET.