page_title: Timeline of STS-121
section_title: July 4 (Launch and flight day 1)
hierarchical_section_title: Timeline of STS-121 / July 4 (Launch and flight day 1)
caption_attribution_description: KENNEDY SPACE CENTER, FLA. – Space Shuttle Discovery kicks off the Fourth of July fireworks with its own fiery display as it rockets into the blue sky, spewing foam and smoke over the ground, on mission STS-121. It was the third launch attempt in four days; the others were scrubbed due to weather concerns. Liftoff was on-time at 2:38 p.m. EDT. During the 12-day mission, the STS-121 crew of seven will test new equipment and procedures to improve shuttle safety, as well as deliver supplies and make repairs to the International Space Station. Landing is scheduled for July 17 at Kennedy's Shuttle Landing Facility.
context_page_description: This is the timeline for STS-121, the mission to the ISS.
context_section_description: At 8:35 a.m. the backup circuit breaker controlling the primary heaters on the segment joints of both solid rocket boosters was noticed to have failed. Consideration was given to a repair on the pad.  However, this was not done as the heaters are only used in colder weather, and the primary heater was functional.
  For the third launch attempt, there was only a 20% chance of weather preventing launch according to NASA.
  At 2:37:55 PM Eastern Daylight Time, Space Shuttle Discovery launched from the Kennedy Space Center at Cape Canaveral, Florida. This was the third attempt at launch. It was also the first (and only) US Space Shuttle to launch on the United States' Independence Day.
  During and after launch much attention was paid to monitoring the external tank for the loss of insulation foam. The shuttle was equipped with a number of new cameras, and video was also taken from spotter planes. Each solid rocket booster contained three cameras - one to monitor the separation, and two focused on the leading edge.  The video from these was not to be broadcast, but recorded for later retrieval from the solid rocket boosters. A further camera was placed on the external tank, as during STS-114, to broadcast live images on NASA TV during launch. The first thing two of the crew from the mid-deck did when the main engines cut out was leave their seats to take photographs and video of the external tank. One of the factors that was potentially affecting the choice of day within the launch window was the lighting conditions for this imagery, as the sun's slightly different angle on the different days affected the location of shadows on the external tank. However, the day to day difference in lighting was determined to be relatively unimportant, particularly as the lighting conditions depended to a great degree on a more unpredictable factor — the degree to which the external tank tumbles on separation.
  During the launch, NASA TV broadcast a view from the external tank camera mounted between the Shuttle and the external tank. Unlike the previous two missions, no foam breakage or foam pieces were easily seen falling off the tank. However, upon close inspection of the many cameras covering the launch NASA has revealed several small pieces of debris were seen jetting away from the tank.  Generally, though, these were seen after the time NASA was most concerned about.
  About 23 minutes into the flight, further debris was observed floating beside the orbiter as reported by Mission Specialist Michael Fossum. His transmission was broadcast live on NASA TV. Fossum initially described the debris as a 4–5-foot-long piece (1.2–1.5 m) with straps attached, a description which would fit a thermal protection system blanket. Such a blanket was noted to be flapping on the previous mission, STS-114, but was not of concern as it protects a section of the vehicle which does not get particularly hot. Analysis of video images on the ground has indicated that the debris observed was strips of ice formed on the outside of an engine nozzle, which sublimated and disintegrated during observation. Very similar ice formations were seen on earlier missions.
  Video analysis also revealed a tile shim which came out during main engine start.
  The orbital maneuvering system thruster in which a heater failed prior to launch (see Pre-launch concerns) was warmed by pointing it at the sun; thus it was possible to be used during ISS rendezvous docking operations.
caption: The launch of STS-121.
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