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Atlas V: Countdown 101

Atlas V lifts off from Launch Complex 41Atlas rockets have evolved over four decades to become a highly reliable launch vehicle. The Atlas V booster family now provides a variety of vehicles capable of launching a wide range of spacecraft and satellites.

The Centaur upper stage uses pressure-stabilized propellant tank design for efficiency. Centaur engines can start and re-start multiple times in space, enabling insertion of a spacecraft into a low Earth parking orbit, followed by a coast period and finally, insertion into geosynchronous transfer orbit. They're also capable of direct insertion into an inter-planetary trajectory.

Image to left: An Atlas V lifts off from Launch Complex 41 at Cape Canaveral Aug. 21, 2002 carrying the HOT BIRD 6 broadcast satellite. Image credit: International Launch Services

Through the Virtual Launch Control Center, you can share in the excitement of each new launch with live coverage directly to your computer via the Web. Coverage usually begins two hours prior to liftoff, and you can use the countdown events below to track the prelaunch milestones and learn about the Atlas V countdown process.

Live launch commentary is also provided on NASA TV.

Here are some countdown milestones and key events that take place after the countdown begins. Keep in mind that event times and lengths are approximate and subject to change.

T-720 minutes (12 hours until launch)
  • Verify countdown clock is set with applicable holds
  • Countdown begins
  • Clear all non-essential personnel from Mobile Launcher Platform (MLP) for jacking
  • Pad Environmental Control System (ECS) preparations, including air-conditioning and gaseous nitrogen purge, are performed
  • Weather briefing
  • Complex 41 and Vertical Integration Facility Area amber warning lights are turned on, indicating hazardous operations are underway
  • Power on searchlights (if launch is set for night or early morning)
  • Complex 41 and VIF track is cleared of non-essential personnel for MLP/Vehicle transport
T-510 minutes
  • MLP transported to launch pad
  • MLP hard down at pad
  • Start Atlas system preparations to support cryogenic tanking
  • Instrumentation checks are completed
  • Atlas liquid oxygen (LO2) system preparations are complete
  • Hazardous Gas Detection System preparations are complete
  • Internal battery checks are performed
  • Centaur liquid hydrogen (LH2) system preparations are complete
  • Public Affairs announcement: All personnel clear complex 41 for cryogenic tanking
T-120 minutes and holding (30 minute hold)
  • Launch Conductor receives reports on vehicle readiness for cryogenic tanking
  • NASA Launch Manager polls team to proceed with tanking
  • Launch Conductor holds a pre-test tanking briefing
Atlas V first stage is lifted in preparation for MROImage to right: In preparation for NASA's Mars Reconnaissance Orbiter mission, the first stage of the Atlas V rocket is raised to a vertical position for lifting into the launch service tower and mated with the second stage, the Centaur. Image credit: NASA/KSC

T-120 minutes and counting
  • Start chilldown procedures on Centaur upper stage's liquid oxygen storage tank
  • Start chilldown procedures on Atlas V's liquid oxygen vault and Mobile Launcher Platform
  • Start Centaur helium bottle charge to flight pressure
  • Begin raising Atlas Pressure Vessels to flight levels
  • Raise Atlas V RP-1 fuel tank to higher pressure
T-115 minutes and counting
  • Safe Arm Device (SAD) cycle test is performed
T-110 minutes and counting
  • Start Centaur liquid oxygen transfer line chilldown
T-103 minutes and counting
  • Start Centaur LO2 tanking
T-93 minutes and counting
  • Pressurize Centaur liquid hydrogen storage tank to chilldown level
T-90 minutes and counting
  • Start filling Atlas V with liquid oxygen
T-85 minutes and counting
  • Start Centaur liquid hydrogen transfer line chilldown
T-60 minutes and counting
  • Start Centaur engine chilldown
T-55 minutes and counting
  • Start flight control final preparations to raise hydraulic pressures
T-45 minutes and counting
  • Pressurize Main Engine Pneumatic System to flight pressure
T-16 minutes and counting
  • Initiate fuel fill sequence
T-10 minutes and counting
  • Weather briefing with Atlas Launch Weather Officer
T-5 Minutes and counting
  • Fuel fill sequence is complete
  • Water deluge system actuation pressure adjustment is performed
  • Atlas L02 at flight level
  • Centaur L02 at Flight level
  • Centaur LH2 at flight level
T-4 minutes and holding (10 minute hold)
  • NAM and NLM final launch polls -- go to continue countdown
  • Spacecraft transfers to internal power
T-4 minutes and counting
  • Hazardous gas monitoring is complete
  • Automatic computer sequencer takes control for all critical events through liftoff
  • Atlas first stage LO2 replenishment is secured, allowing the tank to be pressurized for flight
T-3 minutes and counting
  • Atlas tanks reach flight pressure
T-2 minutes and counting
  • Atlas first stage and Centaur upper stage switch to internal power
  • L02 and LH2 topping for Centaur will stop in 10 seconds
T-90 seconds and counting
  • Launch control system is enabled
  • Ignition and liftoff of the Atlas V!

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