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The countdown has resumed. There's one more planned hold coming up at T-4 minutes.

The OCO-2 mission has special meaning to NASA's Launch Services Program, as we have dedicated it to one of our LSP teammates, Laurie Walls. Laurie began her career more than 30 years ago as a thermal engineer for McDonnell Douglas in Huntsville, Alabama, supporting NASA's Marshall Space Flight Center. She moved to Florida in 1985 …

Countdown clocks are holding at the T-15 minute mark. This is a routine hold scheduled to last 20 minutes.

Up next is a check of the rocket's first- and second-stage engine nozzles, which are put through a pattern of movements, called "slews," to check their steering capabilities. The second-stage engine is tested first, followed by the first-stage engine. A built-in hold in the countdown is coming up at the T-15 minute mark.

The countdown is proceeding smoothly at exactly one hour until liftoff.

Liquid oxygen has been fully loaded into the Delta II first stage, completing the fueling process. The countdown continues to go well and liftoff remains scheduled for 2:56 a.m. PDT, 5:56 a.m. EDT.

A thick fog has settled in over the launch area, surrounding the Delta II rocket awaiting liftoff from Space Launch Complex 2. The fog is not a problem for launch.

Valves are open and liquid oxygen is beginning to flow into the Delta II rocket's first stage. This is a cryogenic propellant, meaning it's cooled to an extremely low temperature — in this case, minus-297 degrees Fahrenheit.

OCO-2's ride into space is the United Launch Alliance Delta II rocket, a vehicle with a lengthy and reliable history. From Earth-observing missions like Aquarius, CloudSat and CALIPSO, to interplanetary voyages such as the Mars Phoenix lander, and astronomy missions like Kepler and WISE, the Delta II has boosted many of NASA's most well-known missions …

NASA Launch Manager Tim Dunn polled his team and gave the go-ahead to begin loading liquid oxygen into the rocket's first stage a few minutes from now.