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Countdown clocks are ticking again. There will be one more built-in hold at the T-4 minute mark.

Launch Weather Officer Joseph Round just briefed the launch team on the current and forecast weather conditions, but there were no surprises. We're still "green" on all launch agency and range safety constraints.

The countdown has paused temporarily at the T-15 minute mark. This is a planned hold lasting 20 minutes. There are no issues being worked in the countdown and liftoff remains scheduled for 2:56 a.m. PDT/5:56 a.m. EDT.

The rocket's second-stage engine is going through a series of steering checks called "slews." The first-stage engine check is up next.

Today's launch window opens in an hour. The countdown continues to go smoothly and weather remains favorable.

The Delta II rocket is fully fueled for launch following the completion of LOX loading activities.

The hexagon-shaped OCO-2 spacecraft was built by Orbital Sciences Corp. and weighs in at almost 1,000 pounds. Two 10-foot-long solar arrays will power all the observatory's systems — including its only scientific payload. This instrument comprises three spectrometers that will break the sunlight reflected up from Earth's surface into its component colors, then focus on …

The OCO-2 spacecraft will be delivered to orbit by a Delta II rocket – an industry workhorse with decades of success for NASA missions. The two-stage vehicle stands 128 feet tall and measures eight feet in diameter, with a 10-foot payload fairing. At T-0, the first-stage engine ignites along with the rocket's three solid-rocket boosters. …

Liquid oxygen loading is in progress.

The next major countdown milestone is the loading of liquid oxygen, or LOX, into the rocket's first stage. LOX is chilled to a very cold minus-297 degrees, making it a cryogenic propellant. NASA Launch Manager Tim Dunn polled his team and received the "go" to start this process a few minutes from now. ULA Launch Conductor …