A heavy blanket of fog and a persistent drizzle couldn't dampen the joy surrounding this week's Delta 2 rocket launch that saw the return of a venerable booster and a comeback for the science team.

Read our full story.

Streaking back into action after a two-and-a-half-year hiatus, a Delta 2 rocket launched NASA's Orbiting Carbon Observatory No. 2 on Wednesday to watch the Earth breathe from space.

Read our full story.

NASA launch director Tim Dunn reports the OCO 2 spacecraft is generating electricity through its solar arrays, and the satellite appears healthy as it undergoes early checkouts in orbit.

This morning's flight was the first Delta 2 rocket launch since October 2011.

"The spacecraft was rock solid in the fairing during the entire countdown," Dunn said. "Delta 2 performed like a champ after almost three years ... There was pure joy in the mission director's center at spacecraft separation."

This is the 97th consecutive successful Delta 2 rocket launch dating back to May 1997. The Delta 2's overall history since debuting in 1989 has achieved 150 successes in 152 flights.

The next Delta 2 launch is planned from Vandenberg on Nov. 5.

The power-generating solar wings on the OCO 2 spacecraft has been unfurled as the satellite begins its life in orbit.

NASA reports ground controllers are receiving data from the OCO 2 spacecraft through the Tracking and Data Relay Satellites.

T+plus 56 minutes, 20 seconds. SPACECRAFT SEPARATION! The Orbiting Carbon Observatory 2 spacecraft has been released from the Delta 2 rocket's second stage, completing the primary objective of today's launch!

T+plus 54 minutes. The rocket is repositioning itself to the proper orientation for release of OCO 2.

The Delta rocket has completed the restart of the second stage engine to put OCO 2 into the planned orbit.

Ignition of the Delta 2's second stage AJ10 engine is scheduled for 3:47 a.m. PDT (6:47 a.m. EDT; 1047 GMT) for a 12-second burn to put the OCO 2 satellite in the correct circular orbit 370 nautical miles above Earth at an inclination of 98.2 degrees.

T+plus 24 minutes. As the rocket coasts in this parking orbit, it performs a "BBQ roll" maneuver to keep the thermal conditions on the vehicle equal.

T+plus 20 minutes. Live telemetry continues to be received via NASA's Tracking and Data Relay Satellite System. Today's launch is the sixth Delta 2 flight to use TDRS during the long mission instead of ordering support from mobile telemetry assets and faraway ground sites after the rocket passes out of view from Vandenberg.

T+plus 13 minutes. The preliminary parking orbit achieved by the Delta 2 rocket is right on the planned mark.

T+plus 11 minutes, 40 seconds. The Delta 2 rocket is now traveling at a velocity of 17,008 mph.

T+plus 10 minutes, 33 seconds. SECO 1 has been confirmed. The second stage's Aerojet Rocketdyne-made engine completed its initial burn for the launch. Delta and OCO 2 have reached orbit.

T+plus 9 minutes, 45 seconds. The second stage continues to fire normally.

T+plus 9 minutes. The rocket is about 101.5 nautical miles up, 942 nautical miles downrange, traveling at at 14,851 mph.

T+plus 7 minutes, 25 seconds. Second stage looking good as this initial burn up gets underway.

T+plus 6 minutes, 50 seconds. The vehicle is 95.6 nautical miles in altitude, 542 nautical miles downrange from the launch pad and traveling at 12,122 mph.

The rocket is at an altitude of 82.7 nautical miles, a downrange distance of 360 nautical miles and a velocity of 11,127 mph.

T+plus 4 minutes, 51 seconds. The rocket's nose cone enclosing the satellite payload has been jettisoned.

T+plus 4 minutes, 44 seconds. The Delta's second stage has ignited! The engine is up and running.

T+plus 4 minutes, 39 seconds. MECO. The first stage main engine cutoff is confirmed and the spent stage has been jettisoned.

T+plus 3 minutes, 34 seconds. The Delta 2 is now passing a speed of Mach 10.

T+plus 3 minutes. The first stage main engine still firing well. The Aerojet Rocketdyne powerplant consumes kerosene fuel and liquid oxygen to produce about 237,000 pounds of thrust.

T+plus 2 minutes, 41 seconds. Delta now traveling at Mach 5.

T+plus 2 minutes. Delta now weighs half of what it did at liftoff two minutes ago.

T+plus 1 minute, 50 seconds. The ATK-made solid rocket boosters have jettisoned from the first stage. They remained attached until the rocket cleared off-shore oil rigs.

T+plus 1 minute, 6 seconds. All three ground-start solid rocket boosters have burned out. The Delta 2's first stage RS-27A main engine is providing the sole thrust for the next couple of minutes.

T+plus 50 seconds. The rocket has flown through the area of maximum aerodynamic pressure in the lower atmosphere.

T+plus 36 seconds. Delta has broken the sound barrier, rapidly accelerating on the power of its first stage main engine and the three ground-lit strap-on solid-fuel boosters.

T+plus 15 seconds. The launch vehicle is departing Vandenberg Air Force Base, heading south for the trek downrange over the Pacific carrying the OCO 2 spacecraft.

LIFTOFF! Liftoff of the Delta 2 rocket with the Orbiting Carbon Observatory 2 to watch the Earth breathe from space!

T-minus 30 seconds. SRB ignitors will be armed at T-minus 11 seconds.

The launch ignition sequence will begin at T-minus 2 seconds when a launch team member triggers the engine start switch. The process begins with ignition of the two vernier thrusters and first stage main engine start. The three ground-lit solid rocket motors then light at T-0 for liftoff.

T-minus 1 minute. All remains "go" for launch.

T-minus 80 seconds. First stage liquid oxygen topping to 100 percent is underway.

T-minus 1 minute, 45 seconds. The launch pad water suppression system is being activated.

T-minus 2 minutes. The first stage liquid oxygen vents are now being closed so the LOX tank can be pressurized for launch. Puffs of vapor from a relief valve on the rocket will be seen in the remainder of the countdown as the tank pressure stabilizes.

T-minus 2 minutes, 30 seconds. The OCO 2 payload has been declared "go" for launch.

T-minus 2 minutes, 45 seconds. Vehicle ordnance is being armed.

T-minus 3 minutes and counting down to the Delta rocket launch dedicated to deploying the OCO 2 spacecraft to map carbon dioxide from space.

T-minus 3 minutes, 45 seconds and counting. The Delta 2 rocket's systems are now transferring to internal power for launch. And the launch pad water system is being enabled.

T-minus 4 minutes and counting! Clocks are ticking down the final moments for liftoff of Delta 367 with the OCO 2 spacecraft. Launch is set for 2:56:23 a.m. local time from California's Central Coast.

Now five minutes from launch! The "go" has been given for release of the hold in one minute.

The OCO 2 spacecraft is configured for launch.

All systems are "go" for on-time launch today!

The launch team is being polled for the final time before liftoff to confirm systems are ready for the Delta 2 rocket and OCO 2 spacecraft.

The OCO 2 spacecraft is now operating on internal power.

Now 10 minutes to go. The rocket's launch opportunity today is 30 seconds long beginning at exactly 2:56:23 a.m. local (5:56:23 a.m. EDT; 0956:23 GMT). The window is dictated by the OCO 2 satellite mounted atop the 12-story booster, and launching at the time ensures optimum insertion of the payload into the desired orbit from the Vandenberg Air Force Base pad.

NASA launch director Tim Dunn has polled his advisory team and the space agency is "go" to continue with the countdown.

T-minus 4 minutes and holding. The countdown has entered the final planned built-in hold. This is a scheduled 10-minute pause leading to today's liftoff time of 2:56:23 a.m. local (5:56:23 a.m. EDT; 0956:23 GMT) for the Delta 2 rocket.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The first stage kerosene fuel tank is being pressurized for flight.

Twenty minutes from launch. The OCO 2 satellite will become the 228th primary payload put into space by the venerable Delta 2 rocket over the past 25 years. Here's a look at some other stats about today's mission. This will be:

• The 367th Delta rocket launch since 1960
• The 234th Delta launch with NASA involvement
• The 152nd Delta 2 rocket mission since 1989
• The 11th Delta 2 to fly in the 7320 configuration
• The 51st Delta 2 mission overseen by NASA
• The 42nd Delta 2 rocket launch from Vandenberg AFB
• The 3rd launch of the Delta family in 2014
• The 7th United Launch Alliance flight this year
• The 84th ULA launch overall

T-minus 15 minutes and counting. Countdown clocks are running again following the planned 20-minute hold. The count will continue to the T-minus 4 minute mark where another hold is scheduled. Launch remains set to occur at precisely 2:56:23 a.m. local (5:56:23 a.m. EDT; 0956:23 GMT).

A poll of the launch team confirms everyone is ready to press ahead with the countdown after this pre-planned 20-minute hold.

Now just 40 minutes away from blastoff. The Delta 2 rocket launching OCO 2 is known as the 7320-10 model. It has a standard first stage with three solid rocket boosters, a hypergolic second stage, no third stage and a 10-foot-diameter payload shroud.

The first stage is fueled by RP-1, a highly refined kerosene, and supercold liquid oxygen. The RS-27A main engine, made by Aerojet Rocketdyne, generates about 237,000 pounds of thrust.

The strap-on solids are known as Graphite Epoxy Motors built by ATK. They generate 109,000 pounds of thrust each.

The second stage features a restartable Aerojet Rocketdyne AJ10-118K engine burning Aerozine 50, which is a mixture of hydrazine and dimethyl hydrazine, reacting with nitrogen tetroxide as an oxidizer. The total thrust is estimated at a 9,750 pounds.

The Delta 2 stands 126.6 feet tall and weighs 165 tons at liftoff.

T-minus 15 minutes and holding. Clocks have entered the first of two planned hold periods during the final portion of the Terminal Countdown. This pause will last 20 minutes in duration.

These holds are designed to give the launch team a chance to deal with any problems and catch up on work that could be running behind schedule.

Now moving into data link checks to verify good telemetry connections.

The first stage engine steering checks are complete.

The second stage engine slews just finished. First stage tests are starting.

The launch team is beginning the "slew" or steering checks of the first and second stage engines. These are gimbal tests of the nozzles on the first stage main engine and twin vernier engines and second stage engine to ensure the rocket will be able to steer itself during launch.

Sixty minutes to launch of the OCO 2 observatory. This morning's liftoff is timed for OCO to just the so-called A-Train constellation of Earth observatories. The spacecraft will head south and enter into a sun-synchronous polar orbit. That limits the available launch window to just 30 seconds each day.

About 15.5 minutes pass between the time the first satellite -- OCO 2 -- and the last -- Aura -- pass the equator in the A-Train. The Orbiting Carbon Observatory-2 precedes GCOM-W1 by 185 seconds, and precedes Aqua, the anchor satellite in the A-Train, by an average of 444.5 seconds.

This nominal separation in satellite spacing between OCO-2 and GCOM-W1 is much greater than that between Aqua, CloudSat and CALIPSO, allowing for looser control and higher margins of safety while still supporting coordinated measurements.

Checks of the rocket's safety system have been accomplished in the past few minutes of the countdown.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The first stage liquid oxygen tank has reached the 95 percent full level. The "rapid load" valve was closed, with the slower "fine load" phase continuing to fill the tank.

Liftoff is just 90 minutes away. Everything is going smoothly for today's launch of the Delta 2 rocket with OCO 2.

The mission will contribute to a number of additional scientific investigations related to the global carbon cycle. These include:

• The dynamics of how the ocean exchanges carbon with the atmosphere
• The seasonal dynamics of northern hemisphere terrestrial ecosystems in Eurasia and North America
• The exchange of carbon between the atmosphere and tropical ecosystems due to plant growth, respiration and fires
• The movement of fossil fuel plumes across North America, Europe and Asia
• The effect of weather fronts, storms and hurricanes on the exchange of carbon dioxide between different geographic and ecological regions
• The mixing of atmospheric gases across hemi- spheres

Ten minutes into the flow. LOX loading is an approximate 25-minute process to fill the first stage liquid oxygen tank.

Cryogenic liquid oxygen, chilled to Minus-298 degrees F, has started flowing from a 28,000-gallon storage tank at Space Launch Complex 2, through plumbing and into the bottom of the Delta 2 rocket. The LOX will be consumed by the first stage main engine during the first four-and-a-half minutes of flight along with the 10,000 gallons of RP-1 kerosene already loaded aboard the vehicle.

The launch team is performing the preparatory steps for filling the liquid oxygen tank.

The official "go" has been given to start first stage liquid oxygen loading right on time this morning.

NASA launch director Tim Dunn has completed a poll to confirm the agency management team is "go" for liquid oxygen loading. No technical problems are being reported in the countdown.

All weather rules are GO for launch, albeit it foggy.. The forecast calls for stratus clouds causing a ceiling at 200 feet and visibilty of 1 to 2 miles with fog.

The countdown is proceeding smoothly and no issues are being worked, NASA says.

Just two-and-a-half hours from launch.

The OCO 2 observatory stands 7 feet by 3 feet tall, has a wingspan of 29 feet and weighs 999 pounds, of which 99 pounds is hydrazine fuel and 288 pounds is the three-channel grating spectrometer science instrument.

Typically at this point in a Delta 2 rocket countdown, the launch team loads 10,000 gallons of highly refined kerosene fuel into the first stage. However, this operation was completed Tuesday afternoon prior to retraction of the mobile service tower.

The kerosene, called RP-1, will be consumed along with liquid oxygen by the first stage RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight.

Filling of the stage with cryogenic liquid oxygen will begin about an hour from now.

COUNTDOWN BEGINS! The three-hour Terminal Countdown sequence has been initiated for today's launch of the Delta 2 rocket from Vandenberg Air Force Base in California. The booster will carry into orbit the OCO 2 climate observatory.

Liftoff from the Space Launch Complex 2 pad is scheduled for 2:56:23 a.m. local (5:56:23 a.m. EDT; 0956:23 GMT). Today's available launch window is open for 30 seconds in duration.

Between now and the launch time, the rocket's guidance system will be activated, onboard helium and nitrogen tanks brought up to pressure, the liquid oxygen supply loaded into the first stage, checks performed on the safety system and steering tests conducted on the engines.

This is a standard countdown for the Delta 2, a typical timeline to ready the vehicle for flight. Pre-planned holds are built into the countdown at the T-minus 15 and T-minus 4 minute points. The first will last 20 minutes; the second will be 10 minutes in length. Those pauses are designed to give the launch team the opportunity to work problems or catch up on items perhaps running behind schedule.

Polls of the various launch team members and managers occur during that T-minus 4 minute hold to ensure everything is ready to proceed into the busy final minutes that see the rocket put on internal power, pressurized and armed for liftoff from California's Central Coast.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

"Man stations for Terminal Count." That's the call to the launch team in preparation for a readiness check to ensure all consoles are "go" for starting the countdown 15 minutes from now.

T-minus 150 minutes and holding. At this time, the countdown is entering a planned 60-minute built-in hold. The Terminal Countdown begins once the hold concludes.

Launch remains targeted for 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT) from Vandenberg Air Force Base in California.

Ready for the second launch attempt, the Delta 2 rocket has been revealed from the retracting mobile servicing gantry at Vandenberg's Space Launch Complex 2 on the California coast.

Rollback began at 5:30 p.m.

The 177-foot tall service tower was used to stack the two-stage vehicle, the three strap-on solid rocket motors and the OCO 2 payload atop the pad's launch mount. The tower also provided the primary weather protection and worker access to the rocket during its stay at the oceanside complex on North Vandenberg.

Ground teams will spend the next few of hours getting the pad configured and secured in advance of the Terminal Countdown. Launch remains targeted for 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT).

A reminder that if you will be away from your computer but would like to receive occasional countdown updates in the morning, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

NASA photos of the Delta 2 rocket on the launch pad:

See our photo gallery.

A faulty valve in the sound suppression water system has been removed and replaced, clearing the way for another launch attempt of the Delta 2 rocket Wednesday at 2:56 a.m. local (5:56 a.m. EDT).

Tests of the water deluge were underway early this afternoon at Vandenberg Air Force Base's Space Launch Complex 2 launch pad.

"The launch team has completed troubleshooting of the launch pad water suppression system that resulted in the scrub of the launch attempt Tuesday. A valve that is part of the pulse suppression water system, which had operated properly during tests shortly before the launch countdown, failed to function properly during the final minutes of the launch attempt. The failed valve has been replaced with a spare, and the system is being tested in preparation for Wednesday's launch attempt," NASA said in a statement.

The weather forecast calls for a ceiling at 600 feet, visibility of 1 to 2 miles in fog and drizzle, northwesterly winds of 8 to 12 knots and a temperature in the mid 50s F.

NASA launch director Tim Dunn says technicians won't know what exactly caused this morning's aborted countdown until they are able to reach the launch pad later today.

Ground crews are waiting for the Delta 2 rocket to be drained of liquid oxygen and kerosene propellants, then they will enter the pad to troubleshoot the problem with the launch pad's water deluge system.

Dunn said an engineer responsible for the water system could not verify that the deluge system was operating normally, resulting in a mandatory hold in the countdown at about T-minus 45 seconds.

"What that deluge system does is it protects the launch mount from the high temperatures of the launch, and it gives some amount of suppression from that huge shockwave from the ignition of the engine," Dunn said.

"It's a bit of a disappointment for the launch team when you have a great countdown up to that point," Dunn said. "However, these are things we prepare for. We're a professional team. We know how to handle this."

Depending on the specific issue that caused the abort, officials say the next launch attempt could occur as soon as Wednesday, July 2.

"We'll have a little more information later, once we're able to evaluate the system properly," Dunn said. "Right now, we've got the team leaning forward ... and preparing for a launch attempt on July 2."

But if the problem requires more extensive repairs, launch could be delayed longer.

Liquid oxygen is being offloaded from the Delta rocket's first stage at this time.

The launch pad water system helps dampen the acoustic pressure of liftoff, protecting the flame duct from damage when the Delta 2 rocket soars skyward.

Officials have directed the launch team to set up for another launch attempt as soon as Wednesday morning, but there has been no decision when to proceed with another countdown. Engineers first want to understand what caused today's abort.

The OCO 2 spacecraft is back on external power. A member of the launch team called a hold less than a minute before liftoff due to a problem with the launch pad's sound suppression water flow system.

The launch team is going through procedures to safe the Delta 2 launch vehicle. The short 30-second duration of the launch window means liftoff will not occur today.

HOLD. The countdown has stopped at T-minus 46 seconds due to an issue with the launch pad water system.

T-minus 1 minute. All remains "go" for launch.

T-minus 80 seconds. First stage liquid oxygen topping to 100 percent is underway.

T-minus 1 minute, 45 seconds. The launch pad water suppression system is being activated.

T-minus 2 minutes. The first stage liquid oxygen vents are now being closed so the LOX tank can be pressurized for launch. Puffs of vapor from a relief valve on the rocket will be seen in the remainder of the countdown as the tank pressure stabilizes.

T-minus 2 minutes, 30 seconds. The OCO 2 payload has been declared "go" for launch.

T-minus 2 minutes, 45 seconds. Vehicle ordnance is being armed.

T-minus 3 minutes and counting down to the Delta rocket launch dedicated to deploying the OCO 2 spacecraft to map carbon dioxide from space.

T-minus 3 minutes, 45 seconds and counting. The Delta 2 rocket's systems are now transferring to internal power for launch. And the launch pad water system is being enabled.

T-minus 4 minutes and counting! Clocks are ticking down the final moments for liftoff of Delta 367 with the OCO 2 spacecraft. Launch is set for 2:56:44 a.m. local time from California's Central Coast.

Now five minutes from launch! The "go" has been given for release of the hold in one minute.

Officials report the OCO 2 spacecraft is configured for launch.

All systems are "go" for an on-time launch today!

The launch team is being polled for the final time before liftoff to confirm systems are ready for the Delta 2 rocket and OCO 2 spacecraft.

The OCO 2 spacecraft is now operating on internal power.

Now 10 minutes to go. The rocket's launch opportunity today is 30 seconds long beginning at exactly 2:56:44 a.m. local (5:56:44 a.m. EDT; 0956:44 GMT). The window is dictated by the OCO 2 satellite mounted atop the 12-story booster, and launching at the time ensures optimum insertion of the payload into the desired orbit from the Vandenberg Air Force Base pad.

NASA launch director Tim Dunn has polled his advisory team and the space agency is "go" to continue with the countdown.

Upper level winds and ground weather are reported acceptable for flight.

T-minus 4 minutes and holding. The countdown has entered the final planned built-in hold. This is a scheduled 10-minute pause leading to today's liftoff time of 2:56:44 a.m. local (5:56:44 a.m. EDT; 0956:44 GMT) for the Delta 2 rocket.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The first stage kerosene fuel tank is being pressurized for flight.

Twenty minutes from launch. The OCO 2 satellite will become the 228th primary payload put into space by the venerable Delta 2 rocket over the past 25 years. Here's a look at some other stats about today's mission. This will be:

• The 367th Delta rocket launch since 1960
• The 234th Delta launch with NASA involvement
• The 152nd Delta 2 rocket mission since 1989
• The 11th Delta 2 to fly in the 7320 configuration
• The 51st Delta 2 mission overseen by NASA
• The 42nd Delta 2 rocket launch from Vandenberg AFB
• The 3rd launch of the Delta family in 2014
• The 7th United Launch Alliance flight this year
• The 84th ULA launch overall

T-minus 15 minutes and counting. Countdown clocks are running again following the planned 20-minute hold. The count will continue to the T-minus 4 minute mark where another hold is scheduled. Launch remains set to occur at precisely 2:56:44 a.m. local (5:56:44 a.m. EDT; 0956:44 GMT).

A poll of the launch team confirms everyone is ready to press ahead with the countdown after this pre-planned 20-minute hold.

Now just 40 minutes away from blastoff. The Delta 2 rocket launching OCO 2 is known as the 7320-10 model. It has a standard first stage with three solid rocket boosters, a hypergolic second stage, no third stage and a 10-foot-diameter payload shroud.

The first stage is fueled by RP-1, a highly refined kerosene, and supercold liquid oxygen. The RS-27A main engine, made by Aerojet Rocketdyne, generates about 237,000 pounds of thrust.

The strap-on solids are known as Graphite Epoxy Motors built by ATK. They generate 109,000 pounds of thrust each.

The second stage features a restartable Aerojet Rocketdyne AJ10-118K engine burning Aerozine 50, which is a mixture of hydrazine and dimethyl hydrazine, reacting with nitrogen tetroxide as an oxidizer. The total thrust is estimated at a 9,750 pounds.

The Delta 2 stands 126.6 feet tall and weighs 165 tons at liftoff.

T-minus 15 minutes and holding. Clocks have entered the first of two planned hold periods during the final portion of the Terminal Countdown. This pause will last 20 minutes in duration.

These holds are designed to give the launch team a chance to deal with any problems and catch up on work that could be running behind schedule.

Now moving into data link checks to verify good telemetry connections.

The first stage engine steering checks are complete.

The second stage engine slews just finished. First stage tests are starting.

The launch team is beginning the "slew" or steering checks of the first and second stage engines. These are gimbal tests of the nozzles on the first stage main engine and twin vernier engines and second stage engine to ensure the rocket will be able to steer itself during launch.

Sixty minutes to launch of the OCO 2 observatory. This morning's liftoff is timed for OCO to just the so-called A-Train constellation of Earth observatories. The spacecraft will head south and enter into a sun-synchronous polar orbit. That limits the available launch window to just 30 seconds each day.

About 15.5 minutes pass between the time the first satellite -- OCO 2 -- and the last -- Aura -- pass the equator in the A-Train. The Orbiting Carbon Observatory-2 precedes GCOM-W1 by 185 seconds, and precedes Aqua, the anchor satellite in the A-Train, by an average of 444.5 seconds.

This nominal separation in satellite spacing between OCO 2 and GCOM-W1 is much greater than that between Aqua, CloudSat and CALIPSO, allowing for looser control and higher margins of safety while still supporting coordinated measurements.

Checks of the rocket's safety system have been accomplished in the past few minutes of the countdown.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

Loading of the Delta 2 rocket's first stage liquid oxygen tank has been accomplished. The process took 26 minutes today. The tank will be replenished through the countdown to replace the super-cold liquid oxygen that naturally boils away.

And now the Delta 2 rocket stands fully fueled its launch of the OCO 2 spacecraft at 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT). The vehicle's first stage was successfully loaded with RP-1 kerosene fuel earlier in the day. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels last week. And the three strap-on booster rockets are solid-propellant.

The first stage liquid oxygen tank has reached the 95 percent full level. The "rapid load" valve was closed, with the slower "fine load" phase continuing to fill the tank.

Liftoff is just 90 minutes away. Everything is going smoothly for today's launch of the Delta 2 rocket with OCO 2.

The mission will contribute to a number of additional scientific investigations related to the global carbon cycle. These include:

• The dynamics of how the ocean exchanges carbon with the atmosphere
• The seasonal dynamics of northern hemisphere terrestrial ecosystems in Eurasia and North America
• The exchange of carbon between the atmosphere and tropical ecosystems due to plant growth, respiration and fires
• The movement of fossil fuel plumes across North America, Europe and Asia
• The effect of weather fronts, storms and hurricanes on the exchange of carbon dioxide between different geographic and ecological regions
• The mixing of atmospheric gases across hemi- spheres

Ten minutes into the flow. LOX loading is an approximate 25-minute process to fill the first stage liquid oxygen tank.

Cryogenic liquid oxygen, chilled to Minus-298 degrees F, has started flowing from a 28,000-gallon storage tank at Space Launch Complex 2, through plumbing and into the bottom of the Delta 2 rocket. The LOX will be consumed by the first stage main engine during the first four-and-a-half minutes of flight along with the 10,000 gallons of RP-1 kerosene already loaded aboard the vehicle.

The launch team is performing the preparatory steps for filling the liquid oxygen tank.

The official "go" has been given to start first stage liquid oxygen loading right on time this morning.

NASA launch director Tim Dunn has completed a poll to confirm the agency management team is "go" for liquid oxygen loading. No technical problems are being reported in the countdown.

The weather outlook continues to be acceptable but very foggy for launch. The forecast calls for stratus clouds causing a ceiling at 300 feet and visibilty down to a half-mile to a mile with fog.

The countdown is proceeding smoothly and no issues are being worked, NASA says.

Just two-and-a-half hours from launch.

The OCO 2 observatory stands 7 feet by 3 feet tall, has a wingspan of 29 feet and weighs 999 pounds, of which 99 pounds is hydrazine fuel and 288 pounds is the three-channel grating spectrometer science instrument.

Typically at this point in a Delta 2 rocket countdown, the launch team loads 10,000 gallons of highly refined kerosene fuel into the first stage. However, this operation was completed Tuesday afternoon prior to retraction of the mobile service tower.

The kerosene, called RP-1, will be consumed along with liquid oxygen by the first stage RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight.

Filling of the stage with cryogenic liquid oxygen will begin about an hour from now.

COUNTDOWN BEGINS! The three-hour Terminal Countdown sequence has been initiated for today's launch of the Delta 2 rocket from Vandenberg Air Force Base in California. The booster will carry into orbit the OCO 2 climate observatory.

Liftoff from the Space Launch Complex 2 pad is scheduled for 2:56:44 a.m. local (5:56:44 a.m. EDT; 0956:44 GMT). Today's available launch window is open for 30 seconds in duration.

Between now and the launch time, the rocket's guidance system will be activated, onboard helium and nitrogen tanks brought up to pressure, the liquid oxygen supply loaded into the first stage, checks performed on the safety system and steering tests conducted on the engines.

This is a standard countdown for the Delta 2, a typical timeline to ready the vehicle for flight. Pre-planned holds are built into the countdown at the T-minus 15 and T-minus 4 minute points. The first will last 20 minutes; the second will be 10 minutes in length. Those pauses are designed to give the launch team the opportunity to work problems or catch up on items perhaps running behind schedule.

Polls of the various launch team members and managers occur during that T-minus 4 minute hold to ensure everything is ready to proceed into the busy final minutes that see the rocket put on internal power, pressurized and armed for liftoff from California's Central Coast.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

"Man stations for Terminal Count." That's the call to the launch team in preparation for a readiness check to ensure all consoles are "go" for starting the countdown 15 minutes from now.

T-minus 150 minutes and holding. At this time, the countdown is entering a planned 60-minute built-in hold. The Terminal Countdown begins once the hold concludes.

Launch remains targeted for 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT) from Vandenberg Air Force Base in California.

Photos from tonight's mobile service gantry rollback:

See our photo gallery.

The Delta 2 rocket has been unveiled from the mobile servicing gantry at Vandenberg's Space Launch Complex 2 on the California coast.

A crowd of news media and VIPs watched over the past few minutes as the cocoon-like tower was wheeled away to get activities underway for tonight's launch countdown.

Rollback began just after 8 p.m. and was completed about 20 minutes later.

The 177-foot tall service tower was used to stack the two-stage vehicle, the three strap-on solid rocket motors and the OCO 2 payload atop the pad's launch mount. The tower also provided the primary weather protection and worker access to the rocket during its stay at the oceanside complex on North Vandenberg.

Ground teams will spend the next few of hours getting the pad configured and secured in advance of the Terminal Countdown. Launch remains targeted for 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT).

A reminder that if you will be away from your computer but would like to receive occasional countdown updates in the morning, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

Good afternoon from just outside the perimeter of Space Launch Complex 2. Reporters and photographers are gathering to head inside the pad to cover the gantry rollback event, which will reveal the Delta 2 rocket for flight. But with no cellphones allowed within the pad, we can't post an update when the tower begins to move. But will give a full status and photo gallery as quickly as possible afterwards.

Photographers for United Launch Alliance, the LA Daily News and Spaceflight Now set up their sound-activated cameras around Vandenberg Air Force Base's Space Launch Complex 2 pad to cover the Delta 2 rocket blasting off with the OCO 2 climate observatory.

See our photo gallery.

The official weather outlook issued before going into the countdown still predicts no chance of violating any of the launch rules for Tuesday's predawn blastoff.

"High pressure will dominate the region through Monday evening with a weak upper level feature helping to destabilize the typical marine layer. For T-0, patchy fog and stratus are forecast along with thin cirrus associated with the weak upper level feature. Visibility will drop to 1 to 2 miles in fog. Surface winds will be from the northwest at 5-8 knots. Temperatures will drop to 50F to 55F.

Watch this page for live coverage overnight, plus live streaming video!

The launch team is resting at this hour before the countdown operations kick off later today. Liftoff of the Delta 2 rocket remains on schedule for tomorrow morning at 2:56 a.m. local time (5:56 a.m. EDT).

It will be the first Delta 2 to fly in two-and-a-half years. The matching of ready payloads with the rocket meant no launches until now.

"I do dearly love this rocket," said NASA launch director Tim Dunn. "So when it appeared three years ago we had flown our final manifested mission with NPP from Vandenberg in the fall of 2011, it was a bit of a sad time for me. But I did ULA had five whitetail Delta 2s that they had manufactured. I knew there was some hope for the future. Our agency did take advantage of those whitetails and have since procured four of them. Knowing that we now have four Delta 2 launches ahead us, I'm thrilled."

The OCO 2 observatory will be delivered to orbit and deployed 56 mintes and 15 seconds after launch. This reflight comes five years after the loss of the original OCO spacecraft.

"It's absolutely fantastic to get another opportunity to actually do these incredibly important scientific measurements. It's been a long, hard road but boy I am glad to be back," said David Crisp, the OCO 2 science team leader

"We've done everything humanly possible...to ensure a safe and successful launch. In addition to that, the launch vehicle that we are riding this time -- the Delta 2 -- is the most reliable launch vehicle in NASA's fleet. I am honored that anybody would consider flying such a small small science experiment on such a highly reliable vehicle.

"It's been a long run up to the starting line of this race. But we are ready to start now."

The Orbiting Carbon Observatory No. 2 to gauge man's impact on climate change and its workhorse booster have been cleared for launch Tuesday morning from California.

The Launch Readiness Review occurred today to verify the OCO 2 spacecraft and United Launch Alliance Delta 2 rocket are standing in perfect shape for blastoff at precisely 2:56:44 a.m. local (5:56:44 a.m. EDT; 0956:44 GMT) from Vandenberg Air Force Base.

"This afternoon we held a successful Launch Readiness Review where we received approval from senior NASA and ULA management, as well as spacecraft and range agencies to proceed into (the) launch countdown tomorrow night," said Tim Dunn, the NASA launch director.

"In summary, the rocket is ready and the launch team is prepared and excited to be back in business with Delta 2 -- and poised to launch this important spacecraft for our nation."

"The Orbiting Carbon Observatory 2, we call it OCO 2, will join our 17 operational Earth science missions in studying the Earth as a system when it launches from the Vandenberg Air Force Base in California on July 1," said Betsy Edwards, OCO 2 program executive with the Science Mission Directorate at NASA Headquarters in Washington.

"OCO 2, in particular, will measure global concentrations of carbon dioxide and watch the Earth breathe as we measure the greenhouse gas that drives climate change."

Out at Space Launch Complex 2 today, pre-flight testing of the Delta 2 vehicle was being wrapped up ahead of the countdown operations.

The C-band link used for tracking the rocket as it flies downrange and the command receiver decoders in the vehicle's safety system were checked with the Western Range.

Technicians also performed a final azimuth alignment for the rocket's RIFCA guidance computer and ran steering checks on the rocket's first and second stage engine nozzles.

With today's LRR giving the okay to continue with preparations and enter into the actual countdown, the team plans to load highly refined kerosene fuel into the first stage Monday afternoon, followed by procedures to ready the mobile service gantry for retraction from around the 12-story rocket. The tower rollback is anticipated at approximately 4:10 p.m. local time (7:10 p.m. EDT).

The Terminal Countdown will start at 11:56 p.m. local time (2:56 a.m. EDT), beginning the formal three-hour process to bring the rocket to life, load the liquid oxygen for the main engine at 1:11 a.m., conduct extensive final testing and then send it into orbit.

Tuesday's launch opportunity extends 30 seconds to ensure OCO 2 reaches the desired orbit. The window closes at 2:57:14 a.m. local (5:57:14 a.m. EDT; 0957:14 GMT)

Watch this page for live updates and streaming video during the countdown and the trek to orbit aboard the Delta rocket.

And if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

And check out Spaceflight Now's Facebook page!

The pre-launch news conference is coming up in a few minutes from Vandenberg Air Force Base. You can watch it live in our streaming video.

The Launch Readiness Review has concluded and given the "go" to continue with Tuesday's liftoff plans. Today's meeting examined the status of the Delta rocket, the OCO 2 spacecraft, the network of ground support and the weather forecast. The review culminated with official consensus to press ahead with countdown operations starting Monday afternoon.

Join us on this page for live streaming video of the pre-launch news conference and payload briefings from Vandenberg starting at 4 p.m. local (7 p.m. EDT; 2300 GMT) today on this page.

Meteorologists have issued their weather forecast for Tuesday's launch opportunity. And that outlook is favorable, albeit foggy, with a 100 percent chance of acceptable conditions.

The forecast calls for only a deck of stratus at 500 feet with tops at 900 feet, some high cirrus at 31,000 feet, visibility of 1 to 2 miles in fog, northwesterly winds of 5 to 8 knots and a temperature between 50 and 54 degrees F.

If the launch slips to Wednesday for some reason, meteorologists say the ideal conditions will continue and the fog will be less prevalent.

Mission officials will convene the Launch Readiness Review today at Vandenberg Air Force Base to confirm all systems are set to enter into the countdown for Friday's early morning liftoff.

After the LRR concludes, The pre-launch press conference is planned for 4 p.m. local (7 p.m. EDT; 2300 GMT) with the NASA launch director, ULA's program manager, OCO 2 officials and the weather officer.

That briefing will be followed by a look at the OCO 2 science objectives.

NASA's first spacecraft dedicated to measuring carbon dioxide in Earth's atmosphere, a critical new data-gathering project for scientists and policy-makers, is awaiting launch Tuesday morning

Read our full story.

A quarter-century since its first launch, preparations are underway to end a hiatus and return next Tuesday to the business of launching Delta 2 rockets, if only for a little while longer.

Read our full story.

The United Launch Alliance Delta 2 rocket first stage, solid rocket boosters, interstage, second stage, payload and nose cone are systematically installed on the launch pad for the Orbiting Carbon Observatory 2.

See a photo gallery.

Meanwhile, the the OCO 2 spacecraft batteries are being charged today and tomorrow the observatory will be powered for a final health check. The Delta 2 rocket's second stage has been fueled and there are no technical issues being worked.

Technicians are finishing the second stage fueling operations today by loading the Aerozine 50 hydrazine into the Delta 2 rocket as activities remain on track for liftoff next Tuesday.

Meanwhile, members of the launch and management teams held a countdown dress rehearsal Wednesday to practice countdown scripts, problem solving and procedures.

"The Delta II launch team of United Launch Alliance and Launch Services Program along with the OCO-2 observatory test team of JPL and Orbital Sciences took part in the exercise," NASA said in a statement.

Storable hypergolic propellants are being loaded into the Delta 2 rocket's second stage today and tomorrow in preparation for Tuesday's predawn blastoff.

Technicians clad in protective suits at Vandenberg Air Force Base's Space Launch Complex 2 began filling the stage with nitrogen tetroxide oxidizer this morning. The hydrazine propellant mixture will be placed aboard tomorrow.

The fuels will power the stage's Aerojet Rocketdyne-made main engine during the two firings needed to propel the OCO 2 satellite into the proper polar orbit.

Officials do not take the decision to start loading the rocket lightly. Commencing the second stage fueling is a major milestone in preparing for launch, one that starts a "clock" of 37 days for the rocket.

That "clock" is based on how long the stage remains certified to fly after the storable propellants begin flowing into the vehicle. Exposure to the fuels limits the lifetime for seals and other parts of the stage.

Liftoff remains on target for 2:56 a.m. local (5:56 a.m. EDT; 0956 GMT) Tuesday morning at the opening of a 30-second launch window.

The Delta 2 rocket and the OCO 2 spacecraft passed the Flight Readiness Review today that assessed the progress of work and granted approval to proceed with loading the storable hypergolics into the second stage.

A NASA spokesperson says there are no outstanding issues from the FRR and the "go" was given for second stage fueling starting tomorrow as planned.

"No significant engineering issues were identified during today's FRR and the OCO 2 observatory was reported to be ready for launch," NASA said in a statement.

"The early preliminary weather outlook at this time, while there were no percentages given, is generally favorable for launch."

Installation of the Delta 2 rocket's nose cone was completed over the weekend in the cleanroom of Vandenberg's Space Launch Complex 2 gantry.

The two halves of the 10-foot-diameter composite fairing will protect the OCO 2 spacecraft during the climb through Earth's atmosphere in the initial five minutes of flight. It is jettisoned in two halves just after ignition of the second stage at altitude of 82 miles while traveling over 11,300 miles per hour.

The spacecraft stands 7 feet tall and weighs 999 pounds.

The Flight Readiness Review is scheduled for Tuesday and the filling of the second stage with storable hypergolic propellants will see the nitrogen tetroxide oxidizer loaded aboard Wednesday and the hydrazine fuel mixture on Thursday.

The Launch Readiness Review will be held on Sunday to grant approval to enter into the countdown operations on the afternoon of next Monday for the middle-of-the-night liftoff on Tuesday, July 1.

Satellite closeouts are being performed today on the Orbiting Carbon Observatory 2 to button up the craft ahead of tomorrow's installation of the aerodynamic nose cone of the Delta 2 rocket.

The shroud will shield the observatory during the climb through the atmosphere and jettison five minutes into flight at altitude of 82 miles.

This week's activities also saw the rocket's ordnance installed, a task that took Wednesday and Thursday to complete.

The Integrated Systems Test for the United Launch Alliance Delta 2 rocket and the OCO 2 spacecraft was run Tuesday to confirm the combined booster and payload are ready for their departure from Earth.

The thorough electrical test simulated the countdown and the rocket's trek from liftoff through spacecraft deployment nearly an hour into flight. Officials reported all went well.

The final two weeks of pre-flight preparations are underway for the OCO 2 spacecraft and Delta 2 rocket.

The Integrated Systems Test is being performed today between the satellite and rocket. The test is a combined electrical and mechanical test for the spacecraft and launch vehicle.

Installation of the booster's nose cone is planned for Friday.

"Prelaunch activities continue on schedule for a launch at 2:56:44 a.m. PDT on July 1 at the opening of a 30-second launch window,: NASA says.

NASA's Orbiting Carbon Observatory, a reflight of the spacecraft by the same name, was mounted atop its Delta 2 rocket booster at Vandenberg Air Force Base in California on Saturday.

Replacing the observatory lost in a 2009 launch failure, OCO No. 2 seeks to understand how the Earth breaths and map global carbon dioxide levels.

The craft was hauled to Space Launch Complex 2 early Saturday morning for hoisting aboard its rocket, the United Launch Alliance Delta 2 that returns to action after a two-year hiatus.

Read our full story.

A NASA satellite dedicated to monitoring carbon dioxide in Earth's atmosphere arrived at Vandenberg Air Force Base in California on Wednesday, ready to begin a two-month launch campaign leading to liftoff July 1 on a Delta 2 rocket.

The refrigerator-sized satellite was shipped via truck convoy from an Orbital Sciences Corp. factory in Gilbert, Ariz.

By 6 a.m. PDT (9 a.m. EDT; 1300 GMT) Wednesday, the spacecraft was at Vandenberg's front gate on California's Central Coast between Los Angeles and San Francisco, according to Ralph Basilio, project manager for the Orbiting Carbon Observatory 2 mission at NASA's Jet Propulsion Laboratory.

Read our full story.