You can watch a US spy satellite launch on a giant Delta IV Heavy rocket tonight. Here's how.

Editor's note: United Launch Alliance successfully launched the Delta IV Heavy rocket carrying the classified NROL-44 satellite for the U.S. National Reconnaissance Office at 8:09 p.m. EST (0109 GMT). Read our full story here

CAPE CANAVERAL, Fla. — A new U.S. spy satellite will launch into space early Thursday (Dec. 10) on the mightiest rocket built by the United Launch Alliance (ULA): the massive Delta IV Heavy.

The booster is set to blast off overnight at 8 p.m. EST (0100 Dec. 11 GMT) from Pad 37 here at the Cape Canaveral Air Force Station to carry the classified NROL-44 satellite into orbit for the National Reconnaissance Office (NRO). You  can watch all the fiery action live online, courtesy of ULA. Launch coverage will begin about 20 minutes prior to liftoff, and you can watch the launch live here and on the homepage or directly via the ULA webcast.

Today's launch attempt will have a four-hour window in which to fly that opens at 5:50 p.m. to 10:30 p.m. EST (2250-0330 GMT), with ULA targeting liftoff for 6:15 p.m. It will carry a sophisticated satellite that will join the NRO's fleet already in orbit, which are designed to capture highly detailed imagery and other types of data, then pass that information on to intelligence agencies and military branches.

Related: Meet the Delta rocket family of the United Launch Alliance 

The mission has been plagued by multiple launch delays, as the flight was originally slated to take off back in August. Three of those attempts were scrubbed due to technical issues with pad infrastructure and the rocket itself. Since the last scrub on Sept. 30, teams have been busy making sure previous problem items, like the swing arm system at Launch Complex 37, are functioning properly and able to support a launch.

Issues with the launch pad infrastructure even triggered a rare pad abort on Aug. 29, in which the rocket ignited and then shut down just three seconds before liftoff. The Delta IV Heavy's three main engines are programmed to ignite in a staggered sequence, with the starboard engine lighting first. 

ULA pinpointed the cause of the abort in ground system equipment that controls the rocket's main engines. Three flow rate regulators, which are part of the launch pad's ground systems, control the helium gas system that spins up the turbines on the main engines. The starboard engine fired as expected, but the regulator for the center engine did not open, prompting an abort.  

ULA  president and CEO Tory Bruno said the root cause of the regulator problem was a torn diaphragm and that the company would replace each regulator out of an abundance of caution. Issues with the pad's swing arm prompted subsequent delays. 

Thursday's planned launch will mark the 12th flight of a Delta IV Heavy rocket since its debut in 2004 and is one of only five Delta IV rockets remaining.  ULA plans to retire the launcher before rolling out its next-generation vehicle, the Vulcan Centaur. (ULA previously retired the Delta II rocket in 2018 and its Delta IV Medium in 2019.) 

Shortly before the launch countdown begins, the 330-foot-tall (100 meters) shroud encasing the rocket — called the Mobile Service Tower, or MST — will roll away, exposing the colossal craft. Composed of three hydrogen-fueled first-stage common core boosters (which are strapped together) and a cryogenic second stage, the Delta IV Heavy stands 233 feet (71 meters) tall and measures approximately 53 feet (16 m) wide. 

The Delta IV Heavy is currently the most powerful rocket in ULA's fleet. Fueled by 465,000 gallons (1.76 million liters) of super-chilled liquid hydrogen and liquid oxygen, the mega launcher generates more than 2 million pounds of thrust. 

Related: Delta IV Heavy: Powerful launch vehicle

 Heavy Competition 

Of the previous 11 Delta IV Heavy missions, seven have carried NRO payloads. Some of the vehicle's other notable missions launched NASA's Orion capsule on an uncrewed test flight to Earth orbit in 2014 and the U.S. space agency's Parker Solar Probe in 2018 on a mission to study the sun. 

All five of the remaining Delta IV Heavy launches will support NRO missions. Three will launch from the Cape, including this one, and two others will launch from Vandenberg Air Force Base in California.

The massive NRO payloads are designed to be installed on top of their rockets (as opposed to horizontally), and due to their bulkiness, launching one is much like launching a school bus. For that reason, military officials say that the Delta IV Heavy is the only launcher on the market today that can accommodate their needs. 

But the Delta IV Heavy is not the only heavy-lift vehicle on the market. 

SpaceX also has a heavy-lifter — the Falcon Heavy — but it uses a horizontal technique to integrate its rockets and payloads. Additionally, the Delta IV Heavy's payload fairing (or nose cone) is larger than the one on the Falcon Heavy, allowing the Delta to better accommodate the massive NRO satellites. 

However, the Falcon Heavy rocket does have one advantage over the Delta: its overall cost. The Falcon Heavy can lift heavier cargo into space for substantially less than the Delta IV Heavy, as evidenced by a coveted launch contract worth $130 million — approximately half the price of a Delta — that was awarded to SpaceX in 2018 to launch a future military payload. 

In August, the Department of Defense announced that ULA and SpaceX will share launch duties for military launches through 2027. ULA was awarded 60% of the contracts, with SpaceX receiving the other 40%.

ULA will rely on its upcoming Vulcan Centaur rocket to launch those missions, while SpaceX will split the duties between Falcon 9 and Falcon Heavy. Estimated to come online next year, the Vulcan Centaur will have the same vertical integration capability as the Delta; however, to make it more competitive and to better accommodate military payloads, SpaceX plans to offer vertical integration capabilities in the future, as well as an extended fairing for its Falcon Heavy rocket. 

 Ready to Fly 

The Mobile Service Tower covering United Launch Alliance's Delta IV Heavy moves away ahead of the NROL-44 launch from Cape Canaveral Air Force Station, Florida on Dec. 10, 2020. (Image credit: United Launch Alliance)

The Delta IV Heavy officially received the go-ahead for launch after teams from ULA, the National Reconnaissance Office and the U.S. Space Force completed a launch readiness review on Wednesday. During the review the teams assessed the health of the vehicle, its payload and pad hardware, as well as  projected weather conditions, which are expected to be 90% favorable for liftoff.

The main concern for launch being the possibility of cumulus clouds over the launch site. There are two backup opportunities if need be, but the weather conditions deteriorate each day. 

"Leadership held the Delta IV Heavy Launch Readiness Review today and gave a unanimous 'go' for Thursday's countdown to liftoff," ULA tweeted Wednesday morning. 

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Amy Thompson
Contributing Writer

Amy Thompson is a Florida-based space and science journalist, who joined as a contributing writer in 2015. She's passionate about all things space and is a huge science and science-fiction geek. Star Wars is her favorite fandom, with that sassy little droid, R2D2 being her favorite. She studied science at the University of Florida, earning a degree in microbiology. Her work has also been published in Newsweek, VICE, Smithsonian, and many more. Now she chases rockets, writing about launches, commercial space, space station science, and everything in between.