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

Update for Sept. 29, 11:55 p.m. EDT: The United Launch Alliance has delayed the launch to Sept. 30 at 11:54 p.m. EDT, as a result of bad weather and a hydraulic leak in the ground system.

Original story: CAPE CANAVERAL, Fla. — A new U.S. spy satellite will launch into space early Saturday (Sept. 27) 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 12:10 a.m. EDT (0410  GMT) from Pad 37 here at the Cape Canaveral Air Force Station to carry the classified NROL-44 satellite into orbit for the  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 Space.com homepage or directly via the ULA webcast. 

The mission has been delayed nearly a month after a rare, last-second abort on the launch pad on Aug. 29. According to ULA, the launch window lasts about 94 minutes.

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This will be the third attempt to get NROL-44 off the ground. Hardware issues plagued the Delta IV Heavy''s previous two launch attempts, with a rare pad abort being called three seconds before liftoff on Aug. 29. The rocket''s three main engines are programmed to ignite in a staggered sequence, with the starboard engine lighting first. 

ULA pinpointed the cause to be with ground system equipment that controls the rocket's three 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 rocket''s three main engines. The starboard engine fired as expected, but the regulator for the center engine did not open, prompting an abort.  

Company 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. 

Saturday''s planned lift off marks the 12th flight of a Delta IV Heavy rocket since its debut in 2004 and will involve 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 m) 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. 

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Of the previous 11 Delta IV Heavy missions, seven 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 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. 

OnAug. 7, 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. 

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