Live coverage of the countdown and launch of a United Launch Alliance Atlas 5 rocket from pad 41 at Cape Canaveral Space Force Station in Florida. The mission will launch the commercial SES 20 and SES 21 communications satellites toward geosynchronous orbit. Text updates will appear automatically below. Follow us on Twitter.

A United Launch Alliance Atlas 5 rocket is counting down for launch Tuesday at Cape Canaveral on a dual spacecraft delivery mission for the commercial telecom satellite operator SES. The Atlas 5 is set to blast off at 5:36 p.m. EDT (2136 GMT) at the opening of a 45-minute launch window.

The SES 20 and 21 communications satellites are stacked one on top of the other inside the Atlas 5’s payload shroud, ready to begin 15-year missions beaming C-band television and radio programming across the United States. The mission will mark the first commercial Atlas 5 launch into geosynchronous orbit, requiring three burns of the rocket’s Centaur upper stage before deploying the SES 20 and 21 satellites more than six hours after liftoff.

Atlas 5 rockets have deployed payloads directly into orbits near geosynchronous altitude on four previous missions for the U.S. military. The maneuvers required to reach such a high-altitude circular orbit must take place over more than five hours, with an initial burn by the Centaur upper stage to reach low Earth orbit, and a second burn to reshape the orbit to an oval shape with a high point stretching more than 20,000 miles above Earth.

A third engine firing by the Centaur upper stage more than five-and-a-half hours into the mission will circularize the rocket’s orbit at an altitude of nearly 22,000 miles, close to the equator. The Centaur stage is one of the few upper stages in the world capable of placing two relatively large satellites close to geosynchronous orbit.

The launch Tuesday will be ULA’s seventh mission of the year, and the sixth using an Atlas 5 rocket. It will be the 96th launch of an Atlas 5 rocket overall since the vehicle’s first flight in August 2002. After Tuesday’s launch there are 20 Atlas 5s remaining in ULA’s inventory before the rocket is retired. ULA, a 50-50 joint venture between Boeing and Lockheed Martin, is developing the next-generation Vulcan Centaur rocket to replace the Atlas and Delta rocket families.

The SES 20 and 21 satellites are nearly identical, and were built by Boeing in El Segundo, California. Each satellite weighs a couple of tons, using all-electric propulsion for in-orbit maneuvers. They are built on Boeing’s 702SP satellite platform.

After separating from the Atlas 5’s Centaur upper stage, SES 20 and 21 will unfurl their solar panels and run through post-launch checkouts. They will use plasma thrusters to fine-tune their orbits and slide into their operating positions in geostationary orbit, where their velocities will match the rate of Earth’s rotation, giving the satellites fixed geographic coverage zones.

SES 20 will maneuver into a position along the equator at 103 degrees west longitude, while SES 21 will head for 131 degrees west longitude. Both satellites will provide C-band television and radio broadcast services over the United States. SES 20 and 21 are part of a program to redirect television communications services to a different part of the C-band spectrum, following the Federal Communications Commission’s decision in 2020 to clear 300 megahertz of spectrum for the roll-out of 5G mobile connectivity networks.

The FCC auctioned U.S. C-band spectrum — previously used for satellite-based video broadcast services to millions of customers — to 5G operators, which are paying satellite operators like SES through multibillion-dollar compensation agreements. SES and Intelsat, another large geostationary satellite operator, purchased new C-band broadcasting satellites to function in the narrower swath of spectrum.

In 2020, SES ordered six new C-band satellites, including a spare, and Intelsat procured seven C-band satellites. SES says the new C-band satellites will enable the broadcast of digital TV services to nearly 120 million homes in the United States.

ULA’s Atlas 5 rocket on the launch pad before liftoff with the SES 20 and SES 21 communications satellites. Credit: Michael Cain / Spaceflight Now / Coldlife Photography

SES launched their first new C-band satellite as part of the program in June on a SpaceX Falcon 9 rocket. Later this week, following the Atlas 5 launch for SES, SpaceX is scheduled to launch another pair of similar C-band television broadcast satellites from Cape Canaveral for Intelsat.

The countdown Tuesday will begin at 10:16 a.m. EDT (1416 GMT) with the power-up of the rocket, checks of the launcher’s guidance system, and preparations to start loading cryogenic propellants into the Atlas 5.

After a few hours of early countdown preparations, if everything is going as planned, ULA’s launch team at Cape Canaveral’s Atlas Spaceflight Operations Center will give the “go” for the start of cryogenic tanking of the Atlas 5 around 3 p.m. EDT (1900 GMT).

Nearly 66,000 gallons of liquid hydrogen and liquid oxygen will be loaded into the two-stage Atlas 5 rocket. The Centaur upper stage’s Aerojet Rocketdyne RL10 engine burns the hydrogen and oxygen propellant mix, and the Atlas first stage consumes liquid oxygen with 25,000 gallons room-temperature kerosene fuel, which was loaded into the rocket Monday, soon after ULA ground crews rolled the Atlas 5 the launch pad from the nearby Vertical Integration Facility.

Two built-in holds occurred in the countdown, one at T-minus 2 hours and another at T-minus 4 minutes, before the final four-minute terminal countdown sequence to prepare the Atlas 5 rocket liftoff.

The rocket’s propellant tanks will be pressurized, and the RD-180 engine will ignite at T-minus 2.7 seconds. After building up thrust on the main engine, the Atlas 5 sent the command to light three Northrop Grumman strap-on solid rocket boosters to power the launcher off pad 41 with nearly 2 million pounds of thrust.

The version of the Atlas 5 flying on the SES 20/21 mission is known as the “531” configuration, with the first number denoting the size of the payload fairing, the second number representing the number of solid rocket boosters, and the third digit the number of engines on the Centaur stage. The payload shroud on the SES 20/21 mission is 5.4 meters (17.7 feet) in diameter, built by Beyond Gravity — formerly called RUAG Space — at ULA’s rocket factory in Decatur, Alabama.

After liftoff, the 196-foot-tall (59.7-meter) Atlas 5 rocket, designated AV-099 for this mission, will head east from Cape Canaveral to target the mission’s geosynchronous injection orbit.

The Atlas 5 will surpass the speed of sound in 40 seconds, then shed its spent strap-on boosters at T+plus 1 minute, 57 seconds. The clamshell-like payload shroud on top of the Atlas 5 will jettison at T+plus 3 minutes, 23 seconds, once the rocket flies above the thick lower layers of the atmosphere.

The first stage’s RD-180 engine will fire until T+plus 4 minutes, 19 seconds. Six seconds later, the first stage will separate from the Atlas 5’s Centaur upper stage, which will ignite its RL10 engine at T+plus 4 minutes, 35 seconds.

Three RL10 engine burns are planned before the Atlas 5 releases the two SES satellites. SES 20, riding in the upper position of the dual-payload stack, will separate at T+plus 5 hours, 38 minutes. SES 21 will follow with deployment at T+plus 6 hours, 18 minutes.

The Atlas 5’s guidance computer aims to release the two spacecraft in a circular orbit roughly 21,750 miles (about 35,000 kilometers), just below geostationary altitude, at an inclination angle of 1.9 degrees to the equator.

SES, based in Luxembourg, says the precise positioning enabled by the Atlas 5 rocket’s Centaur upper stage will expedite the entry into service for the new satellites, which will raise their altitude a few hundred miles and maneuver directly over the equator using their own thrusters. In geostationary orbit orbit, each satellite’s velocity will be fixed with the rate of Earth’s rotation, giving the craft a constant view of the same part of the planet.

The satellite will also extend power-generating polar panels and complete in-orbit checkouts before entering commercial service.

ROCKET: Atlas 5 (AV-099)

MISSION: SES 20/21

PAYLOAD: SES 20 and 21 communications satellites

CUSTOMER: SES

LAUNCH SITE: SLC-41, Cape Canaveral Space Force Station, Florida

LAUNCH DATE: Oct. 4, 2022

LAUNCH WINDOW: 5:36-6:21 p.m. EDT (2236-2321 GMT)

WEATHER FORECAST: 90% chance of acceptable weather

BOOSTER RECOVERY: None

LAUNCH AZIMUTH: East

TARGET ORBIT:

• SES 20: Perigee of 21,741 miles (34,990 kilometers); Apogee of 21,748 miles (35,00 kilometers); Inclination angle of 1.91 degrees to the equator

• SES 21: Perigee of 21,601 miles (34,764 kilometers); Apogee of 21,750 miles (35,003 kilometers); Inclination angle of 1.92 degrees to the equator.

LAUNCH TIMELINE:

T-00:00:02.7: RD-180 ignition T+00:00:01.0: Liftoff T+00:00:05.6: Begin pitch/yaw maneuver T+00:00:40.2: Mach 1 T+00:00:58.7: Maximum aerodynamic pressure (Max-Q) T+00:01:57.7: Solid rocket booster jettison T+00:03:23.5: Payload fairing jettison T+00:04:19.7: Atlas booster engine cutoff (BECO) T+00:04:25.7: Atlas/Centaur stage separation T+00:04:35.7: Centaur first main engine start (MES-1) T+00:11:52.3: Centaur first main engine cutoff (MECO-1) T+00:23:51.2: Centaur second main engine start (MES-2) T+00:28:41.5: Centaur second main engine cutoff (MECO-2) T+05:33:38.6: Centaur third main engine start (MES-3) T+05:35:52.7: Centaur third main engine cutoff (MECO-3) T+05:38:41.7: SES 20 spacecraft separation T+06:18:25.7: SES 21 spacecraft separation

MISSION STATS:

678th launch for Atlas program since 1957 379th Atlas launch from Cape Canaveral 267th mission of a Centaur upper stage 244th use of Centaur by an Atlas rocket 515th production RL10 engine to be launched 3rd RL10C-1-1 engine launched 102nd flight of an RD-180 main engine 96th launch of an Atlas 5 since 2002 3rd SES use of an Atlas 5 20th-22nd GEM-63 solid rocket boosters flown 80th launch of an Atlas 5 from Cape Canaveral 6th Atlas 5 launch of 2022 139th Evolved Expendable Launch Vehicle flight 154th United Launch Alliance flight overall 88th Atlas 5 under United Launch Alliance 111th United Launch Alliance flight from Cape Canaveral 36th 500-series flight of the Atlas 5 5th Atlas 5 to fly in the 531 configuration 107th launch from Complex 41 80th Atlas 5 to use Complex 41 42nd orbital launch overall from Cape Canaveral in 2022

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