WASHINGTON — After years of delays, a group of NASA technology demonstration payloads is finally set to lift off on a Falcon Heavy later this month.

The four NASA payloads are part of the overall Space Test Program (STP) 2 mission scheduled to launch on a Falcon Heavy from the Kennedy Space Center in Florida no earlier than June 24. The launch was previously scheduled for June 22, but officials with the U.S. Air Force, who is overseeing the STP-2 mission, said June 7 that it needed additional time to integrate the satellites flying on the mission.

That slip, though, is minor compared to the overall delays for STP-2. The Air Force procured the launch from SpaceX in 2012, planning for a launch in 2015. Issues wit the development of the Falcon Heavy, which made its first launch in February 2018, delayed STP-2 significantly. This launch will be the third for the heavy-lift rocket, after that inaugural flight and the Arabsat-6A launch in April.

“We’re excited that this flight is almost here,” said Jim Reuter, acting associate administrator for space transportation at NASA, during a June 10 media teleconference. “We’ve spent a lot of time developing these technologies, and we’ve been ready to fly some of these missions for a while as the entire STP-2 mission has been assembled.”

The four payloads include a mix of standalone satellites and payloads hosted on other STP-2 spacecraft. The Green Propellant Infusion Mission is a spacecraft that will demonstrate the use of a “green” propellant called AF-M315E that is safer to handle than hydrazine.

Christopher McLean, principal investigator for the mission at Ball Aerospace, said that the benefits of this propellant go beyond being environmentally friendly. “If I compare this to a standard monoprop hydrazine system, we have 50 percent more total impulse available, mainly due to the density,” he said.

NASA’s Deep Space Atomic Clock will fly as part of the Orbital Test Bed satellite, built by General Atomics. It will demonstrate an atomic clock far more precise than those flown on GPS satellites that could be used on future missions to aid in precise tracking. A spacecraft equipped with such a clock could determine its position simply by receiving signals from Earth.

“One-way tracking with a high-accuracy clock is a much more efficient and flexible way to track a spacecraft and it’s a better way to navigate deep space,” said Jill Seubert, deputy principal investigator for the Deep Space Atomic Clock at the Jet Propulsion Laboratory.

Another NASA hosted payload on STP-2 is the Space Environment Testbeds, which will be a part of the Air Force’s Demonstration and Science Experiments (DSX) spacecraft. The payload consists of four small technology demonstrations to study how spacecraft electronics respond to space radiation.

The test, said Nicola Fox, director of NASA’s heliophysics division, will examine how now just how the electronics hold up but also their margins. “You certainly don’t want to be launching a battleship if a dinghy will do,” she said. “We don’t want to be overdesigning.”

The fourth NASA payload on STP-2 is Enhanced Tandem Beacon Experiment, a pair of three-unit cubesats. The satellites will help scientists study the formation of bubbles in the ionosphere that can disrupt communications by transmitting precise signals at several frequencies that will be recorded on the ground. Similar beacons will be on the six COSMIC-2 GPS radio occultation satellites, also being launched on STP-2.

Those bubbles are linked to large-scale weather patterns in the lower atmosphere, said Richard Doe, payload program manager for the Enhanced Tandem Beacon Experiment at SRI International. “If we understand bubble growth and generation,” he said, “we can generally improve forecasting. If you can forecast, you can take some chances to mitigate their effects.”

Some of those payloads have been waiting to fly for an extended time, Reuter said. “As we had them in storage, on a periodic basis we made sure working,” he said. “We had a little more time to refine the operational phase, and we took advantage of that as well.”

Reuters said that NASA did consider alternative ways of flying those payloads given the extended STP-2 delays. “Ultimately we stayed where we were,” he said, “and we’re really excited now.”