WASHINGTON — NASA hopes to get two long-delayed space science missions launched in the first few months of 2019, one of which will go on one of two back-to-back Falcon Heavy missions.

At a meeting Dec. 18 of NASA’s Heliophysics Advisory Committee here, Nicky Fox, director of the agency’s heliophysics division, said the Space Environment Testbed payload is now scheduled for launch no earlier than April as part of the U.S. Air Force’s Space Test Program 2 mission, launching on a Falcon Heavy rocket.

That long-delayed mission includes a number of satellites, including the Demonstration and Science Experiments (DSX) satellite from the Air Force Research Laboratory, which hosts the Space Environment Testbed. That testbed carries several experiments to study the space radiation environment and how it affects spacecraft electronics.

The other satellites on the vehicle include some from NASA, such as the Green Propellant Infusion Mission, as well as from the Air Force and private organizations, including the LightSail-2 solar sail demonstrator developed by The Planetary Society.

Fox said the mission will launch on the second of two back-to-back launches of SpaceX’s Falcon Heavy rocket, after the launch of the Arabsat-6A spacecraft. “It will launch after the successful launch of Arabsat, which is currently manifested for March,” she said.

The two launches, she said, will use the same set of first stage booster cores. “They will recover and reuse the boosters,” she said, with the second launch taking place about a month after the first. “So we’re kind of watching what happens with that first launch.” SpaceX didn’t comment on its manifest of upcoming Falcon Heavy launches.

Those Falcon Heavy launches will be the first for the heavy-lift rocket since its inaugural launch in February. That launch also made partial use of reflown hardware, with the two side boosters having previously been used as first stages on Falcon 9 missions.

That schedule, with launches in March and April, would fit into other demands for Launch Complex 39A, the pad used for Falcon Heavy launches as well as crewed Falcon 9 missions. The first commercial crew test flight, testing the Crew Dragon spacecraft but without a crew onboard, is currently scheduled for launch no earlier than Jan. 17. That will be followed by a crewed test flight as soon as June.

NASA is also hoping to launch in early 2019 the Ionospheric Connection Explorer, or ICON, satellite, a mission to study the interaction between terrestrial and space weather in the upper atmosphere. That mission has suffered months of delays because of problems with its launch vehicle, the Pegasus XL from Northrop Grumman Innovation Systems.

The investigation into those vehicle problems, which delayed a launch attempt in June and again in November, is ongoing. Fox said the issue is with the “fin rudder” of the launch vehicle, which delayed the most recent launch attempt Nov. 7. “They are continuing to look for root cause,” she said. “We are looking for a launch in early 2019, the first quarter of 2019.”