A replay of the United Launch Alliance Atlas 5 rocket launching Mexico’s Morelos 3 communications satellite from Cape Canaveral, Florida.

NASA managers Thursday cleared the $1.5 billion Parker Solar Probe for launch early Saturday on a daring mission to “touch the sun,” repeatedly flying through its outer atmosphere to find out why the blazing corona is millions of degrees hotter than our star’s visible surface.

The demands of launching the first in an upgraded line of U.S. Air Force GPS navigation satellites, including a late load of extra fuel for the spacecraft and a military policy of reserving fuel to eliminate space junk, will keep SpaceX from recovering the first stage of its Falcon 9 rocket following liftoff Thursday from Cape Canaveral, according to mission managers.