Shielded from view within the protective rocket nose cone, a covert surveillance satellite has been mounted atop a Delta 4-Heavy rocket for launch June 4.

After a six-month voyage from Earth, NASA’s InSight Mars lander, streaking through space at at some 12,300 mph, will slam into the thin martian atmosphere Monday afternoon to begin a nail-biting six-and-a-half-minute descent to the surface, kicking off a billion-dollar mission to probe the red planet’s hidden interior.

Hoisting a 15,000-pound satellite atop an Atlas 5 rocket Wednesday, United Launch Alliance finished assembling its next space-bound booster for liftoff Aug. 31 in support of the U.S. Navy.