Updated Monday: SpaceX called off its early Monday launch of 21 military data satellites in the last minute of the countdown. The Falcon 9 rocket did not leave the pad at Vandenberg Space Force Base in California. Spaceflight Now reported that the abort came as the rocket was moving into its automated startup sequence. TechGenyz and a launch tracker put the halt at about 58 seconds before liftoff.

SpaceX did not give an immediate reason. Because the stop came before liftoff, nothing was launched and no rocket was lost.

The new target is Tuesday, Oct. 6, at 1:17 a.m. Pacific, which is 4:17 a.m. Eastern. Launch times can still slip for weather or technical checks.

The mission is called Tranche 1 Transport Layer A. It is the fourth flight of working satellites for the Space Development Agency, part of the U.S. Space Force. Northrop Grumman built all 21 spacecraft. Three earlier flights have already put 63 of these satellites in orbit.

The satellites form a network in space that passes data between sensors and troops on the ground, at sea and in the air. They link to each other with laser beams, which can move data faster than older radio links. The agency wants the first part of this system to start serving the military in 2027.

The booster for this flight, known as B1103, is set to make its sixth trip. SpaceX plans to land it back at Vandenberg a few minutes after liftoff, which can cause a sonic boom along the nearby coast.

SpaceX streams its launches live, and launch tracking sites update the countdown as the window nears.

What we know: Monday's attempt stopped in the final minute, and SpaceX is aiming for Tuesday at 4:17 a.m. Eastern.
What we don't: Why the countdown stopped, and whether Tuesday's window will hold.