SpaceX scrubbed the launch of its 10th Commercial Resupply Services (CRS) cargo mission to the International Space Station (ISS) today just 13 seconds before liftoff. Two technical problems cropped up with the Falcon 9 rocket during the final phases of the countdown. One was resolved, but the other — involving a steering mechanism on the Falcon 9 rocket’s second stage — worried flight controllers who decided to wait until the problem was better understood. Another launch opportunity exists tomorrow (Sunday) morning, but the company and NASA have not yet announced if they will try to launch at that time.

The Dragon spacecraft on this SpaceX CRS-10 or SpX-10 mission is carrying approximately 5,500 pounds of supplies and experiments to the ISS crew. Among the cargo are 40 mice (jokingly called mousetronauts) that are part of a bone healing experiment conducted by the U.S. Army Center for Environmental Health called Rodent Research IV. They were loaded into Dragon yesterday as part of the “late load” cargo. If the launch does not take place tomorrow, they and other late load items will have to be removed and replaced, so the launch could not occur again until Tuesday at the earliest. However, Russia is launching its own cargo spacecraft, Progress MS-05, early Wednesday morning Eastern Standard Time, so NASA will have to determine how to interweave the schedules.

This will be the first SpaceX launch from NASA’s Launch Complex-39A, which was used for Apollo missions to the moon and space shuttle launches. NASA is leasing the pad to SpaceX. SpaceX also leases launch pads from the Air Force at the adjacent Cape Canaveral Air Force Station (CCAFS) and Vandenberg Air Force Base, California. SpaceX’s prior ISS cargo missions have launched from CCAFS Space Launch Complex-40, but it was badly damaged during a September 1, 2016 explosion that destroyed a Falcon 9 rocket and its Amos-6 commercial communications satellite payload. SpaceX already was preparing LC-39A for launches of Falcon 9 and the Falcon Heavy, which is in development, so was able to rather quickly move this launch to LC-39A. Whenever this launch takes place, SpaceX plans to land the Falcon 9 first stage at a different CCAFS launch complex for a third time. SpaceX routinely tries to recover its first stages so they can be reused. Sometimes they land on autonomous drone ships at sea and sometimes on land depending on the rocket’s trajectory and how much fuel remains after deploying the payload into orbit.

During the countdown this morning, one problem developed with the autonomous flight termination system (FTS) being used as the primary range safety abort system for the first time on a SpaceX launch. Range safety is an Air Force responsibility and the Air Force is transitioning to this new type of automated system for all launches. SpaceX President and COO Gwynne Shotwell said at a press conference yesterday that they have been flying the automated system in “shadow” mode for some time and although they were directed by the Air Force to use it as the primary system for this launch “we would have done it anyway.” NASA Kennedy Space Center Director Bob Cabana said at the same press conference that NASA is in complete agreement with the Air Force. He views the autonomous system as safer and more reliable than the “human-in-the-loop” system that has been used historically. Today’s problem was a software issue that produced “inconsistent data,” but was readily resolved.

The other problem was with a thrust vector control (TVC) system on the rocket’s second stage. The TVC system steers the rocket. The SpaceX team tried to resolve the issue, but decided at T-13 seconds to abort the launch. SpaceX President Elon Musk tweeted in response to a question that he was the one who made the decision.

@Xuan_Thai System was green for launch. I called it off. — Elon Musk (@elonmusk) February 18, 2017

He explained his reasoning in other tweets

If this is the only issue, flight would be fine, but need to make sure that it isn’t symptomatic of a more significant upstream root cause — Elon Musk (@elonmusk) February 18, 2017

Btw, 99% likely to be fine (closed loop TVC wd overcome error), but that 1% chance isn’t worth rolling the dice. Better to wait a day. — Elon Musk (@elonmusk) February 18, 2017

Yesterday, a different problem arose. A small helium leak was discovered in a second stage system that, if it did not work properly, the second stage could not have been deorbited after it placed the Dragon spacecraft into orbit. Rocket stages can pose debris hazards in space if they are not deorbited. SpaceX decided to proceed with the countdown and perform a helium spin-up pressurization test at T-1 minute before liftoff. Musk said today that he did not see a connection between that leak and the TVC problem, but also did not rule it out.