Sunday evening was scheduled to be SpaceX's third attempt at its second rocket launch of the year. But a series of unexpected setbacks, which began with a boat, ultimately led to an abort for the evening.

It was definitely a close call. The countdown clock actually reached T-00:00:00, and the rocket looked as if it were about to lift off at 7:21 p.m. ET, when the computer in control of the rocket suddenly shut the engines down.

At the very last second, you could actually see the engines begin to fire.

Despite Sunday evening's scrub, SpaceX reported that its Falcon 9 rocket was still healthy.

Why the launch was aborted

The problem began at about 6:45 p.m. ET when a boat strayed too close into the danger zone. The launch was temporarily halted to give the boater time to vacate, and SpaceX was still expecting liftoff, albeit a little later than scheduled.

But in the 40-or-so minutes between waiting for the boat to move and restarting the countdown sequence, something had happened to the rocket's fuel, according to SpaceX founder and CEO Elon Mus 

SpaceX began using a new kind of fuel with its upgraded Falcon 9 rockets last December. The rockets now run on deep cryo liquid oxygen.

This fuel has the benefit of being more dense than other rocket propellants, so you can pack more of it into rocket-fuel tanks--which adds to the power--but the trade-off is that you have to chill it at minus 340 degrees Fahrenheit.

The launch team didn't know, however, that the fuel had warmed by the time the rocket was go for launch. That's why the countdown clock reached T-00:00:00.

The rocket's internal computer kicked in and aborted the launch seconds before liftoff.

The rocket's internal computer takes control of the launch sequence with one minute to go before every SpaceX launch. During that time, it analyzes the fuel, the engines, and other aspects of the rocket to make sure everything is functioning properly. This is standard safety procedure.

SpaceX has yet to establish another launch date to try again.

A historic rocket launch

Sunday's launch could have been SpaceX's chance to make history. The launch itself isn't necessarily historic, but the landing could have been.

Since its first attempt in January 2015, SpaceX has been trying to land the first stage of its Falcon 9 rockets on a floating ocean platform. So far, none of the landings have been what you could call a perfect success.

Sunday's attempt would have been SpaceX's fourth attempt.

This story first appeared on Business Insider