Over the weekend, SpaceX completed final preparations for the Starship reusable space launch system and its Super Heavy first stage, readying them for their historic first flight test. On Saturday, a flight cancellation system was installed on them again, which will work if the rocket deviates from the intended course. Then, on Sunday, the company conducted a final launch readiness check, giving the go-ahead.

However, despite careful preparation, there are still some factors that can affect the launch. SpaceX said that while weather conditions at the launch site appear favorable, there are concerns about wind shear in the upper atmosphere. Wind shear occurs when the wind changes direction and speed at different levels of the atmosphere, and if severe enough, can cause structural damage to the rocket.

In addition to weather challenges, this is the first attempt to launch a superheavy first stage, and it’s also the first time the Starship booster will fly in a combined configuration. The combined rocket has a total of 39 Raptor rocket engines, each of which will be carefully monitored before launch. Even a slight anomaly in temperature, pressure, or any other variable detected by one of the thousands of sensors can cause the launch attempt to be aborted.

Despite numerous fueling and static tests, this launch system is still brand new. As a result, the probability of successfully reaching T-0 and taking off within the 90-minute launch window is estimated today to be 50% or less, assuming favorable weather conditions.

If the rocket does take off, it will be an amazing sight. No human had ever seen a rocket of such power and with so many first-stage engines in action.

To witness this historic event, you can watch the webcast of SpaceX’s launch attempt, which is expected to begin approximately 45 minutes before launch today, April 17, at 3:15 p.m. ET.